CANE SUGAR. 165 



boiling being maintained the whole time, and the sugar solution 

 run in slowly towards the end. 



As the reaction takes place somewhat slowly, boiling must be 

 continued for a few minutes before it can be finally decided that 

 the blue colour is permanent. 



It is necessary to repeat the titration, adding a little less of 

 the solution, as with Fehling's solution. This may be added 

 advantageously all at once, and the boiling continued for five 

 minutes. If the boiling be prolonged unduly, the ammonia may 

 be boiled off, and cuprous oxide will then begin to deposit ; in 

 order to avoid this, Shenstone places a tapped funnel in the 

 cork, by means of which an addition of strong ammonia can be 

 made if necessary. 



Stokes and Bodmer strongly recommend this method, and 

 state that the reducing power of milk-sugar is 52 per cent, of 

 that of glucose i.e., 100 c.c. of Pavy solution = 0-0961 gramme 

 of milk-sugar. 



It is advisable to standardise the Pavy's solution on a solution 

 of pure milk-sugar containing 0'5 gramme per 100 c.c. 



Hehner has shown that by varying the proportion of salts in 

 solution, such as alkaline tartrates and carbonates, the accuracy 

 of the results is affected ; but by standardising the solution at 

 the time of using with a solution of pure milk-sugar, the effect 

 of any such variations is eliminated. 



Allen has modified the procedure by placing a layer of petro- 

 leum over the Pavy solution, and dispensing with the cork. 

 This enables an ordinary burette, or even pipette, to be used. 



Estimation of Cane Sugar in Milk. Cane sugar is sometimes 

 added as an adulterant of milk, but the determination is more 

 often required in the case of condensed milks. 



An approximate estimation may be made by estimating the 

 sugar by precipitation with alcohol, and the milk-sugar by 

 Fehling or Pavy solution ; the difference between the two will 

 not be far from the cane sugar. 



Shenstone' s Method. The cane sugar may be estimated 

 by determining the total polarisation of the sample as directed 

 for milk-sugar, and by estimating the milk-sugar gravimetrically 

 or volumetrically by Pavy's or Fehling's solution. The difference 

 between the percentage of anhydrous milk-sugar found by reduc- 

 tion of copper and that deduced from the polarisation divided 

 by T217 will give the percentage of cane sugar. This method 

 yields excellent results with mixtures of fresh milk and cane 

 sugar, and with many samples of sweetened condensed milks, 

 but it is apt to lead to figures below the truth, if the milk has been 

 much heated, owing to the reduction in the rotatory power of 

 milk-sugar under these conditions. 



