Perhaps the most instructive case of this kind, that we can study, is that of iron in ferrous and ferric salts. Exceedingly sensitive tests are known for the ferrous and the ferric ions. Thiocyanates produce an intensely red salt, Fe(SCN)_{3}, when added, for instance, to ‹ferric› chloride; potassium ferrocyanide, K_{4}Fe(CN)_{6}, precipitates ferric ferrocyanide, Fe_{4}[Fe(CN)_{6}]_{3}, Prussian blue, from ferric chloride solutions; ammonium hydroxide precipitates quantitatively the insoluble red ferric hydroxide (exps.). With ‹ferrous› salts, potassium ferricyanide K_{3}Fe(CN)_{6} precipitates ferro-ferricyanide Fe_{3}[Fe(CN)_{6}]_{2}, Turnbull's blue; ammonium sulphide precipitates black ferrous sulphide (‹exps.›). Now, in two of the reagents used, potassium ferro- and ferricyanide, iron is present according to the formulæ given. If one should attempt to demonstrate its presence by means of these tests—among the most sensitive and most reliable tests known in analysis—one would fail utterly. Thiocyanates do not produce even the faintest tinge of pink in potassium ferricyanide solution[163]; ammonium hydroxide does not precipitate any ferric hydroxide (‹exps.›). Ammonium sulphide does not precipitate the least trace of a black sulphide from a ferrocyanide solution, and when the latter is mixed with the ferricyanide solution, no trace, either of Prussian or Turnbull's blue, is shown (‹exps.›). The contrast between the behavior of these salts and ferrous and ferric salts is now sharply and definitely interpreted, as being the result of the contrast in their ionization,—the color tests we use are extremely sensitive tests only for the ‹ferric› and ‹ferrous ions›, Fe^{3+} and Fe^{2+}, respectively,—but potassium ferrocyanide ionizes into potassium ions and the negative ferrocyanide ions Fe(CN)_{6}^{4−}, and shows the actions of ferrous ions as little as chlorate ions ClO_{3}^{−} exhibit the reactions of chloride ions Cl^{−}. Potassium ferricyanide, in turn, gives rise to trivalent, negative ferricyanide ions Fe(CN)_{6}^{3−} and not to ferric [p089] ions.[164] If any doubts arise on this point, one can decide the question readily by experiment. When a concentrated solution of potassium ferricyanide is placed in a U-tube under a solution of some colorless electrolyte, such as sodium sulphate, and plates connected with a battery are inserted, there is no difficulty (‹exp.›) in seeing that the yellow ion,[165] containing the iron, moves to the ‹positive› pole and ‹not› to the ‹negative›. The iron is, therefore, as a matter of experiment, ‹part of a negatively charged substance›. That iron is really present in these compounds can be shown most effectively if we destroy the salts: