The formol titration of bacteriological media

Abstract

The formol titration devised by Malfatti 1 (1908), Sörensen (1907 2 , 1908 3 , and by Henriques and Sörensen 4 (1909) for the titration of the ammonia and amino acids of urine has been more or less modified by bacteriologists for the titration of media and cultures. Ammonium chloride reacts with formaldehyde to produce hexamethylentetramine and hydrochloric acid. Amino acids and polypeptides react with formaldehyde to produce acid methylene derivatives which are stronger acids than the amino acids from which they are derived. The increase in acidity may be titrated against a standard alkali solution and serve as a measure of the amino acids and ammonia present in the sample.

Sörensen (1907, 1908) pointed out that the reaction of amino acids with formaldehyde is a reversible one. A considerable excess of formaldehyde is required to effect complete conversion of the amino acids into their methylene derivatives, and, since water is a product of the reaction, the presence of too much water serves to throw the reaction back in the opposite direction. Sörensen found it necessary to add as much as 10 c.c. of formalin (40 per cent. formaldehyde) to 20 c.c. of amino acid solution.

The amino acids are ampholytes. If a pure monocarboxylic amino acid is dissolved in distilled water the solution will be found to have a hydrogen ion concentration at or near the isoelectric point of the amino acid, a fact also noted by Eckweiler, Noyes and Falk 5 (1921). If it is titrated with alkali and the titration curve plotted with amounts of alkali added as abscissae and hydrogen ion exponents as ordinates the curve will be seen to drop almost vertically toward the alkaline side as the first drops of alkali are added.