The Determination of Urea in Urine by the Urease Method

Marshall’s1 urease method, in the modification proposed by Van Slyke and Cullen, is apparently the most accurate method available for the determination of urea. The method has, however, not yet attained the maximal degree of accuracy to be expected from volumetric processes in general. From the published figures of Van Slyke and Cullen, their modification appears to yield duplicates agreeing within about 1 per cent. Provided the removal of ammonia from the digestion mixture is assumed to be complete, the error must, in all probability, depend upon one of two factors. Either (1) the enzymatic reaction is not absolutely quantitative, or (2) traces of ammonia escape through the acid used for receiving it. As the result of an investigation of this point, the writer has found that, of these two possible sources of error, only the latter is present to a detectable extent. The aim of this paper is to present a further modification of the method which eliminates the above mentioned disturbance. The greater degree of accuracy attainable by the modification to be described, as compared with those previously suggested, depends mainly upon four points. 1. By increasing the volume of fluid from which the ammonia is to be removed, and at the same time decreasing the concentration of potassium carbonate, it is possible so to regulate the rate of removal of the ammonia that every trace of it is held by the acid in the receiver (at the expense, of course, of a certain amount of time). When this part of the process is conducted as described below, it is possible to collect 7 mg. of ammonia nitrogen quanti-