The Complexometric Titration of Micro and Ultramicro Quantities of Calcium in Blood Serum, Urine, and Inorganic Salt Solutions

Abstract

A method for the microdetermination and ultramicrodetermination of calcium in blood serum, urine, and inorganic salt solutions has been developed. Calcium was titrated with disodium ethylenediaminetetraacetate, and Cal-red as the indicator, using a Gilmont ultramicro-buret of 0.1-ml. capacity. Amounts of 5 to 30 µg. of calcium per sample, of 0.05- to 1-ml. volumes, can be determined with a reproducibility of 1 to 2 per cent. The difference between the values found for serum calcium with the Cal-red and the oxalate method had a standard deviation of ±3.32 per cent. Serum magnesium can be determined as the difference of two simple titrations. Phosphate levels up to 30 mg. per 100 ml. P did not interfere with the determination in serum, even when the calcium level was raised to about 20 mg. per 100 ml. Up to products of 80(Ca X P, expressed in mg. per 100 ml.) there was no interference, in inorganic salt solutions. At higher products drift occurred, but the addition of some citrate to the solutions enabled the attainment of sharp end points at Ca X P products up to approximately 1000. Urinary calcium was determined satisfactorily, without the occurrence of drift, if citrate was added to the solution. Magnesium did not interfere appreciably, giving only 1 per cent higher readings at levels of 20 mg. per 100 ml. The interference of copper and iron was removed by the addition of sodium cyanide.