Abstract
A new microcolorimetric method has been developed for the determination of calcium by direct precipitation, in samples of 0.2 cc or less of serum. Although work is now in progress on a simplification of the technic and the use of smaller samples for analysis, the following procedure has given, for some time, such satisfactory results (accuracy within ±2%) as to warrant its present description.
Procedure. In test tubes (10×75 mm), to 0.2 cc samples of serum are added 1.0 cc of water and 0.2 cc of saturated ammonium oxalate. Two blank samples, with water in place of serum, are treated in exactly the same way. After standing overnight, 0.2 cc of 0.1% Triton N E (Röhm and Haas) is added to each tube, 1 the contents stirred, then poured into a pyrex sintered glass filter funnel (No. 2 F, Buchner type) held by a rubber stopper in the mouth of a 500 cc, suction flask. Three successive portions of 0.8 cc of 2% ammonia water are pipetted into the tube, agitated to pick up any remaining crystals of CaC2O4, then poured upon the filter. A receiving tube (7 × 7/8”) is placed in the suction flask under the stem of the filter funnel. The vacuum is turned off, while the walls of the precipitation tube are washed with 1 cc of hot 1 N H2SO4 which is then poured on the filter and allowed to stand for one minute. Suction is reapplied and the acid is caught in the receiving tube. The precipitation tube is discarded. The walls of the filter funnel are then washed with 1 cc and 2 cc portions of hot 1 N H2SO4, allowed to stand for one minute as above.
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