CARBONATE FUSION TO DETERMINE QUARTZ IN RESPIRABLE AND BULK EARTH SAMPLES

The carbonate-fusion technique of Dobreva [Ann. Occup. Hyg., 18 (1975) 121] has been modified to allow the colorimetric assay of solubilized quartz to be done entirely within a 4-ml disposable spectrophotometer cell. An Andreasen Pipette is used to measure respirable size of a sa mple (that with an aerodynamic diameter less than 3.5 mum) and to obta in samples at the 3.5 mum cut for quartz determinations. Amorphous sil ica and silicates are removed from the sample by heating an aliquot wi th 1-2 ml of 48% fluoroboric acid at 70-degrees-C. for 1 h. Quartz is then preferentially solubilized by fusing the filtered and ashed resid ue with a 1:1 mixture of potassium bicarbonate and potassium chloride. After dissolving the carbonate residue with boiling water, a sample i s placed in a polymethacrylate spectrophotometer cell with 0.1 ml of 1 0% ammonium molybdate. This mixture is then adjusted to pH 2.1 and kep t at room temperature for 30 min. Color is then developed for 45 min w ith a solution of citric acid and tartaric acid after which absorbance is measured at 785 nm. This technique recovers 99.8% of 5 mum quartz but retains only 1.1% of amorphous silica. The absolute detection limi t of the colorimetric method is 150 ng for silicon, sufficient to dete ct 8 mug of quartz in a 1-50 mg sample. Analyses of respirable-sized s amples disclose that the major mass of quartz in complex earths can fa ll in the non-respirable range.