Abstract
ALL THE methods for the determination of silica in sea water are based on the reaction of orthosilicic acid with molylxlate forming 1 : 12 silicomolybdic acid. The absorbance of this intensely yellow coloured compound or its blue reduction product is measured at selected wave lengths. Though some of the photometric determinations of silica concerned arestandard methods the data on the condition of formation found out empirically are inhomogeneous. The reaction which leads to the formation of silicomolybdic acid is not known in detail. In numerous papers already published the chemical reaction is only suggested (BRLLIEWITZ, 1933; SWANK and MELLON, 1934; ROaINSON and SpooR, 1936;-ARl~STRONG, 1951 ; MULLIN and RILEY, 1955; BI~, 1958; a summary of methods and papers on the matter is given by JEAN, 1955). STRICKLAND (1952) proved the existence of two different modifications of the silicomolybdic acid which are called the ~and E-species. According to-SrO/RLAC (1956) the two modifications only differ in the water of constitution. Below a pH value of 2.5 the predominating species in the formation is the /3-silicornolybdic acid. It has a somewhat different absorption spectrum and a higher molar extinction. As already mentioned by STRICKLAND, the E-acid is unstable and changes irreversibly into the =-form. The velocity of the change depends on the conditions in the solution. According to GRASSHOrF and HAHN (1959) the formation of the heteropoly acid proceeds reversibly. The formal reaction equation is :
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