90 PROVISIONAL METHODS FOR ANALYSIS OF FOODS. 



(e) DETERMINATION OF TOTAL SULPHUROUS ACID. 



Distill 100 cc of wine 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 decinorinal 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 solu- 

 tion. Twenty-five cc of decinormal iodin solution may be employed, diluted with 

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

 without material loss in accuracy by omitting the current of carbon dioxid, adding 

 10 cc of phosphoric acid instead of 5 cc, and dropping into the distilling flask a piece 

 of sodium bicarbonate weighing not more than a gram immediately before attaching 

 to the condenser. The carbon dioxid liberated is not sufficient to expel the air 

 entirely from the apparatus, but will prevent oxidation to a large extent. The U-tube 

 trap may also be omitted if the end of the condenser tube be made to extend below 

 the surface of the iodin solution, and the distillation conducted with a steady flame. 

 When the distillation is finished, wash the contents of the U-tube into the flask and 

 determine the excess of iodin with standardized thiosulphate solution. On account 

 of its lack of permanence, the iodin solution employed should be titrated from time 

 to time with a decinormal thiosulphate solution (containing 24.8 grams Na 2 S 2 O 3 .5 

 H 2 O per liter). The number of cubic centimeters of decinormal iodin solution 

 employed, less the number of cubic centimeters of thiosulphate solution required at 

 the end of the determination, is multiplied by 0.0032 for the grains of sulphur dioxid 

 per 100 cc of wine. 



Fairly accurate results may also be obtained by the following method: 

 Place 25 cc of a solution of potassium hydroxid containing 56 grams per liter in 

 a flask of approximately 200-cc capacity. Introduce 50 cc of wine by means of a 

 pipette, mix with the potassium hydroxid, and allow the mixture to stand for fifteen 

 minutes with occasional agitation. Add 10 cc of 1-3 sulphuric acid and a few cubic 

 centimeters of starch solution, and titrate the mixture with a N/50 iodin solution. 

 Introduce the iodin solution as rapidly as possible and continue the addition until 

 the blue color will last for several minutes. One cubic centimeter of N/50 iodin 

 solution is equivalent to 0.00064 gram of sulphur dioxid. The number of cubic 

 centimeters of the iodin solution employed, multiplied by 0.00128, gives the weight 

 of the total sulphur dioxid expressed in grams per 100 cc. 



(f) DETERMINATION OF FREE SULPHUROUS ACID. 



Treat 50 cc of wine in a flask, having a capacity of approximately 200 cc, with 

 about 5 cc of 1-3 sulphuric acid, add a small piece of sodium carbonate (about 0.5 

 gram) to expel the air, and titrate the sulphurous acid with N/50 iodin solution, as 

 directed under total sulphurous acid. 



One cubic centimeter of N/50 iodin solution is equivalent to 0.00064 gram of sul- 

 phur dioxid. 



The number of cubic centimeters of iodin solution employed, multiplied by 

 0.00128, gives the weight of the free sulphurous acid expressed as sulphur dioxid in 

 grams per 100 cc. 



(g) DETECTION OF BETA-NAPHTHOL. 



Extract 200 cc of wine with 10 cc of chloroform in a separatory funnel, add a few 

 drops of alcoholic potash to the chloroform extract in a test tube, and place in a 

 boiling water bath for two minutes. The presence of beta-naphthol is indicated by 

 the formation of a deep blue color, which changes through green to yellow. 



