An accurate value of t he faraday has bee n deter min ed by the e lectrolytic dissolution of metall ic silver in aq ueous solu tions of perchloric acid. Standards of electric curren t mass, a nd time a s maintained by t he National Bureau of Standards were utili zed in th~ determinatio ns. Th e electri c c l1l'rent was m easured in terms of th e standards of cle ctromotive force and electrical resistan ce. Silver of hi gh purity, freed fr om oxygen, was used.The value of t he faraday was found to be faraday = 96516.5 ± 2.4 coulombs gram -equival e nt-1 (physical scale). faraday = 96490.0 ± 2.4 coulombs gram -eq ui valent-1 (chemical scale) .These values were obtained usin g 107.9028 ± 0.001 3 and 107.8731 ± 0 .0013 for t h e atomic weigh t of silver on t h e phys ica l and ch e mical scales, resp ectively . ' The electrochemical eq uiva lent of sil ver was found to be electrochemical equivalent of s ilver = 1.117972 ± 0.000019 milligram coulomb-I.This value may be used in a n alternate meth od of defining t h e ampere in absolute value, llamely, t hat steady current which will d issolve 1.117972 milligrams of silver per seco nd a nd de pends only on t he standards of mass and t ime. The indicated un cer tainties are overa ll limi ts of error based on 95 percent co nfidence limi ts for t he mean an d a ll owan ces for t he effects of kn own sources of possible systematic errol'. I . IntroductionThe classic m ethod for the determination of the faraday involves the electrolytic deposition of silver on platinum from an aqueous solu tion of silve)' nitrate. This method h as been extensively studied and tmder closely sp ecified condi tions [1] 1 was used for many years in defining the international amper e. In the original work at th e National Bureau of Standards the national standards of electric current mass, and time were used, and great emphasis wa~ placed on the r eproducibility of the method as a ch eck on the stability of th e existing standards of electromotive for ce (Weston cell) and electrical r esistan ce (mercury ohm). In 1916 Rosa and Vinal [2] sum.-m arized th e work that h ad been done with the silver coulometer not only in yielding a value of the faraday but also in ftttaining conditions wherein reproducible r esults could be obtained. At th at time the consensus was th at tIl e silver coulome ter gave highly accurate as well ftS reproducible r esults and yielded a value of the faraday acc ura te to within 0.001 percent (10 ppm).However , 4 years earlier (in 1912) vVashburn and B ates [3 ] h ad proposed ftn improved 2 iodine coulom-1 F igures in brackets indicate the literature references at th e end of this paper. 2 T he iodine eoulometC!· bad bren studied earlier by H elTOtul [4]. D ann eel 15J, Kreider [6J , and Gallo l7J b ut tbese experimenters did not cla im higb aecm aey for t heir \vork .eter for which high reproducibility and accuracy wer e elaimed. In 1914 Vinal and B ates [8,9] made a direct comparison of the silver and iodine coulometer s in the labor atories of NBS and found that the two...

I .The stabili ty of mill t s ih:er, purified mint silver, and Pyrex fr itted crucibles in aq ueou s solutIO ns of p erchlonc aC I~, In aq ueo us solutions of p erchloric a cid containing silver perchlorate, and 111 cond uctIvI ty water at room t emperature was determined . The stabili t y of the silver in vario us s tates of subdivision was st udied . The corrosion cu rrent-density for mint sil ver i n sheet form is 1.1 X lO-8 amp cnr2 for 20 percent aq ueous solution s of perchloric ncid , 1.3 X 10-9 a mp cm-2 for 20 p ercen t nqueo us solu tions of perchloric acid contallll ng 0.5 p ercent s il ver perchl orate, and withi n the limits of detection is zero for conductivity water. . Pyrex has high stabi li ty at 25° C i n 20 percent aq ueou s sol utions of perchlo nc aCId and In 20 pe rce nt aqu eous solutions of p erchloric acid conta ining 0.5 percen t silv er p crchlorate. Finely-d ivided silver contained in Pyrex crucibles was found to be highly stable whe n the crucibles were filled repeatedly with aqu eou s solutions of perchloric acid containing silver perchlorate, rinsed with conductivity water, and dried a t 105° C. These observations are importa nt in the determination of th e farad ay by the a nodic disso lu tion of silv er in aq ueou s solu tions of perchlor ic a cid which is now underway a t th e K ational Bureau of Standards., .