If the water is already been purified the use of a "Research" Column will provide "polished" water with less than 0.5 ppn Si. The silica may be measured photometricallu using the molybdenum blue method and if you would like to contact me I can send you the specific method which is an adaptation of the CEGB method which ensures that only one form of the silicomolybdic acid if formed.