188 METHODS OF ANALYSIS. 



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 is made to extend below the surface of the iodin solution, and the dis- 

 tillation 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 

 standard thiosulphate solution. On account of its lack of permanence the iodin 

 solution employed should be titrated from time to time with a tenth-normal 

 thiosulphate solution (containing 24.8 grams Na 2 S 2 O 3 .5H 2 O per liter). One 

 cubic centimeter of tenth-normal iodin solution is equivalent to 0.0032 gram of 

 sulphur dioxid (SO 2 ). 



(2) SECOND METHOD (DIEECT TITBATION METHOD). 



In the examination of wine fairly accurate results may also be obtained by 

 the following method. Care must be taken in applying the method to other 

 products than wine to determine whether iodin is decolorized by any substance 

 that may be naturally present. 



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 the sample 

 by means of a pipette, mix with the potassium hydroxid, and allow the mixture 

 to stand fifteen minutes with occasional agitation. Add 10 cc of 1 : 3 sul- 

 phuric acid and a few cubic centimeters of starch solution, and titrate the 

 mixture with a fiftieth-normal 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 fiftieth-normal iodin solution is equiv- 

 alent to 0.00064 gram of sulphur dioxid. 



(c) DETERMINATION OF FREE SULPHUROUS ACID. 

 (Especially adapted to wine.) 



Treat 50 cc of the sample 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 fiftieth- 

 normal iodin solution, as directed under total sulphurous acid. 



One cubic centimeter of fiftieth-normal iodin solution is equivalent to 0.00064 

 gram of sulphur dioxid. 



9. Beta-Naphthol. 



Extract 200 cc of the sample (or of its aqueous extract prepared as directed 

 on page 179) 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 

 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. 



10. Abrastol. 



(a) SINIBALDI'S METHOD." 



Make 50 cc of the sample alkaline with a few drops of ammonium hydroxid 

 and extract with 10 cc of amyl alcohol (ethyl alcohol is added if an emulsion is 



Moniteur scientiflque, 1893, (4), 7: 842. 



