IMPROVED METHODS FOR FAT ANALYSIS. 123 



Iodine Number. 



The iodine number indicates the percentage of iodine chloride absorbed 

 under definite conditions by an oil, fat or wax, expressed in terms of 

 iodine. * 



Reagents. ■ — Iodine solution according to Wijs. ^ Thirteen grams of re- 

 subUmated iodine to 1,000 cubic centimeters ot acetic acid- (99.5 per 

 cent.), free from oxidizable products. After the iodine is completely dis- 

 solved the solution is treated with pure dry chlorine gas ^ mitil the iodine 

 has been converted into monochloride. The completion of the reaction 

 is indicated by a distinct change, the solution becoming transparent cherry 

 red, and its titer "* with thiosulf ate doubled. As it is ad\dsable to have a 

 sUght excess of iodine, a small quantity of untreated solution should be 

 retained and may be added in case of necessity. 



N/10 sodium thiosulf ate (hyposulfite) solution: 24.822 grams* of 

 sodium thiosulfate, dissolved in water and made up to a hter. 



Potassium bichromate solution: 3.8633 grams of dry C. P. potassium 

 bichromate, free from sodium bichromate, dissolved in water and made up 

 to a volume of 1,000 cubic centimeters at 20° C. This solution will keep 

 almost indefuiitely without changing, and is used for standardizing the 

 thiosulfate solution. One hundred cubic centimeters of potassium bi- 

 chromate will Uberate 1 gram of iodine from a potassium iodide solution. 



Potassium iodide solution: 165 grams of neutral potassium iodide, free 

 from iodine and iodate, to 1,000 cubic centimeters of water. lodate is 

 said to be present frequently in commercial potassium iodide, and yields 

 free iodine ^\^th hydrochloric acid. 



Starch paste: 1 gram to 200 cubic centimeters of water. The indicator 

 is prepared by boiling thoroughly, decanting and diluting the solution, 

 and again boiUng to insure a perfect paste free from solid particles. 



Carbon tetrachloride: anhydrous® and tree from oxidizable products. '^ 



Standardizing the Thiosidfate. — Twenty-five cubic centimeters of po- 

 tassium bichromate are accurately'' measured with a burette into a 300 

 cubic centimeter Erlenmeyer flask, and 10 cubic centimeters of potassium 

 iodide and 5 cubic centimeters of concentrated hydrochloric acid added. 

 Simultaneously with the addition of the acid, tliiosulfate is run in until 

 the brownish yellow color (iodine) has been largely destroyed; then 2 cubic 

 centimeters of starch paste are added and the titration continued with 



' Ber. Deut. Chem. Gesell. 31, p. 750 (1898). Wijs' solution, with the same active reagent, 

 iodine monochloride, has largely replaced that of Hubl on account of its far greater stability and 

 more rapid absorption. 



• The acid should be crystallized at 15° C. and the mother liquor discarded. The acid should 

 not react with the bichromate test. 



» Washed and dried by being passed through concentrated sulfuric acid. Gas sufficient for 

 4,000 cubic centimeters of iodine solution can be generated from 44.5 grams of sodium chloride, 

 55.5 grams of manganese dioxide, and 1.50 cubic centimeters of sulfuric acid (1 to 1). 



• With the addition of potassium iodide as usual. 



' Preferably 30 grams to 1,000 cubic centimeters of water. 



• Dried over recently ignited sodium sulfate and distilled. 

 ' Bichromate test. 



