EXP. Potassium ferricyanide, treated with a thiocyanate and ether, does not show the least trace of color; when some concentrated hydrochloric acid is added to the mixture, a perfectly plain, although noticeably faint, pink tint is imparted to the ether solution. Mercuric cyanide, as we have seen (p. 115), is exceptional in its exceedingly small capacity for ionization. Sherrill[461] has found that the dissociation constant for [Hg^{2+}] × [CN^{−}]^4 / [Hg(CN)_{4}^{2−}] has the extremely small value 0.4E−41. Consequently, mercuric-ion must be even more effective than hydrogen-ion (strong acids), in suppressing the cyanide-ion and liberating the ferrous- or ferric-ion, in solutions of ferrocyanides or ferricyanides. In fact, when a solution of potassium ferrocyanide is warmed, for a moment, with some mercuric oxide, the ferrocyanide complex is to some extent decomposed; the liberated ferrous-ion is oxidized, by the excess of mercuric oxide,[462] to ferric-ion, and the presence of the latter, in quantity, is shown by the abundant precipitation of ferric ferrocyanide or Prussian blue, when the mixture is acidified with hydrochloric acid (‹exp.›). [p232] «The Aurocyanide-Ion.»—Gold forms a particularly stable complex ion with cyanide-ion. The constant[463] for the ratio [Au^{+}] × [CN^{−}]^2 / [Au(CN)^{−}_{2}] has the value 10^{−28}. This makes potassium cyanide an excellent solvent for insoluble gold compounds, such as gold sulphide, and the cyanide process for the extraction of gold ores makes use of this property.