Indian Journal of Chemical Technology Vol. 6, March 1999, pp. 63-70

Solid phase electrokinetics oflndion-225 cation exchange resin

Mahendra Prasad', Kaman Singh·b & H S Mishral

'Physical Chemistry Division, National Sugar Institute, Kanpur 208 017, India

bDepartment of Chemistry, Government P.G. College, Tikamgarh 472 001, India

Received 18 March 1998; accepted 22 February 199~

Solid phase electrokinetics of Indion-225 cation exchange resin regenerated in If' form was studied employing different celJ"configurations. At 25°C, the rate was 0.88 cm-3 min- I whereas at 90°C, the observed rate was 0.58 cm-3 min- I for the cell PtJR-HlPt. However, the observed rate was 0.49 and 0.55 cm-3 min- I at 25 and 80°C respectively employing the cell CuJresiniCu. Polarisation kinetics of the resin is governed by hopping of the protons from one si te to another discharge of electrodes giving typical ionic behavi(}ur of current polarisation. Hence, any decrease in proton concentration and non­availability of discharging sites on the surface of the resin are rate detennining factors and consequently the order of the reaction wa,s found to be two with activation energies of 0.42 and 0.20 eV in case of Pt and Cu electrodes respectively. Electronic and ionic regions have been observed as suggested by Rosenbergs in case of proteins. The activation energy seemed to be strongly influenced by electrode materials.

It has become customary to use ion exchange resins in production of deionised water. From the separation point of view' -3, the deve lopment of ion exchange process is of utmost importance, since this may serve as a new tool for studying separation of liquid mixtures and may also have diverse applications in other separation processes. However, recent advances in the development of ion exchange resins4

-6 and the discovery of ion exchange membranes7

-9 have given a

new impetus to look for the prospectus of this technique to demineralisation and decolourisation of cane juices in sugar industry. The regeneration of ion exchange resins by electric current seems to have tremendous potential to become an economical and continuous process for a complete demineralisation of cane juices. Ion exchange resin membranes have an advantage over ion exchange resin in the sense that they can be regenerated by an electric current which would not be o nly economica l but it would also be easier from operational point of view. A literature survey has revealed that there is sti ll ample scope for work on the electrical conducting properties of ion exchange resins in solid phase in respect to temperature and pressure. Sidorova et al. 10 have studied the electrical conductivity of ion exchange resin and resin membranes in differen t ionic forms. Eley" and Saito et al. 12 have also studied the effect of pressure on electrical conduction of ion exchange

*For correspondence

resin. The transport properties of synthetic resin membranes with respect to pressure were studied by Hillrobert' 3 and Greenwood' 4. However, a lim ited information is available on the electrical conductivity of ion exchange resins with respect to temperature. The characterisation of so lid resin in terms of electrochemical kinetics is expected to give better understanding of charge transport in resins. Also the increasing concern of ion exchange and its appli­cations in various fields have attracted attention to investigate the kinetic data of solid resin employ ing Indion-225 cation exchange res in wh ich is regenerated in W form . Such study will be of s ignificant help in understanding electrochemical aspects of ion exchange res ins for developing methods to regenerate resin with the help of electric current.

Experimental Procedure Synthetic ion exchange resin Indion-225 was used

for present study. NaOH, He l, acetone, standard buffer tablets of different pH (BDH) and double distilled conductivity water (I.O x 10-6 mhos) were employed for preparation and purification of resin.

Resin Pretreatment - The resin was pretreated before being employed to make measurements because it is available in different cationic/ ionic forms .

Column Packing - Tndion-225 resin (matrix: cross linked polystyrene, particle mesh size - 120 + 30)

64 INDIAN J CHEM. TECHNOL., MARCH 1999

was completely packed in a pyrex glass tube (2.5 cm i.d., column height, 35 cm) that was thoroughly cleaned, washed and dried. The resin column was provided with grass wool as well as glass beads at the bottom for res in support. In order to eluate the solution, a stop-cock arrangement was provided. The resin was rinsed two to three times with di stilled water to remove any contamination with resins.

Acid Treatment - A HCI (I M) was prepared In a measunng flask with conductivity water for acid treatment.

-5 . 0

Indion-225 Resin Colu mn - With the help of a pipette, 25.0 mL HCI was added to the resin in three operations till the eluent was strongly acidic . After 30 min the resin was completely washed with conductivity water till all the excess acid was removed from the resin columns. The removal of ac id was checked by a pH indicator paper as well as by measuring the pH of the eluant with a digital pH meter. The resin was then transferred to a Buchner fu nnel separately and connected with a vacuum filter pump for removal of water. Acetone was used for

Cell: Pt/R-B/pt

... I~ ... I

! 0

I. 8' ...

.... 1ft

.... 18

~ til 0 ....

• Initial'

-6.0 • Steady

-7.0 0 • 0 • • • 0

-8.0 - « - . . • •

-9.G.

-10.0

Q 2.7 2.9 3 .1 3.3

Fig. ) - Arrhenius plot of d.c. conductivity data of Indion-225 cation exchange resin

-5.0 • Initial

• Steady -6.0

-7.0 I ... • -8 . 0

. " II •

- 9 . 0

- 16 .0

o 2.7 2.9 3 . 1 3.3

Fig. 2 - Arrhenius plot of d.c. conductivity data of Indion-225 catIon exchange resin

PRASAD el af. : SOLID PHASE ELECTROKINETICS OF A RESIN 65

Table I - Activation energies of electrical conductivity of solid Indion-225

Cell' (i) PtJR *-H/Pt (i i) Cu : CU20IR-HJCU20 : Cu

Temperature, OC Activation energy, eV Initial Steady

25-90 0.13 0.21

25-90 0.10 0. 11

R *-H=:Regenerated cation exchange resin .

final drying of the resin. In this manner the resin was regenerated in W form and was stored in a vacuum desiccator for further use.

D.C. Conductivity and Cur rent Measurement - All the moisture was excluded from the conductivity cell. Dry resin (500 mg) was weighed in weighing tube to avoid contamination of moisture. The cell was charged with a weighed quantity of resin giving the cell configurations: PtIR- H-225IPt and CU/R-H/Cu. 1.5V was applied in the c ircuit in which the sample was placed in series with a standard resi stor. The polarization current (P.c.) across the standard resistor was measured with the help of a Keithely 177 DMM digital multimeter under two conditions, viz. , initial and steady state. The mean of P.c. values with both polarity in each case was utilized to calcu late the electrical conductivity. For the measurements at higher temperatures, a thermostat (NBE type 240 I 0) was set-up at the required temperature and the sample was left for three hours to attain thermal equilibrium. Temperature was monitored with the help of a chromel-alumel thermocouple and with an external thermometer.

Results

Polarisation Current

Cell: PtlR - HIPt - The Arrhenius plot of d.c . conductivity obtained with Pt electrodes is shown in Fig. 1. The va lues of d.c. conductivity at 25° and 90°C calculated from the init ial current were found to be 1.79 x 10-8 and 3.97 x 10-8 ohm- ' cm- ' , respectively. Steady-state values at these temperatures were observed as 1.91 x 10-9 and 6.75 x 1O-9 0hm- ' cm- ', respectively. Activation energies obtained from the slope of initial and steady-state conductivity plot were calculated to be 0.13 and 0.21 eV, respectively . Data are shown in Table I.

*A film ofCu(2)O cannot be avoided in experimental condition, hence it is proper to designate the cell configuration as mentioned.

0.0

-1.0

.-'" "-c

'" 0

f -2.0

-3. 0

-4.0 13 . 2 13.6 14.0 14 .8 15 . 2

Lo910 ( a - x,

Fig. 3 - Log dn/dl vs log (a-x) plot

Cell : Cu : CU20 lR - H/CU20 : Cu*- The observed conductivities at initia l state at 25° and 90°C were 5.08 x 10-8 and 9.58 x 10-8 ohm- ' cm- ', respective ly. The corresponding steady-state conductivities were 1.10 x 10-8 and 2.30 x 10-8 ohm- ' cm- ', respectively. The activation energy observed from initial and steady-state conductivities were 0.10 and 0.1 1 e V, respectively. The change of ele~trode material from Pt to Cu showed a decrease in conductivity by 25%. It is also observed from the Arrhenius plot (Fig. I ) that initial conductivity is 75% higher than steady-state conductivity, however, Fig. 2 shows that initial condl,lctivity is about 50% higher than steady-state conductivity.

'Kinetic Studies - Kinetics was studied on the assumption that charge carriers are being discharged at the electrode surface with respect to time . At a given time they represent a concentration of charges which are taking part in conduction process. Kinetic parameters were calculated from current time curve, obtained during. d .c . conductivity measurement.

Cell: PtlR -HIPI- From polarisation curves at 25°·90°C, the rate of disappearance of charge carriers, i.e. , dnl dt , could be equated to KC' , where K is the rate constant, C is the charge carrier concentration at a given time, and n is the order of the reaction . Further, the change in charge carrier concentration with respect to time is approximated by the difference of a and x, where a is the initial concentration of charge carriers. Thus,

dnldt=K (a - x)", where dnldt=slope of the current vs time curve or log, odnldt=log K + n log (a -x)

66 INDIAN J CHEM. TECHNOL., MARCH 1999

Table 2 - Activation energy of Indion-225 (R-H)

Cell T,emperature, TO, K 103rro, K Loglo K Activation Energy,

eV °C

PtJR-H/Pt 25 40 60 70 90

25 50 60 80

+1.0

Cell.Cu.Cu20/R-B!Cu20,Cu

- 0.3 $/ -0.9 Y .,

" "-c

" 0 .... '"

, .3 , ,

-1.5 " , , , , , , , , , , , • 25 ·C , , , , , , j s O· C , / ,

" , D 60·C , ,

, /' 0 SO ·C -2.1 ,

- 2. 7 J.....----,.-----.----...------...--J 14.4 1 4 . 6 14 .8 15.0 1 5.2

Lo9 1 0 (a-x)

Fig. 4 - Log dnJdt vs log (a-x) plot

a or x is evaluated from the conductivity as follows a = nef.1 or n = alell, where n = concentration of charge carriers (cm -3) a = conductivity of resin, e = electronic charge and Il

= mobility of ions

Since the resin was in the H+ form, values of IlH+ were taken as 3.65 x 10-5 cm-2 per volt sec. The specific conductivity of resin with respect to time was also calculated. Values of log dnldl are plotted against 10glO (a - x) at different temperatures as shown in Fig. 3. The nature of positive slope so obtained indicates that the higher the temperatture the lower is the rate value. At 25°C, the rate was 0.88 cm-3 min- I

whereas at 90°C the rate was 0.58 cm -3 min - I. The straight line obtained in a 10glO dnldl vs 10glO(a - x) .

297 3.36 -2.65 0.42 312 3.20 -2.85 332 3.01 -3 .42 342 2.92 -3 .62 362 2.75 -3.80

297 3.36 -1.68 0.20 322 332 352

3.10 -1.95 3.01 -2.08 2.82 -2.20

plot was extrapolated to the y-axis to get the intercept as log K. The average slope of the plot approached to 2.3, indicated that reaction is of the second order. The values of log K obtained at different temperatures were employe9 for evaluation of activation energy which was observed to be 0.42 eV. Kinetic data of the resin at 25° to 90°C are given in Table 4.

Cell: Cu: CU20lR -HICU20: Cu -The data of reaction rate, dn/dl and charge carrier concentration, (a -x) were obtained from the P.c. at 25° to 80°C. The values of 10glOdnidt is plotted against 10glO(a -x) at different temperatures as shown in Fig. 4. It is observed that the higher the temperature the lower is the log K value. At 25°C, the rate was 0.49 whereas at 80°C the value of rate was 0.55. The average slope of straight line graphs are found to be 2.50 which indicated that the reaction is of the second order. Kinetic data of above said cell are given in Table 5.

Discussion Polarisation Kinetics - Polarisation of current of the

resin seems to resemble the typical ionic polarisation behaviour. However, polarisation in ionic salts is primarily governed by point defects that are well­established. Resin on the other hand, differs significantly even from bonding stand po lOt; basically, such materials are covalent sites with exchangable. proton and hydroxyl groups. Therefore, the polarization mechanism in such material is to be understood after consideration of charge transport processes of less developed theories of electrical conduction process · in organic semiconducting materials. On reviewing the results, i.e. the decrease of current with respect to time as shown in Fig. 5 can .be understood on Hie basis of model as shown in Fig. 6. On the application of electric field on the resin the protons experience sweeping action resulting from proton hopping from one site to another.

PRASAD et al.: SOLID PHASE ELECfROKINETICS OF A RESIN 67

Table 3 - Data showing the effect of electrode material on specific conductivity and kinetic parameters of Indion-225 resin

Kinetic data PtIR-HIPt CuIR-HICu

Sp. conductivity, Ohm- I cm- ' , 25°C 90°C

Initial state

I. 79 x 10-8

3.97 X 10-8

Steady state

1.91 x 10-9

6.75 X 10-9

Initial state

5.08 x 10-8

9.58 X 10-8

Steady state

1.10 x 10-8

2.30 X 10-8

Order of reaction

Activation energy 2

0.42eV

2 0.20 eV

Ratio of activation energy 2

dx Rate law equation - = K (proton concentration) (non-availability of discharge sites on the surface of the resin).

dt R-H=Regenerated cation exchange resin.

600 ---------------.......,

500

400 <

'" I~ 300

200

100

'g ~ ~ 1 : 1 ~ 1 ",. , 1;, .~ 1 ~, .... , o ,

0"

5 10 15 20 25 30 35 Time , .in

Fig. 5 - Typical polarization curve of ion exchange resin

Consequently, protons are pushed towards the cathode, first abruptly and then slowly till they begin to experience discharging at the cathode giving a typical ionic behaviour of current polarisation. On reversal of polarity, the instantaneous effect is to reverse the process i.e. , to push some of the protons that could not be discharged and piled upon the electrode surface. Consequently, the electrostatic action is also reversed, causing instantaneous increase of current upon polarity reversal. In this process of charge transport it is proposed that the hopping of protons takes place from site to site via surface protonic site of the resin particles. It is believed that the charge transport through bulk resin is not operative as long as proton is the primary charge transporter. The proton transport in solid phase as well as in liquid phase have been thoroughly investigated by HugginlS who studied, particularly, transport of proton in ice. Eigin et al. 16 also suggested that in ice the proton can jump or tunnel from site to site in the structure through hydrogen bonding. In thi s kind of proton transport, water does not migrate and the charge is transported purely by proton (high

+

00000 -H .-H .-H .-H .-H •

00000 -H .-H .-H e.-H .-H •

U U U U U H++i~tH21' -H+ .-H+ .-H+ .-H+ .-H+ •

00000 -H .-H .-H .-H .-H •

, .- H' R.- Reget...t.d IndIon ·225 I

Fig. 6 - Suggested model for polarization of Indion-225 resin

mobility of proton in ice is interpreted as a sort of quantum tunnelling effect).

Probable Electrical Conduction in Indion-225 Resin­

For understanding the electrical conduction in the resin, it is necessary to classify this material into broad spectrum of p or n type conductor on the basis of well-developed physics and chemistry of Si or Ge. Thus, taking the concept that ionised excess electrons in Si and defficiency of electron in Si band represents electron and hole, respectively. It seems reasonable to purpose that cation exchanger free from cation or proton, i.e . negative polymeric unit as n-type material and either regenerated or exhausted form of cation exchange resin, will be a mixed conductor (ionic­electronic). Referring to Fig. 7 an interesting observa­tion recorded was that there is a definite transition in ionic conductivity at about 40-60°C. The proton is the main charge transporter species, however, above this temperature complete charge transport due to proton only cannot be assigned with 100% certainty. About 90% ionic conductivity below 40°C and about 80%

68 fNOIAN J CHEM. TECHNOL., MAR CH 1999

Table 4 - Kinetic data of Indion-225 (R-H) at 25° to 90°C

Cell : PtlR-HIPt

Temperature Reaction rate dn Charge in loglo (a-x) °C loglo - conc. (a-x ),

dt cm-J

25 0.88 -0.60 2. 14 x 10 14 14.33

0.31 -0.50 1.32 x 10 14 14. 12

0.07 -1. 18 6.84 x l Oll 13 .83

0 .03 - 1.49 5.08 x 10 13 13 .70 0.02 ~ 1.74 4.04 x 10 13 13.60

40 0 .65 -0. 19 2.68 x 10 14 14.43

0 .32 ~0 .49 1.79 x 1014 14.25

0.20 - 0 .70 1.4 1 x 10 14 14.15

0. 12 -0.92 I.I O x lO1 4 14.04

0.04 -1.35 7.34 x 101 1 13 .86

0.05 -1.32 8.76 x 10 13 13.94

60 0.92 - 0.04 2.97 x 10 14 14.47

0.35 - 0.45 1.88 x 10 14 14.37

0. 14 -0.84 1.55 x 10 14 14 .19

0 .10 -1.01 1.36 x 10 14 1-1.13

0 .06 -1.24 1.1 0,,10 11 14.04

0.03 - 1.49 953 x 10 13 13.94

70 1.13 -0.05 3.90 x 101.' 14 .59

0.52 -0.28 2.91 x 1l)14 14.26

0.20 -0.69 2.03 x 1014 14 .30

0.09 -1.05 1.50 x 10 14 14 .18

0.04 -1 .35 1.19 xll)14 I-U)7

0.03 -1.57 1.07 x 10 14 14.02

90 0.58 -0.2-1 4.58x 10 14 14 .6(,

0.35 -0.46 2.75 x 10 14 14.44

0 .11 -0.96 I .R6 x 10 14 14.27

0.07 -115 1.53 x 1014 14.1 8

0.04 -1.37 1.32 x 101.1 14 .12

tOOlC conductivity between 60 to 90°C strongly favour that charge transport in this resin in primarily pro­tonic. The transition above 60°C (Fig. 7) could be due to a shift of proton transport from one mechanism to other. The contribution of electronic conductivi ty of the resin above 40°C cannot be ruled out Elec­tron ic and ionic regions have been observed in thi case as suggested by Rosenbergsl 7 and Prasad el 0 1. 18

in case of proteins and S02 clathrate of hydroquinones respectively.

Arrhenius P lot and Activation Energy of Ionic

Conductivity - Activation energy of the resin turned out to be 0 .1 leV which is ass igned to be activat ion energy of charge transport due to proton. The excellent agreement between this va lue and that of Eigin el 01. 16 gives confidence fo r conduction mechanism. Eig in el of. 16 have thoroughly discussed protonic charge transport in water and in ice. The fact

that when h drogen bonded water can allm-\' tho proton to dissociate to take part in transport process either in water or in ice, then there is no doubt why proton cannot transport the charge in cation exchange such as this. Moreover, it has been suggested that ion­exchange membrane could be a suitable electrode for arriving a t a definite conclusion for proton transport in water and ice. The ir studies, also seems to be consistent to a uthor 's suggestion of p-n junction characteristic for ion exchange resin . Hence it is reasonab le to propose a p-n j unction behaviour of the resin and proton transport with greater certainty.

Effect of C hange of Electrode Materials on K inetic

Parameters of Resin - For the discussion of kinetic data the results are summarized in Table 3 . In this table there are two parameters which need interpretation : (i) The order of di scharging reaction, and (ii) activation

PRASAD el ar: SOLID PHASE ELECTROKINETICS OF A RESfN 69

Table 5 - Kinetic data of Indion-225 (R-H) at 25° to 80°C

Cell : Cu CU201R-HlCU20 Cu

Temperature, °C

Reaction rate dn 10glO -

Charge in 10glO (a-x) conc. (a-x),

dt cm-3

:-. ... . ~ >

. ~ ... (j ~

" c

° (j

(j .... c

° ....

25

50

60

80

0.49 0.18 0.10 0.06 0.03

0.55 0.27 0.17 0.07 0.03

0.47 0.19 0.11 0.07 0.04

0.55 0.27 0.11 0.05

100~--------______________ ~

9U

8 0

70

60

~ O

20 40 60 80 10 0

T empera t ure, °c

-0.31 -0.74 -\.OO -\,21 -1.52

-0.26 -0.57 -0.77 -\,12 -\.46

-0.33 -0.72 -0.95 -\.15 -1.39

--0.26 -0.56 -0.95 - 1.30

Fig. 7 - Graph showing the % of ionic conductivity of Indion-225 resin with temperature

energy influenced by electrode materials. Discharging reaction at the electrode/ion exchange interface seemed to be of second order which can be understood on the fact that both proton concentration and reaction site are main parameters in this process. Hence any decrease in proton concentration and non-

8.56 x 1014 14.93 5.35 x 1014 14.73 4.40 x 1014 14.64 3.68 x 1014 14.56 2.94 x 1014 14.47

7.80 x 1014 14.89 6.91 x 1014 14.84 5.97 x 1014 14.77 4.70 x 1014 14.67 3.83 x 1014 14.58

8.44 x 1014 14.93 6.9 1 x 1014 14.84 5.97 x 1014 14.77 4.94 x 10 14 14.69 4.40 x 1014 14.64

1.28 x 1014 15. 10 8.90 x 1014 14.95 6.33 x 1014 14.80 5.10 x 1014 14.70

availability of discharging sites on the surface of the resin are rate determining factors and consequently the order of the reaction is found to be two.

The change of electrode material from Pt to Cu in initial state at 25 and 90°C does not seem to change the order of magnitude of conductivity, however, under steady state condition the change of electrode material from Pt to Cu changed the conductivity by an order of magnitude. The d.c. electrical c0nductivity with Pt electrode at 25 and 90°C observed from in itial current were 1.79 x 10-8 and 3.79 X 10-8 ohm -I cm - I, respectively. Steady state observed values at these temperatures were 1.91 x 10-9 and 6.75 x 10-9 ohm-I cm- I, respectively. On changing the electrode material from Pt to Cu these values become 5.08 x 10-8

, 9.5 8 X

10-8, I.IO x lO-8 and 2.30 x 10-8 ohm- I cm-I, under

initial and steady state conditions respectively at the same temperatures. The change of electrode material from Pt to Cu shows decrease in conductivity by 25%. The effect of change of electrode materials on kinetic parameters of resin are given in Table 3.

The activation energy seemed to be strongly influenced by electrode material. The activation en~rgy 0.42 and 0.20 eV in case of Pt and Cu electrodes respectively is in the ratio of two. Lower activation energy in case of Cu electrode was perhaps

70 INDIAN J CHEM. TECHNOL., MARCH 1999

due to heterogeneous surface CU20 film on the electrodes.

Acknowledgement The authors wish to thank Professor Ram Kumar,

Former Director, National Sugar fnstitute, Kanpur for providing necessary facilities.

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New York), 1971. 2 Lonsdale H K, J Membr Sci, 10 (1982) 81 .

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5 Tako M & Brahim M, Int Sugar J, 95 (1993) 243.

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Leningr Univ. Fiz Khim, 22 (4) (1976) 120. 11 EleyDD,JPolymSciPartC, 17(1967)73. 12 Saito S, Sasaba H, Nakagima T & Yadak .I , J Polym Sci, 6

(1968) 1297. 13 Hillrobert M, Nato Adv Study Inst Ser B 14 Whys Normet Thin

Films, 1976, 189. 14 Greenwood D A, Nato Adv Study Inst Ser (Phys Lab Uni v

Bristol), 1976. 15 Huggin M L, J Phys Chem, 40 (1939) 723. 16 Eigin M & DeMaeyar L, Proc Royal Soc. A247 (1958) 505 . 17 Rosenbergs B, J Chem Phys, 40 (1968) 4108. 18 Prasad M & Choubey B D, j Indian Chem Soc. 54 ( 1977)

499.