I
”
”
”
”
hydrolyzed surface film, a parallel RC charging curve is obtained which deviates from an exponential growth curve due to dispersion of the double layer capacitance. Electrodes having surface films also show this response at short times, while over a period of seconds concentration polarization within the film is observed. For electrodes without surface films, the time required for the potential to change from the zero current to the constant current steady state will also be equal to the response time of the electrode-i.e., less than 20 msec. The potential-time curve on application of a current step will be different from the potential-time curve obtained by an activity step because in the former, charging of the nonlinear double layer capacitance is involved. In the presence of a surface film it would seem that the response time should be limited by diffusion of ions through the film and it is widely believed that old glass electrodes are sluggish in response. Attempts at the direct measurement of electrode response times (6, 7) do not support this view. The mechanism of ion transport across the film cannot be said to be well understood. Clearly, the film is porous and it may be that under vigorous stirring conditions mass transport is not diffusion limited. For a complete understanding of the time dependence of electrode potentials it is essential that measurements be made of both the kinetics of the ion exchange processes at the membrane interface and of the mobilities of ions within the membrane. It is apparent from this investigation that distinction between these two cannot be made by steady state measurements. The use of pulse perturbations could lead to measurements of kinetic data of the ion exchange processes, providing a more exact description of the membrane double layer capacity can be obtained.
wyTs!rr 2
&
0
1.0
2.0
30
4.0
5.0
6.0
7.0
8.0
4 SEC”2
Figure 14. Test of diffusion control in surface film
Data as in Figure 11 exchange reaction at a membrane permeable to the exchanging ions, R , in fact represents the sum of two resistances in series, one related to the kinetics of the ion exchange reaction and the second representing the mobility of the counter ions in the membrane. Where the glass membrane is coated by a surface-hydrolyzed film, the equivalent circuit has, in addition to that described above, a series Warburg diffusional impedance shunted by a resistance. It is, perhaps, surprising that under an applied electric field the counter ions show any diffusion at all as they are the major charge carrier present in solution. The large resistance in parallel with the Warburg impedance is interpreted as representing the effects of electromigration through ‘pores’ in the surface film. The results of the current step measurements of potentialtime curves are compatible with the equivalent circuit model obtained by impedance measurements. In the absence of a
RECEIVED for review July 7, 1969. Accepted August 21, 1969. This work was supported by a grant from the National Science Foundation. Electronic equipment was provided by the University Program for Scientific Measurement and Instrumentation.
Silver-Silver Nitrate Couple as Reference Electrode in Acetonitrile Byron Kratochvil, Esther Lorah, and Carl Garber Department of Chemistry, University of Alberta, Edmonton,Alberta, Canada The silver-0.01M silver nitrate couple has been investigated as a reference electrodefor potentiometric measurements in acetonitrile. The thermal temperature coefficient of the standard silver-silver(l) couDle in acetonitrile is estimated to be 0.9 mV/OC. The’extent of silver nitrate association was measured and evaluated. The couple is reversible, and its potential is relatively unaffected by those impurities commonly found in acetonitrile.
STANDARD REFERENCE COUPLES normally used in water are not satisfactory in acetonitrile. The hydrogen electrode is unstable and easily poisoned (1). The mercury-mercury(1) halide couples are unsuitable because large mercury(I1) halide formation constants in this solvent cause extensive mercury(1) disproportionation (2). Silver chloride slowly forms a Series of anionic and polynuclear complexes which cause drifting (1) D. J. G. Ives and G. J. Janz, “Reference Electrodes: Theory and Practice,” Academic Press, New York, p 446 (1961). (2) K. Cruse, E. P. Goertz, and H. Petermoeller, 2.Elektrochem., 55, 405 (1951).
potentials in silver-silver chloride electrodes (3); to minimize this drift for polarographic studies, Popov and Geske used a silver-silver chloride reference in which the electrolyte was a saturated solution of silver chloride and trimethylethylammonium chloride (4). We have observed, however, that this electrode is also somewhat subject to drift and, therefore, is not completely satisfactory for potentiometric work (5). Aqueous saturated calomel electrodes have been used as external reference electrodes, particularly for polarography. However, the use of any external reference couple in another solvent gives unknown and variable junction potentials, and introduces the possibility of contamination with the second solvent during accurate measurements (6). (3) G. J. Janz and H. Taniguchi, Chem. Reus., 53, 397 (1953). (4) A. 1. POPOV and D. H. Geske, J . h e r . Chem. SOC.,79, 2074
(1957). ( 5 ) B. Kratochvil and J. Knoeck, unpublished work, Univ. of
Wisconsin (1965). (6) J. F. Coetzee and G . R. Padmanabhan, J. Phys. Chem., 66, 1708 (1962).
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
0
1793
The most widely accepted reference couple for potentiometry in acetonitrile appears to be silver-0.0lM silver nitrate, first used by Pleskov in 1948 (7), and subsequently by others (6, 8-10). We have reassessed the general suitability of this couple as a reference electrode, and report here the effect of temperature variations on the silver-0.01M silver nitrate electrode, along with an estimate of the thermal temperature coefficient for the silver-silver(1) couple. Since conductivity measurements have shown that silver nitrate is partially associated in acetonitrile, even in dilute solutions (ZI), the effect of ion-pairing on the potential has been studied and used to calculate a standard potential for the silver-silver(1) couple in acetonitrile. Also, the effect on the electrode potential of common impurities in acetonitrile, and the reversibility, polarizability, and stability of this electrode are discussed. EXPERIMENTAL
Materials. Anhydrous silver perchlorate was prepared by neutralizing silver carbonate with perchloric acid, recrystallizing the resulting salt from water, and drying in a vacuum at 70 "C for several hours. Silver nitrate (Fisher Certified Reagent) was dried and used as such; analysis by thiocyanate titration showed a purity of better than 99.9%. Tetraethylammonium perchlorate was prepared as described by Kolthoff and Coetzee (12). Acetonitrile (Matheson, Coleman and Bell) was purified according to the procedure described by O'Donnell, Ayres, and Mann (13) except that in some of the work the first step, a preliminary distillation from benzoyl chloride, was omitted since its exclusion did not affect the results obtained. The water content was on the order of 5 X 10-'M by Karl Fischer titration. Apparatus. An H-type cell with two 1-cm ultra-fine glass frits was used for most of the potential measurements. The three cell compartments were interconnected above the liquid levels to equalize any pressure differentials produced during the filling and capping operations. For thermal temperature coefficient measurements a similar cell, having each electrode compartment separately water-jacketed, was used. The electrodes were made by sealing platinum foil into glass and electrodepositing silver onto the platinum from a bath containing 3 grams of AgCN, 2.5 grams of KCN, and 6 grams of K2COI in 100 ml of doubly distilled water. They were stored in a silver nitrate solution in acetonitrile in a drybox when not in use. Spirals of 18 gauge silver wire were used in the polarization measurements; approximately eleven inches of wire was immersed. The bridge solution was 0.1M tetraethylammonium perchlorate in most cases. A Leeds and Northrup Model K-4 potentiometer facility was used for the potential measurements which, except for the temperature coefficient studies, were made at 25.00 f 0.05 "C in a water bath. For temperature coefficient measurements, two circulating water baths were used to control the temperature in each electrode compartment to + O S "C. Procedure. All solutions were prepared in a drybox, and glassware was baked at 150 "C for several hours before introduction into the drybox. For estimation of the temperature coefficient for the silver0.01M silver nitrate electrode, both electrode Compartments -
(7) V. A. Pleskov, Zhur. Fir. Khim., 22, 351 (1948). (8) H. M. Koepp, H. Wendt, and H. Strehlow, 2. Elektrochem., 64,483 (1960). (9) I. M. Kolthoff and F. G. Thomas, J . Pliys. Chem., 69, 3049 (1965). (10)J. F. Coetzee and J. J. Campion, J. Amer. Chem. SOC.,89, 2513 (1967). (11) H.Yeager and B. Kratochvil, J.Phys. Chem., 73, 1963 (1969). (12) I. M. Kolthoff and J. F. Coetzee, J . Amer. Chem. SOC.,79, 870 (1957). (13) J. F. O'Donnell, J. T. Ayres, and C. K. Mann, ANAL.CHEM., 37, 1161 (1965).
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were filled with 0.01M silver nitrate. The bridge compartment was filled with 0.1M tetraethylammonium perchlorate or 0.01M silver nitrate. Silver electrodes were inserted into both electrode compartments ; the closed cell was removed from the drybox and the water jackets were connected to two circulating water baths at different temperatures. After the system had come to equilibrium, the potential difference between the two silver electrodes was recorded as a function of temperature difference between the two compartments. For studies of silver nitrate association, one electrode compartment was filled with silver nitrate and the other with silver perchlorate solution, both of the same formal concentration, and a silver electrode was inserted into each. The bridge solution was 0.1M tetraethylammonium perchlorate. The closed cell was removed from the drybox and placed in a thermostated water bath. Potential readings were taken as a function of time until they either became constant or showed only slight linear drift. A small asymmetry potential, on the order of 0 to 2 mV, was found to exist between the two electrodes when identical solutions were present in the two cell components. The magnitude of this potential changed somewhat with time (see discussion of stability below), so it was determined between each run by measurement of the cell potential with the same solution in both compartments. Temperature effects on silver nitrate association were studied by measuring the cell potential at 25 "C, then changing the bath temperature by 4 to 5 degrees and remeasuring the potential after the bath and entire cell had come to equilibrium at the new temperature. The system was then returned to the initial temperature and the potential m e o w e d once more. The two values obtained at 25 "C were used to draw a base line for drift compensation. To determine the effect on the silver-silver(1) couple of compounds reported to be acetonitrile impurities, both electrode compartments were filled with 0.01M silver nitrate, and solutions of the compound being studied were added in increments to one electrode compartment. The potential difference between the two silver electrodes was recorded initially and after each addition to give a series of values. RESULTS AND DISCUSSION
Temperature Coefficient of SilverSilver Nitrate Reference Electrode. In connection with thermodynamic studies in acetonitrile, the silver-0.01M silver nitrate electrode is being used in this laboratory as a reference for measurements of isothermal temperature coefficients. Since these coefficients involve temperature effects on both half-cell reactions, it is necessary to estimate the thermal temperature coefficient of the silver-0.01M silver nitrate couple. The potentials of the following cells were measured:
(1) Agl0.01M AgN03/10.1M Et4NC10alI0.01M AgN031Ag
TI
Tz
(2) Ag/ 0.01M AgN031 I0.01M AgN031 I0.01M AgN03/Ag
Ti
TZ
(3) Ag10.01MAgN03/ IO.lMEt4NC1O4IIFe(II1,II)-phenl Pt
Ti
Tz
In cell 3, the couple on the right hand side is the trk(1,lOphenanthro1ine)-iron(II1, 11) system. In cells 1 and 2, TI and Tz were varied to give temperature differences up to about 10" C in the region of 25" C. The potential difference between the two silver electrodes was found to be a linear function of temperature difference, with the slope giving the thermal temperature coefficient. Least squares analysis yielded the following values :
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
(dE/di')th. cell 1
=
0.43
* 0.02 mV/"C
(1)
A third estimate of (dE/m)th was obtained by comparing values for the temperature coefficient of cell 3 measured isothermally and thermally. For the isothermal measurements, the temperature of the whole cell was varied, with TI = Tz. For the thermal measurements, TIwas held at 25 " C and Tz varied. The difference between these two values should give the temperature coefficient of the silver-0.0lM silver nitrate couple. Using temperature coefficients measured thermally (14) and isothermally (15) for rnethyl-substituted Iris-( 1 ,lo-phenanthro1ine)-iron(II1, 11) couples, a value of 0.29 f 0.04 mV/"C was obtained. The mean value calculated from the three estimates of (dE/d;r)th is'O.36 =k 0.07 mV/"C. The thermal temperature coefficient measured for the silver0.01M silver nitrate electrode is related to that of the standard silver-silver(1) couple as follows : (dE/fl)th,0.01M
=
+
(dEo/d7)th (R/F)ln (RT/F) (d In U A
U A ~ +f
+ (dE/fl)tljp (3)
. + / ~ )
where (dE/fl),ljPis the temperature coefficient of the thermal liquid junction potential. When the Debye-Huckel limiting law is used to calculate the activity coefficient of silver ion in 0.01M silver nitrate solution, the term (R/F) In U A ~ +is estimated to be -0.43 mV/"C. The change of activity with temperature can be considered negligible compared to the other terms. Because the effect of temperature on silver nitrate association was found to be negligible (see next section), no term incorporating it is included. Therefore, the standard thermal temperature coefficient can be written as
coefficient for the silver couple in water is -0.129 mV/"C (20). The thermal temperature coefficient is directly related to the entropy of electrochemical transport involved in the reduction of silver ion to silver metal:
We see a much more positive entropy change for the reduction of silver ion in acetonitrile than in water, indicating that solvent ordering in acetonitrile solutions of silver nitrate is greater than in aqueous solutions. Several factors may cause this difference. One is that silver ion is known to interact specifically with acetonitrile, as is indicated by the fact that the standard reduction potential is about 0.16 V lower in acetonitrile than in water (see next section), and as is shown by the formation of silver-acetonitrile complexes (8, 21, 22). Another factor is that the presence of a small ion in water decreases ordering by disrupting the high degree of structure normally present. Therefore, any solvent ordering by electrostatic or specific interaction effects would be opposed by initial structure-breaking effects, making the overall entropy change smaller. Although liquid acetonitrile may have some dimeric structure ( 2 3 , it must be considered relatively nonstructured compared to water (24). Thus the structure breaking effects of ions would be negligible and a more positive entropy change would be predicted in acetonitrile. Potentiometric Measurement of Silver Nitrate Association in Acetonitrile. Since silver perchlorate has been shown to be completely dissociated in acetonitrile ( I I ) , the association constant of silver nitrate can be obtained from potential measurements of the cell Aglx M A g N O D /0.1MEt4NC1041 / ( x MAgC104IAg
(4) The thermal liquid junction potential arises because of heats of transfer involved in migration of ions in a thermal gradient (16). Thus the temperature coefficient of the thermal liquid junction potential is a function of the entropies of migration of the ions:
Single ion entropies of migration are not measurable, of course, but in water sufficient data on transport entropies for salts have been obtained so that by making several assumptions, single ion values can be estimated (17-19). However, corresponding data are not available in acetonitrile. The temperature coefficient of the thermal liquid junction potential for 0.01M silver nitrate in water is estimated to be about -0.1 mV/"C, based on single ion migration entropies calculated by deBethune (16) from data of Khoroshin and Temkin (19). Calculations of migration entropies based purely on electrostatics indicate that the value may be slightly higher in acetonitrile than in water, but a purely electrostatic model is not appropriate for the silver ion because of specific interactions in both water and acetonitrile. If, therefore, we take (dE/dZ')tlj,to be -0.1 mV/"C in acetonitrile also, the value for (df?"/mth becomes 0.9 mV/"C with a maximum estimated error of *0.2 mV/"C. The value for the corresponding thermal temperature (14) B . Kratochvil and J. Knoeck, J . Phys. Chem., 70,944 (1966). (15) B . Kratochvil and E. Lorah, unpublished work (1968). (16) A. J. deBethune, J . Electrochem. Soc., 107, 829 (1960). (17) E. D. Eastman, J . Amer. Chem. SOC.,50, 283 (1928). (18) J. N. Agar and W. G. Breck, Trans. Faraday SOC.,53, 167 (1957). (19) A. V. Khoroshin and M. I. Temkin, Zhur. Fiz. Khim., 26, 773 (1952); C A 46, 10967h.
(6)
F(dE"/fl)th= SE*
with the silver perchlorate and silver nitrate solutions having the same formal concentration. Measurements at several concentrations gave the following potential shifts : 0.01 M , 8.5 mV; O.O04M, 4.7 mV; O.O02M, 2.9 mV. The estimated error in these measurements is k 0 . 2 mV. At lower concentrations the potential shift was too small to be measured accurately. The potential shift can be interpreted as due to the difference in concentration of free silver ion in the silver nitrate and silver perchlorate solutions if it is assumed that the silver ion activity coefficients are the same in both solutions. Knowing the free silver ion concentration and the total silver nitrate concentration, a conditional association constant [AgNoal was calculated at each concentration. [Ag+l[NO,-I Plotting log KA' L'S. fi and extrapolating to zero ionic strength gave a thermodynamic association constant value of 74 5 5 . This compares satisfactorily with the value of 70.2 f 0.5 obtained by conductivity measurements (11). The temperature dependence of the potential of the above cell, measured over a range of f5 "C, was found to be negligible. Random changes on the order of 0.06 mV/"C were observed; these may be due to electrode drift during the time KA'
=
(20) A. J. de Bethune, T. S. Licht, and N. Swendeman, J . Electrochem. SOC.,106,616 (1959). ( 2 1 ) C . B. Baddiel, M. J. Tait, and G. J. Janz, J . Phys. Chem., 69, 3634 (1965). (22) G . J. Janz, M. J. Tait, and J. Meier, ibid., 71, 963 (1967). (23) F. E. Murray and W. G. Schneider, Can. J. Chem., 33, 797 (1955). (24) G . J. Janz, A. E. Marchinkowsky, and I. Ahmad, J. Electrochem. SOC.,112, 104 (1965).
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
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required for the cell to come to equilibrium. Thus variations in silver nitrate association with temperature are considered negligible. The potential of the silver-0.0lM silver nitrate electrode can be related to the standard potential by the Nernst equation E"
- Eo.01~=
-0.05916 log U A ~ +
(7)
Kolthoff and Thomas calculated a value for E" - Eo.ol.riof 0.130 V by estimating the activity coefficient for silver ion A oAgN03 taken from from the conductivity ratio h o . ~ l ~ /for data of Walden and Birr (9, 25). Using the potential shift of 8.5 mV measured at 0.01M to calculate a silver ion activity, we find a potential shift of 0.135 V. If the association constant of 70.2 obtained conductimetrically is used to calculate an activity, the value is 0.133 V. In both cases it is necessary to estimate an activity coefficient for the silver ion from the extended Debye-Hiickel equation. The potential of Eo.ol.MA.NO, (acetonitrile) us. the normal hydrogen electrode in water [NHE (water)], based on the rubidium-rubidium(1) couple as a reference between solvents, is 0.503 V (10). Thus EoAg(acetonitrile) us. NHE(water) is 0.637 V. Since E"A, (water) is 0.799 V, the potential of the silver-silver(1) couple in acetonitrile is lower than that in water by 0.162 V. Reversibility, Polarizability, and Stability of Silver-Silver(1) Couple in Acetonitrile. The silver-silver(1) couple has been reported to be reversible in acetonitrile (26, 27). This was confirmed in this study by applying momentarily a 1.5-volt potential across the terminals of the cell described above. The cell potential returned to within a millivolt of its previous reading within two minutes; the polarity of the applied external potential was immaterial. We conclude that the system is adequately reversible for conventional potentiometric measurements. The polarizability was determined by measuring the potential of a cell consisting of two identical silver4.0lM silver nitrate in acetonitrile half-cells as a function of applied external current. Cell potentials on the order of 74 mV were observed for 5.0 pA currents; about 62 mV of this is IR drop, as the cell resistance averaged 12,000 ohms at 1000 Hz. A different cell configuration would undoubtedly reduce this resistance appreciably, but at 5-pA current levels, polarization for a single reference would still amount to about 6 mV, and so for most polarographic work a three-electrode system is recommended when using this electrode. The stability of the couple with time was estimated by following the potential of a cell containing two silver-0.01M silver nitrate half-cells over several days. The half-cells were separated by a fine glass frit. In one run the cell potential varied randomly for the first several hours over a range of 0.3 mV; in a second it drifted unidirectionally about 0.4 mV in this time. On a longer term, the average difference between the two half-cell potentials was 0.16 mV in the first case (5 (25) P. Walden and E. J. Birr, Z . Physik Chem., 144A, 269 (1929). (26) L. Kahlenberg, J . Phys. Chem., 4, 709 (1900). (27) F. K. V. Koch, J . Chem. SOC.1928,524.
1796
days) and 0.3 mV in the second (7 days) with ranges of 0.4 and 1.2 mV, respectively. Effectsof Solvent Impurities on SilverSilver(1) Couple. The most common impurity encountered in acetonitrile is water. Its complete removal is difficult and keeping the solvent in an anhydrous state after purification requires considerable care. Strehlow has reported two formation constants for silver(1) with acetonitrile in aqueous solution (8). Here the inverse system, that is, the interaction of water with silver(1) in acetonitrile, is of interest. By measuring the potential shift produced when weighed amounts of water were added to one side of an H-cell containing identical silversilver nitrate half-cells, a formation constant value of 0.13 f 0.02 was obtained for the first silver(1)-water complex. A second complex was not observed, nor do the data indicate formation of two complexes having K's very close together. As a check on the method, the formation constants for silver(1) with acetonitrile in water were remeasured. Values of 2.9 f 0.3 and 1.1 & 0.3 were obtained for Kl and K z ; the corresponding values reported by Strehlow are 2.7 and 1.1, respectively (8). Other impurities reported to be generally present are unsaturated nitriles (particularly acrylonitrile), acetic acid, and ammonia (28). The latter two form upon hydrolysis of the solvent. All these compounds may be expected to interact with silver(1). Their formation constants with silver(1) were determined in the same way as for water. The following K , values were found [assuming in each case that only a 1 :1 complex was formed with silver (I)] : acrylonitrile, 0.06; acetic acid, 1.6; ammonia (added as ammonium hydroxide), 50; and ammonium acetate, 190. When these constants are used, it is estimated that a concentration of ammonium acetate on the order of 1 X 10-4M will shift the potential of the reference couple about 1 mV, while the effect of the other substances is correspondingly less. In the case of water, a concentration of 0.3M is required to give a shift of 1 mV. Thus, it appears that the couple is relatively insensitive to small amounts of those impurities expected in acetonitrile. In summary, the silver-O.O1M silver nitrate couple appears quite satisfactory as a reference electrode for potentiometric measurements in acetonitrile. Its potential, although affected by silver nitrate association, is sufficiently stable and reproducible. Since the temperature effect on association is negligible, it need not be considered in temperature dependence studies. The effects of impurities commonly found in acetonitrile are small. The couple is reversible, but since it is fairly easily polarized, it is not recommended for polarographic measurements unless a three-electrode system is used. RECEIVED for review May 20, 1969. Accepted August 15, 1969. Support of this work by the National Research Council of Canada and by the University of Alberta is gratefully acknowledged. ~~
(28) J. F. Coetzee, Pure Appl. Chem., 13, 429 (1967).
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
