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 withou t liquid junction: Pt; H 2, HCI (m), AgCI; Ag thro~l gh the range 0° to 95° C, calculations have been made of (1) t he standard potential of the s Ilver-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 a id of punch-card techl1lqu es . 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 t he standard deviation was 0.02 millivolt at 0° C 0.01 milli volt at 25° C and 0.09 millivolt at 95° C. The resul ts from 0° to 60° C are compared with earlier d ete rll1ina~ t lOns of the sta ndard potential and other quantities derived from the electromotive force. 1. Introduction cells tudied ranged from 24 at 45° 0 and 55° 0 to 80 at 60° 0 and 81 at 25° O. The equations used The silvel'- silver-chloride electrode is employed for extrapolation were obtained by the method of extensively in the dctermination of ionization con­ least squares. Punchcard techniques aided in the stants and other thermodynamic data by the electro­ calculatiol1. motive-force method [1].1 It is therefore important that the standard potential of this electrode be 2 . Experimental Procedures known as accurately as possible over a wide range of tempcrature. Hydrochloric acid of reagent grade was distilled Electromotive-force measmemen Ls of cell A in .an all-gla s still ; the middle fraction (about two­ thu'ds) of the distillate wa collected andl'edistilled. Pt; Hi (g, 1 atm ), HCl (m) , AgOl; Ag, (A ) The middle fraction of the distillate from the second distillation was diluted, a needed, with water to at value of m suffi cien tly low to be useful in deter­ about .O.~ m ~nd was s~andard iz ed gravimetrically mining the tandard potential by extrapolation to by wmghmg SlIver chlonde. Test of the undiluted zero molality have been made by a number of acid revealed no bromide [18]. One of th e three investigators [2 to 16]. 2 The measurements of Glin­ O.l-m stock solutions was tandardized three time telberg were made at 20 ° 0, and all of the other over a period of 8 months ; the concentration ap­ investigations, except that of Harned and Ehlers peared to have changed only 0.02 percent in tbat which covered the range 0° to 60° 0, were confined time. to 25 ° O. R ecently, Harned and Paxton [17] have The cell solutions were prepared as needed by calculated the standard potential for the range 0° diluting portions of the stock solutions with water to 50 ° 0 from the electromotive force of cells of that had a conductivity of about 0.8 X 10- 6 ohm- 1 type A containing aqueous mixtures of hydrochloric cm- 1 at room temperature. Dissolved air was re­ acid and stron tium chloridc. In connection wi th moved from most of the solutions by bubbling the establishment of pH standards, the standard mtrogen; the rest of the solutions were saturated potential was n eeded in the range 60° to 95° O. In with hydrogen or boiled under vacuum. When the view of the extensive use of this electrode in electro­ latter procedure was used, the weight of the solution chemical studies, it was deemed desirable to redeter­ was determined after boiling so that the final con­ mine the standard potential at lower temperatures centratiol?- could be calculated accurately. The as well. elec.trolytlC hydrogen, obtained in cylinders, was pUrIfied by passage over a platinum catalyst at The measurements reported here were made at 17 room temperature and then over copper at 500° C. temperatures from 0° to 95 ° 0 and were limited to .Each of the cells, described elsewhere [19], con­ molalities between 0.001 and 0.12. The number of tamed two hydrogen electrodes and two silver­ 1 Figures in brackets indicate the literatlUc references at the end of this paper. silver-chloride electrodes. The latter were of the ' T he calculation of the standard poten Ual from the data of Ham cd and Ehlcrs thermal-electrolytic type [2 , 20]. The silver oxide has been exa mined by Hamcd and Wright flO]. Prentiss and Scatchard [11) Hamel', Bunoll, and Acroe [12). Hills and I ves [13). and Swinehart [14). ' from which theYa:were prepared was wa hecl40 times 283 with distilled water. The ] -1\1£ hydrochloric acid in used in this work, h was about 40 mm. The cor ­ which they were chloridized was a distilled sample rection therefore amounts to 0.02 m v at 25° C, 0.03 free of bromide. The electrodes were prepared at mv at 6Co C, 0.( S mv at 90 ° C, and 0.] 6 mv at 95° C. least 24 hours before use. For the high-temperature Nevertheless, the corrections were not applied to the series (60° to 95° C), the cells were provided with electromotive-force da ta and standard poten tials extra hydrogen saturators eonsistin.g of three cham­ reported here, in order that these results could be bers, as described by Bates and Pinching [21 ). used directly in other studies where the average jet Two calibrated potentiometers were used. The dep th is about the same (namely, 4 em) as in this standards of electromotive force were a pair of satu­ inves tigation. The th ermodynamic constants for rated Weston cells maintained at a temperature hydrochloric acid solutions are unaff ected, as they near 36° C in a th ermostated box of th e type de­ depend upon the difference E - E O and its change scribed by Mueller and Stimson [22) . Three con­ with temperature. stant-temperature baths were employed ; water baths were used from 0° to 60 ° C and an oil bath from 60 ° to 95° C. The temperature was regulated to 'the 3 . Standard Potential of the Cell desired even temperature within the limits of ± 0.02 deg C from 25° to SO° C and ± 0.03 deg C from 0° From the equation for the electromotive force, E , to 20° C and above SO° C. T emperature measure­ of cell A one can write ments wer e made with a platinum r esistance ther­ mometer. The difference of temperature between th e oil bath and the solu tion in a cell immersed in the bath was found to be less th an 0.1 deg C at 90° C. where E O is the standard poten tial of th e cell, 'Y± is The cells from which the data for th e range 0° to the stoichiometric mean ionic molal activity co­ 60° C were obtained were measured initially at 25 ° C. effi cien t of hydrochloric acid, and the other symbols The constant-temperature water th ermostat was h ave their usual significance. H arned and Owen lowered to near 0° C overnigh t , and the measure­ [1, chap. ll) have shown that experimental activity ments from 0° to 30° C were made on th e second coeffi cien ts of uni-univalent strong electrolytes up day , fo llowed on th e third day by th e measure­ to 1 m can be expressed wi th high accuracy by an ments from 30° to 60 ° C. A final check of 34 of the equation of the form cells was made at 25° C. The average differen.ce between initial and final values was O.IS mv. The log 'Y± _ - _A_·V,-,'c_r;: +Cc+ (e xt.)-lO g (1 + 0.03604 m), final value was almost alwavs lower than th e initial I + Ba*, c value, and there was some" indication th at a con­ (2) siderable time was required for equilibrium to be establish ed after the rapid drop from the higher where c is the molar concentration, A and B are temperature. Seven of th e cells were measured constants of the D ebye-Hiickel theory, C is an ad­ only in th e range 25° to 60° C. The data for th e j us table parameter, and a * is the ion-size param eter, high range, 60 ° to 95 ° C, were obtained from a and (ext.) represents the total contribution of the separate group of cells immersed in an oil bath. extended terms in the D ebye-Hiickel theory. The initial measurements of th ese cells were made at 25 ° C or at 60 ° C, and the other temperatures ' 'V'hen m do E' S no t exceed 0. 1, c differs from were studied in ascending order. A final check at J mdo, wh ere dO is the density of pure water, b.\' 60° C was sometimes but not always made. less than 1 par t in 1,000. Substitution of mdo The electromotive-force values were corrected to a for c in eq (2) an d combination with eq (1 ) gives par tial pressure of hydrogen of 1 atm.