Determination of Free Silica: Gravimetric and Spectrophotometry Procedures Applicable to Air-Borne and Settled Dust

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Abstract
Refinements contributing to the precision and accuracy of the phosphoric acid method can be made by the use of commercially available apparatus comprising a rotator and a voltage-controlled heater. Additional refinements include optimizing the conditions of analysis by digestion of orthoclase with phosphoric acid for varying lengths of time, use of correction curves to compensate for loss of quartz, and identification of undissolved minerals by means of a polarizing microscope. An eight-year study of the precision of the phosphoric acid method gave a mean standard deviation of 0.34 in the analysis of typical samples containing from- 3 to 50 per cent quartz. The recovery of quartz from the less-than-5-micron fraction of air-borne dust is about 95 per cent when the digestion time is reduced to 8 minutes. The method can be extended to samples of air-borne dust as small as 2 milligrams by spectrophotometric determination of the quartz as molybdisilicic acid.

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