I was reading someone's biography, where the writer stated his early interest in Faraday's constant measurements (this is set six decades ago). My understanding was that Faraday constant is a true constant but it turns out it is not or it was not considered a true constant for a long time. Different experimental measurements give different results, and the reasons were not fully understood yet! As per the biography, the difference was due to fundamental reasons. Now we are talking about very very small differences but in the world of constants this is not normal. In order to further see the status, as per the author NIST used to state two values for the Faraday's constant Link to the essay on ResearchGate

The Faraday constant as a fundamental physical value has its peculiar features, which make it standing out of the other physical constants. According to the official documents of NIST 1, this constant has two values: $$ \begin{array}{l} \mathrm{F}=96485.33289 \pm 0.00059 \mathrm{C} / \mathrm{mole} \text { and } \\ \mathrm{F}^{*}=96485.3251 \pm 0.0012 \mathrm{C} / \mathrm{mole} . \end{array} $$ The second value refers to the "ordinary electric current". The values are determined according to different experimental techniques. The difference amounts $0.000008 \%$, which is a tiny difference, however, such a discrepancy is not characteristic for other physical constants.

The author's link to NIST is no longer valid, and today the Faraday's constant value is stated to be exact on the NIST website. This change must be fairly recent, but no textbook of physical chemistry, new or old ever stated this problem of getting different results by different methods. Has anyone seen this point in mainstream books? I never saw that and it is hard to trace this issue. One cannot even do a exact search of "ordinary electric current" associated with Faraday's constant.

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    $\begingroup$ Regarding mentioned different measurement techniques, there can be still some not yet addressed mutual bias/systematic error. Similar case is for Gravitational constant, where measurements have much better precission than reproducibility. $\endgroup$
    – Poutnik
    Aug 20, 2021 at 8:33
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    $\begingroup$ Interesting. Since the elementary electric charge is now exact by definition, and likewise the Avogadro number, this appears to force the Faraday constant to a single value. Maybe the former two values of the Faraday constant was due to human beings: dueling committees of experts. ;-) $\endgroup$
    – Ed V
    Aug 20, 2021 at 12:01
  • $\begingroup$ @EdV, I will probably ask the author, he is alive. It seem this was something fundamental. He mentions that this something fundamental related to the solvation of electron. People may have artificially fixed it just like Carbon-12 to avoid these headaches. $\endgroup$
    – AChem
    Aug 20, 2021 at 15:38


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