[membrane science and technology] ion exchange membranes - fundamentals and applications volume 12...

Water Dissociation

Chapter 8

8.1. CURRENT–pH RELATIONSHIP

At an over limiting current, a sufficiently depleted layer is formed on thedesalting surface of an ion exchange membrane, an electric current is carried byH+ and OH– ions (derived from the water dissociation) and the transportnumber of an ion exchange membrane is lowered. The water dissociation wasfirst observed by Kressman and Tye (1956) and Frilette (1956). Since then, thisphenomenon is being widely investigated.

Investigation based on the current–pH relationship was proceeded byRosenberg and Tirrel (1957) using seven-cell assemblies indicated in Fig. 8.1.Cell 1 was bounded by an anode and an anion exchange membrane; cell 7 by acathode and a cation exchange membrane; all other cells were bounded bymembranes as shown. Water dissociation was studied in the central cell (cell 4)which was fed with 0.005–0.05M NaCl solution. Other cells were fed with a salt

1NaCl

NaCl

Cathode

Cation memb.

Anion memb.

Cation memb.

Anion memb.

Cation memb.

Anion memb.

Anode

2

3

4

5

6

7

+

–

Na

Na

Na

Cl

Cl

Cl

Figure 8.1 Cell assembly for studying polarization (Rosenberg and Tirrel, 1957).

DOI: 10.1016/S0927-5193(07)12008-8

dx.doi.org/10.1016/S0927-5193(07)12008-8.3d

10

9

8

7

6

5

4

3

Anion l lim Cation l lim

Cell 4

Cell 5

Cell 3

0.6 0.8 1.0 1.2 1.4

Current (A)

pH

Figure 8.2 Electric current–solution pH relationship (Rosenberg and Tirrel, 1957).

Ion Exchange Membranes: Fundamentals and Applications140

3–10 times more concentrated. At a series of increasing applied voltages,current, influent and effluent concentrations in cell 4, and effluent pH values incells 3, 4 and 5 were determined. The cation exchange membranes were sulfon-ated divinylbenzene polystyrene types, and anion exchange membranes werequaternarized divinyl benzene pyridine types. A typical history of pH in cells 3, 4and 5 is given in Fig. 8.2. As current increases, the interface between the anionmembrane and a cell 4 solution becomes depleted sodium chloride. Transfer ofhydroxyl ions from cell 4 to cell 3 causes pH of the effluent to decrease in cell 4and increase in cell 3. At this current, there is no indication of cation exchangemembrane polarization. As the current is increased, however, the cationexchange membrane interface becomes depleted, hydrogen ion transfer from cell4 to cell 5 occurs and the pH in cell 5 falls. However, the transfer of hydroxylions through the anion exchange membrane interface increases at a faster ab-solute rate, and the pH in cell 4 continues to fall.

The mobility of Na+ ions is less than that of Cl– ions in a NaCl solution.Because of this fact, concentration polarization is generally recognized to occurmore easily on the cation exchange membrane than that on the anion exchangemembrane. Because of this reason, it is expected that the water dissociationoccurs more easily on the cation exchange membrane than that on the anionexchange membrane. However, Fig. 8.2 shows that the water dissociation occursmore easily on the anion exchange membrane than that on the cation exchange

Water Dissociation 141

membrane. This phenomenon has been recognized in many investigationsshowing that the water dissociation is strongly suppressed on the cation ex-change membrane. Accordingly, the pH in cell 5 might not decrease due to thewater dissociation of the cation exchange membrane as described in this study,but would decrease due to the water dissociation of the anion membrane ac-companied by hydrogen ion transfer from cell 4 to cell 5. Whether it is the truthor not, the mechanism of water dissociation is not yet clear. Many investigationsintroduced in this chapter will be concerned with this phenomenon.

Water dissociation reaction is known to be related with the auto-catalyticnature of the functional groups in the membrane as will be described in thesucceeding section in this chapter, although its mechanism is not perfectly clearat present. The difference between water dissociation phenomenon of a cationexchange membrane and that of an anion exchange membrane recognized byRosenberg and Tirrel (1957) is closely related with the catalytic nature.

8.2. DIFFUSIONAL MODEL

An ion exchange membrane is assumed to be placed in a strong 1, 1 valentelectrolyte solution. When an over limiting current is passed across the mem-brane, unstirred boundary layer is formed on the desalting surface of themembrane and the following water dissociation is generated in the boundarylayer:

H2O3k1

k2Hþ þOH (8.1)

where k1 and k2 are the forward and reverse equilibrium constants, respectively.Rubinstein (1977) discussed the steady ion transfer based on the diffusional modelby the following mass conservation with the electroneutrality using subscripts 1,2, 3, 4 to represent, respectively, salt cations, salt anions, H+ ions and OH– ions.

Mass conservation:

dJi

dx¼ 0 or Ji ¼ constant ði ¼ 1; 2Þ (8.2)

dJi

dx¼ RW ði ¼ 3; 4Þ (8.3)

where RW is the generation rate of H+ and OH– ions in Eq. (8.1). The ionicfluxes Ji are given by the Nernst–Planck equations:

Ji ¼ Di

dCi

dxDiCi

dFdx

ði ¼ 1; 3Þ (8.4)


Ji ¼ Di

dCi

dxþDiCi

dFdx

ði ¼ 2; 4Þ (8.5)

where F ¼ jzjFc=RT is the dimensionless electric potential.

Electroneutrality:

C1 þ C3 ¼ C2 þ C4 (8.6)

Equations (8.2)–(8.5) were solved by supplementing the boundary conditions inwhich nonpenetrability for co-ions of an ideal ion exchange membrane is pro-vided. The computation introduced the current–voltage relationship includingthe over limiting current regime, and the profiles of electric potential with theprofiles of H+ and OH– ion concentrations in the unstirred layer.

The conclusion introduced in this research does not explain the differ-ence between water dissociation phenomenon of a cation exchange membraneand that of an anion exchange membrane. This is probably because the auto-catalytic nature is not taken into account in RW (Eq. (8.3)).

8.3. REPULSION ZONE

Patel et al. (1977) developed the theoretical model incorporated with‘‘repulsion zones’’ in the solutions adjacent to the membrane surface. Here,the membrane is assumed to be ideally perm-selective, allowing the passage ofonly cations. In the repulsion zones, it is assumed that the repulsive forcesexerted by the fixed membrane charges upon co-ions dominate the situ-ation. Outside the repulsion zones, the rest of the solution boundary layers

Repulsion zone

Anode Cationexchangemembrane

CathodeDilutedboundary

layer

Concentratedboundary

layer

Figure 8.3 Formation of repulsion zones on the membrane surfaces (Patel et al., 1977).


are present, and outside the boundary layers is the bulk solution phase(Fig. 8.3).

The mass conservation Eqs. (8.2)–(8.5) are applied to the steady stateelectrodialysis system. The relationship between the space charge distributionand the electric potential gradient in the repulsion zones is given by the fol-lowing Poisson’s equation:

d2cdx2¼

4pRTzFðC1 þ C3 C2 C4Þ (8.7)

where e is the dielectric constant. However, away from the membrane surfacelarge changes in the electric potential are not expected, so that the assumption ofelectroneutrality Eq. (8.6) is satisfied instead of Eq. (8.7). The generation rate ofH+ and OH– ions is presented by Eq. (8.1).

Using the above-mentioned equations and the Wien effect theory (cf.Section 8.5), the thickness of the repulsion zone rC, in which the recombinationrate of H+ and OH– ions is strongly affected by the local electric field, wascomputed. The results show that for cation exchange membranes, very highconcentrations of H+ ions are built up at the membrane surface during polar-ization. The decrease in the recombination rate of H+ and OH– ions near themembrane is responsible for this.

The feature of this research is that it takes into account of the Wien effect.However, the Wien effect theory does not include the concept of the auto-catalytic effect, so the conclusion does not explain the difference between thewater dissociation phenomenon of the cation exchange membrane and that ofthe anion exchange membrane.

8.4. MEMBRANE SURFACE POTENTIAL

Ion exchange groups in an ion exchange membrane are electrically neu-tralized by counter-ions and the electroneutrality is satisfied in the membrane.At an over limiting current, however, the counter-ions combine with the ionexchange groups on the desalting surface of the membrane phase and dissolveinto the solution phase. In the cation (anion) exchange membrane, the mem-brane surface is charged negatively (positively), the solution being adjacent tothe membrane is charged positively (negatively) and the membrane surface po-tential is formed. The membrane surface potential is equivalent to the spacecharge (Rubinstein and Shtilman, 1979; Rubinstein, 1981) (cf. Section 7.7.3) andthe repulsion zone (Patel et al., 1977) (cf. Section 8.3), and supposedly suppliescounter-ions from the solution phase toward the membrane phase at an overlimiting current. The membrane surface potential was measured as follows(Tanaka and Seno, 1983a).

The ionic types of ion exchange membranes, CMV, ASV and Aciplex K-102, A-102, were adjusted suitably by immersing the membranes into 1 eq dm3

Table 8.1 Ionic types and electrolyte solutions

Cation Exchange Membrane Anion Exchange Membrane

Ionic Type Electrolyte Solution Ionic Type Electrolyte Solution

H+ HCl OH– NaOHNa+ NaCl Cl– NaClK+ KCl Br– NaBrNH4+ NH4Cl I– NaIMg2+ MgCl2 NO3

– NaNO3

Ca2+ CaCl2 HCO3– NaHCO3

Ba2+ BaCl2 SO42– Na2SO4

Ni2+ NiCl2 SO32– Na2SO3

Co2+ CoCl2 S2O32– Na2S2O3

Source: Tanaka and Seno (1983a).


electrolyte solution shown in Table 8.1. The membrane was incorporated with aset up illustrated in Fig. 8.4 with AgCl electrode E1 fixed on the membranesurface. The potential of E1 is equivalent to that at p in the figure. Anotherelectrode E2 (AgCl or Pt) was placed in a cell keeping the distance from themembrane at 1mm. Supplying water or a 1meq dm3 solution of the electrolytein Table 8.1 into both cells in Fig. 8.4, the potential at p on the basis of E2 wasmeasured while mixing the solution intermittently. From chronopotentiometricchanges of the potential at p, V was detected as shown in Fig. 8.5. After that, theset up was reassembled removing the membrane, and V0 was measured withoutmixing. The membrane surface potential cm evaluated by cm ¼ V–V0 is listed inTable 8.2, showing that:

(a)
The membrane surface potential is more increased when the membraneis placed in water than in electrolyte solutions.
(b)
The membrane surface potential on the cation exchange membrane islarger than that on the anion exchange membrane.
8.5. WIEN EFFECT

The Wien effect is the phenomenon generated in an electrolyte solu-tion under very strong electric potential fields of 106–107Vm1. In thiscircumstance, the ionic mobility is increased and Ohm’s law has only a limitedrange of validity (Wien, 1928). In the case of strong electrolytes, this effect hasbeen successfully interpreted as a destruction of the ‘‘ionic atmosphere’’. Theinitial effect is proportional to the square of the field intensity, and for verystrong fields the equivalent conductance approaches a limiting value, which isnot greater than the limiting equivalent conductance for small concentration.

ZE1

SG G

ME2

Z′(a) (c)

Z-Z′

E1

1 mm

M

p

E1

(b)

Potentiometer

E2

i 3 m

m3

mm

20 mm

E2

Figure 8.4 Experimental apparatus for measuring membrane surface potential (Tanakaand Seno, 1983a).


Weak electrolytes, on the other hand, show much enhanced deviation fromOhm’s law; the conductance increases linearly over a considerable range of thefield intensity, and the limit of the increase, if any, corresponds to completedissociation of the total amount of electrolyte present (Wien, 1931). It wasconsidered an open question whether the prevailing theory for the electrostaticinteraction of the ions could account for this increased dissociation of weakelectrolytes.

Onsager (1934) presented a result which was computed on the basis of theinterionic attraction, and discussed its significance based on theory of the Wieneffect. The agreement with the available measurements of conductance in strongfields was considered satisfactory. In addition, the theory allowed some predic-tions concerning the rates of dissociation and recombination of the ions, and theconsiderations involved in this question were extended with equal ease to thecase of high field intensities. The computed relative increase of the dissociation

∆V

V

V0

Time (min)

Pot

entia

l (m

V)

Figure 8.5 Chronopotentiometric change of membrane surface potential (Tanaka andSeno, 1983a).


rate constant is given by the formula:

k1

k0¼ 1þ bþ

b2

3þ

b3

18þ

b4

180þ

b5

2700þ

b6

56; 700þ (8.8)

with

b ¼ 0:09636E

rT2

(8.9)

Here k1 and k0 are the forward equilibrium constant in Eq. (8.1) under theinfluence of an electric field and that without an electric field, respectively. E isthe electric field density (Vm1), er the relative dielectric constant and T theabsolute temperature. In the case of high field intensities (E4108Vm1), Eq.(8.10) is introduced from Eq. (8.8) being used to calculate the effect of theelectric field on the dissociation rate constant.

k1

k0¼

2

p

1=2

ð8bÞ3=4 eð8bÞ1=2

(8.10)

Table 8.2 Membrane surface potential (mV)

Concentration (meq dm3) Selemion CMV Aciplex K-102

0 1 0 1

Counter–ionH+

241.0 52.7 244.8 44.8Na+ 227.3 69.8 241.0 88.8K+

219.6 11.6 216.7 51.9NH4

+207.1 65.2 220.2 46.4

Mg2+ 109.4 12.2 134.2 8.9Ca2+ 113.0 29.7 114.2 4.7Ba2+ 100.6 26.8 109.6 4.0Ni2+ 103.5 15.7 138.0 4.9Co2+ 119.2 17.7 131.8 10.1

Selemion ASV Aciplex A-102

0 1 0 1

CounterionOH– +48.6 +8.8 +100.4 +0.3Cl– +11.7 0.6 +10.5 +0.1Br– +6.2 +0.6 +22.1 +1.5I– +9.6 +5.1 +16.5 +1.1NO3

– +89.9 +3.7 +149.3 +34.8HCO3

– +55.4 +6.0 +102.0 0SO4

2– +22.2 0 +47.1 +16.3SO3

2– +32.7 +1.7 +81.4 +12.3S2O3

2– +30.3 +2.4 +77.0 +63.2

Source: Tanaka and Seno (1983a).


The discussion mentioned above is not concerned with the auto-catalytic waterdissociation reaction but it is applicable to analysis of the mechanism of thewater dissociation generated in the boundary layer adjacent to an ion exchangemembrane.

8.6. PROTONATION AND DEPROTONATION REACTIONS

Simons (1984, 1985) suggested that with anion exchange membranes thewater dissociation is caused by the following catalytic reversible protonation anddeprotonation reactions of weakly basic groups:

BþH2O3k1

k1BHþ þOH (8.11)

BHþ þH2O3k2

k2BþH3O

þ (8.12)

where B is a neutral base such as tertiary or secondary amine groups.


It has been known that when the applied electric field is high enough,Ohm’s law is no longer valid and the conductance of electrolytes increases rap-idly with the field (Onsager, 1934). For weak electrolytes such as H2O, thisphenomenon is known as ‘‘the second Wien effect’’, and the mechanism ofcatalytic water dissociation reaction described above is presented in two steps asfollows (Simons, 1979):

BþH2Oa

3BHþ OHb

3BHþ þOHc

(8.13)

BHþ þH2O3B H3Oþ3BþH3O

þ (8.14)

where BHþ OH and B H3Oþ are encounter pairs in the same solvent

cage. a–b is a chemical transformation process and b–c a separation and en-counter process. Applying the steady state approximation to the encounter pair,the expressions for the overall forward reaction rate constant k

!and the overall

reverse reaction rate constant k

are

k!¼

kabkbc

kba þ kbc; k ¼

kcbkba

kba þ kbc(8.15)

where kab and kba are, respectively, the forward and reverse reaction rateconstants for the chemical transformations. kbc and kcb are, respectively, theforward and reverse rate constants for the separation and encounter processes.The chemical transformation is so fast that the diffusion-controlled process(separation and encounter process) is the rate-determining step, so that we havekba kbc (kba41012 s–1 while kbc is usually between 1010 and 1011 s–1). Thus, theoverall reaction rate constants defined by Eq. (8.15) are simplified as:

k!¼

kabkbc

kba; k ¼ kcb (8.16)

Onsager suggested that only kbc increases with the electric field, whereas kab, kbaand recombination step kcb are independent of the field (Onsager, 1934; Eigenand Maeyer, 1959). We understand from this suggestion that k

!increases with

the electric field, but k

is not affected by the electric field.We shall consider the following two ways by which the water dissociation

reaction rate might be increased by the strong electric field (Simons, 1979):

(1)
In reaction (8.11), kbc depends on the strength of an external field. Inreaction (8.12), the site in B, for recombination with H3O
+, is a lonepair, so that the charge product is effectively negative when the proton issufficiently close to the reaction site. So, kbc would increase with theelectric field intensity for reaction (8.12) also.

(2)
Actual proton transfer is very fast due to the Grotthuss protonic con-duction (Barrow, 1973) as illustrated in Fig. 8.6. The rate-determiningstep seems to be the necessary reorientation of water molecules between

H H H H

H

E

O H O H H+OHB O

H H H H

H O H O H HOH+B O

H H H H

H O H O H HOBH+ O

Figure 8.6 Deprotonation of amino group and proton conduction along a chain oforiented water molecules (Simons, 1979).


successive transfers (structural diffusion). When reaction (8.12) occurs,it is conceivable that there is a bridge of water molecules in the reactionlayer, already favorably oriented for proton transfer by the strong elec-tric field, leading to the increase of kbc ( k

!).

Simons also suggested that quaternary ammonium groups do not bringabout the reaction in Eqs. (8.11) and (8.12). However, the conversion of thequaternary amino groups into the tertiary form creates the reaction of Eqs. (8.11)and (8.12), and accelerates the water dissociation. Water dissociation at cationexchange membranes is generated when the membrane contains weakly acidicgroups such as carboxylic acid according to the following catalytic reaction:

A þH2O3k3

k3AHþOH (8.17)

AHþH2O3k4

k4A þH3O

þ (8.18)

where AH is a neutral acid group such as carboxylic acid.

8.7. HYDROLYSIS OF MAGNESIUM IONS

Oda and Yawataya (1968) investigated water dissociation for SelemionCSG and CMGmembranes in solutions of NaCl andMgCl2. Electric current–pHrelationship in a concentrating cell is presented in Fig. 8.7 showing that a pH shiftreaches as far as 1.3 in an MgCl2 solution; however, it was less than 4 in a NaClsolution. The pH changes in solutions containing Mg2+ ions are accompanied bya deposition of Mg(OH)2 near or on the desalting surface of the cation exchangemembrane; the greater the pH shift, the more the deposit. The transport numberfor H+ ions through the membranes which are generated by the water disso-ciation was determined to be in the range of 10–1 to 100 in the MgCl2 solution.

CMG-A

CMSG

CMS-A

CMSG

CSG-A

CMG-A, CMP-B

CMP-B

0 10 20 30 40 50Current density (mA/cm2)

7

6

5

4

3

2

1

0

pH

Figure 8.7 pH change phenomena at cation exchange membranes in solutions of NaCland MgCl2 (Oda and Yawataya, 1968).


This phenomenon is in contrast to the case in NaCl solutions, where no signifi-cant pH change is recognized.

They suggested the mechanism of water dissociation on the cation ex-change membrane placed in the MgCl2 solution being caused by the followingtendency toward catalytic hydrolysis of Mg2+ ions similar to the protonationand deprotonation reactions exhibited in Eqs. (8.11) and (8.12):

Mg2þ þ 4H2O3k1

k1MgðOHÞ2 þ 2H3O

þ (8.19)

MgðOHÞ2 3k2

k2Mg2þ þ 2OH (8.20)

This fact indicates that Mg2+ ions become an acceptor of OH– ions generatedfrom the water dissociation, accompanied by the successive water dissociationon the membrane surface. The pH shifts intensified in solutions of CaCl2 andNH4Cl are attributed to the hydrolysis character of the Ca2+ and NH4

+ ions,though it is much weaker than that of the Mg2+ ions. Other heavy metal ions,such as Fe2+ and Cu2+, behave in a similar way in an electrodialysis process.

8.8. EXPERIMENTAL RESEARCH ON THE WATER DISSOCIATION

8.8.1 Current Density–pH Relationship

The apparatus in Fig. 8.8 incorporated with ion exchange membranes(Selemion CSG/ASG, Aciplex CK-2/CA-2, Neocepta CL-2.5T/AVS 4T) was

+

G

P P(a)

De C Con

+ −VA

G

P Con A De P(b)

De : Desalting cellCon : Concentrating cell

C : Cation exchange membraneA : Anion exchange membraneP : Partition (agar)

: Ag, AgCl electrode: Potentiometer

G : Gasket

+ −,,

−Vc

Figure 8.8 Cell arrangement (Tanaka, 1974).


assembled and water dissociation was observed from current–pH curves inFig. 8.9, showing that the water dissociation is strongly suppressed on the cationexchange membrane (Tanaka, 1974). Accordingly, water dissociation in a NaClsolution never prevents the deficit of ions at the cation exchange membrane/solution interface and never contributes to generate H+ and OH– ions at theover limiting current on the surface of the cation exchange membrane. On theanion exchange membranes, however, the strong water dissociation is observed.This phenomenon is probably caused by the auto-catalytic water dissociationreaction of quaternary ammonium groups (Selemion AST, Neocepta AVS-4T)or quaternary pyridinium groups (Aciplex CA-3) in the anion exchange mem-branes in spite of the fact that the auto-catalytic mechanism is unknown. It isestimated further that the auto-catalytic water dissociation is not generated atthe outside of the anion exchange membrane but it is generated at the inside ofthe anion exchange membrane because the reaction is caused by the functionalgroups (quaternary ammonium groups or quaternary pyridinium groups)existing in the anion exchange membrane.

The apparatus in Fig. 8.10 was integrated with commercially availablemembranes (Tanaka, 1975). It consisted of a central concentrating cell (Con)partitioned into cells 1–4 by filter papers, desalting cells (De) and electrode cellsbounded by the desalting cells. Effective area of the membranes was reduced to0.367 cm2 using gaskets and 0.5M NaCl solution was put in the electrode cells.

0.1 0.2 0.3

i (A/cm2)

(c)

(b)

(a)

Concentrating cell

4

6

8Anion exchange

memb.

Desalting cell

Concentrating cell

Desalting cell

8

6

4

pH

8

6

4

Cation exchange memb.

Figure 8.9 Electric current–solution pH relationship (Tanaka, 1974).


Further, 0.1M NaCl solutions and 0.05M NaCl solutions were put in the con-centrating and desalting cells, respectively. After an electric current was passedthrough Ag–AgCl electrodes for 30min, the solutions in cells 1–4 were taken outand pH was measured. The experiment was repeated by changing currentdensities.

Ar G

A 1 2 3 4 K

P De De P

F

Con AnodeCathode

Figure 8.10 Electrodialysis apparatus for measuring water dissociation (Tanaka et al.,1982).

Table 8.3 pH changes of NaCl solutions in each cell (Neocepta CH-60T/AFS-4T)

Cell No., i (A cm2) 1 2 3 4

0.02 6.66 6.72 6.62 6.790.04 6.89 6.71 6.64 6.810.06 10.84 8.08 6.67 6.680.08 11.31 10.40 6.91 6.710.10 11.63 10.85 7.10 6.710.12 11.91 11.22 9.01 6.650.14 12.02 11.68 10.33 6.910.16 12.20 11.71 10.79 7.080.18 12.30 11.89 11.01 8.220.20 12.29 11.86 11.11 8.210.30 12.39 12.19 11.63 10.910.40 12.41 12.21 11.92 11.59

Source: Tanaka et al. (1982).


Table 8.3 shows the results obtained using Neocepta CH-60T/AFS-4T. Itshows that the pH in cell 1 increases at first due to water dissociation of theanion exchange membrane (Neocepta AFS-4T), and the pH in cells 2–4 alsoincreases with increasing current density. The pH in cell 4 decreases slightly atfirst owing to the water dissociation of the cation exchange membrane (Neoce-pta CH-60T). However, it begins to increase with the increase in current densitydue to the water dissociation of the anion exchange membrane. The acceleratedwater dissociation generated on the anion exchange membrane in this exper-iment is attributed to the auto-catalytic reaction of the quaternary ammoniumgroups in the anion exchange membrane.

Table 8.4 pH changes of MgCl2 solutions in each cell (Neocepta CH-60T/AFS-4T)

Cell No., i (A cm2) 1 2 3 4

0.008 5.32 5.93 6.03 6.200.020 5.41 6.00 6.11 6.120.032 5.90 6.12 6.13 5.910.040 6.08 6.00 5.92 5.500.052 6.21 5.98 5.82 5.150.060 6.18 5.97 5.65 4.000.072 6.00 4.00 2.50 1.980.080 6.80 3.19 2.09 1.610.100 6.70 2.69 1.86 1.490.160 3.08 1.92 1.60 1.310.240 2.08 1.68 1.48 1.21

Source: Tanaka et al. (1982).


Next, in the above-mentioned experiment, 0.1M MgCl2 solutions and0.05MMgCl2 solutions were put in the concentrating and desalting cells, respec-tively, and measured pH shifts in cells 1–4 as shown in Table 8.4. In this experi-ment, the pH in cell 4 declined to acidic due to the water dissociation of thecation exchange membrane (Neocepta CH-60T), and succeedingly the pH incells 3 and 2 is also declined to acidic with increasing current density. The pH incell 1 seems slightly declined to alkaline because of the water dissociation of theanion exchange membrane (Neocepta AFS-4T). However, it turns to acidic,being affected by the water dissociation of the cation exchange membrane. Thewater dissociation observed on the cation exchange membrane is brought aboutby the Mg(OH)2 precipitated on the desalting surface of the cation exchangemembrane. This phenomenon demonstrates that the water dissociation occurs inthe Mg(OH)2 layer formed at the outside of the cation exchange membrane andit is the auto-catalytic reaction observed by Oda and Yawataya (1968) in theelectrodialysis for Selemion CSG and CMG cation exchange membranes placedin an MgCl2 solution (cf. Section 8.7).

Finally, in the above-mentioned experiment, the ion exchange membraneswere replaced by Selemion CMV/AST and 10 times as much as diluted seawaterand seawater were put in the concentrating and desalting cells, respectively. Themeasured pH changes in cells 1–4 are presented in Table 8.5 showing that the pHin cells 1 and 2 declines to alkaline caused by water dissociation of the anionexchange membrane (Selemion AST). The pH in cell 4 is decreased at elevatedcurrent density due to the water dissociation of the cation exchange membrane(Selemion CMV). The pH in cell 3 is at first increased owing to the water dis-sociation of the anion exchange membrane. However, it starts to decrease atelevated current density, being affected by the water dissociation of the cationexchange membrane. The water dissociation generated on the cation exchangemembrane at elevated current density is caused by the auto-catalytic effect of

Table 8.5 pH changes of diluted seawater in each cell (Selemion CMV/ASV)

Cell No., i (A cm2) 1 2 3 4

0.01 7.75 7.73 7.67 7.520.02 8.36 7.86 7.78 7.230.036 8.46 7.88 7.86 7.320.05 9.88 7.91 7.74 7.330.07 10.47 8.37 7.67 7.140.08 10.08 9.36 7.59 7.130.09 10.14 8.95 7.46 6.850.10 10.51 9.86 7.50 6.670.11 10.15 9.54 7.86 7.110.12 10.62 9.88 8.05 7.160.13 12.02 9.73 7.86 6.830.15 12.25 9.94 7.95 7.000.17 12.43 10.30 8.20 6.960.20 12.65 10.35 8.22 6.650.21 12.55 9.95 9.18 7.420.22 12.63 10.10 9.33 7.160.24 12.94 10.23 9.74 7.470.25 12.78 9.77 1.15 0.840.26 12.03 11.00 9.86 6.230.27 12.93 11.23 9.75 7.360.28 12.24 10.05 1.65 1.250.30 12.73 10.06 1.16 0.95

Source: Tanaka (1975).


Mg(OH)2 precipitated on the cation exchange membrane and its effect is strongerthan that of quaternary ammonium groups in the anion exchange membrane.

8.8.2 Influence of Ionic Electrolytes in a Solution on the Water Dissociation

Reaction

Fig. 8.11a and b illustrates the apparatus for measuring water dissociationof a cation exchange test membrane K* (Aciplex K-102) and an anion exchangetest membrane A* (Aciplex A-102) integrated in the apparatus. The effective areaof the test membranes was reduced to 0.264 cm2 by a gasket, so that waterdissociation occurs more easily on K* and A* than that on the other membranes.D and C are the desalting and concentrating cells, respectively. A 0.1 eq dm3

solution of several chloride salts or sodium salts was supplied to D by a pump atthe rate of 0.1 cm3 s1. The 1 eqdm3 solution of the same electrolytes whichwere supplied to D was added into D0. A 1 eq dm3 NaCl solution was addedinto C (5 cm3) and D00 as well (Tanaka et al., 1982).

After these preparations, an electric current was passed for 10min be-tween Ag–AgCl electrodes. When water dissociation takes place on the surfaceof K* or A* in D, H+ or OH ions generated are transferred into C. SolutionpH in C was confirmed to be almost steady 10min after applying the electric

Cathode Anode

K A KK* A

G

pHcon pHde

D′′ C D D′

(a)

Anode Cathode

A K A* A K

G

pHcon pHde

D′′ C D D′

(b)

Figure 8.11 Electrodialysis apparatus for measuring water dissociation (Tanaka et al.,1982).


current. The electrodialysis experiment was repeated changing current densitiesincrementally. Current efficiencies for H+ ions ZH and OH ions ZOH werecalculated from the pH changes in the solution in C.

Experimental results are shown in Figs. 8.12 and 8.13 (cation exchangemembrane; Aciplex K-102) and in Figs. 8.14 and 8.15 (anion exchange mem-brane; Aciplex A-102). The changes in these figures are illustrated in Fig. 8.16,indicating that the water dissociation does not occur between A and B, Bpresents the limiting current density ilim, and the water dissociation occurs be-tween B and C. Inspecting the experimental results based on the illustrationFig. 8.16, the following phenomena are recognized:

(a)
When the cation exchange membrane is placed in MgCl2 or NiCl2 so-lutions, ZH is increased largely at above limiting current density andviolent water dissociation occurs. The violent dissociation generated atthe cation exchange membrane is caused by the hydroxides precipitated

10−6

10−5

10−4

10−3

10−2

10−1

1

10−2 10−1 1

i (A/cm2)

: NaCl,

: BaCl2 : NiCl2

: CaCl2, : MgCl2, : KCl,

H

Figure 8.12 Current density vs. current efficiency of H+ ions. Cation exchange mem-brane, Aciplex K-102 (Tanaka et al., 1982).


on the cation exchange membrane. However, when the anion exchangemembrane is placed in the same solutions, ZOH does not increase largelyand the violent water dissociation is not detected. These phenomenamean that the intensity of the auto-catalytic water dissociation causedby quaternary pyridinium groups in the Aciplex A-102 anion exchangemembranes is extremely weaker than that caused by metallic hydroxidesprecipitated on the Aciplex K-102 cation exchange membranes.

(b)
When the cation and anion exchange membranes are placed in the othersolutions, the violent water dissociation does not occur. In this situation,the water dissociation of the cation exchange membranes is more mod-erate (suppressed) than that of the anion exchange membranes, indi-cating that the sulfonic acid groups in the cation exchange membranesare seen not to generate the auto-catalytic water dissociation.

10−6

10−5

10−4

10−3

10−2

10−1

10−2 10−1 1

i (A/cm2)

H

: NaCl,

: NaBr, : NaNO3

: Nal, : Na2SO4

: Na2SO3

Figure 8.13 Current density vs. current efficiency of H+ ions. Cation exchange mem-brane, Aciplex K-102 (Tanaka et al., 1982).


The water dissociation arising on the cation exchange membrane in (b) isestimated to be caused by the Wien effect (cf. Section 8.5) which is generatedunder a strong potential drop in the repulsion zone (Patel et al., 1977) (cf.Section 8.3) or the depleted layer (Gavish and Lifson, 1979) (cf. Section 7.7.4)formed at the interface between the membrane and the solution. The potentialdrop is estimated to be caused by the space charge (Rubinstein and Shtilman,1979; Rubinstein, 1981) (cf. Section 7.7.3). The membrane surface potentialmeasurement suggests that the potential drop at the cation exchange membrane/solution interface is larger than that at the anion exchange membrane/solutioninterface (Tanaka and Seno, 1983a) (cf. Section 8.4).

8.8.3 Influence of Low Electrolyte Concentration and High Electric Potential

Field on the Water Dissociation Reaction

The depleted region formed on a desalting surface of a cation exchangemembrane at an over limiting current is termed the depleted layer (Gavish andLifson, 1979) or the repulsion zone (Patel et al., 1977). It carries a space charge

OH

10−1

10−2

10−3

10−4

10−5

10−6

10−2 10−1 1

i (A/cm2)

: NaCI, : CaCI2, : MgCI2,

: NiCI2: BaCI2,: KCI,

Figure 8.14 Current density vs. current efficiency of OH– ions. Anion exchange mem-brane, Aciplex A-102 (Tanaka et al., 1982).


(Rubinstein and Shtilman, 1979; Rubinstein, 1981) or a membrane surface po-tential (Tanaka and Seno, 1983a). In this regime, the electrolyte concentration isdecreased extremely, an electric potential field is increased drastically and waterdissociation is assumed to be accelerated under the influence of the Wien effect(Onsager, 1934). In order to make sure of the phenomena described above,water dissociation generated in diluted solutions was observed under a highelectric potential field as follows (Tanaka and Seno, 1983b).

An electrodialysis unit consisting of cells I and II was assembled as shown inFig. 8.17. A polyvinyl chloride test plate T (Fig. 8.17a) in which a small hole(diameter 1mm, length 1.2mm) was bored at the center was put between cell I andcell II. A hole was bored in a polyvinyl chloride partition P (thickness 12mm), andagar mixed with a 0.1 eqdm3 electrolyte solution was inserted in the hole,and then was allowed to solidify. These partitions P were put as shown in thefigure. A total of 5 cm3 of the various diluted electrolyte solutions (10–2, 10–3, 10–4

OH

10−1

10−2

10−3

10−4

10−5

10−6

10−2 10−1 1

i (A/cm2)

: NaCI, : CaCI2, : MgCI2,

: NiCI2: BaCI2,: KCI,

Figure 8.15 Current density vs. current efficiency of OH– ions. Anion exchange mem-brane, Aciplex A-102 (Tanaka et al., 1982).


and 10–5 eqdm3) was put in cells I and II with one or two droplets of pH indicators(methyl red and bromo thymol blue), and 1 eqdm3 NaCl solutions were added inthe electrode cells. An electric current was passed applying 500V to both ends of thesmall hole in the test plate. The potential gradient in the hole was estimated to be4 105Vm1. These circumstances reproduce the situations at the cationexchange membrane/solution interface when an over limiting current is appliedand the water dissociation occurring in the hole is observed from the color changesof the pH indicators. From the experiment mentioned above, intensities of waterdissociation were classified and are listed in Table 8.6.

When a 0.1 eq dm3 NaCl solution is added in cells I and II with methylred, the color of the bottom of cell I turned to light pink after 2min of passage ofelectric current. This is because H+ ions generated in the hole caused by the mildwater dissociation transfer into cell II. When the NaCl solution with bromothymol blue is electrodialyzed in the same way, the bottom of cell II turned tolight blue, indicating OH– ion transfer due to the mild water dissociation.

10−5

10−4

10−3

10−2

10−1 H

or O

H

A

C

ilim

B

10−2 10−1 1

i (A/cm2)

Figure 8.16 Current density vs. current efficiency of H+ or OH– ions.

Notch

(b)

+

T(a)

1.2 mm

1mm φ

Ar

P PI IIT

+

−

−

T : Test plateI : Chamber III : Chamber IIP : PartitionAr : Agar

: Anode: Cathode

Figure 8.17 Experimental apparatus for measuring water dissociation (Tanaka andSeno, 1983b).


Table 8.6 Intensities of water dissociation

Concentration of Electrolyte (meq dm3) 102 101 1 10

MnCl2 & J J J

CoCl2 & J J &MgCl2

a & J J DNiCl2 & & J J

MgCl2 & & D

AlCl3 & & D

Cr(NO3)2 & & D

CuCl2 & & D

ZnCl2 & & D

NaBr & & D

NaI & & D

NaNO3 & & D

Na2SO4 & & D

LiCl & &

NaCl & &

CaCl2 & &

BaCl2 & &

Na2S2O3 & &

Fe(NO3)3 & D D

CH3 NH2 HCl & D D

CH3C6H4SO3Na & D D

KCl & D

Na2SO3 & D

Note: (J) Violent, (&) mild, (D) extremely mild and ( ) not occurred.Source: Tanaka and Seno (1983b).aBefore the experimental run, Mg(OH)2 was deposited on the surface in the hole of thetest plate.


When a 0.1meq dm3 MnCl2, CoCl2 or NiCl2 solution was added in cells Iand II with methyl red, the bottom of the cell I turned to deep red just after thepassage of electric current. When the same electrolyte solution was electro-dialyzed with bromo thymol blue, the bottom of cell II turned to deep blue. It isestimated that mild water dissociation occurs at first and precipitates Mn(OH)2,Co(OH)2 or Ni(OH)2 crystals in the hole, and then these hydroxides generate theauto-catalytic violent water dissociation.

The phenomenon observed in the MgCl2 solution electrodialysis wasslightly complicated. Namely, the water dissociation was moderate when innersurface of the hole in the test plate was washed with an aqueous HCl solution(MgCl2 in Table 8.6). However, when Mg(OH)2 was precipitated on the innersurface, the violent water dissociation was observed (MgCl2

a in Table 8.6).In the electrodialysis of other electrolyte solutions, violent water disso-

ciation does not occur, but mild or extremely mild water dissociation takesplace. The mild and extremely mild water dissociations observed in this experi-ment are presumably related to the Wien effect which describes the influence of a


strong electric potential gradient of 106–107Vm1 (cf. Section 8.5) (Onsager,1934). The potential gradient 4 105V cm1 appearing in the hole in this ex-periment is rather small compared to the values caused by the Wien effect. So,the phenomena in this experiment are assumed to be governed by the quasi-Wien effect. It is remarkable that the catalytic violent water dissociation occursin MnCl2, CoCl2 and MgCl2 solutions under such a quasi-Wien effect. Further,it is noticed that the primary amine CH3NH2 HCl does not cause the violentwater dissociation.

The above experiment was achieved without ion exchange membranes.However, we can estimate the mechanism of water dissociation generated on thecation exchange membrane from the above experimental results as follows:

Ta

Ins

MgNiCoMnCuFeAl(MnCaZnCaBaCa

NoSo

(a)

ble 8.

olubl

(OH)(OH)2(OH)(OH)(OH)(OH)3OH)3CO3

(OH)2(OH)2SO4

SO4

CO3

te: (Jurce: T

Mild and extremely mild water dissociation reactions are the same as thephenomena already observed on the cation exchange membrane (Figs.8.9, 8.12 and 8.13), and they are presumably caused by the second Wieneffect in the circumstances of low-concentration and high-potential fieldgenerated at the cation exchange membrane/solution interface.

(b)
Violent water dissociation reactions are caused by the auto-catalyticreactions generated by the metallic hydroxides.
In order to make sure the mechanism in (b), the effect of metallichydroxides on the water dissociation was observed using the apparatus inFig. 8.17, where on the inner surface the test plate was notched in four points(Fig. 8.17b). Various insoluble hydroxides synthesized as shown in Table 8.7were inserted in the notches. A total of 1meq dm3 NaCl solutions was put incells I and II and an electric current was passed as described above. The

7 Effect of insoluble salts on water dissociation

e Salts Synthetic Method Intensities of Water Dissociation

2 Commercial J

NiCl2+2NaOH J

2 CoCl2+2NaOH J

2 MnSO4+2NaOH J

2 CuCl2+2NaOH J

Fe(NO3)3+3NaOH J

AlCl3+3NaOH J

MnSO4+NaHCO3 J

Commercial

ZnCl2+2NaOH

Commercial

BaCl2+2NaOH

CaCl2+NaHCO3

) violent and ( ) not occurred.anaka and Seno (1983b).


intensities of the water dissociation are classified and listed in Table 8.7, showingthat Mg(OH)2, Ni(OH)2, Co(OH)2, Mn(OH)2, Cu(OH)2, Fe(OH)2 and Al(OH)2generate the violent water dissociation. MnCO3 is estimated to convert toMg(OH)2 which arises the auto-catalytic violent dissociation in the notches.

8.8.4 Precipitation of Insoluble Metallic Hydroxides on the Membrane Surface

and Generation of the Water Dissociation (Tanaka, 2007b)

Mg(OH)2 was precipitated on the desalting surface of the cation exchangemembrane K* (Selemion CMV) in the apparatus illustrated in Fig. 8.11a. Sup-plying a 0.1M NaCl solution into D, an electric current was passed. Currentdensity i vs. H+ ion current efficiency ZH exhibits stronger water dissociation onthe cation exchange membrane as shown in Fig. 8.18. Next, Mg(OH)2 wasprecipitated on the desalting surface of the anion exchange membrane A* (Se-lemion ASV). Electrodialysis in this situation resulted in the generation ofweaker water dissociation and the dissolution of Mg(OH)2. The mechanism ofthese phenomena is understandable from the illustration in Fig. 8.19, whichshows that Mg(OH)2 layer on the cation exchange membrane is stable becauseOH ions generated by the water dissociation pass through the Mg(OH)2 layer.

10-3 10-2 10-1 10.0

0.1

0.2

0.3

0.4

0.5

i (A/cm2)

Anion exchange membraneSelemion ASV

Cation exchange membraneSelemion CMV

OH

or H

Figure 8.18 Precipitation of Mg(OH)2 on a membrane surface and generation of waterdissociation.

OH−H+

Mg(OH)2layer

H+

Mg(OH)2layer

OH−

Anion exchangemembrane

Cation exchangemembrane

Water dissociation layer

Figure 8.19 Illustration of Mg(OH)2 precipitation on the membrane surface and gen-eration of water dissociation.


However, Mg(OH)2 layer formed on the anion exchange membrane dissolvesbecause H+ ions pass through the Mg(OH)2 layer.

In order to confirm the influence of inorganic substances on the waterdissociation on the cation exchange membrane (Aciplex K-102), the 0.02MNaCl solutions suspending 0.1% (w/v) of Mg(OH)2, Ca(OH)2, Fe(OH)3,Al(OH)3, MgCO3, CaCO3, CaSO4 or SiO2 were fed into D in Fig. 8.11a. Passingan electric current for 10min, ZH was measured in the same manner as describedin Section 8.8.2. The results show that inorganic hydroxides, Mg(OH)2,Ca(OH)2, Fe(OH)2 and Al(OH)3, generate catalytic strong water dissociation;however, CaCO3, CaSO4 and SiO2 are confirmed not to cause strong waterdissociation. In this experiment, ZOH was evaluated on the anion exchange


membrane (Aciplex A-102) feeding Mg(OH)2 into cell D; however, the accel-erated water dissociation was not detected.

Next, the cation exchange membrane (Aciplex K-102) was incorporatedwith D in the apparatus in Fig. 8.11a. Supplying a 0.02M NaCl solution sus-pending 0.1% (w/v) Fe(OH)3 to D and flowing 50mA of an electric current for5min, Fe(OH)3 was attached to the desalting surface of the cation exchangemembrane K*. After that, 0.02 eq dm3 NaCl, diluted seawater (Cl– ion con-centration 0.02 eq dm3) or 0.02 eq dm3 MgCl2 was supplied into D and ZH wasevaluated by applying current density i during 10min. Plotting ZH against i

indicated in Fig. 8.20 shows that ZH is increased due to the water dissociationcaused by the Mg(OH)2 formed by the combining reaction between OH– ionsgenerated in the Fe(OH)2 layer and Mg2+ ions dissolving in the feeding solution.

Finally, Selemion CMV, Aciplex K-102 or Neocepta CH-45T cationexchange membrane was integrated with D in the apparatus in Fig. 8.11a. Sup-plying a 0.02M NaCl solution suspending 0.1% (w/v) Fe(OH)3 and passing anelectric current I for 5min, Fe(OH)3 was deposited on K*. Then, feeding a 0.02MNaCl solution and passing a 0.1A cm2 of electric current, ZH was measured.

H

1.0

0.8

0.6

0.4

0.2

010−1 110−2

i (A/cm2)

0.02 eq/dm3 NaCl

0.02

eq/

dm3 M

gCl 2

Sea water(diluted)

Figure 8.20 Effect of Mg2+ ions on water dissociation generated on a cation exchangemembrane.

0.5

0.3

0.2

0.1

0

0.4

0 20 40 60 80 100

H

Fe(OH)3 quantity (g/cm2)

Selemion CMV Aciplex K-102 Neocepta CH-45T

Figure 8.21 Precipitation of Fe(OH)3 on the surface of a cation exchange membraneand generation of water dissociation.


Further, disassembling the apparatus, the K* membrane was washed in a 1MHCl solution with ultrasonic waves (35 kHz, 15min). Fe components dissolvedinto the solution were analyzed using atomic absorption spectrometry. Changingthe electric current I, the experiment was repeated. The relationship between ZHand the quantity of Fe(OH)3 attached to the membrane surface was measured asindicated in Fig. 8.21, which shows that ZH increases with Fe(OH)3 quantity.

8.8.5 Adhesion of Bacteria on the Membrane Surface and Generation of the

Water Dissociation (Tanaka, 2007b)

Spherical bacilli (C. bacillus, 1 mm diameter, forming white colony) werecollected from the substances attached on the surface of the membrane inte-grated in the electrodialyzer operating in a seawater concentrating plant. Thespherical bacilli were inoculated in a liquid medium (ORI culture medium), inwhich a cation exchange membrane (Aciplex K-102) and an anion exchangemembrane (Aciplex A-102) were immersed to form bacteria layer on the

0.3

0.2

0.1

0

i (A/cm2)

10−2 10−1 1 10

H o

r O

H

blankAciplex K-102

Aciplex A-102

bacteria attached

Figure 8.22 Bacteria attachment on the surface of an ion exchange membrane andgeneration of water dissociation.


membrane surfaces by means of vibration cultivation (281C, 24 h). The bacterialayer formed membranes were incorporated with D in Fig. 8.11. It was elect-rodialyzed for 10min supplying 0.02M NaCl and current efficiency for thewater dissociation (ZH and ZOH) was measured. At the same time, ZH and ZOH

for the bacteria layer nonformed membranes were measured in the same way.The experimental results are shown in Fig. 8.22.

Inspecting Fig. 8.22, ZOH of the anion exchange membrane increases. Thisphenomenon is presumably caused by the promotion of concentration polar-ization in the bacteria layer and acceleration of auto-catalytic water dissociationreaction of quaternary pyridinium groups in the Aciplex A-102 membrane.However, ZH of the cation exchange membrane does not increase. This phe-nomenon indicates that sulfonic acid groups in the Aciplex K-102 membranedo not accelerate the auto-catalytic water dissociation reaction in spite of thepromoted concentration polarization in the bacteria layer.


8.9. WATER DISSOCIATION ARISING IN SEAWATER

ELECTRODIALYSIS (Tanaka, 2007b)

In an electrodialyzer for concentrating seawater, water dissociation causestroubles such as current efficiency decrease, scale formation and membranebreakage. In this section, water dissociation associated with seawater electro-dialysis operated in salt manufacturing plants in Japan is described with someexperimental works performed to prevent the troubles.

Water dissociation is detected by measuring voltage increases in a stackor pH changes in a solution. Observation in disassembled stacks shows that91% of water dissociation starts at p on the desalting surface of a cationexchange membrane and expands toward an anode with OH– ion transfer asillustrated in Fig. 8.23 (Watanabe et al., 1984). Table 8.8 shows the constituents

p

− +

Figure 8.23 Illustration of water dissociation generation in an electrodialyzer(Watanabe et al., 1984).

Table 8.8 Constituents of substances attached to an ion exchange membrane (g per100 g)

Components Cation Exchange Membrane Anion Exchange Membrane

H2O 84.85 85.62Acid solubleFe(OH)3 4.41 3.85Cu(OH)2 0.03 0.03Others 0.82 0.80Total 5.26 4.68

Acid insolubleSiO2 2.51 2.32Fe2O3 1.42 1.14CuO 0.02 0.03Al2O3 0.12 0.34Others 0.50 0.54Total 4.57 4.37

Ignition loss 3.20 3.49

Source: Watanabe et al. (1980).


of the substances attached to the desalting surface of the membrane whichhad been incorporated with an electrodialyzer in a salt manufacturing plant(Watanabe et al., 1980). From the experiment described in Section 8.8.4,Fe(OH)3 attached on the cation exchange membrane supposedly causes violentwater dissociation.

Membranes integrated in electrodialyzers operating in salt manufacturingplants were taken out, and water dissociation current efficiencies for a sulfonicacid type cation exchange membrane (Aciplex K-102) ZH and for a quaternarypyridinium type anion exchange membrane (Aciplex A-102) ZOH were measuredin the same manner as described in Section 8.8.2. Plotting ZH and ZOH againstoperating period t of the membranes in the electrodialyzers gives Fig. 8.24,showing ZH to be less than ZOH at t ¼ 0 year; the water dissociation is hard tooccur on the cation exchange membrane than on the anion exchange membrane

0 4 8 10 12 140. 0

0. 1

0.2

0.3

0 4 8 10 12 140.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Plant A Plant B Plant CYear

Anion exchange membrane

Year

Cation exchange membrane

2 6

2 6

H (-

) O

H (-

)

Figure 8.24 Changes of ZH and ZOH of membranes integrated in electrodialyzers op-erating for long time in salt manufacturing plants.


at the beginning of the operation. This is supposedly because the water disso-ciation intensities in Aciplex K-102 membranes are weaker than the auto-catalytic water dissociation of quaternary pyridinium groups in Aciplex A-102membranes at t ¼ 0. However, ZH increases gradually at first and increasesextremely after t ¼ 6 years. This is presumably due to the auto-catalyticreaction of Fe(OH)3 deposited on the Aciplex K-102 membranes. It is concludedfrom this experiment that cation exchange membranes having been servedfor six years or more should be replaced with new ones to prevent waterdissociation. Based on this suggestion, the membranes were replaced with newones, and then the water dissociation was suppressed and stack disassemblingfrequencies were diminished from 200 to 40 stacks/month (Akoh KaisuiCo., 1984).

Operating duration of an electrodialyzer during stack disassemblingintervals is related to the frequencies of water dissociation occurrences. Inthis case, the operating duration was 20 days or less because the water disso-ciation occurred frequently as shown in Fig. 8.25 (1). Here, cation ex-change membranes (Aciplex K-102) were replaced to new ones at ‘‘a’’ in thefigure, resulting in an extension of operating duration to 40–80 days. Next,the operating duration was extremely shortened to one to three days asshown in Fig. 8.25 (2). Then, anion exchange membranes (Aciplex A-102)were replaced at ‘‘b’’ in the figure; however, the operating duration was un-changed. Thereupon, cation exchange membranes (Aciplex K-102) were re-placed at ‘‘c’’, causing an extension of operation to 40 days. The experi-mental works described above demonstrate that the replacement of cationexchange membranes is effective to prevent water dissociation (Akoh KaisuiCo., 1984).

In periodical disassembling works of an electrodialyzer in a salt manu-facturing plant (cf. Section 1.5.3 in Applications), the membranes are taken outfrom the electrodialyzer and washed by hand using a sponge at an interval ofusually three to four months. However, such an interval is decreased due to theoccurrence of water dissociation. In order to examine the effect of washing forpreventing the water dissociation, cation exchange membrane (Aciplex K-102)samples were cut in the disassembling works of an electrodialyzer in the saltmanufacturing plant. The samples were sponge washed once or 10 times, andwater dissociation was measured in the same manner as described in Section8.8.2. The result showed that washing once is insufficient but 10 times washing iseffective.

The cation exchange membrane samples were washed in a 5M HCl so-lution for 16 h and substances attached to the membrane were removed. Theeffect of the HCl washing was not clear.

The cation exchange membrane samples were washed for 1 and 16 h in amixed solution of 0.1M ammonium citric acid and 0.03M EDTA. The washingeffect was insufficient (cf. Section 14.2.3).

0

20

40

60

80

100

0

20

40

60

80

a: Cation exchange membranes were replacedb: Anion exchange membranes werereplacedc: Cation and anion exchange membranes were replaced

cb

a

(1)

(2)

Ope

ratin

g du

ratio

ns (d

ays)

Ope

ratin

g du

ratio

ns (d

ays)

Figure 8.25 Replacement of ion exchange membranes in a stack and the preventionwater dissociation in an electrodialyzer (Akoh Kaisui Co., 1984).


The cation exchange membrane samples were washed in an HCl solutionapplying ultrasonic waves. The washing effect was remarkable.

From the experiments mentioned above, Fe(OH)3 suspended in a feedingsolution is estimated to invade into an electrodialyzer, fix to cation exchangemembranes (cf. Table 8.8) and form the water dissociation layers. In the waterdissociation layers the extremely strong auto-catalytic water dissociation occurscaused by the Fe(OH)3.

The Fe(OH)3 fixes also to the anion exchange membrane, but it does notinduce the strong water dissociation (Fe(OH)3 dissolves, cf. Fig. 8.19). The waterdissociation on the anion exchange membrane is estimated to be caused by theauto-catalytic reaction due to the quaternary pyridinium (or ammonium) groupsin the membrane. However, the intensity of the reaction is relatively weakcomparing to the auto-catalytic reaction caused by Fe(OH)3 precipitated on thecation exchange membrane.


8.10. MECHANISM OF WATER DISSOCIATION (Tanaka, 2007a)

8.10.1 Water Dissociation Reaction in the Water Dissociation Layer

Under an unapplied electric potential field, the water dissociation is anequilibrium reaction as shown in Eq. (8.21) (Eigen, 1954).

H2O3ka

kbHþ þOH,

ka ¼ 2 105 s1; kb ¼ 1:5 1014 cm3 mol1 s1 ð8:21Þ

where ka and kb are, respectively, the forward and reverse equilibrium reaction rateconstants. Under an applied electric potential field, ka is assumed to increase withthe electric field due to the Wien effect (Wien, 1928, 1931) and the auto-catalyticreaction (Simons, 1984, 1985; Oda and Yawataya, 1968), whereas kb remainsconstant.

From the experiment described in Section 8.8, it seems reasonable toestimate that the water dissociation occurs in the water dissociation layer formednear the membrane surface and the occurrence of the water dissociation is clas-sified as follows assuming a cation exchange membrane (sulfonic acid type) oran anion exchange membrane (quaternary ammonium or quaternary pyridiniumtype) is placed in a NaCl or a MgCl2 solution:

(1)
The phenomena generated in a cation exchange membrane placed in aNaCl solution.
Sulfonic acid groups in the membrane generate the weak auto-catalytic water dissociation. Water dissociation layer is formed at theinside of the membrance.

(2)
The phenomena generated in an anion exchange membrane placed in aNaCl solution.
Quaternary ammonium groups or quaternary pyridinium groups inthe membrane generate the strong auto-catalytic water dissociation.The water dissociation layer is formed at the inside of the membrane.

(3)
The phenomena generated on a cation exchange membrane placed in anMgCl2 solution.
Mg(OH)2 layer is formed on the membrane and generates extremelystrong auto-catalytic water dissociation. The water dissociation layer isformed at the inside of the Mg(OH)2 layer.

(4)
The phenomena generated in an anion exchange membrane placed in anMgCl2 solution.
Mg(OH)2 layer is not formed on the membrane, so the strong auto-catalytic water dissociation does not occur. However, quaternary am-monium groups or quaternary pyridinium groups in the membranegenerate strong auto-catalytic water dissociation. The water dissociationlayer is formed at the inside of the membrane.


in Section 8.9, it is estimated that the fixing of Fe(OH)3 suspended in feeding
On the other hand, from the seawater electrodialysis experiment described
seawater to the cation exchange membrane generates the extremely strong auto-catalytic water dissociation. Further, the fixing of Fe(OH)3 to the anion ex-change membrane does not induce the strong water dissociation, but the strongwater dissociation occurs due to the quaternary pyridinium (ammonium) groupsin the membrane.

8.10.2 Generation and Transport of H+ and OH– Ions in the Water Dissociation

Layer

When an over limiting current passes across an ion exchange membraneplaced in an ionic solution, the water dissociation layer is formed nearthe membrane surface (the inside of the membrane or the metallic hydroxidelayer) as described in Section 8.10.1. We assume here that H+ and OH– ionsare generated and transported in the water dissociation layer as shown inFig. 8.26.

The generation rate of H+ ions sH at x ¼ 0–x is

sH ¼Z x

0

ðkaCH2O kbCHCOHÞdx

¼ kaCH2Ox kb

Z x

0

CHCOH dx ð8:22Þ

- -

Cathode

H OH

0

Water dissociation layer

-JOH

Anode

JH

x

x axis

l

Figure 8.26 Formation and transport of H+ and OH– ions in a water dissociation layer(Tanaka, 2002).


The generation rate of OH– ions sOH at x ¼ x–l is

sOH ¼

Z l

x

ðkaCH2O kbCHCOHÞdx

¼ kaCH2Oðl xÞ kb

Z l

x

CHCOH dx ð8:23Þ

where x is an axis drawn in the water dissociation layer, CH, COH and CH2O theconcentration of H+ ions, OH– ions and H2O at x, and l the thickness of thewater dissociation layer.

H+ and OH– ions generated are transported by electromigration anddiffusion. The transport rate (flux) of H+ ions JH and that of OH ions JOH at xare obtained from the Nernst–Planck equation as follows:

JH ¼ DHdCH

dx

FDHCH

RT

dcdx

(8.24)

JOH ¼ DOHdCOH

dxþ

FDOHCOH

RT

dcdx

(8.25)

where DH and DOH are the diffusion constants of H+ and OH– ions, F theFaraday constant, R the gas constant, T the absolute temperature and c theelectric potential at x.

From the material balance in Fig. 8.26, the following formulae holdbetween the generation and transport of H+ and OH– ions:

sH ¼ JH ¼i

F

ZH (8.26)

sOH ¼ JOH ¼i

F

ZOH (8.27)

where i is the current density, and ZH and ZOH the current efficiencies for H+

and OH ions, respectively. When the electrolyte solution dissolves cations Aand anions B, total current efficiency for water dissociation reaction Z is

Z ¼ ZH þ ZOH ¼ 1 ðZA þ ZBÞ (8.28)

where ZA and ZB are current efficiencies for A and B ions, respectively.The following equations are obtained from Eqs. (8.22)–(8.28):

i

F

Z ¼ kaCH2Ol kb

Z l

0

CHCOH dx (8.29)

i

F

Z ¼ DH

dCH

dx

FDHCH

RT

dcdx

þDOHdCOH

dx

FDOHCOH

RT

dcdx

ð8:30Þ


8.10.3 Concentration Distribution of H+ and OH– Ions in the Water

Dissociation Layer

Multiplication of the concentrations of H+ and OH– ions (CH and COH)by diffusion constants (DH and DOH), respectively, is expressed by the distri-bution of the converted ionic concentrations XH and XOH as polynomials of x asfollows:

XH ¼ DHCH ¼X1n¼0

anxn (8.31)

XOH ¼ DOHCOH ¼X1n¼0

bnðl xÞn (8.32)

From Eqs. (8.30)–(8.32) we obtain

i

F

Z ¼

dXH

dxþ KXH þ

dXOH

dxþ KXOH (8.33)

i

F

Z ¼

X1n¼0

Kfanxn þ bnðl xÞng

ðnþ 1Þfanþ1xn þ bnþ1ðl xÞng ð8:34Þ

where K is the converted electric potential gradient given by

K ¼ F

RT

dcdx¼

F

RTni (8.35)

where n is specific electric resistance (O cm) of the water dissociation layer.Equation (8.34) indicates that (i/F)Z should be independent of x. There-

fore, with the exception of the first term, all other terms in the polynomials inEq. (8.34) can be regarded as zero: setting n ¼ 0 in the first term

i

F

Z ¼ Kða0 þ b0Þ ða1 þ b1Þ (8.36)

Setting n ¼ 1, 2, 3, y in all other terms

K ¼ 2a2xþ b2ðl xÞ

a1xþ b1ðl xÞ¼ 3

a3x2 þ b3ðl xÞ2

a2x2 þ b2ðl xÞ2

¼ ðnþ 1Þanþ1x

n þ bnþ1ðl xÞn

anxn þ bnðl xÞn¼ ð8:37Þ

Substituting x ¼ l in Eq. (8.37), we arrive at

K ¼ 2a2

a1

¼ 3

a3

a2

¼ ¼ ðnþ 1Þ

anþ1

an

¼ (8.38)


From Eq. (8.38), the coefficients in the polynomials are obtained as follows:

a2 ¼K

2

a1 ¼

K

2!

a1

a3 ¼K

3

a2 ¼

K2

3!

a1

an ¼K

n

an1 ¼

Kn1

n!

a1

(8.39)

In the same way, substituting x ¼ 0 in Eq. (8.37), we get

K ¼ 2b2

b1

¼ 3

b3

b2

¼ ¼ ðnþ 1Þ

bnþ1

bn

¼ (8.40)

b2 ¼K

2

b1 ¼

K

2!

b1

b3 ¼K

3

b2 ¼

K

3!

b1

bn ¼K

n

bn1 ¼

Kn1

n!

b1

(8.41)

Using Eqs. (8.39) and (8.41), Eqs. (8.31) and (8.32) are simplified as

XH ¼ a0 þa1

K

Kx

1!þ

K2x2

2!þ þ

Knxn

n!

¼ a0 þa1

K

expðKxÞ 1

ð8:42Þ

XOH ¼ b0 þb1

K

Kð1 xÞ

1!þ

K2ð1 xÞ2

2!þ þ

Knðl xÞn

n!

¼ b0 þb1

K

½expfKðl xÞg 1 ð8:43Þ

Equations (8.42) and (8.43) still include the polynomials a0, a1, b0 and b1. Fur-ther, the converted ionic concentrations XH and XOH and the converted poten-tial gradient K must be restored to CH, COH and nl. The explanation of thisprocess is present in the literature Tanaka (2007a), and we show only CH and


COH introduced from Eqs. (8.42) and (8.43) as follows:

CH ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiDOH

DHC0

HC0OH

rexp

F

2RT

inl

exp F

RT

inl

þ 1 exp

F

RT

inl

expfðF=RTÞinlxg 1

expfðF=RTÞinlg 1

¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiUOH

UHC0

HC0OH

rexp

F

2RTinl ð2x 1Þ

ð8:44Þ

COH ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiDH

DOHC0

HC0OH

rexp

F

2RT

inl

exp F

RT

inl

þ 1 exp

F

RT

inl

expfðF=RTÞinlð1 xÞg 1

expfðF=RTÞinlg 1

¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiUH

UOHC0

HC0OH

rexp

F

2RTinl ð2x 1Þ

ð8:45Þ

where x ¼ x/l, and C0H and C0

OH are the concentrations of H+ and OH– ions atthe outside of the water dissociation layer.

8.10.4 Electric Resistance of the Water Dissociation Layer

When the solution pH reaches a steady value, the concentration of OH–

ions and a pH value at x ¼ 0 in Fig. 8.26 are given by putting x ¼ 0 in Eq. (8.45)as:

COHjx¼0 ¼

ffiffiffiffiffiffiffiffiffiffiDH

DOH

reðF=2RTÞinl (8.46)

pH ¼ 14þ logCOHjx¼0 (8.47)

The electric resistance nl (O cm2) of the water dissociation layer formed near theanion exchange membrane is introduced from Eqs. (8.46) and (8.47) as

nl ¼pH 14 ð1=2Þ logðDH=DOHÞC

0HC

0OH

ðF=2RT Þi log e; i40 (8.48)

The formulae for a cation exchange membrane are as follows:

CHjx¼1 ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiDOH

DHC0

HC0OH

reðF=2RTÞinl (8.49)


pH ¼ logCHjx¼1 (8.50)

nl ¼ pHþ ð1=2Þ logðDOH=DHÞC

0HC

0OH

ðF=2RT Þi log e; io0 (8.51)

8.10.5 Computation of Electric Resistance and H+, OH Ion Concentration

in the Water Dissociation Layer

Anion exchange membranes (Aciplex A-102) and cation exchange mem-branes (Aciplex K-102) were integrated in assemblies in Fig. 8.11 and a NaClsolution or an MgCl2 solution was supplied (cf. Section 8.8.2). pH values in aconcentrating cell obtained by passing an over limiting current are substituted inEqs. (8.48) and (8.51), and nl is computed as indicated in Fig. 8.27. The figureshows that in the NaCl solution, the nl in the cation exchange membrane is lessthan that in the anion exchange membrane. It is estimated that nl relates to theintensity of the water dissociation reaction. In the MgCl2 solution, the nl on thecation exchange membrane is increased considerably. This event is estimated tobe due to the precipitation of Mg(OH)2 on the membrane surface and resultantacceleration of the water dissociation reaction in the Mg(OH)2 layer.

0.0 0.2 0.4 0.6 0.8 1.0 1.20

2

4

6

8

10

12

14

16

18

20

i (A/cm2)

l (

Ω c

m2 )

:0.1eq/dm3 NaCl, :0.1eq/dm3 MgCl2

Cation exchange membrane (Aciplex K-102)

;0.1eq/dm3 NaCl, :0.1eq/dm3 MgCl2

Anion exchange membrane (Aciplew x A-102)

Figure 8.27 Electric resistance of a water dissociation layer (Tanaka, 2002).

C H

COH

0.0 0.2 0.4 0.6 0.8 1.0

CH

, CO

H(M

)

(-)

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

0.388

1.140

0.502

0.871

0.678

0.568

1.343

0.235

2.950

0.083

l (Ω cm2)

i (A/cm2)

Figure 8.28 Concentration distribution of H+ and OH ions in a water dissociationlayer (anion exchange membrane in a NaCl solution).


Concentration distributions of H+ ions, CH, and OH ions, COH, in thewater dissociation layer are calculated as follows using Eqs. (8.44) and (8.45) withnl obtained above. In the electrodialysis of a NaCl solution, CH and COH changesfor an anion exchange membrane are larger than those for a cation exchangemembrane (Figs. 8.28 and 8.29). In the electrodialysis of an MgCl2 solution,however, the CH and COH changes for a cation exchange membrane becomelarger than those for an anion exchange membrane (Figs. 8.30 and 8.31).

8.10.6 Current Efficiency of H+

and OH–Ions Generated in the Water

Dissociation Layer

For obtaining current efficiency of H+ and OH– ions, Z ¼ ZH+ZOH pre-sented by Eq. (8.29), we must know CHCOH, which is introduced by multiplyingEq. (8.44) by Eq. (8.45) as the following simple constant value:

CHCOH ¼ C0HC

0OH (8.52)

Substituting Eq. (8.52) into Eq. (8.29), Z is introduced as the following simpleform, which is equivalent to an integrated form of Eq. (8.3) presented by

0.0 0.2 0.4 0.6 0.8 1.0

C OH

CH

CH

, CO

H (M

)

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

(-)

0.502

0.871

0.405

1.140

0.669

0.568

0.992

0.239

1.070

0.083

1.522

0.050

l (Ω cm2)

i (A/cm2)

Figure 8.29 Concentration distribution of H+ and OH ions in a water dissociationlayer (cation exchange membrane in a NaCl solution).


Rubinstein (1977):

i

F

Z ¼ ðkaCH2O kbC

0HC

0OHÞl (8.53)

ka is given from Eq. (8.53) as follows.

ka ¼ kbC0HC

0OH þ

iZFl

1

CH2O(8.54)

8.10.7 Electric Conductivity and Thickness of the Water Dissociation Layer and

Potential Gradient in the Water Dissociation Layer

Specific electric conductivity in the water dissociation layer l is given bythe following equation assuming H+, OH, Na+ and Cl ions are dissolved in asolution.

l ¼ lZþ lð1 ZÞ (8.55)

0.0 0.2 0.4 0.6 0.8 1.010-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

C H

COH

CH

,CO

H(M

)

ξ (-)

0.551

0.871

0.463

1.140

0.731

0.568

1.380

0.239

3.091

0.080

3.672

0.049

l (Ω cm2)

i (A/cm2)

Figure 8.30 Concentration distribution of H+ and OH ions in a water dissociationlayer (anion exchange membrane in an MgCl2 solution).


lZ and l(1Z) are respectively contribution of H+, OH ions and Na+, Cl

ions to l, and they are respectively presented by the following equations.

lZ ¼ F ðuHCH þ uOHCOHÞ (8.56)

lð1 ZÞ ¼ F ðuNaCNa þ uClCClÞ (8.57)

Substituting Eqs. (8.44) and (8.45) into Eq. (8.56) gives

l ¼2F

Z

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiuHuOHC

0HC

0OH

qcosh

F

2RTinl 2x 1ð Þ

(8.58)

l is the specific electric conductivity at x in the water dissociation layer, and thespecific electric conductivity of the water dissociation layer L is introduced byintegrating l within the range of x ¼ 01 as follows.

L ¼Z 1

0

ldx ¼4RT

Zinl

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiuHuOHC

0HC

0OH

qsinh

F

2RTinl

(8.59)

0.0 0.2 0.4 0.6 0.8 1.010-14

10-12

10-10

10-8

10-6

10-4

10-2

1

C OH

CH

CH

, CO

H(M

)

ξ (-)

2.910

0.240

5.367

0.120

10.994

0.056

18.241

0.032

l (Ω cm2)

i (A/cm2)

Figure 8.31 Concentration distribution of H+ and OH ions in a water dissociationlayer (cation exchange membrane in an MgCl2 solution).


Thickness of the water dissociation layer l is

l ¼ Lnl Ln ¼ 1 (8.60)

L and l are calculated by substituting nl obtained using Eqs. (8.48) and (8.51)into Eqs. (8.59) and (8.60). Potential gradient in the water dissociation layer iscomputed substituting i and L into Eq. (8.61).

dV

dx¼

i

L(8.61)

8.10.8 Water Dissociation Reaction Generated in the Water Dissociation Layer.

Based on the theory and experimental results using the apparatus shownin Fig. 8.11, L, l, dV/dx and the forward reaction rate constant ka are calculatedusing the following process.

(a)
nl is calculated using Eqs. (8.48) or (8.51). (b) L is calculated using Eq. (8.59). (c) l is calculated using Eq. (8.60)

Figon$an


(d)

ure 8a cat%: dion ex

dV/dx is calculated using Eq. (8.61).
(e) ka1 observed in the electrodialysis experiment is calculated using
Eq. (8.54).
(f) ka2 caused by the second Wien effect is calculated using Eqs. (8.8) and
(8.9) putting E ¼ dV/dx.

Fig. 8.32 shows the current density dependence of the water dissociationreaction on the Selemion CMR cation exchange membrane and the ASR anionexchange membrane. It is seen in this figure that ka1 for both membranes isincreased with current density. ka1 for the anion exchange membrane is largerthan that for the cation exchange membrane. ka2 for both membranes is inde-pendent to current density and identical to 2.70 105 (s1) corresponding theforward reaction rate constant at i ¼ 0. This phenomenon means that the in-tensity of the second Wien effect is negligible for both membranes. In otherwords, the increase of the forward reaction rate constant ka1 observed in theelectrodialysis is due to the auto-catalytic reaction of the functional groups(quaternary ammonium groups in the ASR membrane or sulfonic acid groups inthe CMR membrane) and is not due to the second Wien effect, and further itmeans that the intensity of the auto-catalytic reaction of quaternary ammoniumgroups is stronger than that of sulfonic acid groups.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

2

4

6

8

10

12

14

16

0

2

4

6

8

10

12

14

16Λ

(10-3

S/cm

), l(

10-5m

), d

V/d

x(10

3 V/m

)

k a1(

10-31/

s),k

a210

-51/

s)

i (A/cm2)

.32 Water dissociation reaction generated in a water dissociation layer formedion and an anion exchange membrane. JK: ka1, Wm: ka2, &’: L, BE: l,V/dx Open: Selemion CMR cation exchange membrane Filled: Selemion ASRchange membrane 0.01M NaCl.


L for the ASR anion exchange membrane is less than that for the CMRcation exchange membrane. l in both membranes is independent of currentdensity. l in the anion exchange membrane (4.8 105m) is less than that in thecation exchange membrane (8.4 105m). dV/dx in both membranes tends toincrease with current density, and the values in the anion exchange membraneare larger than those in the cation exchange membrane. It is estimated fromthe above calculation that the second Wien effect does not work because of lowdV/dx values (103104V/m) which is caused by large l values in the membrane.

REFERENCES

Akoh Kaisui Co., 1984, Technical information.Barrow, G. M., 1973, Physical Chemistry, McGraw-Hill, New York.Eigen, M., 1954, Method for investigation of ionic reactions in aqueous solutions with

half-times as short as 109s, application to neutralization and hydrolysis reactions,Discuss. Faraday Soc., 17, 194–205.

Eigen, M., Maeyer, L. D., 1959, Hydrogen bond structure, proton hydration, andproton transfer in aqueous solution, In: The Structure of Electrolytic Solutions, Wiley,New York.

Frilette, V. J., 1956, Preparation and characterization of bipolar ion-exchangemembranes, J. Phys. Chem., 60, 435–439.

Gavish, B., Lifson, S., 1979, Membrane polarization at high current densities, JCSFaraday Trans. I, 75, 463–472.

Kressman, T. R. E., Tye, F. L., 1956, The effect of current density on the transport ofions through ion-selective membranes, Discuss. Faraday Soc., 21, 185–292.

Oda, Y., Yawataya, T., 1968, Neutrality-disturbance phenomenon of membrane–solution systems, Desalination, 5, 129–138.

Onsager, L., 1934, Deviation from Ohm’s law in weak electrolyte, J. Chem. Phys., 2,599–615.

Patel, R. D., Lang, K. C., Miller, I. F., 1977, Polarization in ion-exchange membraneelectrodialysis, Ind. Eng. Chem. Fundam., 16(3), 340–348.

Rosenberg, N. W., Tirrel, C. E., 1957, Limiting currents in membrane cells, Ind. Eng.Chem., 49(4), 780–784.

Rubinstein, I., 1977, A diffusion model of ‘‘water splitting’’ in electrodialysis, J. Phys.Chem., 81(14), 1431–1436.

Rubinstein, I., 1981, Mechanism for an electrodiffusional instability in concentrationpolarization, J. Chem. Soc., Faraday Trans. II, 77, 1595–1609.

Rubinstein, I., Shtilman, L., 1979, Voltage against current curves of cation-exchangemembranes, Faraday Trans. II, 75, 231–246.

Simons, R., 1979, Strong electric field effects on proton transfer between membrane-bound amines and water, Nature, 280, 824–826.

Simons, R., 1984, Electric field effects on proton transfer between ionizable groups andwater in ion exchange membranes, Electrochim. Acta, 29(2), 151–158.

Simons, R., 1985, Water splitting in ion exchange membranes, Electrochim. Acta, 30(3),275–282.

Tanaka, Y., 1974, Concentration polarization and dissociation of water in ion exchangemembrane electrodialysis, J. Electrochem. Soc., Jpn., 42(9), 450–456.


Tanaka, Y., 1975, Concentration polarization and dissociation of water in the ionexchange membrane electrodialysis, J. Electrochem. Soc., Jpn., 43(10), 584–588.

Tanaka, Y., 2007a, Water dissociation in reaction generated in a water dissociation layerformed on an ion exchange membrane, J. Membr. Sci., the article in submitting.

Tanaka, Y., 2007b, Acceleration of water dissociation on ion exchange membranes,J. Membr. Sci., the article in submitting.

Tanaka, Y., Matsuda, S., Sato, Y., Seno, M., 1982, Concentration polarization anddissociation of water in ion exchange membrane electrodialysis III. The effects ofelectrolytes on the dissociation of water, J. Electrochem. Soc., Jpn., 50(8), 667–672.

Tanaka, Y., Seno, M., 1983a, The concentration polarization and dissociation of water inion exchange membrane electrodialysis. V. The acceleration of ionic transport on themembrane surface, J. Electrochem. Soc., Jpn., 51(2), 267–271.

Tanaka, Y., Seno, M., 1983b, Concentration polarization and dissociation of water in ionexchange membrane electrodialysis. VI. The effects of insoluble inorganic materials onthe dissociation of water under conditions of low concentration and high potential,J. Electrochem. Soc. Jpn., 51(6), 465–470.

Watanabe, I., Morimoto, T., Kamaya, M., Tuzura, K., Kawate, H., 1984, Research onion exchange membranes (Part 2), Investigation on water dissociation of ion exchangemembranes, Presented at the 35th annual meeting, Society of Sea Water Science, JPM.

Watanabe, T., Hiroi, K., Azechi, A., Tanaka, Y., Fujimoto, Y., 1980, Concentrationprocess by ion exchange membrane method, Bull. Soc. Sea Water Sci. Jpn., 34(2),61–90.

Wien, M., 1928, Uber die abweichungen der elektrolyte vom Ohmschen gesetz, Phys.Zeits, 29, 751–755.

Wien, M., 1931, Uber leitfahigkeit und dielektrizitatskonstante von elektrolyten beihochfrequenz, Phys. Zeits, 32, 545.

[membrane science and technology] ion exchange membranes - fundamentals and applications volume 12...

Documents