USGS Test Method I-2700-85

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Method Name:
Silica, Colorimetric, Dissolved. Official Name: Silica, colorimetric, molybdate blue, automated-segmented flow

Silica reacts with molybdate reagent in acid media to form a yellow silicomolybdate complex. The complex is reduced with ascorbic acid to form the molybdate blue color. Two polymorphic forms may form, alpha or beta, or a mixture of both. The pH is kept below 2.5 during analysis to favor the beta polymorph. Analysis is performed on an automated-segmented flow instrument.

This method may be used to determine concentrations of silica in surface, domestic and industrial waters.

Methods for the Determination of Inorganic Substances in Water and Fluvial Sediments, Techniques of Water-Resources Investigations of the United States Geological Survey, Book 5, Chapter A1 Edited by Marvin J. Fishman and Linda C. Friedman

Interferences from phosphate, which forms a phosphomolybdate complex, is suppressed by the addition of oxalic acid. Hydrogen sulfide is removed by boiling the acidified sample. Large amounts of iron interfere.

QC Requirements:
Quality-control samples area analyzed at a minimum of one in every ten samples. These QC samples include at least one of each of the following: blanks, quality control samples, third party check solutions, replicates, and spikes. Correlation coefficients for calibration curves must be at least 0.99. QC samples must fall within 1.5 standard deviations of the mean value. If all of the data-acceptance criteria in the SOPs are met, then the analytical data are acceptable.

Maximum Holding Time:
180 days from sampling



0.1-40.0 (undiluted)

The standard deviation for dissolved silica within the range of 4.70 to 17.4 mg/L for twenty samples was found to be independent of concentration. The 95-percent confidence interval for the average standard deviation of 1.10 mg/L ranged from 0.97 to 1.26 mg/L.

Method detection limits (MDLs) were determined for this method using the procedures of the U.S. Environmental Protection Agency (1997).

Revision Number:

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