Indian Journal of ChemistryVol. 22A, December 1983, pp. 1033-1036

Preparation & Characterisation of an Ion SelectiveElectrode for Copper (In

S K SRIVASTAVA·, NARESH PAL, R P SINGH & SUSHMA AGARWALDepartment of Chemistry, University of Roorkee, Roorkee 247667

Received 6 June 1983; revised and accepted II August 1983

Copper tungstoarsenate membrane selectively responds to copper (II) ions and can be used to determine the activity ofthese ions in the concentration range 10 -I to 10 -5 M. The response time of the electrode is a few seconds and the potentialsgenerated are reproducible. The membrane shows more selectivity to copper (II) ions as compared to many other uni-, bi- andter-valent cations. Anions, except sulphate and chloride also show no interference. Addition of small amount of a cationicsurfactant, like cetylpyridinium bromide, causes large shifts in membrane potentials but a membrane after being treated withthis surfactant gets desensitized with respect to potential shifts and shows response to copper (II) ions even in the presence ofsurfactant cation. The membrane electrode has also been used in the estimation of copper in industrial discharge.

A number of ion selective electrodes for copper (II),prepared from copper sulphide, chalcocite etc., havebeen reported in literature 1.2. Normally silver, ironand mercury are found to affect the electrode systemand interfere with its working even when present insmall concentrations. The results of our successfulefforts in developing a selective solid membraneelectrode for copper (II), based on coppertungstoarsenate, are reported in this paper.

Materials and MethodsAll the reagents used were of A R grade. Copper

tungstoarsenate was prepared by adding excess ofcopper nitrate solution with constant stirring to asolution of sodium tungstoarsenate, which in turn wasobtained by mixing sodium tungstate and arsenicpentoxide in sodium hydroxide solution and thenacidifying it. The precipitate was separated, washed bycentrifugation and dried at 90°C. Copper tungsto-arsenate thus prepared analysed for CU3(W12As04oh(Found: W, 73.18; Cu, 3.22; As, 2.27 Calc. W, 73.15;Cu, 3.15; As, 2.48%). The sample was found to beanhydrous and decomposed at 550°C as revealed byTG data.

Preparation of the membranesThe membranes were prepared by mixing copper

tungstoarsenate (0.7 g) with araldite (0.3 g). Themixture was spread out thinly (about I mm) on a pieceof filter paper and left in air to cure. The hardenedmembrane was cut into a circular disc (diam 1.5em)and equilibrated with IM copper nitrate solution forfour days. Membranes equilibrated for lesser time didnot show a stable potential. Between measurements,the electrodes were washed with distilled water toprevent cross contamination.

Potential measurementsThe electrode assembly used for potential

measurements was the same as reported earlier", thereference solution being 0.1 M copper nitrate.Potentials between test and internal referencesolutions were measured with a sensitive poten-tiometer (accuracy ±O.I mY) through a pair ofsaturated calomel electrodes. Leakage of KCI fromcalomel electrode was minimized by the use of doublejunction electrodes. All measurements were made at 25±O.I°C.

Results and DiscussionThe plots of variation of potential versus log [Cu2 +]

or log activity of Cu2+ ion, the potentials beingmeasured with copper tungstoarsenate membrane incontact with copper nitrate (10 -1 to 10 -7 M) solutionson both the interfaces, are shown in Fig. 1(curves a andb). Activity coefficients in dilute solutions werecalculated with the extended form of the Debye-Hiickel equation while at larger concentrations thecomplete Hiickel equation was used". The observedpotentials are quite close to Nernstian value in theconcentration range 10 -5 to 10 -I M of Cu (II) and theplots (Fig. I, curves a and b) show slopes of 28 mV perdecade of concentration. The useful concentrationrange in which the electrode can be used for copper ionestimation is 10 -5 to 10 -I M. The working of thiselectrode system has also been checked at constantionic strength by observing potentials ill the presenceof NaN03 and in the presence of sodium acetate andacetic acid buffer (see Fig. I, curves c and d). Theseplots are similar to original calibration plots having thesame slope values.

The response time of this membrane electrode isreasonably fast (20 to 30 see), stable for 15 min and

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-40.0

"> -60.0g.~--CII

& -80.0

-100.

INDIAN J. CHEM., VOL 22A, DECEMBER 1983

(alIbI leI (dl

-120.0~ ~ ~=-__~~ ~ __--~~----~----~~----L-----~---0.0

0.0 1.0I I I

2.0 3.0 4.0 5.0 6.0 7.0+loq eoncentrotion ICu2+) (in presenceof Nar.l)31tc I

0.0 1.0I I I

7.0

Fig. I-Plots showing variation of potential versus -log concentration or activity of copper ions (curves a and b), in the presence ofNaN03 (curve c) and sodium acetate and acetic acid (curve d)

reproducible within ±0.5 mY. The membrane can beused for six months without observing any drift inpotentials. The useful pH range for this electrode is 2 to6. At higher pH (> 7.0) potential drifts are observedprobably due to depolymerisation of the hetero-polyacid salt.

The response of membrane electrode in non-aqueous medium is non-Nernstian, although a lineardependence (plots not shown) of potentials withactivity of copper ions (concentration range 1 x 10-5

to 10 -1 M) is observed in solutions containing up to30% ethyl alcohol or acetone. In solutions havinghigher non-aqueous content the response time rises to5-6 min and a drift in potential occurs. Thus, theelectrode, under investigation can also be used insolutions having non-aqueous contents up to 25%.

The performance of electrode in the presence ofvarious interfering ions and its selectivity for copperover other ions have been tested by the fixedinterference method" and the selectivity coefficients,

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kr.~obtained are given in Table 1.The data in Table 1show that the electrode, except for Ba 2 + and Fe3 + , ismore selective to copper ions as compared to other uni-, bi- and tri-valent cations. It is observed that NO 3"andCH3COO - do not interfere in the performance of thiselectrode while SOi - and Cl- do interfere even whenpresent in smaller concentrations.

The electrode exhibits interesting behaviour in thepresence of surfactant, cations. Addition of smallamount of cationic surfactant, like cetylpyridiniumbromide (CPBH 5 x 10-4 M) to 10 -3 or 10 -4 M coppersolution causes a shift in potential by 10to 15mV. Thisshift is too large to be attributed to complex formationor to factors like changes in junction potential etc. It isfurther observed that the membrane after being treatedwith the surfactant solution (10 -4 M) for about 4-6hr,becomes desensitized with respect to the drifts causedby the surfactant. The potentials of the surfactanttreated membrane are -64.5 and -92.4 mV in thepresence of 10 -3 and 10 -4 M Cu 2+ and the potentials

SRIVASTAVAet al.: ION SELECTIVEELECTRODE FOR Cu(II)

Table I-Selectivity Coefficients for Copper-selective SolidMembrane Electrode

[Cone. of interfering ion= 10-2M]

Interfering 10'x kpot Interfering 10'x kpotA.B A.B

ion ionLi+ 0.21 Cd2+ 0.032Na+ 0.21 Mn2+ 0.25K+ 0.17 Pb2+ 0.44Rb+ 0.16 8a2+ 10.9Cs+ 0.15 Sr2+ 0.23NH! 0.22 Ca2+ 0.18Ti+ 0.26 Mg2+ 0.032Ag+ 0.46 Fe2+ 0.016Hg2+ 0.17 AI3+ 0.20Zn2+ 0.056 Ce3+ 0.32Ni2+ 0.33 Fe3+ 2.6Co2+ 0.32

in the presence of Cu2+ and surfactant cation are-64.5 and -92.5 mV respectively. Thus the electrodecan be used to estimate copper even in the presence ofsurfactant ions, although the potentials of thesurfactant treated membrane shift towards morenegative side as compared to those observed with theuntreated membrane. If conditioning of the membraneis done with surfactant at higher concentrations, itexhibits erratic behaviour. In the present case, theexchanger material is negatively charged and there is apossibility of exchange of surfactant cations or theiradsorption only at the interface, when the membrane istreated with 10-4 M surfactant solution. Completeuptake of surfactant by the membrane phase is ruledout due to the fact that the behaviour of the treatedmembrane towards surfactant cations at variousconcentrations is erratic. Moreover, we have earlierobserved" that the crystal lattice of such heteropolysalts is not sufficiently open to permit the exchange ofvery large cations such as CPB.

The presence of proteinous matter seriouslyhampers the working of this electrode assembly. Themembrane surface gets poisoned and it can not beregenerated even by equilibration with coppersolution.

The electrode has also been used to estimate copperin metal processing and plating wastes, containing 370mg and 220 mg of copper respectively. The amounts ofcopper found using the present ion selective membranecome out to be 368 mg and 225 mg respectively, thusestablishing the utility of this membrane for theestimation of copper in industrial discharges.

Ion selective electrodes are mainly used for directpotentiometric determination but they can also beapplied in potentiometric titrations. as indicatorelectrodes. The titration of 0.001 MCu2+ with 0.01 M

-57

-; -61E

o

~0-63a,

o-65

-67.L- ~----~~--~~--~~-0.0

Volumeof EDTA (mil

Fig. 2-Potentiometric titration of 10-3 M Cu solution(20ml)with10-2 M EDTA, usingcopper tungstoarsenatemembraneelectrode

(AL 20m1110-3M Cut 1Cf;" Col><10-2M EDTA

(81- 20ml (10-3M Cu+ 1O-5MPbl> <1O-2M EDTA

(AI~ -58"0

1a:: -62

• (BI-66

0.0

Fig. 3-Potentiometric titration of (A)of 20 ml of 10-3 M Cu+ 10-5 Co with 10-2 M EDTA; and (8)of 20 ml of 10-3 M Cu+ 10-5 M Pb with 10-2 M EDTA using copper tungtoarsenate

membraneelectrode

EDT A is shown in Fig. 2. The potential gradually fallswith the addition of EDTA and remains almostconstant after the end point. The break in the curve(end point) corresponds to the stoichiometric ratio.The titration curves of binary mixtures of copper withcobalt and lead with 10 -2 M EDT A, presented in Fig.3, show the utility of this electrode in titrationsinvolving Cu2+ as indicator ions. The shapes oftitration curves are not the same as normally observedin potentiometric titrations. The removal of Cu 2 + ionsresults in a decrease in potential. The constancy ofpotential after the end-point may be due to (a) low

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INDIAN J. CHEM., VOL. 22A, DECEMBER 1983

permselectivity of the membrane, (b) co-ion diffusion,(c) selectivity for other cations or (d) low conductance.

References1 Buck R P, Thompsen J C & Melroy DR, Ion selective electrode in

analytical chemistry, Vol 2, edited by Henry Freiser (plenumPress, New York) 1978, 175.

1036

2 Meyerhoff ME & Fracticelli Y M, Analyt Chern, 54 (1982) 27R.3 Malik W U, Srivastava S K & Bansal Amla, Analyt Chern, 54

(\ 982) 1399.

4 Moody G J & Thomas J D R, Talanta, 19 (\972) 623.5 Guilbault G G, Ion Selective Rev, I (1979) 139.6 Malik W U, Srivastava S K & Kumar Satish, Talanta, 23 (\976)

323.