By comparison with the equations given on p. [292], it is evident, that the only difference lies in the fact that the positive charges, in the present case, are carried to the formaldehyde salt through metal wires and a metal electrode, while previously they were discharged directly by silver ions on the formaldehyde salt.
[587] Cf. Nernst, Theoretical Chemistry (1904), p. 739, and the applications mentioned there.
[588] Le Blanc, Elektrochemie, p. 215; v is the valence of the ion.
CHAPTER XVI SYSTEMATIC ANALYSIS FOR ACID IONS
The systematic analysis for acid ions is made on a plan differing in an important particular from the systematic analysis for metal ions. The latter, as has been seen, are divided into groups, which, by precipitation or solution of characteristic salts, are successively separated from subsequent groups, before the isolated groups are analyzed. That is, in general, a group of metal ions is examined in the absence of the ions of all other groups. Acid ions are also divided into groups, but, as a rule, the groups are not separated from each other for analysis. The reason for this difference in procedure is found, chiefly, in the fact that foreign acid ions interfere[589] to a smaller degree with the specific tests for the ions of a group, than is the case in the analysis for metal ions.
Grouping of Acid Ions.
If acids were present only in the form of the free acids or of their alkali salts, the division into groups could naturally and profitably be made to include groups, which are identified by reactions carried out in neutral or alkaline solution, as well as by such as are made in acid solutions. Now, cations other than the alkali ions are liable to interfere with tests designed for alkaline or neutral solutions. For instance, a group of acid ions, in which the phosphate-ion is included, is characterized by the fact that the acids form barium salts, which are soluble in acid but not in neutral or alkaline solutions. The absence or presence of such a group may be recognized, if no cations other than the alkali metal ions are present, by the addition of barium chloride to the solution and by careful neutralization of any free acid, by ammonium [p300] hydroxide. Barium phosphate and the barium salts of the other acids of the group will be precipitated, if their ions are present. It is clear, however, that a number of metal ions must interfere with the test. For instance, a solution of aluminium nitrate or of ferric chloride, treated with barium chloride, and with ammonium hydroxide to neutralize the acid present in the solution as a result of the hydrolysis of the salt, would give a precipitate of aluminium hydroxide or of ferric hydroxide, and not of barium salts. The formation of a precipitate under these conditions evidently will not constitute any basis whatever for reaching a conclusion as to the presence or absence of acid ions, such as can form precipitates of barium salts under the same circumstances.
Systematic Analysis for Acid Ions, Based on the Removal of Metal Ions other than the Alkali Metal Ions.
The treatment with sodium carbonate, while advantageous in certain cases, is not uniformly successful[590]: it is also frequently complicated by the presence of amphoteric bases or of organic substances, and frequently demands treatments and tests beyond those made with the solution thus prepared. Furthermore, if a group is found to be present, say a group of acid ions forming barium or calcium salts insoluble in water, but soluble in acids,[591] all the acids of the group, which have not been found to be present or absent in the analysis for metal ions,[592] must be specifically [p301] tested for, although they may all be absent and the only representative of the group present may be an ion previously found in the analysis for metal ions.[593] Then, too, when the group is found to be absent by the group test, more sensitive tests for some of the acid ions of the group must still be made to insure their complete absence.[594]