A Liquid Ion-Exchange Membrane Electrode for Polyvalent Cations Jack B. Harrell,' Alan D. Jones,2 and Gregory R. Choppin Department of Chemistry, Florida State University, Tallahassee, Fla. 32306
INTHE PAST few years, a number of new ion-selective electrode systems have been studied (1-4). At the moment, such electrodes have been devised for many anions and mono- and divalent cations and are available commercially. However, no electrodes which are suitable for ions of higher valence have been reported. In this investigation we have attempted to develop a versatile electrode system which will respond to tri- and tetravalent cations. Because of the recent development of ion-selective electrodes based on the liquid-liquid membrane principle, we have investigated the dinonylnaphthalenesulfonic acid (DNNS) membrane system.
Calomel electrode
MZ+ Reference
i
Ion-exchange membrane
~
~
MZ+ Test
Calomel electrode
EXPERIMENTAL Apparatus. Two types of cells were constructed to support DNNS membranes for use in concentration cells. One cell was milled from a block of Teflon (Du Pont) and contained two 50-cc compartments which were separated by insertion of a tapered Teflon rod. A hole through the rod connected both compartments oia a membrane supported in the hole by two small Teflon spacers. The second cell was made from two glass cells (75-cc capacity) which could be bolted together over a Teflon plate supporting the membrane. This complete cell could be immersed in a water bath for temperature-controlled experiments. The temperature was controlled to 23 f 1 "C. The potentials across the cells were measured using Beckman Calomel electrodes and a Beckman Research pH meter. Reagents. Dinonylnaphthalenesulfonic acid (DNNS) was chosen as the cation-exchanger because of the similarity of its exchange properties to those of Dowex-50 (5). The DNNS was obtained from the R. T. Vanderbilt Co. as a 3 9 x solution in naphtha. All electrolyte solutions were prepared from reagent grade chemicals. Metal ion solutions were standardized by passing aliquots through cation-exchange resins (Dowex-50) in the hydrogen form. The acid released was titrated with standard NaOH. Procedure. The DNNS had to be converted into the salt of the cation under study before it could be used in a concentration cell. This was achieved by agitating a sample of the DNNS/naphtha solution with a 1.OM solution of the cation for a period of 12 hours at 25 "C. The naphtha solutions of the DNNS salts obtained were very viscous and were diluted with Nujol to give a more convenient consistency. The DNNS salt solutions were supported on thin, porous, inert membranes (Orion Research Corp.) by soaking the membrane in the Nujol solutions of the salts and wiping off the excess with filter paper. Each newly impregnated mem-
Present address, Department of Chemistry, University of Houston, Houston, Texas, U. S . A. * Present address, Department of Chemistry, St. Salvator's College, University of St. Andrews, St. Andrews, Fife, Scotland. G. A. Rechnitz, Chem. and Eng. News, 45, No. 25, 146 (1967). G. A. Rechnitz, ibid., 44, No. 5, 24 (1966). J. W. Ross, Science, 156, 1378 (1967). C. J. Coetzee and H. Freiser, ANAL.CHEM., 40,2071 (1968). J. M. White, P. Kelley, and N. C. Li, J. Znorg. Nucl. Chem., 16, 337 (1961).
(1) (2) (3) (4) (5)
RESULTS AND DISCUSSION
The potential of an ideal cation-selective membrane system can be expressed by the Nernst equation in the form:
E = constant
+ 2*303RT log adMz+ ZF ~
where Z and a are the charge and activity of the cation M . The constant includes terms related to choice of reference electrode, reference electrode junction potential, and metal ion activity in the reference solution. The Effect of pH on Potential. The variation of potential with the pH of the aqueous metal ion solutions was measured for various membrane systems. From Figure 1, the potentials of the La(ClO&DNNS system are seen to be essentially independent of hydrogen ion from pH 4.0-5.5 for a range of lanthanum concentrations from 10-1-10-4M. An increase in potential is observed below pH 4.0 and the potential becomes unstable above pH 5.5. The behavior at the higher pH limit (6) P. K. Glasoe and F. A. Long, J. Phys. Chem., 64, 188 (1960). (7) W. E. Keder and D. R. Kalkwarf, ANAL.CHEM.,38, 1288 (1966). VOL. 41, NO. 11, SEPTEMBER 1969
1459
L\
->
40
b
30
-
20
-
10
-
E
Y
-I
0 -
4
g
-10
-
W I-
o a
-20-
-30
I
-50‘ 1.5
I
I
2.5
I
I
4.5
3.5
5.5
Figure 1. Effect of pH on potentials of La(CIOa),/ DNNS membrane system at various lanthanum concentrations 0 1.184 X 10-lMLa(C104)3 0 5.92 X 10-3M La(C10& 0 1.184 X 10-2MLa(C104)3 Q 1.184 X 10-4MLa(C104)3
is presumably due to hydrolysis of the La(II1) ions. At pH < 4, the electrode response is related to the concentration of hydrogen, rather than lanthanum, ions. A similar behavior was observed for other metal/DNNS membrane systems except that the range of pH over which a plateau region was observed for the measured potential varied for each system. Table I summarizes the effective pH ranges over which hydrogen ions had little effect on the measured potentials of various systems. Effect of Concentration on Potentials. The variation of the potential with activity of metal ion for various membrane Table I. Optimum pH Range for M Z‘/DNNS Membrane Operation pH range 2.0-3.0 4.0-5.5 4.0-5.5 3.0-4.5 4.0-6.0 3.0-5.5 4.0-11.0
System Th(Cl)d/DNNS La(ClO&/DNNS La(Cl),/DNNS Cr(C1)3/DNNS NiCL/DNNS CuClz/DNNS CaCL/DNNS
1460
- 1.0
0
PH
log
ANALYTICAL CHEMISTRY
-3 0
a-
Figure 2. Potentials as a function of M z + activity for ThC14, CrCI3, and LaCI,/DNNS membrane systems 0 ThCI4,pH 3.0; 0 CrCI3,pH 4.0; 0 LaCI,, pH 4.0 Lines of Nernstian slope for tetra- and trivalent cations drawn through data
systems is shown in Figures 2 and 3. The reference solution was 0.01 molal in each case and the potential values were the average of three separate measurements. The potential was corrected for the asymmetry of the system, defined as the potential difference between the calomel electrodes when both reference and test solutions were at a concentration of 0.01 molal. The magnitude of this correction was from 0.2 mV to 2.0 mV. The values of mean ionic activity coefficients used were taken from the values of Harned and Owen (8) and Robinson and Stokes (9). From Equation 1, a cation-selective membrane having an ideal Nernstian response will lead to a linear plot of potential us. log uMz+ with a slope of (2.303RTIZF). For a tenfold increase in uMz+,the slope for di-, tri- and tetravalent cations should be +29.38, +19.58, and +14.69, respectively, at 23 “C. The data in Table I1 give the slopes of potential cs. (8) H. S. Harned and B. B. Owen, “The Physical Chemistry of Electrolytic Solutions,” Reinhold Pub. Corp., N. Y., 1958. (9) R. A. Robinson and R. H. Stokes, “Electrolyte Solutions,” Academic Press, N. Y., 1955.
Table 11. MZ+/DNNSMembrane Behavior at 23 i 1 “C Concentration range Slope for linear response, PH mV/log UMZ+ molar 0.1-10-3 3.0 15.0 4.0 4.0 4.0 4.0 4.0 6.0
-2.0
21.4 20.0 20.0
16.7 16.7 17.8
0.2-10-4 0.2-4 x 10-4 0.3-101.e10-3 1 .O-2 10-2 1.0-4 X
x
Useful concentration, range, molar 0.1-10-4 0.5-10-‘ 0.5-5
x
0.5-10-4 1.0-10-4 1 .o- 10-3 1.0-10-8
10-4
,Ot
J
20
4
10
I-
\
-
z w
I-
0-
B -10
-10
-zol
\
-30 -40
-
-20 -1.0
-2 0
log
t
-I 0
-2 0
-3 0
log a Figure 3. Potentials as a function of MZ+ activity for CuCI2, NiCl,, and CaC12/DNNS membrane systems L? CuC12, pH 4.0; A NICI?, pH 4.0; 0 CaCL, p1-I 6.0 Line of Nernstian slope for divalent cations drawn through dots
log U.,~Z+ plots for a number of MZ+/DNNS membrane systems. As can be seen from Table 11, the DNNS membranes with C I ~ C ~La(C1)3, )~, La(C10J3, and Th(Cl), salts give a nearNernstian response over a considerable concentration range for the pH conditions studied. In the case of the divalent metal ions, the DNNS membranes show a linear response over a considerable concentration range, but there are appreciable deviations from the theoretical (Nernst) behavior. The deviations from linear behavior observed at lower concentrations may be caused by two factors: the aqueous solubility of the DNNS salt may be comparable to the concentration of the metal salt; the solubility of the metal salt in the organic phase may be comparable to the concentration in the aqueous phase. It seems unlikely that the deviations at higher concentrations are attributable to anion permeability in the membrane because there is little evidence to support appreciable anion invasion in liquid cation exchangers. The effect of different anions on the potential of the La(III),IDNNS membrane system was studied by measuring the concentration potentials in the presence of C1-, C104-, and NO1- ions. Figure 4 is a plot of the concentration potentials as a function of the logarithm of the lanthanum activity for the three anion systems. Also, the effect of substituting a La(C10,):: reference solution (0.01 molal) for the La(C1)3 reference solution in the La(C1)3/DNNS membrane system is shown in Figure 5. These data indicate that the membrane systems are essentially reversible with respect to the cations in solution. Water Transport. A more likely explanation of the deviations of membrane potentials from the Nernstian values at higher concentrations is that water transport through the membrane is occurring, as reported previously with other ion exchange membranes (10). Because DNNS is known to form (10) J. Greyson, J . Pliys. Chem., 71, 259 (1967).
-3.0
LO^+]
Figure 4. Potentials as a function of lanthanum activity for LaC13, La(NOJ3, and La(C104)3/DNNS membrane systems at pH 4.0 C La(C10& A La(N03)3 0 Lac&
J
52
0-
k-
Z
w
-
I0
a
-eo -
I _
-0.5
-I 5
-2.5
-35
log [ ~ a ~ + ] Figure 5. Potentials as a function of lanthanum activity for LaC13 with LaC13 and La(C104)3 0 as reference solutions at pH 4.0
micelles in contact with aqueous solutions, this suggests a likely mechanism for water transport through the membranes (10, 11). The results obtained from the study of the transfer of D20 across the La(ClO&/DNNS membrane (Figure 6) indicate that only a small amount of DzO (cu. 4 7 3 had been transferred even after 14 hours. The greater transfer of water through the Ca(C1)2/DNNS membrane (ca. 8%) in the same period of time may account for the considerable deviation of the system from Nernstian behavior, as noted in Table 11. The infrared spectra of the DNNS solutions also indicated that the calcium salt contained about twice as much water as the lanthanum salt. Response of Membrane Systems to Foreign Ions. A measure of the effect of foreign cations on the behavior of a particular membrane system can be obtained from the selectivity coefficient ( K ) , which is defined as (3, 12) : (11) A. S. Tombalakion and W. F. Greydon, ibid., 70, 3711 (1966). (12) R. M. Garrels, M. Sato, M. E. Thompson, and A. H. Truesdell, Science, 135, 1045 (1962). VOL. 41, NO. 11, SEPTEMBER 1969
1461
Table 111. Selectivity Coefficients for MZ+/DNNSMembrane Systems Foreign Concencation tration, M PH B"+ B"+ 0.039 6.40 Naf 0.016 6.40 K+ 6.40 NH4+ 0.037 6.40 Mgz+ 0.039 0.037 3.00 Na+ 0,039 3.00 NH4+ Na+ 0.068 3.00 0.039 3.00 NH4+ 0.037 3.00 Na+ 3.00 NH4+ 0.039 0.068 4.00 Na+ 0.039 4.00 NH4+ 0.039 3.96 Na+ 0.039 2.95 Na+ 0.016 3.96 K+ 0.016 2.95 K+ 0.037 3.96 NH4+ 0.037 2.95 NH4+ 0.039 3.96 Mg2+ 2.95 Mg2+ 0.039
40
K 10.7 8.5 22.8 0.4 2.4 0.4 0.7 4.1 91.9 13 17.6 107.9 30.0 24.7 85.3 74.4 75.6 69.5 0.96 0.92
Table IV. MZ+/DNNSMembrane Resistance in 0.01M Solutions DNNS Resistance system Mz' PH (ohms) Th(C1)4 3.0 1 . 5 x 109 La(CIOS3 4.0 1.6 x 107 Cr(C1h 4.0 2.0 x 107 Ni(C1)Z 4.0 4.5 x 106 Ca(CI)z 6.4 1.0 x 106 Cu(C1)z 4.0 2.0 x 105 Table V. Variation of Resistance with La Content for the ~+ 0.01 Molar La(C104)3/DNNS System at pH 4.0 and C L ~ = La in DNNS Resistance (Ohms) 6.12 1 . 6 x 107 0.84 1 . 3 X log 0.098 1 x 109
E
=
constant
+ ___ 2*303RTloglo ZF
[UMZ+
f K ( A B ~ + ) . I ~ ](2)
where A B n + refers to the activity of a foreign cation Bn+. The selectivity coefficients for various membrane systems were obtained by measuring the change in potential on adding a foreign cation Bn+ to membrane cells containing 0.01M solutions of Mz+ in both compartments. Table I11 summarizes the selectivity coefficients ( K ) obtained for various membrane systems. Small values for K enable the activity of the primary ion M Z + to be measured with a minimum of interference and it may also result in a high electrode sensitivity to MZ+(1). Values of K vary somewhat with solution composition, but for the purpose of comparing different membrane systems this variation has been ignored. From the K values given in Table 111, the MZT/DNNS membrane systems do not appear to be highly selective to the trivalent primary ions in the presence of other cations. The values of K can be somewhat misleading in the case of M3+-B1+ competition because the (ABn+)zinterm is quite small for low concentrations of Bnf. For a concentration of 10-2M in M 3 + ,the B1+ ions do not interfere below 10-2M 1462
ANALYTICAL CHEMISTRY
t I
0
2000
4000
8000
6000
TIME (MINS)
Figure 6. 6% transfer of D 2 0across La(C104)3/DNNS membrane as a function of time at pH 4.0 and 23 =t1OC
for K = 100 and even for K = 1000, the change in EMF is only 2 - 3 x ; . Above 10-2M, Bn+ ions interfere seriously so the electrode is useful only at low concentrations of foreign cations. One use of this electrode would be in determining the solubilities of lanthanide compounds of low solubilities since this could be done in weakly acidic (pH 5 ) solutions, avoiding foreign cation interference. Lanthanide solutions could be analyzed by titration if dilute solutions (0.010.001M) of both titrants are used-e.g., lanthanum chloride and sodium oxalate solutions. In the case of Orion calcium selective electrodes, the following K values at pH 6 have been quoted (3): ANa+
=
KK+ = K m , +
=
Kir,z+
=
0.014
(3)
Thus the Orion calcium ion electrode, which is based on an organophosphorus acid exchanger, is much more selective to calcium ions than the CaC12/DNNS membrane described here, However, no details on the response of Orion electrodes to tri- and tetravalent cations are available at the present time. Membrane Resistance. The resistance data given in Table IV were obtained assuming that the capacitive impedance component of the resistances was constant for the various DNNS membrane systems studied. Because only relative resistance measurements were required for the present study, this assumption is reasonable. In general, the data indicate that the resistance across the membrane system increases with the valence of the cation present in the DNNS salt. Spiegler (13) noted a similar behavior with resistance measurements made on synthetic ion-exchange resins. It is difficult to establish relative binding strengths from the resistance measurements because of the variation in resistance with concentration of the cation in the DNNS salt and with the amount of water present in the membrane. The results in Table V show that the resistance across the La(C104),/DNNS system increases as the lanthanum content of the DNNS salt is reduced. Summary. This investigation has shown that although MZ+/DNNS membrane electrodes are not very selective in the presence of foreign cations, they respond reversibly over a useful range of concentrations of the cations. Because reversible electrodes are not commercially available for the trivalent lanthanides and Th(IV), the electrodes described here could be useful for these ions. RECEIVED for review July 3 , 1968. Accepted June 26, 1969. The assistance of the U. S . Atomic Energy Commission through Contract AT-(40-1)-1797 is gratefully acknowledged. (13) K. S. Spiegler, J. Electrochem. SOC.,100, 303c (1953).
