In the paper of 1887, M. Moissan adopted the view that the first
action of the electric current was to effect the decomposition of the
potassium fluoride contained in solution in the hydrofluoric acid,
fluorine being liberated at the positive pole and potassium at the
negative terminal. This liberated potassium would at once regenerate
potassium fluoride in presence of hydrofluoric acid, and liberate its
equivalent of hydrogen:
KF = K + F.
K + HF = KF + H.
But when the progress of the electrolysis is carefully followed, by
consulting the indications of the amperemeter placed in circuit, it is
found to be by no means as regular as the preceding formulæ would
indicate. With the new apparatus, the decomposition is quite irregular
at first, and does not attain regularity until it has been proceeding
for upward of two hours. Upon stopping the current and unmounting the
apparatus, the platinum rod upon which the fluorine was liberated is
found to be largely corroded, and at the bottom of the U-tube a
quantity of a black, finely divided substance is observed. This black
substance, which was taken at first to be metallic platinum, is a
complex compound containing one equivalent of potassium to one
equivalent of platinum, together with a considerable proportion of
fluorine.
Moreover, the hydrofluoric acid is found to contain a small quantity
of platinum fluoride in solution. The electrolytic reaction is
probably therefore much more complicated than was at first considered
to be the case. The mixture of acid and alkaline fluoride furnishes
fluorine at the positive terminal rod, but this intensely active gas,
in its nascent state, attacks the platinum and produces platinum
tetrafluoride, PtF_{4}; this probably unites with the potassium
fluoride to form a double salt, possibly 2Kl.PtF_{4}, analogous to the
well known platinochloride 2KCl.PtCl_{4}; and it is only when the
liquid contains this double salt that the electrolysis proceeds in a
regular manner, yielding free fluorine at the positive pole, and
hydrogen and the complex black compound at the negative pole.