Determination of Amino-Nitrogen in Urine

Abstract

Despite the importance of an understanding of amino-acid metabolism, there has been no trustworthy method of amino-nitrogen determination that could be easily applied as a routine in a clinical laboratory. The chief difficulty has been in the preparation of filtrates of blood and urine for the actual titrations. In this note a method is described for the removal of NH₃ and CO₂ from urine filtrates enabling one to determine amino-nitrogen expeditiously by the Sörensen titration. Many determinations may be carried out in parallel with little supervision by the analyst.

The Sörensen 1 titration as described by Northrop 2 is used. The preparation of urine filtrates follows the procedure of Van Slyke and Kirk. 3 Instead of distilling the NH₃ and CO₂ successively in vacuo, the filtrates are exposed in shallow layers in vacuo in a desiccator over dilute H₂SO₄. This means of collecting or removing NH₃ has been known for a long time and occasional reference is made to it in the current literature. It requires no further effort on the part of the analyst, in contrast to the individual distillations in vacco as described by Van Slyke and Kirk or as further improved by Kirk. 4 Quantitative transfer of solutions from the large inaccessible surface of the flask is likewise avoided.

In preliminary trials it was found that NH, could be removed from ammonium chloride in about 3 hours. With urine filtrates, however, 5-7 hours was required for constant titration figures. The alkaline filtrate remains unchanged in the refrigerator for several days so that analyses may be repeated.

Urine filtrates after Van Slyke and Kirk (5 ml.) are placed in 50 ml. beakers with as little exposure to air as possible. The lips of the beakers may be paraffined to facilitate pouring.