164 THE ESTIMATION OF SUGARS. 



are dissolved in hot water, mixed with 142 grammes of caustic soda, 

 also dissolved in water, and, after cooling, made up to 1 litre. 



Equal volumes of these two solutions are mixed for use as 

 wanted, and the mixed solution should not be kept longer than 

 a day. 



For the estimation of milk-sugar 10 c.c. of the freshly mixed 

 Fehling's solution is measured into a 200 c.c. flask and raised 

 to boiling. The solution of milk from which the proteins have 

 been removed (about 5 c.c. of milk per 100 c.c.) is then run into 

 the boiling solution in small amounts, commencing with 5 c.c. 

 After each addition the mixture is boiled, the solution being 

 kept rotated. About a dozen drops of the indicator (see below) 

 are placed on a porcelain or opal glass plate, and when it is 

 judged that the precipitation of cuprous oxide is complete, a 

 drop of the liquid is withdrawn, and brought into contact with 

 a drop of the indicator. The test must be carried out rapidly, 

 and it is essential to keep as far as possible an atmosphere of 

 steam in the flask, to exclude atmospheric oxygen. When a 

 red coloration is no longer produced, all the copper has been 

 precipitated. A second or third titration should be made to 

 establish the end point accurately. The Fehling's solution should 

 be standardised on a solution containing 0-25 gramme pure sugar 

 in 100 c.c. 



The indicator is prepared by dissolving 1 gramme of ferrous 

 ammonium sulphate and 1 -5 grammes of ammonium thiocyanate 

 in 2-5 c.c. of concentrated hydrochloric acid and 10 c.c. of water. 

 The solution then has a brownish-red colour, which is removed 

 by the addition of a trace of zinc dust. On keeping, a red colour 

 develops, which may be removed by the addition of a further 

 quantity of zinc dust ; the delicacy is, however, impaired after it 

 has been reduced several times. When freshly prepared it is 

 almost too delicate, and the indicator is most useful after it has 

 *been reduced twice. 



Pavy's Solution Method. Pavy's ammoniacal cupric solu- 

 tion may be substituted for Fehling's solution. This is prepared 

 by mixing 120 c.c. of Fehling's solution, 400 c.c. of 12 per cent, 

 caustic soda solution, and 300 c.c. of strong ammonia (specific 

 gravity 0-880), and diluting the whole to a litre. 



One hundred c.c. of this solution are placed in a small flask, 

 which is closed by an india-rubber stopper with two holes ; 

 through one passes the nozzle of a Mohr's burette, and through 

 the other a bent tube, which dips into a flask containing cold 

 water to absorb the ammonia given off. Hydrogen or coal gas 

 may be passed through the flask containing the Pavy solution. 



The solution is brought to boiling, and the sugar solution run 

 in gradually, till the blue colour of the liquid is destroyed, the 



