Identifying Fluorine Minerals

Etching Test. The ordinary test for fluorine consists in converting it into hydrofluoric acid and observing the latter’s etching effect upon glass. A watch glass or other piece of glass may be covered with paraffin and then the coating removed in spots. Upon this is placed the powdered mineral with a few drops of concentrated sulfuric acid. The action of the acid upon the fluoride will serve to liberate hydrofluoric acid, which will in turn etch the glass where it has been exposed. The action should be allowed to continue for some time, when on cleaning the glass the etched spots will be visible.
A modification of the above test can be made in a closed tube. Take a closed tube of about ¼ inch diameter and made preferably of hard glass. Into this introduce a powdered mixture of the mineral, glass, and acid potassium sulfate, and then heat in the.

Bunsen burner flame. When heated, acid potassium sulfate is converted into the normal potassium sulfate with the liberation of sulfuric acid. The acid attacks the fluoride and sets free hydrofluoric acid. This in turn acts upon the glass present and etches it. The etching, however, is not readily apparent and etches it. The etching, however, is not readily apparent on account of the conditions of the experiment. As a secondary reaction, however, there will be formed in the upper part of the tube a white sublimate of silicon dioxide. This sublimate is volatile because of the presence with it of small amounts of hydrofluosilicic acid. If the bottom of the tube is broken off and its Interior gently washed with water, this acid will be dissolved and removed. If the tube is now dried again, the white coating will prove to be no longer volatile. This silicon dioxide coating is a proof of the action of hydrofluoric acid in the bottom of the tube and therefore of the presence of fluorine in the mineral. A similar test may be made by substituting sodium metaphosphate for acid potassium sulfate. The sodium phosphate may be prepared by fusing salt of phosphorus to glass.