Kansas Geological Survey, Bulletin 202, pt. 1, originally published in 1971

Determination of Suspended Silica by Atomic Absorption

by Larry M. Magnuson and O. Karmie Galle

Originally published in 1971 as part of Kansas Geological Survey Bulletin 201, pt. 1, p. 17. This is, in general, the original text as published. The information has not been updated. An Acrobat PDF version of the complete bulletin (7 MB) is also available.

Abstract

Described is a method for analyzing small quantities of suspended silica, either airborne or suspended in water. The silica is entrapped on a membrane. The membrane is ashed and the silica fused with Na₂CO₃ and dissolved in water. The quantity of silica is determined by atomic absorption.

The following steps are used in preparing samples:

1. Transfer membranes and silica to platinum crucibles and add 5 ml of 50-percent (V/V) H₂SO₄ to each.
2. Heat crucibles on a hot plate until fuming ceases.
3. Heat crucibles on a Meeker burner until all carbon is oxidized.
4. Fuse sample with 0.5 g Na₂CO₃ by heating at full flame on the Meeker burner. Heat until a clear melt results.
5. Cool crucibles, add 10 ml of water, and set aside overnight.
6. Dilute contents of crucibles to 100 ml in plastic volumetric flasks. The contents are quantitatively transferred to the flasks by washing the crucibles first with water, then 1 ml of 8 N NaOH, and then again with water.
7. Transfer resulting solutions to 4-ounce polypropylene bottles, cap bottles lightly, and digest on a steam bath for 1 to 2 hours. The solution can be stored indefinitely at this point; but, the digestion must be repeated before proceeding with the analysis if they are stored. The measurement by atomic absorption should be made 30 minutes to 3 hours after digestion of the solutions.

Care must be exercised at all steps to avoid loss of silica or contamination. HF cleaning of all platinum and plastic prevents contamination from those sources. Keeping the samples covered at all times prevents airborne contamination. All reagents should be silica free, and use of glassware should be avoided.

Atomic absorption analysis is performed with a nitrous oxide-acetylene flame using the 2516 Å line of silicon. A standard curve is prepared using standards containing 100 to 0.3 ppm Si. A new curve should be prepared after every 10 samples.

The range of the method is 20 to 0.6 mg SiO₂. This may be extended by dissolving the fused sample in a greater or lesser volume of water. However, if the amount of silica is much less than 0.6 mg, a colorimetric analysis should be performed or another method more sensitive than atomic absorption should be used.

The standard deviation based on 20 samples of 10-mg quartz was 0.5 mg, and the standard deviation based on 20 samples of 1-mg quartz was 0.06 mg. Recovery was 98 to 99 percent in both cases. After storage for three weeks recovery dropped to 90 percent but was restored to 98 to 99 percent after digestion on the steam bath for 1 hour.

Kansas Geological Survey, Determination of Suspended Silica by Atomic Absorption
Placed on web May 7, 2009; originally published in May 1971.
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