Modified procedures for the colorimetric estimation of arginine and histidine.

In the estimation of very small q-qantities of material there is no question that colorimetric is preferable to gravimetric analysis. The colorimetric method is generally quicker and simpler to carry out-an important factor when numerous analyses are involvedand it is not subject to the manipulative errors incurred in dealing gravimetrically with minute amounts of material. The method, however, must be reliable; it must be specific, sensitive and capable of giving consistent and reproducible results. These conditions are not always fulfilled and the colorimetric method is very often neglected in favour of a relatively more dependable gravimetric method. Such is the case with the estimation of arginine and histidine for, although several colorimetric methods are available, unless certain conditions are rigidly observed and considerable experience has been gained, the methods are prone to give erroneous results and many workers prefer the more laborious gravimetric technique. With a view to rendering the colorimetric estimation ofarginine and histidine thoroughly reliable under less stringent conditions, the modified procedures described in this paper are put forward. It is proposed to employ the methods in the ahalyses of solutions of the basic amino-acids obtained by electrodialysis or precipitation with phosphotungstic acid or. by a combination of both. A subsequent paper will be published in this connection.