Differential Spectrophotometric Determination of Calcium

Abstract

Abstract The appearance of kits for the determination of micro amounts of calcium stimulated our interest in the use of Eriochrome Blue SE for this analysis. A study of spectrophotometric curves indicates that, at pH values above 13.7, calcium will complex and cause a change of dye absorbance while magnesium does not complex with the dye. A differential spectrophotometric technic is described in this paper in which the spectrophotometer is set at zero absorbance with a dye-calcium standard solution or a dye-calcium sample solution as reference and the absorbance of the dye solution then measured. For a set of standards the absorbance-calcium relationship is linear. With the Beckman spectrophotometer, Model DU, it has been found that 1 part of serum to 100 parts of alkaline dye solution (100 λ of serum to 10 ml. alkaline dye solution) can be used. The technic shows greater sensitivity and accuracy than do previous methods using Eriochrome Blue SE.