526 SIMPLE CHEMICAL MANIPULATIONS 



5. After the completion of the reaction, read off from the burette 

 the number of c.c. of sugar solution used and calculate from this 

 figure the percentage of sugar present in the original sugar-containing 

 fluid. 



Example. As an example, suppose that a 3 per cent, starch bouillon 

 has been inoculated with a microorganism, secreting a diastatic 

 ferment, and that the amount of sugar formed after three days' 

 incubation is to be ascertained. The solution must first be filtered. 

 The preliminary qualitative test, also made with Fehling's solution, 

 shows considerable sugar present. The filtrate is, therefore, diluted 

 in the proportion of 1 to 3 parts of distilled water and the fluid so 

 obtained filled into a clean burette in which it is so regulated that 

 40 c.c. are present when the addition to the boiling copper solution 

 is begun. When all the copper hydrate has been reduced it is found 

 that 24 c.c. have been used out of the burette. This amount of fluid 

 contains 100 milligrams of sugar, which are necessary to reduce all of 

 the copper hydrate in 20 c.c. of Fehling's solution; hence, 100 c.c. .of a 

 fluid like the one used in the burette contain 416 milligrams. Since 

 the fluid in the burette represents one-fourth of the concentration of 

 the original filtered liquid, the latter contains four times as much sugar 

 as the dilute fluid used in the burette, or 1664 milligrams= 1.664 grams 

 of sugar per 100 c.c., or 1.664 per cent. If n is the number of c.c. 

 used out of the burette then the following equation results : 



n : 100 = 100 : x 

 and x = 10,000 

 n 



In other words: To find the amount of sugar in milligrams present 

 in 100 c.c. of the fluid used in the burette, divide 10,000 by the number 

 of c.c. used out of the burette. This gives the amount of sugar in 

 milligrams per 100 c.c. for the dilute fluid, and it must be multiplied 

 by the number of times diluted to express the amount of sugar in 

 milligrams present in 100 c.c. of the original fluid. The sugar in this 

 example has been calculated as glucose or dextrose. In the case of 

 maltose the division is made into 16,000 in the same manner and in 

 the case of lactose into 13,500, because these sugars reduce copper 

 hydrate in a different manner. Saccharose does not reduce copper 

 hydrate, and when it is to be determined by the aid of Fehling's 

 solution it must first be changed into invert sugar by boiling with 

 dilute acids or by the action of the enzyme invertin. 



