The differential titration of mixed strong and weak acids. A method for "free HCl" in gastric contents, etc

It is frequently convenient to be able to distinguish between strong inorganic acids and weaker acids present in the same aqueous solution. Biochemically, such a distinction is required in analysing gastric contents for "free HCl," in titrating blood for "organic acids," in determining free acidity during the course of peptic digestion, and so on. Chemically, it is useful to be able to estimate by direct acid titration moderately strong acid groups combined as salts (such as sodium acetate, sodium potassium tartrate, potassium citrate, lead acetate, etc.). Available methods were either entirely approximate or even positively inaccurate. To remedy this position, a simple modification affording considerable quantitative improvement is now proposed. The difficulty common to current methods (see Hawk, 1931; Hollander, 1931) is chiefly that of distinguishing strong acids (fully dissociated) from other unknown acids only slightly less strong (not fully dissociated), by an endpoint which depends on pH alone. No new choice of indicator can improve the quantitative uncertainty as to how far these unknown moderately strong acids have participated as "strong acids" in an estimation, and, in fact, no improvement is possible unless the acidity of the moderately strong acids is so depressed as to minimize the extent of their interference. The inherent advantage of titration in non-aqueous solvents over ordinary aqueous titrations in this direction has long been realized (Bishop, Kittredge, and Hildebrand, 1922); but the procedure cannot be used in biochemical estimations, where the experimental material is in aqueous solution. Yet it is strange that the obvious device—the basis of which was established by Cray and Westrip (1925)—of adding miscible organic solvents (acetone) to these aqueous solutions, has not been adequately exploited as a simple practical method. Preliminary notice of such adaptation has already been given (Richardson, 1932), but detailed presentation has been delayed to permit treatment of other aspects of mixed solvent titration (p. 121).