150 METHODS OF ANALYSIS [Chap. 



acid has been added to decompose carbonates, filter, ignite the insoluble portion, 

 extract with dilute acetic acid, and again filter. The insoluble portion now con- 

 tains calcium silicate and fluorid, while the filtrate will contain all the boric acid 

 present. 



QUALITATIVE TESTS. 



27 Method 7.1° 



Incinerate the filter, from 26, containing the insoluble portion, mix with a little 

 precipitated silica, transfer to a short test tube, attached to a small U-tube contain- 

 ing a few drops of water and add 1-2 cc. of concentrated sulphuric acid. Keep 

 the test tube in a beaker of water on the steam bath for 30-40 minutes. If any 

 fluorin 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. 



Next test the filtrate as directed under 14. If both hydrofluoric and boric acids 

 be present, it is probable that they are combined as borofluorid. If, however, 

 silicon fluorid is detected and not boric acid, the operation should be repeated with- 

 out the introduction of the silica, in which case the formation of the silicon skeleton 

 is conclusive evidence of the presence of fluosilicate. In an ash containing an ap- 

 preciable amount of silica, sulphuric acid will liberate silicon fluorid rather than 

 hydrofluoric acid. The presence of a fluosilicate is indicated, therefore, and not 

 the presence of a fluorid. 



28 Method II. 



Incinerate the filter, from 26, containing the insoluble portion, in a platinum 

 crucible, mix with a little precipitated silica, and add 1 cc. of concentrated sul- 

 phuric acid. Cover the crucible with a watch glass, from the underside of which 

 a drop of water is suspended, and heat for an hour at 70°-80°C., keeping the watch 

 glass cooled. The silicon fluorid which is formed is decomposed by the water, leav- 

 ing a gelatinous deposit of silica and etching a ring at the periphery of the drop 

 of water. Test the filtrate for boric acid as directed under 14. 



SULPHUROUS ACID. 



29 Qualitative Test.^^ 



Add some sulphur-free zinc, and several cc. of hydrochloric acid to about 25 

 grams of the sample (with the addition of water, if necessary) in a 200 cc. Erlen- 

 meyer flask. In the presence of sulphites, hydrogen sulphid will be generated 

 and may be detected with lead acetate paper. Traces of metallic sulphids are 

 occasionally present in vegetables, and will give the same reaction as sulphites 

 under the conditions of the above test. Positive results obtained by this method 

 should be verified by the distillation method under 30. 



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 sufficient indication of the presence of sulphur dioxid either as a 

 bleaching agent or as a preservative. 



TOTAL SULPHUROUS ACID. 



30 Method I. — Distillation Method. 



Distil 20-100 grams of the sample (adding recently boiled water if necessary) 

 in a current of carbon dioxid, after the addition of about 5 cc. of a 20% glacial phos- 



