This preparation is a double salt of potassium and iron; it is therefore wrongly called tartrate of iron as is commonly heard.
With sulphate of potassa it forms a double salt (manganese alum).
Let us compare, now, the relations between the solubility curve for the double salt, and those for the two constituent salts.
At this point, the double salt can exist together with the single salts in contact with solution.
As can be seen from the diagram, however, the solution in E contains less of component A than is contained in the double salt.
This diagram explains very clearly the phenomenon of the decomposition of a double salt at the transition point.
It must, of course, be understood that the temperature is on that side of the transition point on which the double salt is stable.
In the case of solution Z, first component B and afterwards the double salt will be deposited.
The point of intersection of these lines then gives the composition of the double salt.
If the evaporation is discontinued before the solution has attained the composition F, only double salt will have separated out.
double salt |
A salt that crystallizes from an aqueous solution of a mixture of two different ions. The mineral dolomite (CaMg(CO3)2), for example, is a double salt that crystallizes from a solution containing both calcium and magnesium ions. Double salts exist only as solids. Compare complex salt, simple salt.