A Study of the Volatilisation from Potassium Oxide-Silica Glasses

JSGT_1933_V17_T122_T144

The volatilisation of K₂O from potash–silica glasses of composition ranging from 89·88% SiO₂ 10·12% K₂O to 50·68% SiO₂ 49·32% K₂O has been studied at temperatures ranging from 1100° to 1400° and in five cases to 1450°. Within the temperature range studied the silica is non-volatile. At 1400° the loss of K₂O during the first 20 hours varied from 5·88 mg per hour per sq. cm. of surface for the glass containing 49·32% K₂O to 0·27 mg for that containing 19·61% K₂O; at 1100°, for the 49·32% K₂O glass the loss was 0·56 mg. The rate of volatilisation could be represented, as is the case with the Na₂O–SiO₂ glasses, by the formula log(A−2x)/A=kt, where A is the amount of K₂O initially present and x the loss after t hours; whilst a linear relationship holds good for each glass between log (initial rate of loss) and 1/T° (Abs.). The vapour tension of the 49·32% K₂O glass ranged from 3·10 mm Hg at 1300° to 0·90 mm at 1100°; for the 43·46% K₂O glass from 1·82 mm to 0·50 mm; and for these two glasses the heat of volatilisation of K₂O was calculated as −37,000 cal. per gm.-mol. On plotting total volatilisation loss at different times against K₂O content in the glass, there was evidence of a break at the composition corresponding to the compound K₂O.4SiO₂, although less sharp than for Na₂O.2SiO₂ in the soda–silica glasses. Several other relationships however, gave clear evidence of the existence in the molten glasses, of the compound K₂O.4SiO₂ and indicated the possible presence also of the compound K₂O.2SiO₂ at the appropriate concentration.

Eric Preston & W. E. S. Turner