Spectrophotometric determination of trace silicate in highly purified water by 1-butanol extraction of Heteropoly Blue.

Abstract

To determine trace silicate in highly purified water, conditions for the extraction of Heteropoly Blue into 1-butanol were investigated. A sulfuric acid concentration of 1.3 M was proposed for the extraction. The recommended procedure is as follows. Two hundred milliliters of a sample solution is taken in a 300 ml Teflon separating funnel and 4 ml of an ammonium molybdate (20%) -H2SO4 (2.5 M) solution is added. The solution is left standing for 20 min to form molybdosilicic acid completely. After the addition of 25 ml of H2SO4 (2+1) and 2 ml of L-ascorbic acid solution (10%), it is left standing for 10 min to develop Heteropoly Blue. Twenty five milliliters of 1-butanol is added to the solution which is shaken for 2 min. The absorbance of the Heteropoly Blue extracted into the organic layer is measured at 800 nm. For the reagent blank test, 4 ml and 8 ml of the ammonium molybdate-H2SO4 solution are added respectively to two 200 ml portions of water. The absorbances, A1 and A2, for the two solutions are measured as mentioned above. The difference, A2-A1, corresponds to the absorbance due to 4 ml of the ammonium molybdate-H2SO4 solution. The absorbance obtained on a sample is corrected by subtracting the A2-A1 value. Silicate in some highly purified waters was determined to be over the range of 225μg of SiO2/l.