Application to the Characterization of Racemates.—The form of the isothermal solubility curves is also of great value for determining whether an inactive substance is a racemic compound or a conglomerate of equal proportions of the optical antipodes.[[363]]
As has already been pointed out, the formation of racemic compounds from the two enantiomorphous isomerides, is analogous to the formation of double salts. The isothermal solubility curves, also, have a similar form. In the case of the latter, indeed, the relationships are simplified by the fact that the two enantiomorphous forms have identical solubility, and the solubility curves are therefore symmetrical to the line bisecting the angle of the co-ordinates. Further, with the exception of the partially racemic compounds to be mentioned later, there is no transition interval.
In Fig. 111, are given diagrammatically two isothermal solubility curves for optically active substances. From what has been said in the immediately preceding pages, the figure ought really to explain itself. The upper isothermal acb represents the solubility relations when the formation of a racemic compound is excluded, as, e.g. in the case of rubidium d- and l-tartrates above the transition point (p. [265]). The solution at the point c is, of course, inactive, and is unaffected by addition of either the d- or l- form. The lower isothermal, on the other hand, would be obtained at a temperature at which the racemic compound could be formed. The curve a′e is the solubility curve for the l- form; b′f, that for the d- form; and edf, that for the racemic compound in presence of solutions of varying concentration. The point d corresponds to saturation for the pure racemic compound.
From these curves now, it will be evident that it will be possible, in any given case, to decide whether or not an inactive body is a mixture or a racemic compound. For this purpose,
two solubility determinations are made, first with the inactive material alone (in excess), and then with the inactive material plus excess of one of the optically active forms. If we are dealing with a mixture, the two solutions thus obtained will be identical; both will have the composition corresponding to the point c, and will be inactive. If, however, the inactive material is a racemic compound, then two different solutions will be obtained; namely, an inactive solution corresponding to the point d (Fig. 111), and an active solution corresponding either to e or to f, according to which enantiomorphous form was added.
Partially racemic compounds.[[364]] In this case we are no longer dealing with enantiomorphous forms, and the solubility of the two oppositely active isomerides is no longer the same. The symmetry of the solubility curves therefore disappears, and a figure is obtained which is identical in its general form with that found in the case of ordinary double salts (Fig. 112). In this case there is a transition interval.
The curves acb belong to a temperature at which the partially racemic compound cannot be formed; a′dfb′, to the temperature at which the compound just begins to be stable in contact with water, and a″ed′f′b″ belongs to a temperature at which the partially racemic compound is quite stable in contact with water. Suppose now solubility determinations, made in the first case with the original material alone, and then with the original body plus each of the two compounds, formed from the enantiomorphous substances separately, then if the original body was a mixture, identical solutions will be obtained in all three cases (point c); if it was a partially racemic compound, three different solutions (e, d′, and f′) will be obtained if the temperature was outside the transition interval, and two solutions, d and f, if the temperature belonged to the transition interval.