About two years ago, Hench and Aldrich1 described their method for determining blood urea. The procedure, which in principle is the same as their earlier method for the determination of urea in saliva2 (salivary index to blood urea), is as follows:
Five cubic centimeters of whole blood is mixed with 5 cc. of 10 per cent trichloracetic acid, and the protein precipitate is removed by filtration or, preferably, by centrifugation. Five cubic centimeters of the filtrate is titrated with a 5 per cent solution of mercuric chloride, which combines with the nonprotein nitrogenous constituents present. The end-point is reached when a drop of the mixture gives within three seconds a definite brownish coloration with saturated sodium carbonate solution on a spot plate.
The number of cubic centimeters of mercuric chloride solution required is multiplied by 40 (giving the amount that would be required by 100 cc. of whole