Differential Spectrophotometric Determination of Calcium.

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 Dli, it has been found that 1 part of serum to 100 parts of alkaline dye solution (100 A 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.