FOOD PRESERVATIVES. 



187 



phuric acid, in a short test tube, which is attached to a small U tube containing 

 a few drops of water. Place the test tube in a beaker of water and keep it hot 

 on the steam bath for from 30 to 40 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. 



Now test the filtrate as directed under boric acid. 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 is 

 repealed without the introduction of the silica, in which case the formation 

 of the silicon skeleton is conclusive evidence of the presence of fluosilicate.<* 



(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 the underside of which a drop 

 <f water is suspended, and heat an hour at the temperature of 70 to 80 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 section 4, page 183. 



8. Sulphurous Acid, 

 (a) QUALITATIVE DETECTIONS 



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

 placed in a 200 cc Erlennaeyer flask, add some sulphur-free 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 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 sufficient indication of the presence of sulphur dioxid either 

 as a bleaching agent or as a preservative. 



(b) DETERMINATION OF TOTAL SULPHUROUS ACID. 







(1) FIRST METHOD (DISTILLATION METHOD). 



Distil 100 grams (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 tenth-normal iodin 

 solution in a flask closed with a stopper perforated with two holes, through 

 one of which the end of the condenser passes and through the other a U-tube 

 containing a portion of the standardized iodin solution. Twenty-five cubic 

 centimeters of tenth-normal iodin solution may be employed, diluted with water 

 to give the desired volume. The method and apparatus may be simplified with- 

 out material loss in accuracy by omitting the current of carbon dioxid, adding 



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, therefore, and not the presence of a fluorid. 



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



c U. S. Dept. Agr., Division of Chemistry, Bui. 13, pt. 8, p. 1032. 



87404 Bull. 107- 



