96 ANALYSIS OF MILK. 



solution, which is prepared by dissolving 34'639 grammes of 

 pure, crystallised copper sulphate in water, and diluting to 

 500 c.c. ; 173 grammes of pure sodium-potassium tartrate 

 (Rochelle salt), and 51 to 55 grammes of sodium hydroxide of 

 good quality are also dissolved in water and made up to 500 c.c. 

 Equal parts of these solutions are mixed (preferably at the time 

 of making the test) to form Fehling's solution. It is convenient 

 to use a 50 per cent, solution of caustic soda solution, which has 

 been filtered clear through asbestos, for making the alkaline 

 tartrate solution. The percentage of sodium hydroxide is esti- 

 mated in this by titration, and such a quantity is weighed out 

 as will give 51 grammes. Most of the impurities in ordinary 

 caustic soda are insoluble in a 50 per cent, solution, so that this 

 affords a ready means of purification. 



Before estimating the milk-sugar in milk, the fat and protein must 

 be removed ; this ma}' be accomplished ))y the following methods : — 



(1) By diluting 10 c.c. of milk to about 100 c.c, adding 1'5 c.c. 

 of 10 per cent, acetic acid solution and boiling ; after cooling, 

 the whole is made up to 100 c.c. (Citric acid may be substituted 

 for acetic acid.) 



(2) Add to "25 c.c. of milk about 2(>0 c.c. of water and 10 c.c. 

 of copper sulphate solution, as above ; carefully neutralise with 

 dilute caustic alkali solution, and make up to 250 c.c. This 

 solution contains a small amount of copper. 



(3) Carefully neutralise 10 c.c. of the filtrate from the milk 

 which has been treated with acid mercuric nitrate with caustic 

 alkali till exactly neutral to phenol-phthalein, filter, and pass 

 sulphuretted hydrogen through the filtrate ; filter, to separate 

 the precipitated mercuric sulphides, and boil the filtrate to expel 

 sulphuretted hydrogen. Make up to 100 c.c. (The mercury 

 may be precipitated by phosphoric acid ; add a small quantity 

 of phosphoric acid or a soluble phosphate to the filtrate from 

 mercuric nitrate ; exactly neutralise, filter and wash the pre- 

 cipitate, and make up to 100 c.c.) 



(4) Deniges' method, as above described. 



Each of the methods of separating the fat and protein gives a 

 solution, of which 50 c.c. contains the milk-sugar in 5 c.c. of milk, 

 which, in a normal milk, represents about J gramme of milk-sugar. 

 The following modes of manipulation are among those in use : — 

 (1) O'SuUivan's Method. — Measure 50 c.c. of the filtrate into 

 a beaker, and place this in a briskly-boiling water-bath ; dilute 

 a mixture of 30 c.c. of the copper solution, and 30 c.c. of the 

 alkaline tartrate solution, with about twice its volume of water, 

 and boil over a flame. When the milk-sugar solution has 

 attained the temperature of the water-bath, pour into it the 

 Fehling's solution, and keep on the water-bath for 13 to 15 



