Abstract
This method involves the digestion of urea by urease (Marshall-Van Slyke), the precipitation of the proteins (Folin-Wu), the direct Nesslerization of the filtrate (Myers) and determination of the color in the micro-colorimeter previously described. 1 For this color comparison a wedge, containing 1 per cent. potassium dichromate, mounted on a deep yellow ground-glass plate, is used.
The technique is as follows: A 0.2 c.c. pipette is rinsed with 20 per cent. potassium oxalate solution. The residual fluid is blown out well and 0.2 c.c. of blood is drawn up from the pricked finger or ear-lobe and discharged into a small test-tube. The pipette is then rinsed twice with exactly 0.2 c.c. of water and the washings are added to the blood. Three or four milligrams (knifepoint) of powdered urease are now added to the blood and, after shaking, a stopper is inserted and the tube is kept at 50° for 10 minutes or at room temperature for 30 minutes or longer. Then 1.0 c.c. of water is added, followed by 0.2 c.c. of 10 per cent. sodium-tungstate solution and 0.2 c.c. of 2/3 N sulphuric acid.
The mixture is immediately shaken and, after the precipitate has darkened, it is filtered into another small test-tube, using a 2.5-3 cm. funnel and small thin filter paper. With a dry 1 C.C. pipette, graduated in 1/100ths, a definite volume of the filtrate is discharged into one of the 5 C.C. graduated test-tubes with which the micro-colorimeter is provided. It is convenient to take 0.5 C.C. but one need not wait for this amount to filter through. Two volumes of water are added and one volume of Nessler's solution (BockBenedict formula diluted I :5). After thorough mixing the tube is placed in the micro-colorimeter, and matched to the standard “nitrogen” wedge, described above.
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