[Determination of free silica in airborne dust by phosphoric acid method].

Free silica content in airborne dust was measured with a modified procedure collecting sample dust on glass fiber filter and heating the dust together with the filter in phosphoric acid. The insoluble phosphoric acid residue of the glass fiber filter used by us was of small amount. Fifteen mg of dust was enough to determine its free silica content correctly. On dust collected at pottery and foundries works, satisfactory coincidence was found between the values of free silica content by our procedure and by X-ray diffraction. It can be concluded that this is a reliable method for determination of free silica content of airborne dust as well as of deposited one.