128 



THE CARBOHYDRATES. 



[CH. V. 



the total volume 1000 cc. The solution appears to keep indefinitely, 

 without any special precaution, such as exclusion of light, etc. 



Method of Analysis. Fit a 150 cc. flask 

 into a ring of a retort s f and of such a si/e that 

 it is fairly firmly held. There is no need to 

 use a wire gauze. Arrange the flask at such a 

 height over a Bunsen burner that the reagent 

 can be kept briskly boiling by means of a 

 small flame. In the flask place 3 to 4 grams, 

 of anhydrous sodium carbonate. This can be 

 roughly measured by taking a depth of I inch 

 in a dry test-tube. Then add 25 cc. of the 

 reagent and heat till most of the carbonate is 

 in solution. Run the sugar solution in from 

 a burette, which is best held in the hand. 

 Run the sugar in slowly, till a bulky chalk- 

 white precipitate is formed and the blue colour 

 lessens perceptibly in intensity. From this 

 point the sugar is added more and more 

 slowly, with constant boiling, until the dis- 

 appearance of the last trace of blue colour, 

 which marks the end-point. If the volume of 

 the sugar is less than 5 cc., dilute it accurately 

 with water till about idee, are judged necessary. 

 Repeat the titration with this as before. 



NOTES. There is a tendency to run in an excess of the sugar, unless 

 special care is exercised throughout the titration, and particularly at the end. 

 The solution must be kept steadily boiling during the entire process, and 

 towards the end the sugar must be added in portions of a drop or two, with an 

 interval of about 30 seconds after each addition. Should the mixture become 

 too concentrated, boiled distilled water may be added to replace that lost by 

 evaporation. 



The titration can also be carried out in a white porcelain dish of 10 to 

 15 cm. in diameter, but the risk of reoxidation of the cuprous compound is 

 greater than in a flask. 



Should the solution bump excessively, two or three small pieces of broken 

 porcelain may be added. 



The 3 to 4 grams, of anhydrous sodium carbonate are added to produce the 

 necessary alkalinity. This proportion of alkali cannot be added to the bulk 

 of the standard solution, for it would crystallise out at room temperature. 



Fig. 1 3 A. 



Apparatus for 

 Benedict's Method. 



