186 METHODS OF ANALYSIS. 



6. Fluorids. 

 (a) MODIFIED METHOD OF BLABEZ.O 



Thoroughly mix the sample and heat 150 cc to boiling (in the case of solid 

 foods the filtrate prepared as directed under salicylic acid may be employed. 

 Add to the boiling liquor 5 cc of a 10 per cent solution of potassium sulphate 

 and 10 cc of a 10 per cent solution of barium acetate. Collect the precipitate 

 in a compact mass (a centrifuge may be used advantageously) and wash upon 

 a small filter. Transfer to a platinum crucible and ash. 



Prepare a glass plate (preferably of the thin variety commonly used for 

 lantern slide covers), as follows: First thoroughly clean, polish, and coat on 

 one side by carefully dipping the plate while hot in a mixture of equal parts 

 of Canauba wax and paraffin. Near the middle of the plate make a distinctive 

 mark through the wax with a sharp instrument, such as a pointed piece of wood 

 or ivory, which will remove the wax and expose the glass without scratching 

 the latter. 



Add a few drops of concentrated sulphuric acid to the residue in the crucible 

 and cover with the waxed plate, having the mark nearly over the center and 

 making sure that the crucible is firmly embedded in the wax. Place in close 

 contact with the top or unwaxed surface of the plate a cooling device, consisting 

 of a glass tube considerably larger in diameter than the crucible, the bottom 

 of the tube being covered tightly with a thin sheet of pure rubber. A constant 

 stream of cold water is passed through the tube. Heat the crucible for an hour 

 at as high a temperature as practicable without melting the wax (an electric 

 stove gives the most satisfactory form of heat). 



Remove the glass plate and indicate the location of the distinguishing mark jj 

 on the unwaxed surface of the plate by means of gummed strips of paper, then 

 melt off the wax by heat or a jet of steam, and thoroughly clean the glass with 

 a soft cloth. If fluorin be present, a distinct etching will In- apparont on the] 

 glass where it was exposed. 



(b) SECOND METHOD. 



If it is desired the preceding method may be varied by mixing a small nmoui 

 of precipitated silica with the precipitated calcium fluorid and applying 

 method given below for the detection of fluosilicates. 



This method is of value in the presence of foods whose ash contains a coi 

 siderable amount of silica, which unites with fluorin and forms fluosilicntc 

 The sulphuric acid then liberates hydrofluosilicic acid, which would esca] 

 detection by the Blarez modified method. 



7. Fluoborates and Fluosilicates. 



Make about 200 grams of the sample alkaline with lime water, evaporate 

 dryness, and incinerate. Extract the crude ash first obtained with water, 

 which sufficient acetic acid has been added to decompose carbonates, filter, bui 

 the insoluble portion, extract with dilute acetic acid, and again filter, 

 insoluble portion now contains calcium silicate and fluorid, while the filtrat 

 will contain all the boric acid present. 



(a) FIRST METHOD.* 



Incinerate the filter containing the insoluble portion, mix with a little pr$J 

 cipitated silica, and place, with the addition of 1 or 2 cc of concentrated sul 



Chem, News, 1905, 91 : 39 ; Ann. Rcpt. Mass. Stnt- I'.onnl of Health. 1'ior,, p. 498. 

 Niviere and Hubert, Moniteur scientiflque, 1895 [4], 9: 324. 



