Penfield uses a similar arrangement, but passes his silicon fluoride into an alcoholic solution of potassium chloride. Silica and potassium silico-fluoride are precipitated, and hydrochloric acid is set free.[100] The acid thus liberated is titrated, with a standard solution of alkali, in the alcoholic solution, and from the amount of free acid found the fluorine is calculated. The weight of hydrochloric acid (HCl) found, multiplied by 1.562, gives the weight of the fluorine. With this method of working, fewer U-tubes are required. The exit tube from the flask is bent so as to form a small V, which is kept cool in water; this is directly connected with the U-tube containing the alcoholic solution of potassium chloride. The flask with the assay is heated for about two hours, and a current of dry air is aspirated throughout the determination. Fluoride of silicon is a gas not easily condensed to a liquid: but is immediately decomposed by water or moist air.
FOOTNOTES:
[95] This will require two or three hours to thoroughly complete. It is best to powder the oxide first produced, and recalcine.
[96] No magnetic oxide was formed.
[97] For example:—
CaO + 2HCl = CaCl2 + H2O.
PbO + H2SO4 = PbSO4 + H2O.
MgO + 2HNO3 = Mg(NO3)2 + H2O.
Al2O3 + 6HCl = Al2Cl6 + 3H2O.
Fe2O3 + 3H2SO4 = Fe2(SO4)3 + 3H2O.