The Estimation of Non-protein Nitrogen in Blood.*

When Folin and Denis’ described their method for the estimation of non-protein nitrogen in blood, it became possible to make such determinations quickly, accurately, and with the use of only small amounts of blood. The method rapidly came into general use and has proved to be of great service. It has, however, not escaped modification in the interest of increased accuracy and convenience. The present author2 pointed out that the protein precipitant should be employed in aqueous solution, so as to obtain more complete extraction of the water-soluble constituents and more complete precipitation of the fats, lipoids, etc. Trichloroacetic acid, in 2.5 per cent solution, was recommended for this purpose, the trace of protein remaining in the filtrate being removed by shaking with kaolin. Shortly afterward, Bock3 showed that no appreciable amount of nitrogen is split off from the blood proteins in the process of coagulation by heat. It seemed that it might be possible to free the filtrate from the coagulum of the residual protein by means of kaolin. This was found to be the case. If the filtrate from the coagulum was shaken with kaolin until the foam, which was first very voluminous, had almost completely disappeared and was then filtered, the filtrate was water-clear and gave no precipitate with HgCl,, HgI,.SKI, picric acid, etc., even after evaporation to a very small volume (one-tenth that of the original blood). Determinations of the total nitrogen in such filtrates gave results that agreed with those obtained by the trichloroacetic acid-kaolin method (Table I). However, upon closer study, it was found that kaolin removed some of the basic constituents of the liquid. Crentininc was found to be practically completely removed.’ The results of some experiments upon pure materials are presented in Table II. It was evident that the use of kaolin was inadvisable.