180 PRACTICAL ORGANIC AND BIO-CHKMISTRY 



Determination of the Acid Value. 



A weighed amount of fat, from 3 or 5 to 10 gm., is dissolved in neutral 

 alcohol 1 or a mixture of alcohol and ether, a few drops of phenolphthalein 

 are added and the solution is titrated with 'iN or ^N potassium hydroxide 

 until it is pink in colour. The pink colour should be permanent for about 

 2 minutes : after this time it usually disappears. 



The acid value depends on the purity and age of the fat, i.e. on the 

 amount of hydrolysis and oxidation that has taken place. 



Determination of the Saponification Value. 



The fat is saponified with an approximately ^N solution of alcoholic 

 potassium hydroxide, standardised against *5N hydrochloric acid. 



The alcoholic potassium hydroxide is prepared by dissolving 28 gm. of 

 pure potassium hydroxide in a little water and diluting to 1000 c.c. with 

 alcohol or purified methylated spirit ; after 24 hours the solution is filtered 

 into a litre bottle, which is closed by a rubber stopper carrying a 25 c.c. 

 pipette. The pipette is closed by a piece of rubber tubing and a glass rod. 



In measuring out the volume it is convenient to allow the liquid to run out 

 of the pipette without touching the sides of the vessel and then to let three 

 drops fall from the end. 



A weighed amount of fat (1*5 to 2 gm.) is heated in a 200 c.c. flask on a 

 water-bath under a reflux condenser with 25 c.c. of the alcoholic potash solu- 

 tion for 30 minutes. At the same time a blank experiment (i.e. the same 

 experiment without the fat) is carried out. The contents of the flasks are 

 heated so that they boil gently and are shaken from time to time. 



When the saponification is complete and the fat has dissolved giving a 

 clear solution, i c.c. of phenolphthalein solution is added and the mixture is 

 titrated with ^N hydrochloric acid. 



The difference between the values found in the blank experiment and the 

 one with fat is the volume of ^N alkali required to saponify the fat. Hence 

 the amount in mgm. required to saponify i gm. of fat can be calculated. 



Determination of the Iodine Value. 



There are two methods of determining the iodine value, (a) Hiibl's, (b) 

 Wijs. The process is the same in the two methods, the difference being in 

 the iodine solution. Hubl used a mixture of iodine and mercuric chloride, 

 Wijs iodine trichloride. 



The following reagents are required : 



(i) Iodine solution. 



(a) Hiibl's. Equal volumes, e.g. 30 c.c. of a solution of 25 gm. of iodine 

 in 500 c.c. of pure 95 per cent, alcohol and of a solution (filtered if necessary) 

 of 30 gm. of mercuric chloride in 500 c.c. of pure 95 per cent, alcohol are 

 mixed 12-24 hours before use. The mixture should not be used if it has 

 been prepared longer than 24 hours. 



(b) Wijs. 9*4 gm. of iodine trichloride are dissolved in 200 c.c. of glacial 

 acetic acid contained in a 300 c.c. flask heated on the water-bath and closed 

 with a cork carrying a calcium chloride tube; at the same time 7*2 gm. of 

 finely powdered iodine are dissolved in glacial acetic acid in a similar way. 

 The two solutions are poured into a 1000 c.c. flask and any undissolved 

 iodine dissolved in a fresh portion of acetic acid. The mixed solutions are 

 cooled and made up to i ooo c c. 



This solution may also be made by dissolving 13 gm. of iodine in 1000 c.c. 

 of glacial acetic acid and passing washed and dry chlorine through the 

 solution. A colour change occurs at the point when iodine trichloride is 

 formed. In preparing the solution, the iodine content of the solution is de- 

 termined before the passage of the chlorine ; the chlorine is passed into th? 

 solution until the iodine content is doubled (Lewkowitch). 



Methylated spirit which has been kept in contact with sodium hydroxide and distilled 

 may be used. If it should contain aci 1 it maybe neutralised with -iN potassium hydroxide 

 until it shows a faint pink colour t? phenolphthalein, a few drops of which are added as 

 jndjcatpr, 



