Reference Electrode for Electrochemical Studies in Fused Alkali Metaphosphates Roy D. Caton, Jr., and Clinton R. Wolfel Department of Chemistry, The University of New Mexico, Albuquerque, N. M. 87106
Concentration cells of the following type were studied: Corning
Corning
Ag!AgPO3(ml), solvent solvent, AgP03(0.40m)/Ag where the solvent was equimolar NaP03-KP03 melt at 702 OC. Concentrations ranged from 0.04 to 1.225 molal and the Corning 1720 glass served to isolate electrode compartments while maintaining electrolytic contact with the melt. The data yielded a Nernst slope of 0,1944 (=k0.0017) in the concentration range 0.04 to 0.7 molal compared to a theoretical value of 0.1935 at 702 O C . A negative deviation from Nernstian behavior occurred above 0.7 molal. Twelve separate electrode pairs each containing 0.4m Ag(1) and Constructed over a period of four months were reproducible to within 1.9 mV. The asymmetry potential of the reference electrode was found to be 16.6 & 1.8 mV, obtained by measuring cells of the type Ag 1 AgPO3(ml)j 1720 glass lAgP03(ml) I Ag. Constant current coulometric studies established the fact that silver is not transported through the Corning 1720 glass membrane. ALTHOUGH SEVERAL ELECTROCHEMICAL studies have been made in fused alkali metaphosphates ( I ) , no description of a stable reference electrode for EMF studies in such solvents has been described. For this reason, a search was made for a suitable half-cell which could serve as a convenient and stable reference electrode. In the present work, the behavior of the Agl Ag (I) couple in fused equimolar NaPOrKPOs was investigated at 702 OC using concentration cells of the type Ag 1 AgPOdml) (1 1 :1 NaP03-KP0311 AgPO3(m2)1 Ag, where the double slashes consist of a Corning No. 1720 glass membrane. The Nernstian response of such cells was established, and further studies were made to establish the stability of any given reference half-cell, the reproducibility of response among several identical electrodes, the existence of an asymmetry potential for individual electrodes, and the mode of transport through the glass membrane. EXPERIMENTAL
Apparatus. The fused salt assembly employed in this work is shown in Figure 1. The electrode manifold contained four holders arranged around the central thermocouple holder, such that four electrodes could be accommodated during any given experiment. Neoprene O-ring seals contained in the electrode holders permitted the raising and lowering of electrodes while the system was under vacuum or under an argon atmosphere. A Hevi-Duty, Type 86, electric furnace was used in conjunction with a West Gardsman Model JP temperature controller. A chromel-alumel thermocouple located between the furnace coils and the cell served as the sensing element to Present address, Research and Development Center, Westinghouse Electric Corporation, Pittsburgh, Pa. 15235. (1) R. D. Caton, Jr., and H. Freund, ANAL.CHEM., 35,2103 (1963). 660
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, M A Y 1971
the controller, the controller being at the proper setting to maintain the melt inside the cell at the desired temperature. Melt temperatures were measured using the thermocouple immersed in the melt, and they deviated less than & 2 "C during experimental measurements. EMF measurements were made using a Rubicon Model 2745 potentiometer. Reagents. Potassium and sodium metaphosphates were prepared as described elsewhere (I, 2). Silver metaphosphate was prepared by addition of a 1M solution of NaP03 to a cold AgN03 solution. The resulting white precipitate was filtered, washed with water, and dried under vacuum at 35 "C for several days. The dried product was assayed for silver content by potentiometric titration with a standard NaCl solution. Solutions of A g W 3 in the equimolar NaP03-KP03 glass were prepared by addition of the appropriate amounts of the salt to weighed portions of the 1 :1 glass, followed by melting in a platinum dish at 850 "C for at least 3 hours. The resulting molten glasses were poured onto a stainless steel slab, crushed, and ground to about 10-50 mesh. Potentiometric silver determinations of such glasses were performed to confirm their original composition by weight. Electrode Construction. Silver wire was of 99.999 purity. Construction details of the reference electrode employed in this work are shown in Figure 2. Reproducible silver surfaces were obtained by heating the silver portion of the electrode with a natural gas-oxygen torch until the surface appeared bright and on the verge of melting. Such treatment also provided an intimate contact at the platinum silver junction. The outer envelope of Corning No. 1720 glass served both as a salt bridge and a means of isolating the electrode contents from the solvent. When properly constructed, the entire electrode assembly should not have a resistance exceeding 3000 ohms when dipped into the 1 :1 solvent glass at 700 "C. A similar technique in electrode construction was employed by Bockris and coworkers for their AgIAg(1) reference in LiC1-KC1 (3). Procedures for Concentration Cell Studies. The platinum crucible was charged with the required amount of 1 : 1 glass solvent at room temperature. Several electrodes were then filled with solid 1 :1 glass of known AgP03 concentration, and the electrodes were then mounted in the manifold at a sufficient height above the crucible to prevent their contents from melting when the solvent in the crucible was later brought up to the temperature of the experiment. The manifold was securely fixed into position and the cell was alternately evacuated and flushed with dry argon several times before the furnace was turned on. A positive pressure of argon was then maintained for the remainder of the time the cell was in use. After the solvent had reached the desired temperature, the electrodes were lowered into the solvent and allowed to equilibrate for at least 1 hour before EMF data were taken. Procedures for Asymmetry Potential Studies. The procedure was identical to that described above with the following exceptions. The solvent melt in the platinum crucible was (2) Clinton R. Wolfe, Ph.D. Thesis, The University of New Mexico, Albuquerque, N. M., 1966. (3) J. O'M. Bockris, G. J. Hills, D. Inman, and L. Young, J. Sci. Instrum., 33, 438 (1956).
-
A
EPOXY
SEAL
CORNING # 1720 G L A S S , 6 MM. OD.
B
C D
CORNING .zL 7740
GLASS
Figure 1. Fused salt cell assembly A . Electrode holder B. Vent to argon or vacuum C. Stainless steel manifold
Figure 2. Construction of the Ag/ A g P 0 3 reference electrode
D. Neoprene O-ring E. Cooling coils F. Furnace liner G. Reference electrode H . Thermocouple well I. Silver wire electrode J. Nickel A cylinder K. Platinum dish L. Ceramic disk replaced by a melt of known AgP03concentration which was identical to that in the self-contained electrodes. The potential of each electrode was measured us. a silver wire immersed in the bulk solution. A typical arrangement is shown by the placement of electrodes in Figure 1. Procedure for Transport Studies. A silver electrode with a 1 1-mm diameter Corning No, 1720 glass bulb was constructed in the same manner as the smaller electrode described above. After filling with a 0.40m AgP03 glass, the electrode was immersed in the 1 :1 solvent at 700 “C along with a 2 sq. in. platinum counter electrode. A known, constant current of nominally 1.0 mA was passed between the two electrodes for a period of over 8 hours, the silver electrode serving as the anode. At the conclusion of the experiment, the bulb was withdrawn from the melt, crushed, and its contents leached out with hot distilled water. The silver wire with its contact wire was cleaned and weighed to determine the loss in weight of silver. The glass fragments were leached with hot dilute HNOI and the leachings added to the original solution. The total Ag(1) content of the combined leachings was then determined by potentiometric titration with NaCl. RESULTS AND DISCUSSION Preliminary Studies. To ascertain the conditions under which the Ag I Ag(I) half-cell would be suitable as a reference electrode, the reaction of silver metal with the solvent in inert atmosphere and in air was briefly studied. In the presence of air, silver metal was observed to dissolve quite readily. The following reaction, which is analogous to the dissolution of noble metals in the presence of strong complexing agents and oxygen, can account for the dissolution :
4 Ag
+ O2 + 2 M P 0 3
700 OC ___f
2 Ag2MPO4(M
=
Na or K).
The formulas of the compounds are empirical and are not meant to imply the actual nature of the species. The 1 :1 solvent is complex in nature, and the Ag(1) in the melt is undoubtedly highly complexed by the metaphosphate polymers present (4). Under an argon atmosphere, the silver slowly dissolves until an equilibrium concentration is attained. Current voltage curves described elsewhere ( I ) indicate that silver cannot be deposited from melts containing less than 0.04m Ag(1) without reducing the solvent itself. Consequently all Ag(1) concentrations in this work were made up to be greater than 0.04m. Since a 0.4m silver half-cell appeared to be quite stable and well poised, one side of all concentration cells studied in this work consisted of a 0.4mAg(I) solution. Concentration Cells. The results of concentration cell studies at 702 “C indicated Nernstian behavior within the concentration range of 0.04 to 0.7m. Negative deviation from Nernstian behavior was observed above 0.7m. The least squares equation of the Nernst straight line plot obtained from this study up to 0.7m is
E
=
0.0783 (+0.0010)
+ 0.1944 (+O.O017)10g
~ A ~ ( I )
where the numbers in parentheses are standard deviations. The theoretical Nernst slope at 702 “C is 0.1935. The intercept of the plot corresponds to the potential of a 1.OOm Ag(1) half-cell us. a 0.40m Ag(1) half-cell. Asymmetry Potential. When a half-cell is isolated from the solution by means of a membrane, an asymmetry potential may be exhibited. In the concentration cell studies, asymmetry potentials would be expected to roughly cancel out because both half-cells used to construct a cell were each contained in separate glass bulbs. The following cell was used for the measurement of the potential difference between the two surfaces of the glass bulb: Ag 1 AgP03, (mJ I Corning No. 1720 glass 1 AgPOs, (mJ 1 Ag (4) 3. F. Van Wazer, “Phosphorus and Its Compounds,” Vol. I, Interscience, New York, N. Y., 1958.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
661
Table I. Asymmetry Potential Study AEA (mV) f std deva m 0.046 15.7 =t0.5 0.067 20.8 f 1.1 0.084 17.2f 0.6 0.400 15.4 f 0.1 0.400 14.6 f 0.4 0.400 15.8 f 0.0 0.400 17.3 f 0.1 0.400 16.4 f 0 . 2 Average 16.6 f 1.8 a Average of four or more observations per electrode taken over a period of 3-5 hours. Table 11. Comparison of Identical 0.40m Electrodes Experiment No. AE, mVa 1 2.6 2 2.1 3 2.2 4 1.2 5 0.6 6 0.6 I 3.5 8 0.5 9 3.4 10 0.3 11 4.2 12 1.8 Average 1.9 Uncertainty in AEA 1.8 a Average of four or more observations taken over a period of 3-5 hours.
The results are shown in Table I. The potential differences are reasonably reproducible, always identical in sign, and independent of concentration. The effect of the asymmetry potential is to make the electrode encased in the glass bulb 16.6 mV more negative than a silver wire immersed in a solution of the same composition but without the glass membrane.
662
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
Table 111. Typical Coulometric Study Milliequivalents of &(I) Added Found Electrode compartment (as AgP03) 2.113 Constant current anodization 0.300 Loss in electrode weight 0.300 Total Ag(1) 2.413 2.409
Reproducibility. Results obtained during the comparison of identically prepared 0.40m half-cells are given in Table 11. It is interesting to note that the average potential difference between identical electrodes is equal to the uncertainty in the asymmetry potentials. It is quite reasonable, then, to ascribe these small potential differences to differences in asymmetry potential. Transport Study. Since the glass membrane employed in this work served to isolate the half-cell from a solution or a solvent, as well as serving as a salt bridge, it was of interest to see if Ag(1) is readily transported through the glass membrane. The constant current electrolysis of one electrode yielded the results given in Table 111. Virtually all the silver can be accounted for in the bulb itself, indicating that the transport of Ag(1) through the glass membrane is insignificant compared to transport of solvent ions. A qualitative test for Ag(1) in the bulk melt outside the glass bulb was negative. The fact that the constant current anodization of the silver metal occured at 100% current efficiency indicates that Ag(1) concentrations in half-cells could be conveniently adjusted by this method. RECEIVED for review August 18, 1970. Accepted January 18, 1971. Paper presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 7, 1969. This investigation was supported by Contract No. 53-0196 from Sandia Corporation, Albuquerque, N. M. and was performed under auspices of the United States Atomic Energy Commission.
