Relations between ionization and solubility. Determination of ionization constants by phase equilibria

Abstract

In the preparative laboratory, maximal yields can be obtained by utilizing the ionization constant of the substance being made. When, as is often the case, the medium is water and the substance is present as a dissolved salt, the maximal yield is obtained by adjusting the pH to the value that is, at least, 2 units on the far side of the pK _a. Thus an acidic solution of p-toluidine may be at pH 1, but the pK_a is 5 1 and hence the solution should be adjusted to pH 7 1. If the substance is insoluble in water, this rule gives the nearest pH at which the maximal yield of precipitate can be obtained; if the substance is soluble in water, this rule indicates the nearest pH at which an immiscible solvent will extract the desired substance most efficiently. The principle involved is simply that a neutral molecule is less water-soluble than the corresponding ion, and, if the neutral molecule is too water-soluble to be precipitated, it is more easily shaken out of solution than an ion. Reference to Appendix V shows that when the pH is made equal to the pK_a the substance is still 50% ionized, but when the pK has been exceeded by 1 unit of pH, the substance is about 10% ionized, and when it has been exceeded by 2 units, it is only 1% ionized.