Quantitative Estimation of the Carbohydrates. 1 1 1 



being used to wash out the flask if much ciilorophyl! or fat is present. 

 It is now necessary to remove tannins, amino-acids, basic 

 substances, etc. This is effected by addition of the exact quantity 

 of basic lead acetate required to precipitate the whole of these 

 substances. Any excess of lead is removed by hydrogen sulphide 

 (Brown and Morris) or by solid sodium carbonate (Davis, Daish and 

 Sawyer). A solution so prepared can be kept for several weeks if a 

 little toluene is added, provided no excess of basic lead acetate is 

 present and that the solution is just alkaline. Any excess of basic 

 lead acetate or much alkali brings about a rapid destruction of 

 fructose. 

 6. Estimation of Sugars, 



The methods employed by various workers for the estimation 

 of sugars vary in details, but the general principles underlying all 

 of them are the same. We will consider first the method used by 

 Parkin, for as this worker concluded that sucrose, glucose and 

 fructose are the only sugars present in the snowdrop leaf, his 

 analysis is simpler than that of Brown and Morris, and Davis, Daish 

 and Sawyer who analysed their extracts for other sugars as well. 



(a) Parkin's Method. The cupric-reducing power and optical 

 rotation of a definite volume of the purified extract is first measured. 

 The cupric-reducing power is due to the hexoses alone, the optical 

 rotation to the hexoses and sucrose together. A further volume of 

 the extract is inverted with invertase and the cupric-reducing power 

 and optical activity again measured. The increase in reducing power 

 and change of optical activity must be due to the inversion of the 

 sucrose, and from these numbers the quantity of sucrose is obtained. 

 From the reducing power and optical activity due to the hexoses, 

 the quantities of fructose and glucose can be calculated. 



Parkin also found that after fermentation with brewers' or 

 bakers' yeast, the cupric-reducing power and optical activity 

 became negligible. This indicates the absence of pentoses and the 

 I forms of the hexoses. 



(6) Method of Brown and Morris. 



(i.) The cupric-reducing power and optical rotation are first 

 measured. 



(ii.) For the estimation of cane sugar the solution is inverted 

 with invertase at 50" to 55"C. The increase in cupric-reducing 

 power and change in optical rotation will both give the quantity of 

 sucrose. 



(iii.) 50 c.c. of the 1% solution is heated with 3 c.c. of concen- 



