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The author's link to NIST is no longer valid, and today the Faraday's constant value is stated to be exact on the NIST website.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.
