There exists the possibility of loss of material during transfer. The weighing inevitably required the collection and the transfer of this anodic residue to a filter before the final weighing. It was counted as part of the final weight of the anode. In the experiments of Craig this finely divided material was recovered, filtered and weighed. When one electrolyzes pure silver anodically into a solution of perchloric acid, small quantities of finely divided solid silver fall away from the anode. No further purification was attempted other than an effort to remove oxygen by heating in a high vacuum, a process described elsewhere in the paper. The preparation of the silver sample by the manufacturer is proprietary, but NBS was informed by the manufacturer that high purity silver was subjected to dissolution and electrolysis, and was then melted in vacuum and formed. The last two pieces were near the end of section G. The pieces furnished to us were labeled thus 55: A−7, B−7, D−7, E−7, F−7, G−Y2, G−Y3. These lengths were cut into segments 50 mm long numbered again in the direction of the draw. Rod 55 was divided into 45 cm lengths labeled by letter sequentially in the direction of the draw. The samples furnished to us by the Office of Standard Reference Materials of NBS were from Rod 55 of the lot used for Standard Reference Material 748 Silver Vapor Pressure Standard. From this lot, most of which was issued as a high temperature vapor-pressure standard, were selected a few rods which were characterized by residual resistivity ratio, and set aside for research purposes at NBS. The silver samples were part of a large lot purchased by the National Bureau of Standards for certification and issue as a Standard Reference Material. The formation of Ag 2O and water from silver and aqueous hydroxyl ion during the reaction: No such variation is found, as will be seen. We shall take the view that if the overpotential of dissolution has measurable effect, the values of the electrochemical equivalent of the samples should vary with current density or dissolution rate. The arguments, which our experiment demonstrates to be correct, are presented in Craig’s paper and need not be repeated here. Hence, unless the overpotential of the dissolution of silver is high, the contribution from reactions other than the desired reaction is negligible, i.e. From thermodynamic arguments he shows that the reactions of the constituents of the solutions either follow eq (2) namely at a potential of −0.799 V with respect to a normal hydrogen electrode, or have reduction potentials of at least 0.4 V lower. In his paper, Craig postulated a set of possible reactions which involved the chemical entities Ag +, ClO 4 −, OH −, H +, H 2O, O 2, and so forth, which are known to be present in the solutions where the reaction of eq (2) takes place. The work establishing these desirable conditions was done at the National Bureau of Standards (NBS), by Craig et al in preparation for his determination 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. Where e − represents an electron and s indicates the silver is metallic. At this level, it was hoped, systematic errors which may be present in electrochemical Faraday determinations would yield to more precise analysis than has hitherto been possible. It was our hope that, with a few innovations in the method, the statistical scatter in the measurements could be reduced to something approaching 1 ppm. We have undertaken an electrochemical determination of the Faraday constant using the silver dissolution coulometer. A recent recomputation of this latest measurement now places it in good agreement with the value recommended in but the estimated errors in the measurement are too large to allow the results to be definitive. Since 1973, a new and novel electrochemical Faraday determination has been made. 1 These authors view the statistical evidence as providing an indictment of the electrochemical measurements. The situation is summarized by Cohen and Taylor in their 1973 least squares adjustment of the fundamental constants. In fact, however, the modern electrochemical measurements of the Faraday constant are self-consistent but fail to agree with the constant calculated from eq (1). Where M p is the proton rest mass in atomic mass units, γ p ′ ( low ) is the proton gyromagnetic ratio measured by the so-called low field method, μ p ′ / μ N is the proton magnetic moment in units of the nuclear magneton, and K is the ratio of the as-maintained ampere in terms of which γ p ′ ( low ) is determined to the SI ampere.īoth methods of arriving at the Faraday constant should, of course, agree.
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