156 MILK AND DAIRY PRODUCTS PART 



Another way of preparing the solution is to dissolve 9 g. of 

 commercial iodine trichloride in 1 litre of glacial acetic acid, and 

 then titrate 5 c.c. of it with N/W sodium thiosulphate, first 

 adding a few cubic centimetres of 10 per cent, potassium 

 iodide solution and a little water. Afterwards sufficient finely- 

 powdered iodine is dissolved in the solution of iodine tri- 

 chloride to make the contents one and a half times as much as 

 before. Instead of chloroform, Wijs uses carbon tetrachloride, 

 which ought to be first shaken with a solution of potassium 

 bichromate and concentrated sulphuric acid, the colour of which 

 should not disappear even on standing for some time in contact 

 with the tetrachloride. Wijs prefers carbon tetrachloride to 

 chloroform because he has found that chloroform always contains 

 some alcohol. The procedure is the same as in Htibl's method, 

 save that only 10 c.c. of 10 per cent, potassium iodide solution 

 are required, and half an hour is sufficiently long for the 

 reaction to take place. 



Wijs' method for determining the iodine number has come 

 more and more into use during the last few years, for it has 

 the advantage over Hiibl's method in being simpler, quicker, 

 and more constant in its results. 



V. DETERMINATION OF THE AMOUNT OF FREE ACID. 



As it is interesting in many respects to know what 

 quantity of free acid is present in butter, the following 

 analytical method, prescribed by the German Imperial Health 

 Authorities, is most generally used : 



About 6 g. of melted and filtered butter fat are weighed out 

 and dissolved in an acid-free mixture composed of 15-20 c.c. of 

 alcohol and the same quantity of ether. The solution is then 

 titrated with N/W alkali, and a 1 per cent, solution of alcoholic 

 phenolphthalein used as an indicator. Every cubic centimetre 

 of normal alkali which is required to saturate this free acid in 

 100 g. butter fat is known as the degree of acidity. Some 

 analysts calculate the total amount of free acid into oleic acid, 

 and state what quantity of oleic acid, expressed in percentages 

 of the weight of the butter fat, is equivalent to the free acid. 

 One c.c. N/10 alkali, as used in the above method, is equivalent 

 to 0-0282 per cent, oleic acid (C J8 H 34 O 2 = 282). 



