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6445353 
Journal Article 
On the determination of silica in sea water 
Grasshoff, K 
1964 
11 
597-604 
Methods for the determination of silica in sea water are all based on the reaction of orthosilicic acid with molybdate, forming 1:12 silicomolybdic acid, and measurement of the absorbance of this yellow compound, or its blue reduction product, at selected wavelengths. With a view to developing an analytical procedure for the determination of dissolved reactive silica in sea water suitable for use on board ship, and applicable over the whole range of silica concentrations occurring in natural waters, studies have been made on the chemical reactions leading to the formation of the heteropoly acids and the change from the b- to the a-silicomolybdic acid. Detailed procedure for the method developed is described. Two samples of the water are filtered and acidified with monochloroacetic acid; the molybdate reagent is added to one sample, and both are shaken and stored for 6 hours or more (up to 3 days), and the absorbance is then measured at 390 mu. The sample without molybdate reagent is used for comparison. If low concentrations of silica are present, the procedure can be continued by addition of a reducing reagent, and after about 8 hours the absorbance of the samples is measured at 640 mu. Some results obtained with the method under different conditions are tabulated.