The Colorimetric Determination of Blood

In 1926 we (1) published a calorimetric method for the estimation of blood calcium, in which the calcium is precipitated from an alkalinized trichloroacetic acid filtrate as calcium phosphate and determined as phosphate by the Benedict and Theis (2) method for the estimation of inorganic phosphorus. We have since simplified and shortened this method and in this paper we wish to report our improved procedure. In our new procedure the calcium phosphate is precipitated, washed, dissolved in molybdic acid, and treated for color production in the same tube, thus eliminating all transfers which would require time and offer chances for error; the Fiske and Subbarow (3) method of determining inorganic phosphorus is used instead of the Benedict and Theis method, the latter procedure being retained as an optional method where a reducing agent of greater keeping qualities is desired; a more successful washing mixture for the Ca3(PO& precipitates, which eliminates one washing, has been developed; the calcium is precipitated at a higher alkalinity to remove any possibility of interference by unusual amounts of magnesium; and a number of minor changes in technique are introduced.