| Salt. | Normality of acid. | Temperature. |
| HgCrO4 | 1.41 | 50° |
| 3HgO.CrO3 | 2.6 × 10-4 | 50° |
| Hg(NO3)2.H2O | 18.72 | 25° |
| 3HgO.N2O5 | 0.159 | 25° |
| HgSO4 | 6.87 | 25° |
| 3HgO.SO3 | 1.3 × 10-3 | 25° |
| HgF2 | 1.14 | 25° |
| HgNO3.H2O | 2.95 | 25° |
| 5Hg2O.3N2O5.2H2O | ca. 0.293 | 25° |
| 2Hg2O.N2O5(?) | 0.110 | 25° |
| 3Hg2O.N2O5.2H2O(?) | 1.7 × 10-3 | 25° |
| Hg2SO4 | 4.2 × 10-3 | 25° |
| 2Hg2O.SO3.H2O | 5.6 × 10-4 | 25° |
Mercuric fluoride does not form any basic salt.
Since two succeeding members of a series can coexist only in contact with a solution of definite concentration, we can prepare acid solutions of definite concentration by bringing an excess of two such salts in contact with water.
Indirect Determination of the Composition of the Solid Phase.—It has already been shown (p. [228]) how the composition of the solid phase in a system of two components can be determined without analysis, and we shall now describe how this can be done in a system of three components.[[376]]
We shall assume that we are dealing with the aqueous solution of two salts which can give rise to a double salt, in which case we can represent the solubility relations in a system of rectangular co-ordinates. In this case we should obtain, as before (Fig. 120), the isotherm adcb, if we express the
composition of the solution in gram-molecules of A or of B to 100 gram-molecules of water.
Let us suppose, now, that the double salt is in equilibrium with the solution at a definite temperature, and that the composition of the solution is represented by the point e. The greater part of the solution is now separated from the solid phase, and the latter, together with the adhering mother liquor, is analyzed. The composition (expressed, as before, in gram-molecules of A and B to 100 gram molecules of water) will be represented by a point (e.g. f) on the line eS, where S represents the composition of the double salt. That this is so will be evident when one considers that the composition of the whole mass must lie between the composition of the solution and that of the double salt, no matter what the relative amounts of the solid phase and the mother liquor.
If, in a similar manner, we analyze a solution of a different composition in equilibrium with the same double salt (not necessarily at the same temperature as before), and also the mixture of solid phase and solution, we shall obtain two other points, as, for example, g and h, and the line joining these must likewise pass through S. The method of finding the
composition of an unknown double salt consists, therefore, in finding, in the manner just described, the position of two lines such as ef and gh. The point of intersection of these lines then gives the composition of the double salt.