The outline of this figure represents four ternary solutions in which the component salts have a common acid or basic constituent; viz. sodium chloride—sodium sulphate, sodium sulphate—potassium sulphate, potassium sulphate—potassium chloride, potassium chloride—sodium chloride. These four sets of curves are therefore similar to those discussed in the previous chapter. In the case of sodium and potassium sulphate, a double salt, glaserite [K3Na(SO4)2] is formed. Whether glaserite is really a definite compound or not is still a matter of doubt, since isomorphic mixtures of Na2SO4 and K2SO4 have been obtained. According to van't Hoff and Barscholl,[[389]] glaserite is an isomorphous mixture; but Gossner[[390]] considers it to be a definite compound having the formula K3Na(SO4)2. Points VIII. and IX. represent solutions saturated with respect to glaserite and sodium sulphate, and glaserite and potassium sulphate respectively.
The lines which pass inwards from these boundary curves represent solutions containing three salts, but in contact with only two solid phases; and the points where three lines meet, or where three fields meet, represent solutions in equilibrium with three solid phases; with the phases, namely, belonging to the three concurrent fields.
If it is desired to represent a solution containing the salts say in the proportions, 51Na2Cl2, 9.5K2Cl2, 3.5K2SO4, the difficulty is met with that two of the salts, sodium chloride and potassium sulphate, lie on opposite axes. To overcome this difficulty the difference 51 - 3.5 = 47.5 is taken and measured off along the sodium chloride axis; and the solution is therefore represented by the point 47.5Na2Cl2, 9.5K2Cl2. In order, therefore, to find the amount of potassium sulphate present
from such a diagram, it is necessary to know the total number of salt molecules in the solution. When this is known, it is only necessary to subtract from it the sum of the molecules of sodium and potassium chloride, and the result is equal to twice the number of potassium sulphate molecules. Thus, in the above example, the total number of salt molecules is 64. The number of molecules of sodium and potassium chloride is 57; 64 - 57 = 7, and therefore the number of potassium sulphate molecules is 3.5.
Another method of representation employed is to indicate the amounts of only two of the salts in a plane diagram, and to measure off the total number of molecules along a vertical axis. In this way a solid model is obtained.
The numerical data from which Fig. 124 was constructed are contained in the following table, which gives the composition of the different solutions at 0°:—[[391]]
| Point. | Solid phases. | Composition of solution in gram-mols. per 1000 gram-mols. water. | Total number of salt molecules. | |||||
| Na2Cl2. | K2Cl2. | Na2SO4. | K2SO4. | |||||
| I. | NaCl | 55 | — | — | — | 55 | ||
| II. | KCl | — | 34.5 | — | — | 34.5 | ||
| III. | Na2SO4,10H2O | — | — | 6 | — | 6 | ||
| IV. | K2SO4 | — | — | — | 9 | 9 | ||
| V. | NaCl; KCl | 46.5 | 12.5 | — | — | 59 | ||
| VI. | NaCl; Na2SO4,10H2O | 47.5 | — | 8 | — | 55.5 | ||
| VII. | KCl; K2SO4 | — | 34.5 | — | 1 | 35.5 | ||
| VIII. |
| Glaserite; Na2SO4,10H2O |
| — | — | 10 | 10 | 20 |
| IX. | Glaserite; K2SO4 | — | — | 7.5 | 10 | 17.5 | ||
| X. |
| Na2SO4,10H2O; KCl; NaCl |
| 51 | 9.5 | — | 3.5 | 64 |
| XI. |
| Na2SO4,10H2O; KCl; glaserite |
| 40.5 | 13 | — | 3.5 | 57 |
| XII. | K2SO4; KCl; glaserite | 18 | 23 | — | 3 | 44 | ||
From the aspect of these diagrams the conditions under which the salts can coexist can be read at a glance. Thus,
for example, Fig. 124 shows that at 0° Glauber's salt and potassium chloride can exist together with solution; namely, in contact with solutions having the composition X—XI. This temperature must therefore be below the transition point of this salt-pair (p. [314]). On raising the temperature to 4.4°, it is found that the curve VIII.—XI. moves so that the point XI. coincides with point X. At this point, therefore, there will be four concurrent fields, viz. Glauber's salt, potassium chloride, glaserite, and sodium chloride. But these four salts can coexist with solution only at the transition point; so that 4.4° is the transition temperature of the salt-pair: Glauber's salt—potassium chloride. At higher temperatures the line VIII.—XI. moves still further to the left, so that the field for Glauber's salt becomes entirely separated from the field for potassium chloride. This shows that at temperatures above the transition point the salt-pair Glauber's salt—potassium chloride cannot coexist in presence of solution.
