11 



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

 of silica, which unites with fluorin with the formation of fluosilicates. The sulphuric acid 

 then liberates hydrofluosilicic acid, which would escape detection by methods 1 and 2. 



7. FLUOBOBATES AND FLUOSILICATES 



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

 incinerate. Extract the crude ash first obtained with water, to which sufficient acetic acid 

 has been added to decompose carbonates, filter, burn the insoluble portion, extract with 

 dilute acetic acid, and again filter. The insoluble portion now contains calcium silicate and 

 fluorid, while the filtrate will contain all the boric acid present. 



(a) FIRST METHOD.** 



Incinerate the filter containing the insoluble portion, mix with a little precipitated silica, 

 and place, with the addition of 1 or 2 cc of concentrated sulphuric acid, in a short test tube, 

 which is attached to a small U-tube containing a few drops of water. The test tube is now 

 placed in a beaker of water, which is kept hot on the steam bath for from thirty to forty 

 minutes. If any fluorid be present the silicon fluorid generated will be decomposed by the 

 water in the U-tube and will form a gelatinous deposit on the walls of the tube. 



The filtrate is now tested, as directed under boric acid. If both hydrofluoric and boric 

 acids be present, it is probable that they were combined as borofluorid. If, however, silicon 

 fluorid be detected and not boric acid, the operation is repeated without the introduction of 

 the silica, in which case the formation of the silicon skeleton is conclusive of the presence of 

 fluosilicate.6 



(b) SECOND METHOD. 



Incinerate the filter containing the insoluble portion in a platinum crucible, mix with a 

 little precipitated silica, and add 1 cc of concentrated sulphuric acid. Cover the crucible 

 with a watch glass to whose underside a drop of water is suspended, and heat an hour at the 

 temperature of 70 or 80 C.c The silicon fluorid which is formed is decomposed by the 

 water, leaving a gelatinous deposit of silica, and etching a ring at the periphery of the drop 

 of water. Test the filtrate for boric acid as described under (a). 



8. SULPHUROUS ACID. 



(a) QUALITATIVE DETECTION .d 



To about 25 grams of the sample (with the addition of water, if necessary) placed in a 

 200-cc Erlenmeyer flask, add some pure zinc and several cubic centimeters of hydrochloric 

 acid. In the presence of sulphites, hydrogen sulphid will be generated and may be tested 

 for with lead paper. Traces of metallic sulphids are occasionally present in vegetables, and 

 by the above test will indicate sulphites. Hence positive results obtained by this method 

 should be verified by the distillation method. 



It is always advisable to make the quantitative determination of sulphites, owing to the 

 danger that the test may be due to traces of sulphids. A trace is not to be considered suffi- 

 cient either as a bleaching agent or as a preservative. 



(b) DETERMINATION OF TOTAL SULPHUROUS ACID. 



(1) First method. 



Distill 100 grams of the sample (adding water, if necessary) in a current of carbon dioxid 

 after the addition of about 5 cc of a 20 per cent solution of glacial phosphoric acid until 50 cc 

 have passed over. Collect the distillate in a decinormal iodin solution in a flask closed with 



a Neviere and Hubert, Moniteur scientifique, 1895, [4] 5:324. 



b It must be remembered that, in an ash containing an appreciable amount of silica, sulphuric acid will 

 liberate silicon fluorid rather than hydrofluoric acid. The presence of a fluosilicate is indicated, there- 

 fore, and not the presence of a fluorid. 



c The watch glass may be kept cool by means of a piece of ice. 



d U. S. Dept. Agr., Division of Chemistry, Bui. 13, pt. 8, p. 1Q32. 



