The basic ferric acetate precipitation procedure used for removing phosphate from solutions in which silica is to be determined has been critically investigated. I t has been found that unless strict control of the pH is observed the method may give inaccurate results, owing to (a) incomplete removal of phosphate or (b) removal of silica with phosphate.Previous work has been confirmed that both phosphate and silica couple with molybdate in weak acid, but that only the silicomolybdate complex is reduced in strong acid. On this principle a new procedure for determining silica in biological material has been developed, in which phosphate is left in solution.Iron, in amounts greater than those occurring in tissue, has been found not to interfere.In urines with high phosphate concentrations two methods have been evolved to overcome interference by precipitation of ammonium phosphomolybdate. In one, phosphate is partly or completely removed by precipitation with calcium hydroxide; in the other, ammonia and urea are removed with nitrous acid and sodium molybdate is used to couple with silica.The new colorimetric procedure gives good agreement with gravimetric analyses, and good recoveries of added silica have been obtained with tissue, blood and urine. The procedure likewise gives results in good agreement with gravimetric methods,. when applied to small samples of mineral dusts.SILICON may be determined chemically by gravimetric, titrimetric and colorimetric procedures. All methods depend on the decomposition of siliceous material to liberate silicic acid. In the gravimetric method silicic acid is subsequently dehydrated to the oxide of silicon, and the silica, SiO,, is isolated and weighed. The silica so isolated often contains small amounts of metallic salts, and for this reason it is usual to treat the impure silica with hydrofluoric acid, which completely volatilises the silicon as silicon tetrafluoride. The gravimetric procedure is described in most textbooks of quantitative analysis, e.g., Hillebrand,l Treadwe11 and Ha112 and Scott.3 Gravimetric procedures as ordinarily performed are not sufficiently sensitive for the accurate determination of less than 5 mg of silica. Micro-gravimetric procedures based on similar chemical principles to the above, but with the apparatus and techniques of Emich4 and Pregl,5 have been described by Morgan and King6 Gerstel7 and HolL8

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Holt and Kiqzg. 773 The Chemistry of Silica Surfaces.By P. F. HOLT and D. T. KING.[Reprint Order No. 5820.1Solubility measurements indicate that an incomplete monolayer normally exists on silica surfaces which is more soluble than the rest of the material. This layer has been identified as silicic acid by following the exchange reaction between a quartz surface and dissolved p1Si]silicic acid. The layer is removed by sodium hydroxide and re-formed when the silica is equilibrated with a silicic acid solution.LENHER and MERRILL ( J . Amer. Chem. Soc., 1917, 39, 2630), using platinum apparatus, found that the solubility of gelatinous silica at 25" is 16.2 mg. and at 90" is 41.8 mg. of SiO, in 100 ml. of conductivity water.Equilibrium between the gelatinous silica and water was established after about 8 days a t 25" and after 24 hours at 90". Lenher and Merrill found that ignition of this silica reduced the rate of dissolution but concluded that " the true solubility of ignited silica is probably the same as that of gelatinous silica, but as saturation is not reached in any short period of time, the apparent solubility is somewhat less than that of the gelatinous silica." Alexander, Heston, and Iler ( J . Phys. C l m . , 1954, 58, 453) recently re-examined the values for the solubility of silica in water. Measuring the solute concentration by a colorimetric method, they found that amorphous silica produced by the combustion of silane, wet silica gel, and sols produced by treating sodium silicate with cation-exchange resins all had the solubility, 10-14 mg./100 ml. at 25".The values found for the solubility of quartz and other forms of silica are mostly much lower than Lenher and Merrill's values for silica gel ; for example, Briscoe, Matthews, Holt, and Sanderson (Trans. Inst. Min. Met., 1936-37, 46,291) found that, when 1 g. of quartz dust was shaken with 100 ml. of water at Z O O , the solute contained only 1.6 mg. of SiO, per 100 ml. after 50 days. Other observers give widely Mering values (see King and Belt, Physiol. Rev., 1938, 18, 329).The rates of dissolution of various forms of silica have also been studied (see, e.g.,

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Silicic acid reacts with insulin, albumin and amorphous nylon over a wide pH range but monolayers are tanned only near pH 6. Tanning requires (i) the combination of silicic acid with the protein or polyamide, and (ii) further polymerization of the adsorbed silicic acid. The roles of ionic links and hydrogen bonds are discussed.