A CONVENIENT METHOD FOR THE FORMOL TITRATION

The formol titration, as described by Srrensen, 1 has been most useful in the determination of amino-acids and especially in following the course of hydrolysis of proteins. The method as originally described could not be used for accurate determinations of small quantifies of amino-acids owing to the difficulty of determining the exact end-point. The value obtained also depends on the point at which the titration is started. In the light of our present knowledge the success of the method depends upon the fact that in the presence of formalin the titration curve is displaced to the acid side to such an extent that the end-point of the titration is reached at a pH of about 9.0, where a sharp end-point is easily obtained, instead of at about 12.0, where the end-point is very indefinite owing to the buffer effect of the alkali itself. With a solution of a pure amino-acid or peptide, therefore, the titration gives directly the alkali equivalent of the compound. In the case of an unknown solution, however, it is necessary to select some arbitrary pH as the starting point. I t so happens that practically all the amino-acids and peptides whose titration curves have been studied, have an isoelectric zone around pH 6 to 7, and that the proteins also have a flat place in the titration curve in this region although it is not the isoelectric point. In practically all solutions of proteins and their split products, therefore, it is possible to obtain a sharp end-point in this range of pH. I t is, consequently, a convenient point from which to start the titration. The difficulty with the alkaline end-point is largely due to the fact that the formalin affects the color of the indicators so that it is difficult to match the standard exactly. This may be overcome by taking advantage of the property of a one color indicator which makes it possible to vary the end-point