Direct flame-photometric determination of calcium in soil and plant extracts,water and serum with special reference to sodium,potassium and phosphate interference

An investigation of the possibilities for the direct determination of calcium with a single-cell filter flame photometer, without preliminary separation as oxalate, led to the following conclusions:Calibration curves for 100 p.p.m. of calcium are rectilinear.Sodium causes an appreciable positive error.The error due to potassium interference is relatively much smaller than that due to sodium. Up to a certain concentration it is positive and increases with the concentration, then the emission seems to be gradually depressed and the error finally becomes negative.Phosphates cause a very significant negative error, which tends to be asymptotic. The “radiation buffer” method of correction is not practical here because the max. error is too high (—92.5%).Except for extreme cases, calcium can be determined by direct spraying, without preliminary separation as oxalate: (a) in drinking and irrigation water, correction being necessary only for sodium interference, (b) in soil extracts, correcting only for phosphate effects, (c) in normal blood sera, without clinically significant corrections at 1+3 dilution.Owing to the large variations in the contents of Na, K and PO₄^-3 in plants, calcium cannot be determined in plant-ash extracts without preliminary separation as oxalate. This applies also to abnormal sera, although there the variations in Na and PO₄^-3 content are not so large.