degree the power of influencing the solubility of other sub- stances present in the solution. It might well happen that the undissociated part of the silver acetate in the example cited did not remain constant after increasing quantities of sodium acetate had been added. As a matter of fact, experi- ments indicate that the solubility of undissociated silver acetate is appreciably depressed by the addition of sodium acetate.
These two disturbing factors act in opposite directions. They thus partially compensate each other (in cases so far investigated almost exactly), so that the view of the constancy of the ionic products is far more applicable than might have been supposed.
Distribution of a Base between Two Acids (Avidity). — The condition of equilibrium
is of very great importance for determining the relative strengths of acids and bases. If, for instance, acetic acid is added to a solution of sodium formate, some formic acid and some sodium acetate will be formed. By applying the above equation we obtain as result that the ratio between the quantity of formate and that of acetate is equal to the ratio of the square roots of the dissociation constants of the corre- sponding acids. The ratio between the degrees of dissociation of the two acids at equal concentrations is also the same. The distribution of a base between two acids can easily be <jalculated by means of this rule. Thomsen and Ostwald have experimentally ascertained the value of this distribution for several cases. Thomsen (4) observed the heat change which accompanied the addition of an acid to the solution of a salt of the other acid, whilst Ostwald (o) determined the change of volume or of the refractive power under the same conditions. The fact that a change does take place indicates that a rea<;tion has occurred, and from the magnitude •of the change the extent of the reaction can be calculated. The following table contains some distribution ratios. The
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