Micro Determination of Calcium

Abstract

I AM grateful to Dr. G. J. van Kolmeschate, of the Analytical Institute, Delft, and to Drs. H. A. Flaschka and A. J. Barnard, jun., of the Georgia Institute of Technology, and the Baker Chemical Co., New Jersey, respectively, for directing my attention to the fact that the indicator used by me1 for the micro-determination of calcium is Eriochrome Black T, and therefore well known in this connexion. For this I express my regret: the error was due in part to discrepancies in the literature. However, although the use of this indicator is evidently not novel, my results show that it is possible to use it at pH 12–13, where other workers have found it unsatisfactory; I have therefore found it possible to determine calcium directly in the presence of magnesium. Possibly my ability to obtain a satisfactory end-point is due partly to carrying out the titration in essentially a thin film, and partly to using a high concentration of indicator—5λ of a 0.4 per cent methanol solution, or about 40 times the concentration used by Hildebrand and Reilly with ‘Calcon’2. In this respect I find Eriochrome Black T to be superior to the latter indicator at such micro-levels. With the very small volumes involved, the coloration of ‘Calcon’ is undetectable at the recommended concentration: and using this indicator sufficiently concentrated to produce a visible coloration leads to gross inaccuracies which the use of blank determinations cannot correct at the calcium-levels it is desired to estimate. On the other hand, the high concentration of Eriochrome Black T required to give a detectable colour apparently does not affect the accuracy of the results.