EPA-NERL: 370.1:  Silica by Colorimetry

  • Summary
  • Analytes
  • Revision
  • Data and Sites
Official Method Name
Silica, Dissolved (Colorimetric)
Current Revision
Issued 1971; Editorial Revision 1978
Spectroscopy (Colorimetry; Photometry)
Method Subcategory
Method Source
  Methods for the Chemical Analysis of Water and Wastes (MCAWW) (EPA/600/4-79/020)
Brief Method Summary
A well-mixed sample is filtered through a 0.45 um filter. The filtrate, upon addition of the molybdate ion in acidic solution, forms a greenish-yellow complex proportional to the dissolved silica concentration, and its absorbance is measured spectrophotometrically. If concentrations of silica are exceptionally low, the greenish-yellow complex is reduced to a more intense heteropoly blue complex using 1-amino-2-naphthol-4-sulfonic acid.
Scope and Application
This method determines silica in drinking, surface, and saline waters; domestic and industrial wastes.
Applicable Concentration Range
2 to 25 mg silica/L (standard).
Less than 1 mg/L with reducing agent modification.
(A) Color/Turbidity: Correct for color or turbidity error by running blanks prepared without addition of the ammonium molybdate color reagent solution.
(B) Tannin and phosphates: Interferences can be reduced or eliminated with oxalic acid.
(C) Iron and sulfide: Iron and sulfide will interfere.
(D) Contamination: Use silica-free reagents and minimize contact with glass. A blank also should be run.
Quality Control Requirements
Sample Handling
Maximum Holding Time
28 days (MCAWW, Table 1).
Relative Cost
Less than $50
Sample Preparation Methods