The colorimetric microdetermination of urea nitrogen by the xanthydrol method.

Although many methods for the determination of urea nitrogen have been described, few have proved highly convenient for the accurate determination of small amounts of urea nitrogen without having to make use of special apparatus or techniques. Among the older methods, the Van Slyke gasometric method (1) is generally considered the most accurate, with an allowable error of 1 per cent. This method, however, is not applicable to the determination of very small amounts of urea. The microdiffusion technique of Canway and O’Malley (2) and its recent modification by Kinsey and Robison (3) make possible the measurement of as little as 1 y of urea nitrogen, with an error of about 5 per cent. We have had no experience with the latter methods. Because of its simplicity and specificity the old xanthydrol method of Fosse et al. (4) has been reinvestigated for its usefulness and accuracy at the lower levels of urea nitrogen concentrations. Originally designed as a gravimetric technique, this method has been modified by Beattie (5) and by Lee and Widdowson (6) for the visual calorimeter. The method depends upon the precipitation of urea as dixanthylurea by a methyl alcoholic solution of xanthydrol, the separation of the dixanthylurea precipitate, and the formation of a yellow color upon the addition of 50 per cent sulfuric acid to the dixanthylurea. Making use of the Lumetron model 402-E calorimeter with the 420 rnp monochromatic filter, we have been able to measure urea nitrogen levels down to 2 y with an accuracy of about 1 per cent and recover accurately as little as 0.43 y of added urea nitrogen.