FATS AND OILS. WAXES. LECITHINS 181 



(2) -iN thiosulphate solution. This is prepared by dissolving 24-823 gm. 

 of the pure salt in 1000 c.c. of water or by dissolving 25 gm. of the salt in 1000 

 c.c. and standardising after 24 hours against potassium bichromate as follows : 



20 c.c. of a bichromate solution containing 3*8657 gm. in 1000 c.c. are 

 placed in a bottle containing 10 c.c. of the potassium iodide solution. 5 c.c. 

 of concentrated hydrochloric acid are added. The brown solution is titrated 

 with the thiosulphate solution, using starch solution as indicator. 



Since 20 c.c. of the bichromate solution yields 0-2 gm. iodine, i c.c. of 



^ 



thiosulphate = gm. of iodine, if x c.c. are required in the titration. 

 oc 



(3) 10 per cent, potassium iodide solution. The quantity of iodate pre- 

 sent must be taken into consideration. 



(4) i per cent, starch solution. 



(5) Pure chloroform or carbon tetrachloride. Their purity is tested by 

 adding 10 c.c. of iodine solution and titrating after 2-3 hours. The value 

 should be same as for 10 c.c. iodine solution. 



Pure glacial acetic acid (purified by recrystallisation). It should not give 

 a green colour on heating with bichromate and sulphuric acid after prolonged 

 standing. 



Procedure : 



15 to -i 8 gm. of marine animal oil, *2 to -3 gm. of semi-drying oil, "3 to 

 4 gm. of non-drying oil, or *8 to i'o gm. of solid fat is placed in a 500-800 

 c.c. bottle with well-fitting stopper and dissolved in 10 c.c. of chloroform or 

 carbon tetrachloride. 1 25 or 50 c.c. of iodine solution are added, the pipette 

 being allowed to drain for 2 or 3 drops after it has emptied. Iodine solution 

 and solvent must give a clear solution on shaking, otherwise more solvent is 

 added. The mixture is kept in a dark place for 6-8 hours in the case of fat 

 and non-drying oils, for 8-10 hours in the case of semi-drying oils or for 12-18 

 hours in the case of marine animal oils. The mixture must contain 3 times the 

 amount of iodine necessary and should be deep brown after 2 hours. More 

 iodine solution is added, if the colour be paler. 15-20 c.c. of potassium iodide 

 solution and 400 c.c of water are added and the iodine titrated with -iN 

 thiosulphate. If a red precipitate should form, more potassium iodide must 

 be added. 2 5 c.c. of the iodine solution are titrated previously or subsequently. 

 The difference gives the amount of iodine absorbed. The result is calculated : 



e.g. 25 c.c. iodine solution = 3 5'5 <=.c. - lN Thiol > iodine used = J*g 

 0-983 gm. fat + 25 c.c. iodine solution = 18-6 c.c. 'iN Thio j " " . 



. 



.-. 100 gm. fat = 21-8 gm. iodine. ' lodme 



Determination of the Reichert-Wollny Value. 



The standard apparatus for this estimation is shown in Fig. 38. 



chloride, 



FIG. 38. 

 1 Glacial acetic acid in the case of oxidised oils not completely soluble in carbon tetra- 



