[161] ‹Vide› Jorgensen, ‹J. prakt. Chem.›, «16», 349 (1877).
[162] See Chapter XII in regard to the stability of (PtCl_{ 6}^{ 2−}) as a complex ion.
[163] Freshly prepared solutions must be used.
[164] See Chapter XII as to the decomposition of the "complex ions."
[165] K^{ +} and CN^{ −} are ‹colorless› ions. The yellow color of the ion moving to the positive electrode shows the presence of the iron in it—a fact that can be confirmed by testing the solution round the anode for ferricyanide by the method discussed further on in the text.
«CHEMICAL EQUILIBRIUM. THE LAW OF MASS ACTION»
The theory of ionization, as studied so far, gives us simple, rational explanations of many of our qualitative reactions—explanations which agree with phenomena taken from separate fields of investigation. But, if our study of the theory ceased at the present stage without further elaboration, we should fail to find in it a satisfactory explanation of a number of other important facts of analysis—notably, why certain reactions, the occurrence of which we might anticipate, ‹do not take place›. For instance, the addition of a soluble carbonate to a barium chloride solution precipitates almost all the barium as barium carbonate (‹exp.›); we have 2 Na^{ +} + CO_{ 3}^{ 2−} + Ba^{ 2+} + 2 Cl^{ −} → BaCO_{ 3} ↓ + 2 Na^{ +} + 2 Cl^{ −}. But the addition of carbonic acid to barium chloride solutions fails to produce the slightest precipitate (‹exp.›), although carbonic acid also gives rise to the carbonate-ion, CO_{ 3}^{ 2−}. In the same way silver nitrate readily precipitates silver phosphate from sodium phosphate solutions (‹exp.›), but not from a solution of phosphoric acid (‹exp.›). Hydrogen sulphide precipitates zinc sulphide from a zinc sulphate solution (‹exp.›), Zn^{ 2+} + SO_{ 4}^{ 2−} + 2 H^{ +} + S^{ 2−} → ZnS ↓ + 2 H^{ +} + SO_{ 4}^{ 2−}; but the addition of hydrochloric acid effectually prevents the precipitation (‹exp.›), although the hydrogen sulphide is still ionized, as is apparent from the precipitation of copper sulphide when copper sulphate is added to the mixture (‹exp.›). In the negative results, we have instances of a very large number of cases which require closer study, and a further development of the theory, if we wish to interpret them satisfactorily. The line of development to be followed is indicated perhaps most sharply by the following experiment.