Ccxci. the Biochemistry of Silicic Acid Vii. Depression of the Apparent Solubility of Silica in Body Fluids

THE depression of solubility of SiO2 through the agency of various metals and metallic oxides, notably Fe and Al, was briefly referred to in a former communication [King & McGeorge, 1938]. It was suggested that the apparent decrease in solubility of quartz and amorphous SiO2 was due to precipitation of the dissolved silicic acid as the insoluble silicate of the metal. The phenomenon has now been explored further, and the effects of various mineral dusts on the solution of SiO2 from quartz have been investigated. Most of the mineral silicates are much less soluble in blood serum and ascitic fluid than are the free forms of SiO2. Kaolin is particularly insoluble. In a mixture of kaolin and quartz powder suspended in a liquid it might be expected that the amount of SiO2 dissolved would be conditioned by the most active member ofthe mixture, and that in the presence ofan excess ofthe mixed powder the solubility would approach that of quartz powder taken alone. Naturally occurring kaolins, such as those represented in shales, contain variable, sometimes large, amounts of free SiO2, and yet these natural stone dusts have an exceedingly low solubility. The amount of free SiO2 represented in a sample of stone dust would lead to much more solution of silica if suspended alone in the same volume of liquid. This fact was brought out in the previous communication where it was shown that a stone dust (shale-used for mine dusting) had a solubility of less than 1 mg./100 ml. although it contained about 40% free SiO2, whereas the 0-8 g. free SiO2 present in the 2 g. stone dust taken for 100 ml. ascitic fluid would if taken separately dissolve to the extent of 9 mg./100 ml. The figures set out in Table I demonstrate these points more fully. The methods used in this investigation-preparation and examination of dusts, solution experiments, separation of solid from liquid phase, determination of dissolved SiO2 etc.-were those described in the previous communication [King & McGeorge, 1938]. The silicate minerals are much less soluble than quartz in the ascitic fluid used,' and the stone dusts are only slightly soluble despite their free SiO2 contents which in some cases are large. The SiO2 dissolved from the coal and coal-mine dusts is, of course, related to the silicate minerals contained in them, and is of the same order as that from the stone dusts. The apparent depression of the solution of free SiO2 contained in the stone dusts suggested that the effect might be extended to quartz admixed with the stone dust. This proved to be the case as is shown in Table II. The addition of increasing amounts of stone dust to 2 g. quartz suspended in 100 ml. ascitic fluid led to a progressive decrease in the SiO2 dissolved, from 7 mg. SiO2/100 ml. to