Treasure of the Past VI: Standard Potential of the Silver-Silver-Chloride Electrode from 0[degrees] to 95[degrees] C and the Thermodynamic Properties of Dilute Hydrochloric Acid Solutions - Statistical Data Included

Journal of Research of the National Institute of Standards and Technology, March-April, 2001 by Roger G. Bates, Vincent E. Bower

From electromotive-force measurements of the cell without liquid junction:

Pt; [H.sub.2], HCl (m), AgCl; Ag

through the range 0[degrees] to 95[degrees] C, calculations have been made of (1) the standard potential of the silver-silver-ehloride electrode, (2) the activity eoeffieient of hydrochloric aeid in aqueous solutions from m (molality) = 0 to m=0.1 and from 0[degrees] to 90[degrees] C, (3) the relative partial molal heat content of hydrochloric acid, and (4) the relative partial molal heat capacity of hydrochloric acid.

The extrapolations were made by the method of least squares with the aid of punch-card techniques. Data from at least 24 cells were analyzed at each temperature, and 81 cells were studied at 25[degrees] C. The value of the standard potential was found to be 0.22234 absolute volt at 25[degrees] C, and the standard deviation was 0.02 millivolt at 0[degrees] C, 0.01 millivolt at 25[degrees] C, and 0.09 millivolt at 95[degrees] C. The results from 0[degrees] to 60[degrees] C are compared with earlier determinations of the standard potential and other quantities derived from the electromotive force.

1. Introduction

The silver-silver-chloride electrode is employed extensively in the determination of ionization constants and other thermodynamic data by the electromotive-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 temperature.

Electromotive-force measurements of cell A

Pt; [H.sub.2] (g, 1 atm), ECl (m), AgCl; Ag, (A)

at values of m sufficiently low to be useful in determining the standard potential by extrapolation to zero molality have been made by a number of investigators (2 to 16]. [2] The measurements of Guntelberg were made at 20[degrees] C, and all of the other investigations, except that of Harned and Ehlers which covered the range 0[degrees] to 60[degrees] C, were confined to 25[degrees] C. Rccently, Harned and Paxton [17] have calculated the standard potential for the range 0[degrees] to 50[degrees] C from the electromotive force of cells of type A containing aqueous mixtures of hydrochloric acid and strontium chloride. In connection with the establishment of pH standards, the standard potential was needed in the range 60[degrees] to 95[degrees] C. In view of the extensive use of this electrode in electrochemical studies, it was deemed desirable to redetermine the standard potential at lower temperatures as well.

The measurements reported here were made at 17 temperatures from 0[degrees] to 95[degrees] C and ware limited to molalities between 0.001 and 0.12. The number of cells studied ranged from 24 at 45[degrees] C and 55[degrees] C to 80 at 60[degrees] C and 81 at 25[degrees] C. The equations used for extrapolation were obtained by the method of least squares. Punchcard techniques aided in the calculation.

2. Experimental Procedures

Hydrochloric acid of reagent grade was distilled in an all-glass still; the middle fraction (about two-thirds) of the distillate was collected and redistilled. The middle fraction of the distillate from the second distillation was diluted, as needed, with water to about 0.1 m and was standardized gravimetrically by weighing silver chloride. Test of the undiluted acid revealed no bromide [18]. One of the three 0.1-m stock solutions was standardized three times over a period of 8 months; the concentration appeared to have changed only 0.02 percent in that time.

The cell solutions were prepared as needed by diluting portions of the stock solutions with water that had a conductivity of about 0.8X[10.sup.-6] [ohm.sup.-1] [cm.sup.-1] at room temperature. Dissolved air was removed from most of the solutions by bubbling nitrogen; the rest of the solutions were saturates with hydrogen or boiled under vacuum. When the latter procedure was used, the weight of the solution was determined after boiin so that the final concentration could be calculated accurately. The electrolytic hydrogen, obtained in cylinders, was purified by passage over a platinum catalyst at room temperature and then over copper at 500[degrees] C.

Each of the cells, described elsewhere [19], contained two hydrogen electrodes and two silver-silver-chloride electrodes. The latter were of the thermal-electrolytic type [2, 20]. The silver oxide from which they were prepared was washed 40 times with distilled water. The 1-M hydrochloric acid in which they were chloridized was a distilled sample free of bromide. The electrodes were prepared at least 24 hours before use. For the high-temperature series (60[degrees] to 95[degrees] C), the cells were provided with extra hydrogen saturators consisting of three chambers, as described by Bates and Pinching [21].

Two calibrated potentiometers were used. The standards of electromotive force were a pair of staturated Weston cells maintained at a tempaerature near 36[degrees] C in a themostated box of the type described by Mueller and Stimson [22]. three constant-temperature baths were employed; water baths were used from 0[degrees] to 60[degrees] C and an oil bath from 60[degrees] to 95[degrees] C. The temperature was regulated to the desired even temperature within the limits of [+ or -] 0.02 deg C from 25[degrees] to 80[degrees] C and [+ or -] 0.03 deg C from 0[degrees] to 20[degrees] C and above 80[degrees] C. Temperature measurements were made with a platinum resistance themometer. The difference of temperature between the oil bath and the solution in a cell immersed in the bath and the solution ain a cell immeresed in the bathe was found to be less tha 0.1 deg C at 90[degrees]C.