State of silicic acid in solution, and methods for its colorimetric determination

Abstract

1. In acid solutions, polymerization of silicic acid occurs, and this results in a reduction in the amount that can be found by colorimetric analysis. An equilibrium between molecularly dispersed and colorimetrically undeterminable forms of silicic acid is established in accordance with the acidity of the medium. The presence in the solution of sodium, calcium, or aluminum chloride increases the rate at which equilibrium is established, but is practically without effect on its position. 2. In the colorimetric analysis of solutions containing silicic acid partly in the undeterminable form, a continuous increase with time in the intensity of color is to be observed. This is to be explained by the partial displacement of the equilibrium in the direction of the molecularly dispersed form brought about by the addittion of ammonium molybdate. 3. In order to determine silicic acid in those cases when it is present in solution in the polymeric form, it is necessary first to convert it into the molecularly dispersed form. In order to achieve this we have heated the solutions with sodium carbonate or with caustic alkali, and we have also made use of addition of sodium fluoride at room temperature. All three methods gave quite satisfactory results. The choice of one or another of these methods depends on the nature of the other substances present in solution.