Determination of Free Amino Acids by Titra- Tion of the Carbon Dioxide Formed in the Reaction with Ninhydrin

In the present method the CO2 evolved by decarboxylating amino acids with ninhydrin is transferred to standard barium hydroxide and titrated. The conditions for making the ninhydrin reaction quantitative are those previously worked out for applicat,ion in the manometric method (6, 7). The transfer of the CO, to the barium hydroxide is done by a distillation in vacua which is completed in 2 to 3 minutes. The titration, with added barium chloride to insure complete precipitation of the barium carbonat,c formed, follows a principle used by Krogh and Rehberg (2). After the present method was completed Christensen, West, and Dimick (1) published a procedure in which Van Slyke and Dillon’s (6) conditions for the quantitative ninhydrin reaction were applied, and, as in the present method, the CO2 was distilled into standard barium hydroxide and tit.rated. We nevertheless publish the present method because it is carried out somewhat more rapidly, and with cheaper and simpler apparatus. A pair of small Erlenmeyer flasks attached to a U-tube provides all the special apparatus required for the reaction, the distillation, and the titration. The titration yields results of the same order of constancy as the manometric method in the macroand microanalyses. In the submicroanalysis, however, (carboxyl carbon under 0.1 mg.) the titration shows less constancy than the manometric procedure. When the manometric apparatus is available, the analysis based on it will usually be preferred, particularly for small amounts. The manometric procedure requires fewer precautions against