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%) -H
2SO
4 (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 H
2SO
4 (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-H
2SO
4 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-H
2SO
4 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 SiO
2/l.
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