The determination of calcium in blood and plasma

Abstract

The method consists in a direct precipitation of the calcium without removal of the blood porteins. The only reagents necessary are ammonium chloride (to prevent the precipitation of magnesium), and ammonium oxalate, both adjusted approximately to P_H 7.4.

Blood.—Citrated blood (5 c.c. in a 50 c.c. centrifuge tube), is hemolyzed by the addition of four volumes (20 c.c.) of warm water and after standing 15 to 20 minutes is centrifuged to remove the stroma. An aliquot (20 c.c.) of the clear, red liquid is transferred to a 50 c.c. centrifuge tube. One c.c. of 5 per cent. ammonium chloride is added and, after mixing, 3 c.c. of 3 per cent. ammonium oxalate are added. The oxalate should be added slowly and the contents of the tube must be well mixed. After standing at least 16 hours (the much shorter time recently reported by de Waard 1 results in an incomplete precipitation of the calcium), the mixture is centrifuged and the supernatant liquid siphoned off. The tube is washed by the addition of 25-30 c.c. of cold water and immediately centrifuged. After removal of the wash water the precipitate is dissolved in 5 C.C. of approximately normal sulfuric acid and, after heating in a water bath to 75° C., the contents are titrated with 0.01 normal potassium perrnanganate. (After precipitation of the calcium the method follows, with minor modifications, that described by Halverson and Bergeim. 2

Plasma.-Citrated plasma (5 C.C. in a 50 C.C. centrifuge tube) is diluted with an equal volume (5 c.c.) of one per cent. ammonium chloride and 10 C.C. of one per cent. ammonium oxalate are slowly added. After standing 16 hours the precipitate is centrifuged, washed and titrated as in the procedure for blood.