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

Membrane Property Measurements

Chapter 2

Ion exchange membranes were developed at first for advancing the elect-rodialysis technology of seawater and brackish water, which have been a mainsubject to be discussed at present. Because of this reason, the membrane prop-erty measuring method is standardized referring to electrodialysis of seawaterand brackish water (Kosaka and Emura, 1963; Yamabe and Seno, 1964; Take-moto, 1966; Seno and Tanaka, 1984). Now, the application of ion exchangemembranes is extended to many fields such as electrodeionization, bipolarmembrane electrodialysis, electrolysis, fuel cells etc. The circumstances in whichthe ion exchange membrane is placed differ in each field, so that the measuringconditions defined in this chapter might not be reasonable in the other fields.However, the method described here is assumed to be applicable to the otherfields by changing appropriately the measuring conditions, because the basictheory of the measuring method is universal.

2.1. SAMPLING AND PRETREATMEMT OF MEMBRANES

A membrane sheet about 5 cm� 5 cm breadth is cut off, confirmed pin-holes not to be detected and preserved in a 0.5M NaCl solution for over one dayand night, and then pretreated as follows.

DO

(1)

I: 10.10

Cation exchange membrane: A sample membrane is immersed in a 4%HCl solution for 2 h. In the meantime, the solution is stirred every10min, renewed every 20min, and then washed three to four times withwater. Next, it is immersed in a 2M NaCl solution for 2 h; meanwhile,the solution is stirred every 10min, renewed every 20min, and thenwashed with a 0.5M NaCl solution three to four times and immersed ina 0.5M NaCl solution.

(2)
Anion exchange membrane: A sample membrane is immersed in a 4%NaOH solution or 4% NH4OH solution for 1 h. In the meantime, thesolution is stirred every 10min, renewed every 20min and washed three tofour times with water. Next, the membrane is washed with a 0.5M NaClsolution three to four times and immersed in a 0.5M NaCl solution.
16/S0927-5193(07)12002-7

dx.doi.org/10.1016/S0927-5193(07)12002-7.3d

Ion Exchange Membranes: Fundamentals and Applications18

2.2. ELECTRIC RESISTANCE

2.2.1 Alternating Current Electric Resistance

Electric resistance is practically important because this parameter relatesdirectly to energy consumption in an electrodialysis process. The relationshipbetween electric resistance R (O), area S (cm2), thickness d (cm) and specificresistance n (O cm) is expressed as follows:

R ¼ndS

(2.1)

where n is a membrane characteristic and equivalent to R at d ¼ 1 cm andS ¼ 1 cm2. Practical electric resistance r ¼ nd (O cm2) is equivalent to r atS ¼ 1 cm2.

Electric resistance is measured as follows. A membrane sample is im-mersed in a 0.5M NaCl solution for about 2 h and the membrane surface iswiped with a filter paper, and then it is incorporated with the apparatus shownin Fig. 2.1 (effective membrane area: 1 cm2, electrodes: platinum black). A 0.5MNaCl solution is supplied into the cells in the apparatus, which is left in a 251Cthermostat. After temperature in the cells becomes 251C, electric resistance (R1)is measured with an alternating current bridge (frequency: 1000Hz). Next, themembrane sample is taken away, the apparatus is re-integrated without a mem-brane, and electric resistance (R2) is measured. n is calculated substitutingR ¼ R1�R2, S ¼ 1 cm2 and d measured with a micrometer into Eq. (2.1).

2.2.2 Direct Current Electric Resistance

The electric resistance of an ion exchange membrane described in Section2.2.1 is measured passing an alternating current, and is referred here as thealternating current electric resistance ralter. In the electrodialysis of an electrolytesolution, a direct current is passed across the membrane, so that the electricresistance in this circumstance which is referred as the direct current electricresistance rdire is different from ralter. Accordingly, it is necessary to estimaterdire from ralter, which is explained in this section (Tanaka, 2000).

An ion exchange membrane is set in a two-cell apparatus (Fig. 2.1), and alow-concentration NaCl solution (specific conductivity, klow (mS cm�1)) is sup-plied into the cells. Electric resistance of the membrane (r0dire (O cm2)) is meas-ured at 251C applying a direct current. The relationship between klow andr0dire=ralter is expressed by the following empirical equation:

logr0direralter

� �¼ a1 þ a2 log klow þ a3ðlog klowÞ

2 (2.2)

Next, a low-concentration NaCl solution is supplied to the desalting sideand a high-concentration NaCl solution (specific conductivity, khigh) is suppliedto the concentrating side of the apparatus (Fig. 2.1). The electric resistance ofthe membrane rdire is measured at 251C by applying a direct current and

1. Pt electrode 2. Pt wire 3. Terminal4. Solution injection hole; Thermometer inserting hole5. Air extracting hole 6. Rubber gasket

1

2

445

3

24

6 6

Figure 2.1 Electric resistance measuring apparatus (Takemoto, 1966).

Membrane Property Measurements 19

subtracting the effect of membrane potential. The empirical relationship be-tween khigh/klow and rdire=r

0dire is obtained as follows:

rdirer0dire

¼ 1:000þ b logkhighklow

� �(2.3)

rdire is estimated from ralter measured in Section 2.2.1 using rdire/ralter obtainedby multiplying Eq. (2.2) by Eq. (2.3). It should be noticed that rdire includes theelectric resistance of a boundary layer formed on the desalting surface of themembrane due to the concentration polarization.

2.3. ION EXCHANGE CAPACITY AND WATER CONTENT

A membrane consists of cross-linked polyelectrolytes and forms gel struc-ture that absorbs water in an aqueous solution. The water content of an ion


exchange membrane is defined as the weight of water included in a 1 g swelledmembrane in pure water (g H2O (g wet membrane)�1). The ion exchange ca-pacity is defined as milli-equivalent of ion exchange groups included in a 1 g drymembrane (meq (g dry membrane)�1). The ion exchange group concentration isobtained from the ion exchange capacity divided by the water content in 1 g drymembrane (meq (g H2O)�1).

In order to measure the ion exchange capacity of a cation exchangemembrane, a sample membrane is left at first in a 1M HCl solution for over 6 h,changing the solution and converting perfectly the exchange groups to an Htype, and then it is washed with water sufficiently until the washed water doesnot exhibit acidity recognized by the reaction with methyl red. Next, the mem-brane is immersed in a 2M NaCl solution (about 30ml) which is changed twotimes every 1 h and is further immersed in a 2M NaCl solution for over 6 h andwashed sufficiently with water. The immersed solutions and washed water arecollected, and finally, H+ ions dissolved into the collected solution are analyzedwith a 0.1M NaOH and a phenolphthalein indicator (a meq).

In order to measure the ion exchange capacity of an anion exchangemembrane, a sample membrane is left at first in a 2M NaNO3 solution for30min.Then it is immersed in a 2M NaCl solution for over 6 h, changing the 2MNaCl solution until the exchange groups convert perfectly to a Cl type. Next, theCl type membrane is washed with water until the washed water does not exhibitwhite muddiness owing to the reaction with AgNO3. Next, the membrane isimmersed in a 2M NaNO3 solution (about 30ml) which is changed two timesevery 1 h and is further immersed in a 2M NaNO3 solution for over 6 h andwashed sufficiently with water. The immersed solutions and washed water arecollected, and Cl� ions dissolved into the collected solution and the washed waterare analyzed with a 0.1M AgNO3 and a K2CrO4 indicator (a meq).

In order to measure the water content, the moisture adhered on the samplemembrane is wiped with a filter paper and the membrane is weighed (d g). Themembrane is dried in a 651C thermostat until the weight becomes constant (c g).

The ion exchange capacity AR and water content W are calculated asfollows:

AR ðmeq ðg dry membraneÞ�1Þ ¼

a

c

W ð%Þ ¼100ðd � cÞ

c

2.4. TRANSPORT NUMBER

When an ion exchange membrane is placed in an electrolyte solution,concentration ratio of counter-ions in the membrane is extremely larger than


that of co-ions, so that the greater part of an electric current is carried across themembrane by counter-ions. The transport number of an ion exchange mem-brane ti is defined as a ratio of an electric current Ii carried by specific ions i

against total electric current I:

ti ¼I i

I(2.4)

An electrolyte solution dissolves cations and anions. A cation and ananion exchange membrane pass mainly cations and anions, respectively. Thisevent is represented by the following equation introduced from Eq. (2.4):

tþ ¼zþJþ

zþJþ þ z�J�

t� ¼z�J�

zþJþ þ z�J�

(2.5)

where z is the electric charge of cations (+) and anions (�), and J the ionic fluxacross the membrane.

The transport number is classified into the apparent transport number andthe actual transport number. The apparent transport number is applied widelybecause it is measured easily. However, the actual transport number is impor-tant for theoretical discussion of transport phenomena.

2.4.1 Apparent Transport Number

When both surfaces of an ion exchange membrane are in contact with asolution of different concentrations dissolving monovalent cations and anions,membrane potential E generated between the solutions contacting with bothmembrane surfaces is expressed by the following Nernst equation:

E ¼ ðtþ � t�ÞRT

F

� �ln

a2

a1

� �(2.6)

where tþ and t� are the apparent transport number of a cation and an anionexchange membrane, tþ þ t� ¼ 1; R the gas constant, T the absolute temper-ature and a1/a2 the electrolyte activity ratio in the solutions in contact with bothmembrane surfaces.

The apparent transport number is measured using a two-cell apparatus(Fig. 2.2), in which a sample membrane immersed in a 1M NaCl for over 2 h isintegrated between the cells. A 251C 0.5M NaCl solution and a 2.5M NaClsolution are poured into the cells, and the apparatus is left for about5min.Membrane potential (a V) is measured by connecting both cells and ca-lomel electrodes through KCl bridges and letting both solutions flow towardthe membrane surface. The apparent transport number is calculated using the

1. Membrane 2. Solution inlet 3. Solution outlet4. Salt bridge inserting hole 5. Rubber gasket

45 2

4

3

1

Figure 2.2 Apparent transport number measuring apparatus (Takemoto, 1966).


following equations introduced from Eq. (2.6):

t ¼aþ b

2ab

b ðVÞ ¼ 0:5915 logc1f 1c2f 2

� �

where c1 and c2 are the NaCl concentrations in both cells (M) and f1 and f2 theNaCl activity coefficients in both cells.

2.4.2 Actual Transport Number

A sample membrane is incorporated with a two-cell apparatus as shown inFig. 2.3 (effective membrane area: 4 cm2). Accurately measured 30 cm3 of a 0.5MNaCl solution is put in a desalting cell and 30 cm3 of a 0.5M NaCl is put in aconcentrating cell. Next, Ag–AgCl electrodes washed sufficiently with water andleft for over one night are put in both cells. The apparatus is set in a 251Cthermostat, and an electric current of 40mA (1A dm�2) is passed for 40min and10 s (1mF) through the electrodes, making ions in the desalting cell transfertoward the concentrating cell. Then, the solution in the concentrating cell isdiscarded and that in the concentrating cell is taken out into a 250 cm3 measuringflask. Further, the inside of the desalting cell is washed sufficiently with water andthe washed water is collected into a 250 cm3 measuring flask. The electrode(anode) used in the desalting cell is washed sufficiently with water and immersedin water in a 30 cm3 beaker. Both washed water and immersed water are collectedinto the 250 cm3 measuring flask. Finally, the volume of the solution in the

1. Membrane 2. Desalting cell 3. Concentrating cell4. Stirrer 5. Ag-AgCl electrode 6. Rubber gasket

6

56 4

1

2 3

Figure 2.3 Actual transport number measuring apparatus (Takemoto, 1966).


measuring flask is adjusted to 250 cm3 by adding water, and the quantity of Cl�

ions in the solution is analyzed using an AgNO3 standard solution. The actualtransport number is calculated using the following equations:

Transport number of a cation exchange membrane; tþ ¼ a� b

Transport number of an anion exchange membrane; t� ¼ 1þ a� b

where a is the analytical value of Cl� ions in the desalting cell before passing anelectric current (meq) and b the analytical value of Cl� ions in the desalting cellafter passing an electric current (meq).

In case of transport number evaluation of a cation exchange membrane,NaCl concentration in the desalting cell decreases and that in the concentrating cellincreases during passage of electric current, resulting in NaCl diffusion across themembrane. In order to decrease the experimental error due to the event describedabove, the NaCl concentration in the desalting cell is adjusted beforehand at largerthan 0.5M, and that in the concentrating cell is at less than 0.5M. Further, theNaCl concentrations in both cells are devised to be reversed during passage ofelectric current and the experimental error to become a minimum. In case of ananion exchange membrane, the device mentioned above is not necessary.

2.5. SOLUTE PERMEABILITY COEFFICIENT

When an ion exchange membrane is placed in an electrolyte solution andthe ionic concentration difference between both surfaces of the membrane is


maintained at DC, ions in a concentrating side diffuse toward a desalting side.The ionic flux JS in this system is represented by the following Fick’s diffusionequation:

JS ¼DS DC

d(2.7)

where d is the thickness of the membrane and DS the diffusion constant. DS/d isdefined as the solute permeability coefficient, which is measured as follows.

A sample membrane immersed into a 0.5M NaCl solution is washed withwater, wiped with a filter paper and integrated in the two-cell apparatus asshown in Fig. 2.4 (effective membrane area: 4 cm2). Accurately measured 15 cm3

of water is put in the desalting cell and a 4M NaCl solution is supplied into theconcentrating cell so that the solution level in both cells is the same. The ap-paratus is left in a 251C thermostat for 30min exactly, during which the solutionin the desalting cell and that in the concentrating cell are stirred violently by airblowing and stirrer, respectively. Afterward, the solution in the desalting cell istaken out immediately and NaCl concentration is measured using a flame spec-trochemical analyzer (a M). The solute permeability coefficient is calculated

1. Membrane 2. Desalting cell 3. Concentrating cell4. Stirrer 5. Air blowing inlet 6. Rubber gasket

1

3 2

46

5

Figure 2.4 Solute permeability coefficient measuring apparatus (Takemoto, 1966).


using the following equation:

DW

dðcm s�1Þ ¼

15a

4� 4� 1800

2.6. ELECTROOSMOTIC COEFFICIENT

When ions are transferred across an ion exchange membrane, a solution istransported under applying current density i. The solution flux JV is expressedby the function of the electroosmotic coefficient b as follows:

JV ¼ bi (2.8)

A sample membrane is immersed for over a whole day and night in a0.5M NaCl solution. b is measured using a two-cell apparatus (Fig. 2.5) in

1. Membrane 2. Measuring cell 3. Membrane holding plate4. Pipette holding hole 5. Solution injecting hole 6. Cock7. Electrode inserting hole 8. Air extracting hole 9. Rubber gasket

2 27

1

5

7

5

4

8

3

8

9

6

Figure 2.5 Electroosmotic coefficient measuring apparatus (Takemoto, 1966).


which the sample membrane is incorporated with Ag–AgCl electrodes. Theapparatus is placed in a 251C thermostat. Electroosmotic coefficient of a cationexchange membrane is measured, putting a 251C 0.5M NaCl solution in bothcells and an electrode (cathode) in the measuring cell, and then applying currentdensity of 1 A dm�2. In this case, NaCl concentration in the measuring cellincreases and that in the other cell decreases during passage of electric current,resulting in solution permeation across the membrane due to an NaCl concen-tration difference generated in the cells. In order to decrease the experimentalerror due to the event described above, the NaCl concentration in the measuringcell is adjusted beforehand at less than 0.5M, and that in the other cell is atlarger than 0.5M. Further, the NaCl concentrations in both cells are devised tobe reversed during passage of electric current, resulting in minimum experi-mental error (cf. Section 2.5). In case of an anion exchange membrane, thedevice mentioned above is not necessary. The solution volume increase in themeasuring cell is measured accurately for 60min after 10min from the begin-ning of passage of electric current using a measuring pipette (a cm3). b is cal-culated using the following equation:

b ðcm3 A�1 s�1Þ ¼ð102=3600Þa

S

where S is the effective membrane area (cm2).Temperature changes and bubbles generated from a cathode during pas-

sage of electric current cause an experimental error, so the temperature must beadjusted accurately and the cathode should be prepared perfectly to be an AgCltype.

2.7. WATER PERMEATION COEFFICIENT

When an electrolyte concentration difference DC is established betweenboth membrane surfaces, water permeates across the membrane owing to os-motic pressure. Water flux JW in this situation is expressed by the followingequation:

JW ¼DW DC

d(2.9)

where d is the thickness of the membrane and DW the concentration differencepermeation constant. DW/d is defined as the water permeability coefficient,which is measured as follows.

A sample membrane immersed into a 0.5M NaCl solution is washedwith water, wiped with a filter paper and integrated in the two-cell apparatus(Fig. 2.6). Then 251C water is put in the measuring cell, and a 4M NaClsolution is put in the other cell so that the solution level in both cells is the

1. Measuring cell 2. Membrane holding plate 3. Magnetic stirrer4. Pipette holding plate 5. Solution injecting hole 6. Cock7. Air extracting hole 8. Rubber gasket

57 7

3

8

3

6

4

1

5

1

4

7 7

2

Figure 2.6 Water permeation coefficient measuring apparatus (Takemoto, 1966).


same. The apparatus is left in a 251C thermostat and the solutions in both cellsare stirred violently using magnetic stirrers. After 10min, a decrease of thevolume in the measuring cell is observed for 60min using a measuring pipette(a cm3). The water permeation coefficient is calculated using the followingequation:

DW

dðcm4 s�1 mol�1

Þ ¼a� 103

4� 3600S


2.8. SWELLING RATIO

The measuring procedure is as follows:

(1)
A sample membrane is immersed in a 0.5M NaCl solution for 24 h,during which the solution is substituted more than three times.
(2)
A vertical (a mm) and a horizontal (b mm) length of the membrane aremeasured using a ruler, and a thickness (c mm) is measured using amicrometer.
(3)
The sample membrane is immersed in a 3.5M NaCl solution for 24 h,during which the solution is substituted more than three times.
(4)
A vertical (a0 mm) and a horizontal (b0 mm) length and a thickness (c0
mm) is measured as described in (2).
(5) The swelling ratio is calculated as follows:
Vertical swelling ratio ð%Þ ¼ða� a0Þ � 100

a

Horizontal swelling ratio ð%Þ ¼ðb� b0Þ � 100

b

Thickness swelling ratio ð%Þ ¼ðc� c0Þ � 100

c

2.9. MECHANICAL STRENGTH

A sample membrane is immersed in a 0.5M NaCl solution for 24 h, duringwhich the solution is substituted more than three times. Then, the burstingstrength and tensile strength are measured as follows.

2.9.1 Bursting Strength

A sample membrane sheet (about over 6 cm� 6 cm width) is fixed into aMullen tester and inflated injecting glycerol. The maximum pressure when themembrane bursts is the bursting strength.

2.9.2 Tensile Strength

A sample membrane sheet (about 10 cm length� 1 cm width) is fixed to aSchopper’s tension tester. The tensile strength is the maximum strength when thesample is cut off. The tensile strength value is influenced by the thread (rein-forcement) put in the membrane. So it is reasonable to cut off the samplemembrane, for example, as an angle of the thread to the sample length to be 451.

The characteristics of commercially available ion exchange membranes arelisted in Table 2.1.

Table 2.1 Commercially available ion exchange membranesa

Company Product Name Type (counter-ions) Thickness[mm]

Electricresistanceb

[O cm2]

Transportc

numberBurstingstrengthd

[kg cm�2]

Features

Ionics Nepton CR61CMP-447

Cation membrane 0.6–0.7 10 17 Desalination

CR67HMR-412

Cation membrane 0.56–0.58 2 0.89 7.0 Desalination

AR103-QDP Anion membrane 0.5 3.4 0.95 22 DesalinationAR204-UZRA-412

Anion membrane 0.57 3 0.95 7.0 Desalination

AsahiChemical

Aciplex K192 Strong acidic cationmembrane (Na+)

0.13–0.17 1.5–1.9 1.0–2.5 Monovalent cationpermeable,concentration

K501 Strong acidic cationmembrane (Na+)

0.16–0.20 2.0–3.5 3.5–6.0 High strength,desalination,concentration

K541 Strong acidic cationmembrane (Na+)

0.25–0.40 5.0–8.0 6.0–8.0 High strength, lowresistance, electrolysis

A192 Strong basic anionmembrane (Cl�)

o0.15 1.8–2.1 >2 Monovalent anionpermeable,concentration


0.22–0.24 3.6–4.2 2.6–3.8 High acid diffusion,desalination


0.17–0.19 1.4–1.7 2.5–3.5 High acid diffusion,diffusion dialysis



Asahi Glass Selemion CMT Strong acidic cationmembrane (Na+)

0.20–0.25 4.0–6.0 >0.94 6–8 Desalination

CMV Strong acidic cationmembrane (Na+)

0.13–0.15 2.5–3.5 >0.94 3–5 Concentration

Mem

braneProperty

Measurem

ents

29

Table 2.1. (Continued )


Electrresistan[O cm

Transportc


[kg cm�2]

Features

HSV Cation membrane(Na+)

0.13–0.15 2.27 3–5 H+ permselective, acidconcentration

HSF Cation membrane(Na+)

H+ permselective,corrosion resistant, acidconcentration

AMT Strong basic anionmembrane (Cl�)

0.20–0.25 3.5–5.5 >0.96 6–8 Desalination

AMV Strong basic anionmembrane (Cl�)

0.13–0.15 2.0–3.0 >0.96 3–5 Concentration

ASV Strong basic anionmembrane (Cl�)

0.13–0.15 3.0–3.5 >0.97 3–5 Monovalent anionpermselective,concentration

AAV Anion membrane 0.11–0.14 4.0–6.0 >0.95 1.5–2.0 H+ low permeable, acidconcentration

AMP Anion membrane 0.15–0.20 8–10 2–3 Alkaline resistantDSV Strong basic anion

membrane (Cl�)0.13–0.17 0.9–1.2 1.5–2.0 Acid diffusion permeation

APS Strong basic anionmembrane (Cl�)

0.13–0.18 0.2–0.5 2–3 High acid diffusionpermeation

Dupont Nafion N-117 Fluoro-sulfonic acidmembrane (H+)

0.183 2.0 Water hydrochloric acidelectrolysis, fuel cell

N-324 Fluoro-sulfonic acidmembrane (H+)

4.8 Composite membrane,sodium chlorideelectrolysis

NE-424 Fluoro-sulfonic acidmembrane (H+)

Waste acid recovery, metalrecovery, KOHproduction

NE-2010-WX

Fluoro-sulfonic/carboxylicmembrane (K+)

Sodium chlorideelectrolysis

IonExchangeMem

branes:

FundamentalsandApplica

tions

30

icceb2]

N-981-WX Fluoro-sulfonic/carboxylicmembrane (K+)

Sodium chlorideelectrolysis

Tokuyama Neocepta CM-1 Strong acidic cationmembrane (Na+)

0.13–0.16 0.8–2.0 1.5–3.0 Low electric resistance,desalination,concentration

CM-2 Strong acidic cationmembrane (Na+)

0.12–0.16 2.0–3.5 1.5–3.0 Low diffusion,desalination,concentration

CMX Strong acidic cationmembrane (Na+)


CMS Strong acidic cationmembrane (Na+)

0.14–0.17 1.5–3.5 2.0–3.5 Monovalent cationspermselective, acidremoval

CMB Strong acidic cationmembrane (Na+)

0.22–0.26 3.0–5.0 5.0–8.0 High strength, alkalineresistant, electrolysis

AM-1 Strong basic anionmembrane (Cl�)

0.12–0.16 1.3–2.0 2.0–4.0 Low electric resistance,desalination,concentration

AM-3 Strong basic anionmembrane (Cl�)

0.11–0.16 2.8–5.0 2.0–4.0 Low diffusion,desalination,concentration

AMX Strong basic anionmembrane (Cl�)


AHA Strong basic anionmembrane (Cl�)

0.18–0.24 3.5–5.0 6.0–10.0 High strength, alkalineresistant, electrolysis

ACM Strong basic anionmembrane (Cl�)

0.10–0.13 3.5–5.5 1.5–3.5 Low acid permeability,acid concentration

ACS Strong basic anionmembrane (Cl�)

0.12–0.20 3.0–6.0 2.0–4.0 Monovalent anionspermselective,desalination

ACS-3 Strong basic anionmembrane (Cl�)

0.09–0.12 1.5–2.0 1.3–2.0 Monovalent anionspermselective, saltproduction

Mem

braneProperty

Measurem

ents

31

Table 2.1. (Continued )


Electrresistan[O cm

Transportc


[kg cm�2]

Features

AFN Strong basic anionmembrane (Cl�)

0.15–0.18 0.2–1.0 2.0–4.0 High acid diffusion,diffusion dialysis,desalination

AFX Strong basic anionmembrane (Cl�)

0.14–0.17 0.5–0.7 2.5–4.5 High acid diffusion,diffusion dialysis

BP-1 Bipolar membrane 0.20–0.35 4–7 Organic/inorganic acidproduction

Source: Fukuda, K. (2004), Representative commercially available ion exchang mbranes, In: Seno, M., Tanioka, A., Itoi, S.,Yamauchi, A., Yoshida, S. (Eds.), Functions and Applications of Ion Exchange M brane, Industrial Publishing & Consulting Inc.,Tokyo, pp. 279–280.aQuoted from maker’s catalogs and technical data.bNepton: measured in 0.1M NaCl (CR61CMP-447; 0.01M NaCl); Aciplex, Neoce measured in 251C 0.5M NaCl using an alternatecurrent bridge; Selemion: measured in 0.5M NaCl under 1000Hz (AAV; 0.5M H ; Nafion: measured in 0.6M KCl.cNepton: measured from current efficiency in 1.0M NaCl; Selemion: measured fr embrane potential in 0.5M NaCl 1.0M NaCl.dMullen bursting strength; however, Aciplex A201 and A221 possess tensile stren (kg cm�2).

IonExchangeMem

branes:


tions

32

icceb2]

e meem

pta:Cl)

om mgth


2.10. ELECTRODIALYSIS

Main components of an electrodialyzer are desalting cells (Fig. 2.7 (1)),concentrating cells (Fig. 2.7 (2)), solution feeding frames (Fig. 2.7 (3)), electrodecells (Fig. 2.7 (4)) and spacers (Fig. 2.7 (5)). An electrodialyzer is assembledintegrating these components with cation and anion exchange membranes(Fig. 2.8). An electrodialysis system is formed using a circulating tank, a reservetank, a pump, a flow meter and the electrodialyzer described above (Fig. 2.9).The circulating tank is immersed in a thermostat adjusted at a constant tem-perature. An electrolyte solution is put into concentrating cells through a de-flating tube and the head of a concentrated solution extracting tube is adjustedat a reasonable level h. A 0.5M NaCl solution is put into electrode cells. Theelectrolyte solution is put into the reserve tank.

The electrolyte solution in the reserve tank is supplied into the circulatingtank through the flow meter F1 and further supplied to the electrodialyzerthrough the flow meter F2 adjusting the solution velocity in desalting cells at 4–5 cm s�1. This situation is left as such for several hours, during which it isconfirmed that an outflow from the concentrated solution extracting tube due tosolution leakage is negligible. When the outflow is recognized, the assemblingwork is estimated to be imperfect, so that the electrodialyzer is disassembled andassembled again. A slight level change in the concentrated extracting tube isprevented by regulating the height h indicated in Fig. 2.9.

Next, an electric circuit is formed putting an Ag electrode into the anodecell and an AgCl electrode into the cathode cell. A 0.5M NaCl solution isdropped into the electrode cells and the electrolyte solution is supplied into thecirculating tank at the amount Q (Eq. (2.10)) through the flow meter F1.

Q ¼NSZiFaC

(2.10)

where Q is the amount of flow indicated by F1 (cm3 min�1), N the number ofmembrane cell pairs, S the effective membrane area (cm2 per pair), a the de-salting ratio, Z the current efficiency, i the current density (A cm�2), F theFaraday constant (1608 A min eq�1 and C the ionic concentration in the feedingelectrolyte solution (eq cm�3).

An electric current is passed, an overflow from the concentrated solutionextracting tube is collected and its concentration is observed by a refractionmeter. Confirming that the electrolyte concentration of the concentrated solu-tion has become constant, the volume q (cm3 s�1), ionic concentration C00 (eqcm�3) and pH are measured. At the same time, the solution at the inletsof desalting cells and that at the outlet are collected, and their ionic concen-tration and pH are measured. Further, voltage difference V between the con-centrating cells integrated at both ends of the electrodialyzer is measured using

1. Desalting cell: Rubber 0.75 mm thick 2. Concentrating cell: Rubber 0.75 mm thick 3. Feeding frame: Transparent PVC4. Electrode cell: Transparent PVC 5. Spacer: Polyethylene diagonal net 0.7-0.8mm thick

1 2 3

5

4

Figure 2.7 Components of an electrodialyzer (Seno and Tanaka, 1984).

IonExchangeMem

branes:


tions

34

K: Action exchange membrane A: Anion exchange membraneD: Desalting cell C: Concentrating cell F: Solution feeding frame

Figure 2.8 Electrodialyzer (Seno and Tanaka, 1984).

R: Reserve tank of an electrolyte solution, Cir: Circulating tank,E: Electrodialyzer, P: Pump, F: Flow meter,Con: Concentrated solution, De: Desalted solution

Figure 2.9 Electrodialysis system (Seno and Tanaka, 1984).


Pt electrodes put in these cells. During the passage of current, Ag and AgCl areconverted to AgCl and Ag, respectively, so that both electrodes must be replacedappropriately. A time limit of replacement is decided from abrupt increase of thevoltage difference.


REFERENCES

Kosaka, Y., Emura, T., 1963, General characteristics and performance measuring meth-ods, In: Kosaka, Y., Shimizu, H. (Eds.), Ion Exchange Membrane, Kyoritu-ShuppanCo. Ltd., Tokyo, Japan, pp. 117–177.

Seno, M., Tanaka, Y., 1984, Ion exchange membrane experimental method, In:Nakagaki, M. (Ed.), Membrane Science Experimental Method, Kitami Shyobo Co.,Tokyo, Japan, pp. 191–207.

Takemoto, N., 1966, Measuring methods of ion exchange membrane characteristics, In:Scientific Paper of the Central Research Institute, Japan Monopoly Corporation, Ja-pan, pp. 295–303.

Tanaka, Y., 2000, Current density distribution and limiting current density in ion ex-change membrane electrodialysis, J. Membr. Sci., 173, 179–190.

Yamabe, T., Seno, M., 1964, Ion Exchange Resin Membrane, Gihodo Co, Tokyo, Japanpp. 213–240.

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