Micro-determination of soluble silica in urine.

Silicon may be determined in solution by the spectrophotometric method of Dienert & Wandenbulche (1923), as modified by King & Stantial (1933). This involves the formation of the yellow silicomolybdate and the reduction of this complex to molybdenum blue by 1-amino-2-naphthol-4sulphonic acid reagent. Measurement of this colour provides a means of determining silicon. A disadvantage of this method of determining silicon is that phosphate interferes by forming phosphomolybdate, which is similarly reduced to molybdenum blue. Various methods have been used for preventing phosphate interference during silicon determination. Khaler (1941) used mineral acids to prevent the formation of phosphomolybdate. Complex-forming agents such as oxalic acid (Schwartz, 1942; De Sesa & Rogers, 1954) or citric acid (Case, 1944; De Sesa & Rogers, 1954) were used to bleach the interfering phosphomolybdate while leaving the silicomolybdate unaffected. Paul