12-Molybdosilicic acid analysis

Various approaches to the analysis of dissolved silicon have been tried. Most of them are based on the formation of /J-molybdosilic acid [ 199-203 ]. Dissolved silicon exists in seawater almost entirely as undissociated orthosilicic acid. This form and its dimer, termed reactive silicate , combine with molybdosilicic acid to form a- and /I-molybdosilicic acid [180]. The molybdosilicic acid can be reduced to molybdenum blue, which is determined photometrically [206]. The photometric determination of silicate as molybdenum blue is sufficiently sensitive for most seawater samples. It is amenable to automated analysis by segmented continuous flow analysers [206-208]. Most recent analyses of silicate in seawater have, therefore, used this chemistry. Furthermore, reactive silicate is probably the only silicon species in seawater that can be used by siliceous organisms [204]. 

This analytical procedure is based on an optimum analysis condition for segmented continuous flow analysis. The sample is combined with a molybdate solution at a pH between 1.4 and 1.8 to form the //-molybdosilicic acid. After an appropriate time for reaction, a solution of oxalic acid is added, which transforms the excess molybdate to a non-reducible form. The oxalic acid also suppresses the interference from phosphate by decomposing phosphomolyb-dic acid. Finally, a reductant is added to form molybdenum blue. Both ascorbic acid and stannous chloride were tested as reductants. 

Molybdenyl acetylacetonate, 6 147 12-Molybdosilicic acid, 1 127 analysis of, 1 128 Monazite, extraction of, 2 38 separation of rare earths from, 2 56... 

A method for the determination of silica in seawater using solvent extraction is described. EtOAc extraction was used in the determination of molybdosilicic acid 3 M H2SO4 was chosen as the extraction condition, and the absorbance measurements were made at 3350 A. At this acid concentration the interference by phosphate was small. The solvent extraction method was quick and eliminated the necessity for filtering turbid waters before analysis. 

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