Published: Journal of Analytical Toxicology, Volume 21, Number 6, October 1997, pp. 503–505.

TECHNICAL NOTE: A Sensitive Spectrophotometric Assay for the Measurement of Soluble Silica in Water
Barbara R. Manno, Imad K. Abukhalaf, and Joseph E. Manno

The National Committee for Clinical Laboratory Standards (NCCLS, 1) recommends that Type I water used in clinical laboratories must be virtually silica (SiO₂) free (¾ 0.05 mg/L). The recommendation has been adopted by the College of American Pathologists (CAP) with the inclusion of a question regarding soluble silicate in water in CAP Laboratory Accreditation Program checklists (2). These requirements are based upon silica’s direct interference with spectrophotometric measurements at specific wavelengths of the light spectrum. Therefore, its presence in water not only interferes with trace metal and electrolyte analyses but also could adversely affect many enzymatic determinations such as those procedures using alcohol dehydrogenase and various phosphatases (1).

A commercial silica testing kit (catalog # 22550-00, Hach Silica Test Kit, Hach, Loveland, CO) was evaluated. Kit specifications indicated that it measured silica concentrations in the range of 0–1 mg/L. Measurement of the color intensity of the resulting silicate-molybdate complex required visual comparison of the developed color to a color disc directed toward an artificial or natural light source and depended on color comparison by the naked eye. Hach Silica Testing Kit reagents were used in the development of a modified procedure for the detection and more accurate quantitation of low concentrations (¾ 0.05 mg/L) of soluble silica in water.

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