The Estimation of Silica in Tissues

The estimation of silica in animal organs by the gravimetric method is attended with considerable difficulty. Large amounts of tissue are necessary, and ashing by the method of Stolte (I), or even with the aid of an electric muffle furnace is a tedious process requiring several hours for completion. Analysis of the ash for silica in the conventional way also takes considerable time, so that an estimation is a fairly lengthy process. Isaacs (2) seemed to have supplied the need for a rapid and easily manipulated procedure in his calorimetric method for the estimation of silica in tissue. Discordant and inaccurate results, however, attended the use of Isaacs’ method in this laboratory. The colors produced in both standard and test solutions were not proportional to the concentrations, and in many cases, even with the utmost care, duplicate estimations did not check. In the case of tissues high in calcium, there was invariably formed, when the test solutions were heated in the water bath, a heavy precipitate of calcium molybdate. This precipitate was a source of constant trouble, as the test solutions had to be filtered before they could be read, and in several instances it was thought that some of the blue color was being carried down by the calcium molybdate. At any rate the results obtained by this method did not agree with gravimetric analyses done as a check on the same tissues, and the discrepancy was largest where the precipitate in the solution had been heavy. In the present paper a micro method is proposed which makes use of the intense yellow color formed when a silicate solution is treated with ammonium molybdate and sulfuric acid. The method is quick, and easily applied, and the proportionality of