Faraday's Constant
Dieter Britz
> Would anyone know Faraday's Constant and/or its derivation? The constant
> describes the quantity of charge (coulombs) needed to plate 1 mole of
> singly-charged ions out of solution.
It isn't derived, it's measured. The last paper I have seen about its
measurement is by Bower and Davis, J. Res. Natl. Bur. Stand. (US) 85
(1980) 175-191 "The electrochemical equivalent of pure silver - a value
of the Faraday". As the title says, they did it by depositing silver.
Look it up, it's fascinating reading. They followed it up two years later
in Bower, Davis and Murphy, ibid 87 (1982) 21-22, "Recalculation of the
Faraday constant due to a new value for the atomic weight of silver",
which sort of explains itself.
You can't derive everything, some things need to be measured.
-- Dieter Britz alias br...@kemi.aau.dk
Louis A. Coury
Along the same lines, there is an article by M. Towns and Derek Davenport
in Bulletin for the History of Chemistry 11 (1991) 92-100 entitled
"From Electrochemical Equivalency: the Evolution of the Faraday."
They show a plot of the "accepted" value of Faraday's constant versus
year, spanning 1880-1990. One of the points that they make is that
the definition of F is the product of N(a) and e, both of which are
"fundamental" constants whose recommended values are independent
of any electrochemical measurements. (The quantities which are
ultimately involved are the proton rest mass, the gyromagnetic ratio
of the proton, and the proton magnetic moment; each of which are
experimentally determined quantities.)
Anyway, they review the history of the electrochemical methods which
have been used to determine F (silver dissolution coulometry,
iodine coulometry, electrolysis of benzoic and oxalic acids, etc.).
Their conclusion is that the physics and electrochemistry communities
are asymptotically converging on the same value for F, which is
essentially the value measured by Rayleigh in 1884 by electrochemical
means!
--
Lou Coury
Department of Chemistry
Duke University
tho...@ro.com
> On Mon, 30 Oct 1995, Shiran Pasternak wrote:
>
> > Would anyone know Faraday's Constant and/or its derivation? The constant
> > describes the quantity of charge (coulombs) needed to plate 1 mole of
> > singly-charged ions out of solution.
>
> It isn't derived, it's measured. The last paper I have seen about its
> measurement is by Bower and Davis, J. Res. Natl. Bur. Stand. (US) 85
> (1980) 175-191 "The electrochemical equivalent of pure silver - a value
> of the Faraday". As the title says, they did it by depositing silver.
> Look it up, it's fascinating reading. They followed it up two years later
> in Bower, Davis and Murphy, ibid 87 (1982) 21-22, "Recalculation of the
> Faraday constant due to a new value for the atomic weight of silver",
> which sort of explains itself.
>
> You can't derive everything, some things need to be measured.
>
> -- Dieter Britz alias br...@kemi.aau.dk
Faraday's Constant is simply Avagadro's number multiplied by the charge of
an electron (or singly-charged ion):
F = (6.023e23 electrons/mol) * (1.6e-19 Coulombs/electron) = 96500 Coul/mol.