92 MILK AND DAIRY PRODUCTS PART 



This precipitate is insoluble in water, but soluble in concen- 

 trated mineral acids, so it is dissolved in hydrochloric acid, 

 excess of ammonia added, as well as a known quantity of 

 N/10 KCN, and then titrated with N/W AgN0 3 solution, KI 

 being used as indicator. The difference expresses the amount 

 of N/10 AgNO 3 which is equivalent to the mercury in the 

 precipitate. Beau's l directions for carrying out the estimation 

 are as follows : 50 c.c. of milk are put into a 200 c.c. graduated 

 flask, about 75 c.c. of distilled water and 50 c.c. of mercuric 

 sulphate 2 are then added, and the flask moderately shaken, 

 This causes the precipitation of the casein, and the flask is 

 then filled to the mark and the contents filtered, the liquid 

 being passed several times through the paper until it possesses 

 only a slight opalescence. It is impossible to obtain an 

 absolutely clear filtrate. 



One hundred c.c, of the filtrate (equivalent to 25 c.c. milk) 

 are oxidised with a 1 per cent, solution of potassium perman- 

 ganate, the liquid being heated to boiling, then the flame 

 removed and permanganate solution added drop by drop, the 

 flask being shaken after each addition. The liquid soon 

 becomes turbid, after which a yellowish-white precipitate 

 forms. The addition of potassium permanganate is, however, 

 continued until the precipitate sinks rapidly to the bottom, 

 and the liquid above is perfectly clear. To reach this point 

 5-10 c.c. of potassium permanganate are required. A slight 

 excess is of no disadvantage, for otherwise the oxidation is apt 

 to remain incomplete. 



The precipitate has a coffee-brown colour, on account of small 

 quantities of manganese dioxide, but this can be removed by 

 heating the liquid until it boils, then removing the flame and 

 adding small amounts of hydrogen peroxide until the 

 precipitate is quite colourless. Generally five to ten drops of 

 hydrogen peroxide are required for this. After cooling, the 

 liquid is filtered through an asbestos filter (Fig. 47, p. 86), 

 attached to a filter flask and water-pump ; the residue being 

 washed with water until the filtrate gives no cloudiness with 



1 Loc. cit. 



' 2 The mercuric sulphate is prepared by mixing 50 g. of red oxide of mercury 

 with 400-500 c.c. of distilled water in a litre flask, and then gradually 

 adding sulphuric acid (sp. gr. 66 Beaume) until all the mercuric oxide is dis- 

 solved. This will take about 75 c.c. of H 2 S0 4 , and the flask is then made up 

 to the mark, heated, and filtered. 



