Bi2O3—N2O5—H2O.—Although various systems have been studied in which there is formation of basic salts,[[373]] we shall content ourselves here with the description of some of the conditions for the formation of basic salts of bismuth nitrate, and for their equilibrium in contact with solutions.[[374]]
Three normal salts of bismuth oxide and nitric acid are known, viz. Bi2O3,3N2O5,10H2O(S10); Bi2O3,3N2O5,4H2O(S4); and Bi2O3,3N2O5,3H2O(S3). Besides these normal salts, there are the following basic salts:—
| Bi2O3,N2O5,2H2O | (represented by B1-1-2) |
| Bi2O3,N2O5,H2O | ( ,, ,, B1-1-1) |
| 6Bi2O3,5N2O5,9H2O | ( ,, ,, B6-5-9) |
| 2Bi2O3,N2O5,H2O | ( ,, ,, B2-1-1) |
Probably some others also exist. The problem now is to find the conditions under which these different normal and basic salts can be in equilibrium with solutions of varying concentration of the three components. Having determined the equilibrium conditions for the different salts, it is then possible to construct a model similar to that for MgCl2—KCl—H2O or for FeCl3—HCl—H2O, from which it will be possible to determine the limits of stability of the different salts, and to predict what will occur when we bring the salts in contact with solutions of nitric acid of different concentrations and at different temperatures.
For our present purpose it is sufficient to pick out only some of the equilibria which have been studied, and which are represented in the model (Fig. 119). In this case use has been made of the triangular method of representation, so that the surface of the model lies within the prism.
This model shows the three surfaces, A, B, and C, which represent the conditions for the stable existence of the salts B1-1-1, S10, and S3 in contact with solution at different
temperatures. The front surface of the model represents the temperature 9°, and the farther end the temperature 75.5°. The dotted curve represents the isotherm for 20°. The prominences between the surfaces represent, of course, solutions which are saturated in respect of two solid phases. Thus, for example, pabc represents solutions in equilibrium with B1-1-1 and S10; and the ridge qdc, solutions in equilibrium with S10 and S3. The point b, which lies at 75.5°, is the point of maximum temperature for S10. If the temperature is raised above this point, S10 decomposes into the basic salt B1-1-1 and solution. This point is therefore analogous to the point M in the carnallite model, at which this salt decomposes into potassium chloride and solution (p. [284]); or to the point at which the salt 2FeCl3,2HCl,12H2O decomposes into 2FeCl3,12H2O and solution (p. [294]). The curve pab has been followed to the temperature of 72° (point c). The end of the model is incomplete, but it is probable that in the neighbourhood of the point c there exists a quintuple point at which the basic salt B1-2-2 appears. In the neighbourhood of e also there probably exists another quintuple point at which S4 is formed. These systems have, however, not been studied.