THE DETERMINATION OF REDUCING SUGARS 537 



The precipitated cuprous oxide is dissolved in nitric acid, the excess of 

 acid partly removed by evaporation, neutralized with a slight excess of sodium 

 carbonate, and the precipitate redissolved with acetic acid. A slight excess 

 of potassium iodide over that indicated as necessary from the above equa- 

 tion is added, and the iodine determined in the usual way with sodium thio- 

 sulphate, using starch as an indicator. 



The thiosulphate solution should be standardized against pure electric 

 copper or a pure preparation of a copper salt. 



Choice of Form in which the Copper is estimated. If the material analysed 

 is a pure reducing sugar, the reduced copper may be estimated as cuprous 

 oxide, as cupric oxide, as metallic copper or volumetrically with identical 

 results. With materials such as cane molasses, a precipitate other than that 

 of cuprous oxide may be thrown down and hence the weight found will be in 

 excess of that due to the reduced cupric salt. If the cuprous oxide be burnt 

 to cupric oxide, the only contamination will be that due to ash. The most 

 exact methods are probably the estimation as copper deposited electroly- 

 tically, and as copper estimated iodometrically. The estimation by per- 

 manganate is likely to be falsified by the presence of organic matter in the 

 cuprous precipitate. Meade and Harris 18 have shown that the results 

 with cane molasses are almost identical when the estimation is made as cupric 

 oxide, as reduced copper or iodometrically. 



Glucose Ratio of Sugars and Munson and Walker's Table. Munson 

 and Walker's table concerns invert sugar, glucose, lactose and maltose. By 

 establishing the reducing ratio of the sugars, that for invert sugar may be 

 used for any sugar. Accordingly, only the values for invert sugar and invert 

 sugar and sucrose are recorded in their table in the Appendix. These are 

 specially applicable to cane sugar work, where the mixture of reducing sugars 

 is never far removed from invert sugar. The table has also been altered 

 by substituting the weight of cupric oxide for cuprous oxide. 



The reducing ratios of the commoner reducing sugars compared with 

 anhydrous glucose as unity are thus given by Browne 19 : 



Glucose, i-ooo; Fructose, 0-915; Xylose, 0-983; Arabinose, 1-002; 

 Invert Sugar, o 957 ; Galactose, o 898 ; Lactose, H 2 O, o 678 ; 

 Maltose, H 2 O, o 620. 



The Effect of Cane Sugar on the Determination of Reducing Sugars. 



Cane sugar, by itself, has only a very small reducing power, but in the presence 

 of reducing sugars,, especially when the cane sugar is in great excess, the effect 

 is sufficient to invalidate the analysis. This behaviour is allowed for in 

 Munson and Walker's tables, and the analysis should be so conducted that the 

 quantities of material taken are substantially those for which these tables 

 are drawn up. 



Preparation of Materials for Reducing Sugar Assay. Many cane sugar 

 products, without previous treatment, afford a copper precipitate which is 

 incapable of filtration. Certain formal directions specify a clarification 

 with basic lead acetate, the use of which is, of course, irrational (cf. Chapter 

 XXV). Further, the precipitated lead-reducing sugar compounds are not 

 broken up by the addition of sodium sulphate, but the error is accentuated, 

 since, with an excess of lead, a further precipitation of lead-reducing sugar 



