80] Estimation of Sugar 51 



which can be represented by equations and calculated from 

 atomic and molecular weights. The determination of the 

 quantity of sugar in any substance is an operation of a different 

 order. No calculation can be made from equations. The 

 test for grape sugar known to elementary students (red 

 precipitate of Cu 2 on boiling with Fehling's solution) is not 

 strictly quantitative, as a slight variation of the conditions 

 under which the test is made will entirely alter the results 

 obtained. Certain sugars, when heated with an alkaline 

 solution of copper tartrate, give red precipitates of cuprous 

 oxide. The quantity of that precipitate varies greatly not 

 only with the quantity of the sugar, but also with its chemical 

 constitution and with the conditions of temperature and 

 strength of the solution. 



It is therefore evident that however carefully the standard 

 solution of copper tartrate may be made, it will be useless 

 unless carefully standardised by the especial sugar and under 

 the especial condition of temperature and dilution which will 

 apply when the actual estimation is carried out. 



The two sugars dealt with in this article are cane sugar and 

 glucose. Glucose is estimated by the method described below, 

 but cane sugar must first be ' inverted ' by boiling with an 

 acid which converts it into invert sugar. This (dextrose 

 and laevulose) has practically the same action on Fehling's 

 solution as glucose (dextrose). 



80. Preparation of Fehling's Solution.— Weigh out 

 exactly 34*64 grams of copper sulphate which has been ground 

 finely and pressed between folds of filter paper. Dissolve with 

 the usual precautions in about 300 c.c. of distilled water, prefer- 

 ably in a 500-c.c. flask. Make up to the 500-c.c. mark, mix 

 well, and keep in a well-stoppered bottle marked ' Fehling's 

 solution I.' Next weigh out 173 grams of Rochelle salt (sodium 

 potassium tartrate) and 160 grams of caustic potash. Dissolve 



E 2 



