DETERMINATION OF TRACE SILICON IN HIGH-PU RITY TITANIUM AND CHROMIUM BY MOLYBDOSILICIC ACID BLUE SPECTROPHOTOMETRY AFTER FLUORIDE SEPARATION

Fluoride separation-molybdosilicic acid blue spectrophotometry was app lied to the determination of trace silicon in high-purity titanium and chromium samples. The analytical procedure used is as follows: dissol ve 1.0 g of the sample in 6 ml of hydrochloric acid by heating. After replacing the hydrochloric acid with 20 ml sulfuric acid by the aid of a nitrogen gas flow, add 0.5 ml of 0.05% hydrofluoric acid for the ch romium matrix, or the same amount of 10% acid for the titanium matrix, to yield volatile silicon tetrafluoride. Transfer the fluoride comple tely into a boric acid solution by purging it with nitrogen. Then, mov e the solution into a 25 ml flask, and add hydrochloric acid and ammon ium molybdate to form molybdosilicate. After adding oxalic acid and as corbic acid, measure the light absorbance of the solution at 810 nm. B y this method, ppm level silicon in commercial high-purity titanium an d chromium samples were successfully determined. Recoveries (n = 10) o f 10 mu g of silicon were 98% and 97%, respectively, for The detection limit (3 sigma) was 0.07 ppm for 1 g of both kinds of sample.