1989 Volume 38 Issue 12 Pages 729-733
Effects of concentrations of ammonium molybdate and sulfuric acid for the formation of molybdosilicic acid (Si-Mo) and heteropoly blue (Si-Mo blue), as well as for the extraction of Si-Mo blue, were investigated in detail. Under the concentrations suitable for the formation of Si-Mo, molybdate added in excess was also reduced to blue substance with ascorbic acid. Selective reduction of Si-Mo only, was carried out by increasing the acidity of the solution after the formation of Si-Mo blue, it was found that the concentration of sulfuric acid had to be raised above 1 M. The procedure of the experiment is as follows. Fifty milliliters of a sample solution containing silicate (up to 40 μg SiO2) in a 100 ml separating funnel is added with 1 ml of an ammonium molybdate (20%)-H2SO4 (2.5 M) solution. The solution is left standing for 20 min to form Si-Mo completely. After 5 ml of H2SO4 (2+1) and 1 ml of L-ascorbic acid solution (5%) are added, the solution is left standing for 10 min. Twenty milliliters of 1-butanol is added to the solution which is shaken for 2 min. An absorbance of the organic layer is mesured at 800 nm. The accuracy of the method is about 2.1% for 5 μg of SiO2.
