Liv. the Biochemistry of Silicic Acid

THE presence of silica in blood, and the much larger amounts sometimes found in urine, indicate that silica is appreciably soluble in body fluids. The determination of such solubility, however, is a matter of considerable difficulty and different results have been obtained by various workers, owing to the operation of such variable factors as particle size, concentration of suspension, method of separation of solid from liquid phase and method of estimation of silica in solution. In addition, soluble silica is present partly in true molecular form and partly as a colloidal dispersion; and higher figures for solubility will be obtained if colloidal silica be included in addition to that present in true molecular solution. It has been shown by Lucas & Dolan [1936: see also Briscoe et at. 1937] that under standard conditions up to periods of at least 2 weeks, both concentration of suspension and particle size have an important influence on the rate of solution of quartz. The amount of silicic acid which a mineral form of silica such as quartz will yield in solution is exceedingly small when the mineral is present as crystals of ordinary dimensions, or as coarse fragments. But if it be reduced to a particle size of the order of the dust in the air, i.e. < l-lIO,, then the rate at which silica appears in solution becomes appreciable. While the evidence does not warrant the statement that the smaller the size of the particles the more quickly they dissolve, it is true that with "sized" particles more silica is yielded in solution in a given time by small particles than is obtained with the same weight of particles of a larger size. The amount dissolved also varies with the total amount of dust suspended in the liquid: at sufficiently low concentrations of dust the silica in solution is roughly proportional to the total amount of the solid phase. This simple proportionality is not maintained at higher concentrations of the solid, where a condition of apparent saturation may be approached.' The very low solubility figures recently recorded by Titus [1937] for quartz and sericite were obtained with small amounts of solid (10 mg. of dust per 100 ml.) and would undoubtedly have been greater had larger amounts of dusts been used. The total surface of solid exposed to the solvent probably determines the rate, if not the final amount of solution. The phenomenon of the solution of mineral particles might, in fact, be better considered as a leaching of material from the surfaces of the particles. In some cases (e.g. certain of the silicates)