Browning — Detection of Silicates, Fluorides, etc. 2-i9 



Art. XXIX. — A Modified Procedure for the Detection of 

 Silicates, Fluorides, and Fluosilicates ; by Philip E. 

 Browning. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — ccxxv.] 



The method for the detection of silica depending upon the 

 formation of the insoluble skeleton of silica in the metaphos- 

 phate bead is not always satisfactory in the hands of the aver- 

 age student, and the method which depends upon the observa- 

 tion of the jelly of silicic acid on evaporation with hydrochloric 

 or nitric acid is of course available only when the silicate is 

 decomposed by those acids. 



The formation of silicon fluoride by the action of hydro- 

 fluoric acid or a fluoride and sulphuric acid upon a silicate is 

 often applied to the detection of silica, since the silicon fluoride 

 when acted upon by water gives a white precipitate of silicic 

 acid. The usual procedure in making this test is to allow the 

 gas to come in contact with a moistened glass rod, but the con- 

 densation of steam or sulphuric acid on the rod often makes 

 the results uncertain. 



The work to be described was undertaken to devise if 

 possible a method by means of which the reaction could be 

 made more delicate and trustworthy. It was found that when 

 moistened black paper was brought in contact with the fumes 

 of silicon fluoride the deposit of silicic acid was very easily 

 detected. Accordingly the following method was tried. 



A small lead cup about one centimeter in diameter and 

 depth was made by running the melted metal into a mould, 

 and a flat piece of lead with a small hole in the center was 

 used as a cover. Into this cup a small amount of finely 

 powdered calcium fluoride, generally about Ol grm., was placed 

 together with the silicate, and the mixture covered with a few 

 drops of concentrated sulphuric acid, added by means of a 

 medicine dropper or fountain-pen filler. Upon the upper side 

 of the cover a piece of moistened black Alter paper was placed 

 and upon this a small moistened pad of ordinary Alter paper 

 was laid to keep the black paper moist during a heating of 

 about ten minutes upon a steam bath. At the conclusion of 

 the heating a white deposit was found on the underside of the 

 black paper over the opening in the cover, if silica was present 

 in appreciable amount. 



Talbot* has shown that very small amounts of fluorides may 

 be detected by the etching test. The converse of the method 

 just described for the detection of silicates was applied to the 

 detection of fluorides, and while extreme delicacy cannot be 

 claimed, the results are fairly satisfactory when a milligram of 

 fluorine is present. 



* J. Amer. Chem. Soc, xxviii, 1437. 



