state of monolayer palladium on gold obtained from Figure 4 to literature values for the average oxidation state of bulk palladium. First, complete surface oxidation of bulk palladium electrodes a t a given potential takes approximately 10 minutes. The data in Figure 4 were obtained from current-potential curves recorded a t 100 mV/sec. Longer oxidation times of palladium on gold are not practical because of the significant dissolution of the monolayer palladium. Second, both oxidation and reduction of monolayer palladium occur at more positive potentials than the corresponding processes on bulk palladium (Figure 2). Third, dissolution of Pd(I1) occurs during the potential cycle. Experiments showed that approximately 10% of the palladium deposit was dissolved during a potential cycle between 0.0 V and $1.0 V a t 0 rpm. All of these considerations indicate that the average oxidation state of the monolayer palladium deposits on gold under these experimental conditions may be less than the average oxidation state of bulk palladium a t f 1 . 0 V. To verify the above result, a n experiment based on the following reasoning was performed. The deposition of copper a t underpotential has been studied on platinum (9, 10) and gold (11, 12) electrodes. One monolayer of the metal is deposited a t underpotential with a ratio of approximately one atom of copper per atom of platinum or (9) S.H . Cadle and S Bruckenstein. Anal. Chem., 43, 1858 (1971). (lo) M . W. Breiter, Trans. FaradaySoc., 65,2197 (1969). ( 1 1 ) E. Schmidt, P. Beulter, and W. J. Lorenz. Ber. Bunsenges. Phys. Chem., 75,71 (1971). (12) W. J. Lorenz, I Moumtzes. and E. Schmidt, Electroanal. Chem., 33,121 (1971).
gold. A similar phenomenon is found for several other metals (13, 14). If copper plates a t underpotential on palladium, it would be reasonable to expect that one monolayer of copper will be deposited. The charge required for this process can be compared to the charge required to oxidize the palladium electrode a t various potentials. The potential at which the two charges are equal should correspond to the potential a t which a monolayer of oxygen has been adsorbed-Le., an average oxidation state of 2.0. Copper was deposited on a palladium electrode from a 2 x 10-5M Cu(II), 0.2M H2S04 solution. Underpotential deposition of Cu(0) was observed. The maximum quantity of Cu(0) which could be deposited a t underpotential was 290 pC at +0.04 V. Comparison of this value to the oxidation of the electrode indicated that monolayer oxygen formation occurred a t +1.05 V. This result is in reasonable agreement with the above data on submonolayer palladium deposits on gold, although it does indicate a lower oxidation state than expected. The results of this work support the data of Burshtein et al. (4) who found that the average oxidation state of palladium is 2.0 a t +1.2 V us. RHE in 1N HzS04. I t is suggested that Burshtein’s results be used to estimate the roughness factor of palladium electrodes. Received for review July 30, 1973. Accepted November 16, 1973. (13) S. H. Cadle and S. Bruckenstetn, J. Electrochem. S O C , 119, 1166 (1972). (14) E Schmidt and N. Weithuck, J. Electroanal. Chem., 40, 400 (1972).
New Methods for the Preparation of Perchlorate Ion-Selective Electrodes T. J. Rohm and G. G. Guilbault Department of Chemistry, Louisiana State University in New Orleans, New Orleans, La. 70722
The increased interest in ion-selective electrodes has led to the development of new sensor materials which show selectivity for a variety of anions and cations and new methods for the construction of electrodes from these materials. Recently, Davies, Moody, and Thomas incorporated a commercially available liquid ion exchanger in a poly(viny1 chloride) matrix to prepare a nitrate selective electrode ( I ) . Griffiths, Moody, and Thomas have prepared calciumselective electrodes by mixing a liquid ion exchanger which is sensitive to calcium with poly(viny1 chloride) ( 2 ) . A potassium-selective electrode was reported by Davies, Moody, Price, and Thomas based on the same principle ( 3 ) .Kneebone and Freiser coated a platinum wire with a nitrate-selective liquid ion exchange in a poly(methy1 methacrylate) and used the electrode to determine nitrogen oxides in ambient air ( 4 ) . Ansaldi and Epstein prepared a calcium-selective electrode by coating a graphite rod with a calcium exchanger in poly(viny1 chloride) ( 5 ) . Davies, G . J. Moody, and J. D . R. Thomas, Analyst (London), 97,87 (1972) (2) G. H Grtffiths, G . J. Moody, and J. D . R . Thomas, Analyst, (London), 97,420(1972). (3) J E. W. Davies. G . J. Moody. W. M . Price, and J. D . R. Thomas, Lab. Pract.. 22,20 (1973) (4) E. M Kneebone and H . Freiser, Anal. Chem.. 45,449 (1973). (5) A . Ansaldi and S. I Epstein, Anal. Chem., 45,595 (1973). ( 1 ) J . E. W.
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These innovations greatly reduce the cost of ion-selective electrodes and provide insight for the study of charge transport through the membrane. Furthermore, these “solid” electrodes are reported to have longer lifetimes than the liquid electrodes ( I ) . In this study, we have prepared perchlorate-selective electrodes by mixing a commercially available (Orion) exchanger for perchlorate in PVC and used the material to construct an electrode in which the membrane is used with a reference solution and internal reference electrode, and an electrode in which the exchanger is coated on a platinum wire. The performance of these electrodes is compared to the commercial perchlorate electrode. EXPERIMENTAL The commercial electrode was prepared according to the manufacturer’s manual (Orion Perchlorate Ion Activity Electrode-9281) (6). The PVC perchlorate material was prepared by mixing 360 mg (20 drops) of the commercial liquid ion exchanger with 170 mg of PVC (Breon 119) dissolved in 5 ml of THF. When mixed, the solution was poured into a glass ring (32-mm i.d.) resting on a glass plate. The ring was then covered with a piece of filter paper and a watch glass. After 24 hours, the glass ring and membrane were turned over t o permit the solvent to evaporate from the underside of the membrane. Circles 1 m m in thickness (6) Orion Research Instruction Manual 92-17/92-81, Cambridge, Mass. 02139.
I
I
350
I
I
1
r 2001
5 .__. *...
____ % .' . . . . . L
2 40
1
2
3
4
I
I
I
6
I
5
- l o g aC104-
1 >
I
E
-
100
i 1
-
2
3
-log a
0
L
1
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-loot
Figure 1. Calibration curve for the commercial perchlorate ionselective electrode
-
C
.--
i
0)
0
a 0 -
t
-loo I
,/ 1
3
2 log
4
5
"clod
Figure 2. Calibration curves for PVC perchlorate ion-selective electrodes: PVC-disk, W PVC-wire
and 8 m m in diameter, were cut from the large membrane and glued to the ends of glass tubes (8-mm o.d.1. T h e internal reference solution used with these electrodes was 0.05M (2104- in 0.0c5.V CI ~, Sodium chloride was used as use of potassium chloride would result in precipitation of potassium perchlorate. A silver/silver chloride electrode was used as the internal reference. Alternatively a platinum wire (0.32-mm diameter, 5-cm length), which had been heated until a small drop of platinum formed at the tip. was dipped into the PVC-exchanger mixture and let dry. The uncoated portion of the wire was coated with silicone rubber. Coaxial cables with t h e outer shield grounded. were used with both the PVC-disk electrode and PYC-wire electrode. All potential measurements were made L'S. a S C E using a Corning Digital 110 Expanded Scale p H meter.
4
5
c I 04-
Figure 4. Selectivity coefficient evaluation of the PVC-disk ionselective electrode in 0.1M NaOH
All chemicals were reagent grade and were used without further purification. Deionized water was used in the preparation of all solutions. Perchlorate standards were prepared by serial borate buffer dilution of a 10-1MNaC104 solution in 0.01M borate buffer. Solutions used for the determination of selectivity coefficients ( K l , ) by t h e two solution method. were prepared by dissolving the potassium salts of the various anions in borate buffer and diluting t o volume with buffer. Solutions used for the study of selectivity coefficients by the mixed solution method were prepared by diluting a 10-'M C104 in 0.1M N a O H with 0 . 1 M S a O H . All perchlorate electrodes were soaked for 24 hours in 10- 1M C l o d before use and were stored in this solution when not in use. Measurements were made in stirred solutions at ambient temperature (23 "C). -
RESULTS AND DISCUSSIOS Results of measurements of the potential of perchlorate solutions with the commercial electrode assembly, PVCdisk, and PVC-wire electrodes are shown in Figures 1 and 2. The different internal references of each of the electrodes are responsible for the potential differences for a given perchlorate solution. The commercial electrode and the PVC-disk electrode show linear near-Sernstian response from 10-1 to lO-4M solutions. The linear range of the PVC-wire electrode is smaller, but still near-NernANALYTICAL CHEMISTRY, VOL. 46, NO. 4 , APRIL 1974
591
stian. The commercial electrode and the PVC-disk electrode gave stable responses for one month. The PVC-wire electrode could be used for a period of two weeks before it lost linear response, Hydroxide ion is reported to interfere with the measurement of perchlorate (6). This interference is shown in Figures 3 and 4 by the mixed solution procedure in which the activity of the interfering ion is constant and the activity of the ion of interest is varied (7). The selectivity coefficients of these electrodes were calculated from
where a, and a, are the activities of the perchlorate and the hydroxide ion, respectively. K,, equals 1.2 x for the commercial perchlorate electrode and 1.3 X for the PVC-disk electrode in 0.1MS a O H solution. Interference by iodide, bromide, and nitrate ions was determined by the separate solution method and calculated from,
303RT / Z F
=
logK,,
+ log a
(2)
The selectivity coefficients determined for these ions using the PVC-disk electrode are 5.0 X l W 3 (lO-lM I - j , ( 7 ) G . J Moody a n d J. D. R. Thomas. Taianta. 1 9 , 6 2 3 ( 1 9 7 2 )
1.0 x 1 0 - 6 (10-2M Br-) and 2.9 x 10W5 (10-2M N O s - ), respectively. The selectivity coefficients for the same ions are reported to be 1.2 X lo-* for iodide, 5.6 X for bromide, and 1.5 x for nitrate, with the commercial liquid electrode (6). The response times for measurements of the more concentrated solutions of perchlorate were of the order of 30 to 60 seconds. The response time for measurements of the 10-5M solution was approximately 120 seconds. CONCLUSION Electrodes prepared by using liquid ion-exchangers in PVC show approximately the same characteristics as the commercially available electrode. The PVC-disk perchlorate electrode has the same linear range as the commercial electrode, but the PVC-wire perchlorate electrode had a shorter linear range. The selectivity of the electrode was improved by incorporating the exchanger in a PVC matrix.
ACKNOWLEDGMENT The authors wish to thank J . D. R. Thomas for a generous supply of poly(viny1 chloride). Received for review June 6, 1973. Accepted October 25, 1973. The financial assistance of the Environmental Protection Agency (Grant No. R-800359) is gratefully acknowledged.
Change in Potential of Reference Fluoride Electrode without Liquid Junction in Mixed Solvents Kathleen M . Steltingl and Stanley E. Manahan2 Department of Chemistry. University of Missouri-Columbia.
Columbia. Mo. 6520 1
A major problem in the determination of formation constants of metal-organic solvent complexes is liquid junction potentials in mixed solvent systems. This is particularly true in the determination of the formation constants of weak complexes where it is necessary to add organic solvent ligand to such a n extent that an appreciable fraction of the solvent medium no longer is water. The fluoride electrode ( I ) used in cells without liquid junction provides some unique possibilities as a reference electrode. The electrode is quite stable: it has a low impedance; it is relatively interference-free: and. in many cases. the low concentrations of fluoride ion required to poise the potential of the electrode can be tolerated in a medium without detrimental effect upon the system. This paper describes the use of the fluoride electrode as a reference in a cell without liquid junction for the determination ofsilver-acetonitrile complexes. It shows that discrepancies in this system can be explained on the basis of altered solubility of lanthanum fluoride in a medium containing an appreciable mole fraction of solvent as acetonitrile. Present address. Department of Chemisrry. California State University. Fresno. Calif. 93710. .4uthor to whom inquiries should be addressed. ( 1 ) Stanley E. Manahan.Ana/. C h e m . . 42, 128 (1970)
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ANALYTICAL CHEMISTRY, VOL. 46, NO. 4 , APRIL 1974
EXPERI-MENTAL Apparatus. For the attempred deterniinarion or formation constants in a cell wirhout liquid junction. a dual electrode sysrem consisting of a solid-stare silver, sulfide indicating electrode (Orion S o . 94-16)and a solid-state fluoride elecrrode (Orion S o . 94-0921) was used. The elecrrode system was conlained in a jackered glass cell regulated to 25.00 i 0.05 "C with a constant temperarure bath. The solution in the cell was srirred rnagrietically during nieasurernents. The silver $sulfide elecrrode wab curinected to the measuring side of a Corning Model 12 expanded scale p H meter and rhe fluoride electrode was connected directly to the reference input. a configuration permitted by the low impedance of the fluoride elecrrode. An electrode system with liquid junction consisted ot a silver sulfide electrode and a conventional aqueous calomel electrode with a 1 . O S S a C l filling solution bridged with an agar bridge made up with O,lOOAbf sodium perchlorate. For studies involving the glass sodium electrode (Beckman S o . 39137). the glass electrode was connected to the high impedance terminal of the merer and the fluoride or calomel electrode to the low impedance terminal. Subsequent mention of "reference electrode" in this work does not imply the terminal t o which the electrode was connected--e.p.. the glass electrode used as a reference was actually connected to the measuring terminal. Reagents. Acetonitrile. Eastman Chromatoqualit:,- Reagent Grade. was used without additional purification. Sodium perchlorate supporting electrolyte was prepared by neutralization of perchloric acid (hlallinckrodt AR Grade) with sodium hydroxide. Solutions were stored in polyethylene bottles.
