Determination of the value of the faraday with a silver-perchloric acid coulometer

Abstract

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...

The electrochemical equivalent of pure silver - a value of the Faraday

“…The silver perchlorate is present in the solution to reduce to insignificant proportions the already meager spontaneous dissolution of silver into perchloric acid. The work establishing these desirable conditions was done at the National Bureau of Standards (NBS), by Craig et al [4] in preparation for his determination [5] of the Faraday.…”

Electrochemical analysis section :

“…Over a large number of experiments, n, however, n i™ Z AI? = (36) The errors of type (1) include such things as day-to-day variation of the standard frequency, diurnal variation of the standard resistor, standard cells, air density (in corrections of mass to vacuum), sample transfer losses, etc. Errors of type (2) may include end-point uncertainty, sensitivity of the balance, switching error, etc.…”

Pilgrims' progress in search of the fundamental constants

“…The horizontal lines correspond to determinations by Bender and Driscoll line is the most recent, very accurate, re-measurement of the Faraday constant at the National Bureau of Standards. 35 Figure 11 shows what happens to this diagram if we adopt the value of a calculated from the hyperfine splitting in hydrogren with the uncertain proton-structure correction terms. The diagram is the same as before, save that the ellipse of error and the entire complex of lines moves to higher values of e and lower values of N by 3 times 26 ppm.…”

“…31 and 32) Ne = C, (Ref. 35) a'e-1 = C t (Refs. 29 and 30) wherein the C's are stochastic numerics depending only on the results of the physical measurements referred to above by bibliographical reference number and on "auxiliary constants".…”