Standard Potential of the Silver-Silver-Chloride Electrode

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Journal of Research of the National Bureau of Standards Vol. 53, No.5, November 1954 Research Paper 2546 Standard Potential of the Silver-Silver-Chloride Electrode from 0° to 95° C and the Thermodynamic Properties of Dilute Hydrochloric Acid Solutions Roger G . Bates and Vincent E. Bower From electromotive-force measurements of the cell without liquid junction: Pt; H2, HCI (m), AgCI; Ag thro~lgh the range 0° to 95° C, calculations have been made of (1) t he standard potential of the sIlver-silver-chloride electrode, (2) the activity coefficient of hydrochloric acid in aqueous solutIOns from m (molality) = 0 t.o m ~ O.l and from 0° to 90° C, (3) the relative partial molal hcat content of hydrochIol'lc aCId, and (4) the relatIve partial molal heat capacity of hydrochloric acid. The extrapolati?ns were made by the method of least squares with the aid of punch-card techl1lques. Data from at least 24 cells were analyzed at eaeh temperature and 81 cells were studied at 25° C. The value of the standard potential was [ound to be 0.22234 absolute volt at 25° C, and the standard deviation was 0.02 millivolt at 0° C 0.01 millivolt at 25° C and 0.09 millivolt at 95° C. The results from 0° to 60° C are compared with earlier deterll1ina~ tlOns of the standard potential and other quantities derived from the electromotive force. 1. Introduction The silvel'- silver-chloride electrode is employed extensively in the dctermination of ionization con- stants and other thermodynamic data by the electro- motive-force method [1].1 It is therefore important that the standard potential of this electrode be known as accurately as possible over a wide range of tempcrature. Electromotive-force measmemen Ls of cell A Pt; Hi (g, 1 atm), HCl (m), AgOl; Ag, (A) at value of m sufficiently low to be useful in deter- mining the tandard potential by extrapolation to zero molality have been made by a number of investigators [2 to 16].2 The measurements of Glin- telberg were made at 20° 0, and all of the other investigations, except that of Harned and Ehlers which covered the range 0° to 60° 0, were confined to 25° O. Recently, Harned and Paxton [17] have calculated the standard potential for the range 0° to 50° 0 from the electromotive force of cells of type A containing aqueous mixtures of hydrochloric acid and stron tium chloridc. In connection wi th the establishment of pH standards, the standard potential was needed in the range 60° to 95° O. In view of the extensive use of this electrode in electro- chemical studies, it was deemed desirable to redeter- mine the standard potential at lower temperatures as well. The measurements reported here were made at 17 temperatures from 0° to 95° 0 and were limited to molalities between 0.001 and 0.12. The number of 1 Figures in brackets indicate the literatlUc references at the end of this paper. 'ThecalculationofthestandardpotenUalfrom thedataofHamcdandEhlcrs has been examined by Hamcd and Wright flO]. Prentiss and Scatchard [11) Hamel', Bunoll , and Acroe [12). Hills and I ves [13). and Swinehart [14). ' cells tudied ranged from 24 at 45° 0 and 55° 0 to 80 at 60° 0 and 81 at 25° O. The equations used for extrapolation were obtained by the method of least squares. Punchcard techniques aided in the calculatiol1. 2. Experimental Procedures Hydrochloric acid of reagent grade was distilled in.an all-gla s still; the middle fraction (about two- thu'ds) of the distillate wa collected andl'edistilled. The middle fraction of the distillate from the second distillation was diluted, a needed, with water to about .O.~ m ~nd was s~andardized gravimetrically by wmghmg SlIver chlonde. Test of the undiluted acid revealed no bromide [18]. One of the three O.l-m stock solutions was tandardized three time over a period of 8 months; the concentration ap- peared to have changed only 0.02 percent in tbat time. The cell solutions were prepared as needed by diluting portions of the stock solutions with water that had a conductivity of about 0.8X10-6 ohm-1 cm- 1 at room temperature. Dissolved air was re- moved from most of the solutions by bubbling mtrogen; the rest of the solutions were saturated with hydrogen or boiled under vacuum. When the latter procedure was used, the weight of the solution was determined after boiling so that the final con- centratiol?- could be calculated accurately. The elec.trolytlC hydrogen, obtained in cylinders, was pUrIfied by passage over a platinum catalyst at room temperature and then over copper at 500° C. .Each of the cells, described elsewhere [19], con- tamed two hydrogen electrodes and two silver- silver-chloride electrodes. The latter were of the thermal-electrolytic type [2, 20]. The silver oxide from which theYa:were prepared was wa hecl40 times 283

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