Cxxix. the Biochemistry of Silicic Acid

OF the several colorimetric methods which have been described for the estimation of small amounts of silica, probably that of Dienert and Wandenbulche [1923] has met with most favour. The principle of this method is the production of a yellow silicomolybdic acid complex; it was first studied by Schreiner [1903] and Lincoln and Barker [1904]. Winkler [1914] used a similar method for studying the silicic acid content of natural waters, but the procedure of Dienert and Wandenbulche, variously modified by Atkins [1923], King and Lucas [1928], King [1928], Steffens [1930], Thayer [1930], and Liebknecht et al. [1931], has been most generally used during recent years. Reduction of the yellow silicomolybdic acid complex to give a blue colour has been made the basis of other colorimetric methods proposed by Isaacs [1924] who used sodium sulphite at the temperature of boiling water to effect the reduction, by Oberhauser and Schormuller [1929] who used stannous chloride, by Nemec, Lanic and Koppova [1931] who used sodium thiosulphate, by King [1930] and Pincussen and Roman [1931] who used quinol, and by Berg and Teitelbaum [1930] who used pyrrole as the reducing agent. Most of these methods have been developed with a view to determining the silica content of various natural waters. Rather large amounts of water (usually 100 cc.) are used for the test solutions and the colour comparisons are made in Nessler or Hehner tubes. While yellow-coloured solutions are difficult to compare accurately in a Duboscq colorimeter, blue colours can be matched relatively easily and very much more accurately. An attempt has been made to develop a blue colour method, which would be accurate in practice though simple in principle and in detail, and which could readily be applied to a variety of biological materials, particularly blood, urine and tissue.