Part 26 (1/2)
[Ill.u.s.tration: FIG. 118.]
Bi_{2}O_{3}--N_{2}O_{5}--H_{2}O.--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 bis.m.u.th nitrate, and for their equilibrium in contact with solutions.[374]
Three normal salts of bis.m.u.th oxide and nitric acid are known, viz.
Bi_{2}O_{3},3N_{2}O_{5},10H_{2}O(S_{10}); Bi_{2}O_{3},3N_{2}O_{5},4H_{2}O(S_{4}); and Bi_{2}O_{3},3N_{2}O_{5},3H_{2}O(S_{3}). Besides these normal salts, there are the following basic salts:--
{299}
Bi_{2}O_{3},N_{2}O_{5},2H_{2}O (represented by B_{1-1-2}) Bi_{2}O_{3},N_{2}O_{5},H_{2}O ( ” ” B_{1-1-1}) 6Bi_{2}O_{3},5N_{2}O_{5},9H_{2}O ( ” ” B_{6-5-9}) 2Bi_{2}O_{3},N_{2}O_{5},H_{2}O ( ” ” B_{2-1-1})
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 MgCl_{2}--KCl--H_{2}O or for FeCl_{3}--HCl--H_{2}O, 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.
[Ill.u.s.tration: FIG. 119.]
This model shows the three surfaces, A, B, and C, which represent the conditions for the stable existence of the salts B_{1-1-1}, S_{10}, and S_{3} in contact with solution at different {300} 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 B_{1-1-1} and S_{10}; and the ridge _qdc_, solutions in equilibrium with S_{10} and S_{3}. The point _b_, which lies at 75.5, is the point of maximum temperature for S_{10}. If the temperature is raised above this point, S_{10} decomposes into the basic salt B_{1-1-1} and solution. This point is therefore a.n.a.logous to the point M in the carnallite model, at which this salt decomposes into pota.s.sium chloride and solution (p. 284); or to the point at which the salt 2FeCl_{3},2HCl,12H_{2}O decomposes into 2FeCl_{3},12H_{2}O 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 B_{1-2-2} appears. In the neighbourhood of _e_ also there probably exists another quintuple point at which S_{4} is formed. These systems have, however, not been studied.
The following tables give some of the numerical data:--
ISOTHERM FOR 20.
----------------------------------------------------------------------
Composition of the solution. Gram-mols.
in 1000 gm.-mols. of water.
Solid phase.
----------------------------------------
Bi_{2}O_{3}
N_{2}O_{5} -----------------------------
-----------------
---------------------- B_{1-1-1}
10.50
38.65 --
27.20
83.84 B_{1-1-1}; S_{10}
30.15
97.97 S_{10}
29.70
96.57 --
19.65
98.76 --
10.51
162.58 --
33.51
355.87 S_{10}; S_{3}
51.00
403.0 S_{3}
14.35
492.0 --
7.45
592.9 ----------------------------------------------------------------------
SYSTEMS IN EQUILIBRIUM WITH B_{1-1-1} AND S_{10} (CURVE _pabc_).
------------------------------------------------------------
Composition of the solution. Gram-mols.
in 1000 gm.-mols. of water.
Temperature.
----------------------------------------
Bi_{2}O_{3}
N_{2}O_{5} -------------------
---------------
------------------------ 9
26.7
88.2 20 (point _a_)
30.15
97.97 30
33.6
112.3 50
41.8
148.4 65
57.21
190.8 75.5 (point _b_)
87.9
288.4 72 (point _c_)
96.0
327.0 ------------------------------------------------------------
SYSTEMS IN EQUILIBRIUM WITH S_{10} AND S_{3} (CURVE _qde_).
--------------------------------------------------------
Composition of the solution. Gram-mols.
in 1000 gm.-mols. of water.
Temperature.
-----------------------------------------
Bi_{2}O_{3}
N_{2}O_{5} --------------
---------------