Plastic Electrodes Specific for Organic Ions SIR: Ion specific electrodes responsive to inorganic ions have undergone rapid expansion in recent years. Electrodes are now available for selective potentiometric determination of many cations including the alkali metals (I),some divalent cations, e.g. CaZ+ and Cuz+ ( I ) , and recently polyvalent cations (2). A wide variety of anion selective electrodes are also available ranging from the precipitate impregnated halide membrane electrodes of Pungor (3) to those based on ion association extraction systems (4, 5). The latter have been shown to be useful for the determination of small organic as well as inorganic anions. Aside from these organic anions, however, there has been relatively little published for systems which respond primarily to organic ions. [While this manuscript was in preparation two additional papers appeared in the literature (6, 7).] In this communication we wish to describe our experience gained over the past five years with simple plastic membrane electrodes which appear to be largely specific for relatively hydrophobic organic cations and anions. A typical cell used for measuring the potentials developed was : SCE
I
0.1MKBr 0.1M KH2P04
Plastic membrane
1 R4N+ 1 test soln
SCE
It is our opinion that nearly any organic plastic matrix of limited hydrophilicity can be used as the gelling component of the membrane, the selection being primarily guided by its compatibility with the desired liquid “plasticizer” components. The liquid components were chosen for their ability to solvate the particular ions of interest. To confer the highest degree of electrode specificity, it is necessary to provide the liquid solvent components with a particularly high degree of specificity in solvating ability. In Figures 1-4 are shown the observed response of some of the systems which were examined. The response time for the electrodes was rapid, stable potentials being obtained in less than a minute for concentrations greater than 1 X M. Figure 1 shows the potential values obtained for a polyvinyl chloride (PVC) membrane plasticized with N,N-dimethyl oleamide (Hallcomid 18-OL) at different concentrations of tetrabutylammonium ion, the slope of the plot corresponding within experimental error to the expected Nernst relationship. In Figure 2, comparable results are given for the same electrode responding to potassium bromide concentration. It is evident that this system can differentiate significantly between the two cationic types. The data in Figure 3 for the same system responding to tetraphenylboron anion suggest that the membrane shows some response to very high concentrations of the organic anion but essentially no response to lower levels. This specificity to organic cations is ascribed to the lower solvating ability of electron-donating systems, such as the amide, toward anions. Unfortunately, the amide plasticized membrane cannot be widely useful for aqueous solutions (1) For a review see G. A. Rechnitz, Chem. Eng. News, 45 (25), 146 (1967). (2) J. B. Harrell, A. D. Jones, and G . R. Choppin, ANAL.CHEM., 41,1459 (1969). (3) E. Pungor, ibid.,39, (13) 28A (1967). (4) C. J. Coetzee and H. Freiser, ibid.,40,2071 (1968). (5) Zbid.,41,1128(1969). (6) G. Baum, Atial. Lett., 3,105 (1970). (7) M. Matsui and H. Frieser, ibid.,p 161. 1674
E(Mv)
-LOG (TBA+)
Figure 1. Response of PVC-amide membrane to tetrabutylammonium bromide in distilled water
-ao -60
1
I
E(Mv)-120 -100
-140 2,O
4,O
3,O -LOG
5,O
(K’)
Figure 2. Response of PVC-amide membrane to KBr
since it is relatively sensitive to H* as shown in Figure 4, amides being relatively good proton carriers. The sensitivity toward hydrogen ion is not present in PVC plasticized with dioctylphthalate as shown in Table I. This membrane still shows an excellent response, however, to organic cations as may be expected from its ability to share two pairs of electrons with cationic species. The excellent Nernst response towards organic species is evident in the data presented in Figure 5 . The selectivity constants calculated according to Eisenman (8)for the two electrode systems are shown in Table 11. (8) G. Eisenman, “Glass Electrodes for Hydrogen and Other Cat-
ions: Principles and Practice,” Marcel-Dekker, New York, 1967.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
450
-2\
3%
-40
25C E(Mv)
150
5c
-LOG
(QUAT.)'
Figure 5. Response of PVC-DOP membrane to quaternary ammonium ions 1
Tetrapropylammonium: slope = -57.5 Tetrabutylammonium: slope = -58.5 C. Tetrapentylammonium: slope = -59.5 D. Tetrahexylammonium: slope = -58.5
A. B.
Figure 3. Response of PVC-amide membrane to sodium tetraphenylboron
1
-120
Table I. Response of PVC-DOP Membrane to HCl Concn HC1 ( M ) --log[H+] E (mV) 1
x
5 x 1 x 5 x 1x 1x 1x
-114 -76 -76 -79 - 85 - 60 - 32
5.0 4.3 4.0 3.3 3.0 2.0 1.0
10-6 10-6 10-4 10-4 10-3 10-2
10-1
t
-160 -Iqo
E(Mv)
60
-180
.
-200
-
-LOG
(TPB-)
Figure 6. Response of nylon-phenol membrane sodium tetraphenylboron (slopefor lowest concentrations = 50.5)
40
20
0
Table 11. Representative Selectivity Constants K Defined by Eisenman as: Mzi = 0.1 M M2+ = 0 E - E = - (RT/F)lnK.+f,+ / M : + Mi+ = 0 Mi+ = 0.1 M PVC-amidemembrane: KTBA+,K+ = 1 . 9 x 104 KTBA+,H+ = 12.8 PVC-DOP membrane: KTBA+,K = 7 . 4 x 105 KTBA+/H+ = 2 . 3 X 106
-20 E(Mv)
-40 -60 SLOPE. -56.0
+
-80 -100
-120
3.0
4,O -LOG
5,O
(H')
Figure 4. Response of PVC-amide membrane to HCI
The role of hydrophobic or lipophilic forces in determining the sensitivity and specificity of these electrode systems is also evident in the data shown in Figure 5. The response to tetrahexylammonium ion, for example, appears to be consistently 350 mV greater than that to tetrapropylammonium ion. This greater effect is qualitatively in agreement with the
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
1675
60
120
40 90
60
E(Mv) 30 -LOG (TBA+)
Figure 7. Response of nylon-phenol membrane to tetrabutylammoniumbromide
0
-30 -60 -90 NL
NATPB TITRANT
Figure 9. Titration of dextromethorphan. HBr with sodium tetraphenylboron Sample 50 ml5 X 10-3MDMHBr, O.02M Na+, 0.001M (Et)sNH+ Titrant 1 X 10-2MNaTPB PVC-amide indicating electrode
One of the more potentially interesting applications of these plastic membranes is their use in titrimetric analysis. In Figure 8 a titration curve of a drug cation, diphenhydramine, with tetraphenylboron anions is shown. The behavior of -1601 0
5
1
15
10 ML
20
25
30
I
NATPB TITRANT
Figure 8. Titration of benadryl.HC1 with sodium tetraphenylboron Sample 50 m15 X 10-3MBn.HC1 Titrant 1 X 10-2MNaTPB PVC-amide indicating electrode
free energy requirement for transfer of twelve -CH2- groupings from a lipoidal to an aqueous environment. The average value obtained for the free energy of transfer of a -CHzgroup was - 680 cal, calculated from the relationship of AG = - nFE. This compares with the value of - 800 cal per -CHZgroup obtained independently in our laboratory for transfer of the group from aqueous to lipoidal environment. Utilization of proton-donating plasticizers may be expected to provide preferential solvation of negatively charged species. That this is indeed true is evident from Figures 6 and 7 where the membrane responses of a phenolic plasticized nylon electrode toward anionic and cationic organic ions are shown. It is quite apparent that at the lower levels the system responds more actively to tetraphenylboron anion than to the cations.
1676
another drug species, dextromethorphanium ion, in the presence of a less hydrophilic system is illustrated in Figure 9. It is evident that these electrodes offer interesting analytical possibilities. TAKERU HIGUCHI C. R. ILLIAN J. L. TOSSOUNIAN'
Department of Analytical Pharmaceutical Chemistry and Pharmaceutics School of Pharmacy University of Kansas Lawrence, Kan. 66044 RECEIVED for review May 28,1970. Accepted September 16, 1970. 1
Present address, Hoffman-LaRoche, Inc., Nutley, N. J. 07110
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
