Manual and automated methods for measuring urea based on a modification of its reaction with diacetyl monoxime and thiosemicarbazide.

Since the reaction between urea and diacetyl monoxime was first described (Fearon, 1939) it has formed the basis of most laboratory methods of estimating urea. Many modifications of the technique as originally described have since been suggested in order to try and overcome some of the difficulties of the strongly acid solutions required, light sensitivity of the coloured product of the reaction, and the deviation of the reaction from Beers Law. The advent of automatic systems of analysis and in particular of the AutoAnalyzerl has reduced the severity of some of these problems but has by no means eliminated them. The method until recently advocated by the Technicon Company, and widely used in Britain, is basically that of Marsh, Fingerhut, and Kirsch (1957) but includes the modified ferric alum acid reagent suggested by Richter and Lapointe (1962). This method utilizes an acid reagent consisting of 33 % v/v sulphuric acid and 33 % v/v phosphoric acid, and a diacetyl monoxime reagent in 15 % w/v sodium chloride. These reagents are expensive to produce and dangerous to use. A further complication is that there is no simple manual procedure based on this reaction which is available for use in emergency determinations. Hence laboratories tend to use the urease method for emergency work resulting in two entirely different chemical principles being used for one determination which one would very much wish to avoid. Recently further modifications of the basic method have been suggested by the introduction of other reagents into the reaction. In Australia Beale and Croft (1961) used phenylanthanilic acid while in the United States thio-semicarbazide and phenazone have proved popular In this paper the thiosemicarbazide method of Marsh, Fingerhut, and Miller (1965) has been modified to provide automated and manual methods suitable for use in a routine laboratory. In the automated method the 10 mm colorimeter flow cell has been replaced by a 15 mm tubular flow cell and one of the air lines has been omitted. These modifications have enabled the sample pump tubing to be reduced in size from 0-035 in. ID to 0-025 in. ID while still achieving an increase in sensitivity sufficient to warrant dilution of all specimens containing urea in excess of 150 mg/ 100 ml which was considered desirable. In the manual procedure concentrations of reagents have been adjusted to give maximum colour development without developing the turbidity which is found if an excess …