CH. v.] BENEDICT'S METHOD. 127 



D. Polarimetric Method. 



ii. This is of great value. The relationships between reduc- 

 ing and rotatory powers of solutions before and after hydrolysis 

 must be determined for the identification and analysis of mixed 

 carbohydrates. 



Of the various methods proposed, the Author is of the opinion that for 

 ordinary routine work and for urinary analysis, Benedict's direct volumetric 

 method is the most reliable in the hands of the majority of workers. The 

 recent method of Folin and McEllroy has certain advantages, especially in the 

 cost of materials, and may eventually supersede Benedict's. The Wood-Ost 

 process is worthy of more extended recognition. In spite of the 10 minutes' 

 boiling that is necessary, the method is a rapid one, and when completed one is 

 left with a sense of confidence that is somewhat lacking in the direct methods. 



For accurate research work the method of Amos Peters is extremely 

 satisfactory. 



When a large series of diabetic urines have to be examined sufficiently 

 approximate results can be obtained by polarisation, after removal of the 

 pigment by blood charcoal in the presence of 10 per cent, of acetic acid, a little 

 freshly prepared metaphosphoric acid being added if proteins are present. 



158. Benedict's Method. 



Principle of the Method. An alkaline solution of copper sul- 

 phate, containing thiocyanate is boiled and the sugar solution run 

 in from a burette till the blue colour just disappears. The thio- 

 cyanate forms a white insoluble compound with the cuprous hydroxide 

 formed by the reduction of the copper, and so there is no red cuprous 

 oxide precipitated to obscure the blue tint. A little potassium 

 ferrocyanide is also added to prevent any possibility of the deposi- 

 tion of the cuprous oxide. 



Preparation of the Solution. With the aid of heat dissolve 

 Sodium citrate . . . .. . . 200 grams. 



Sodium carbonate (cryst.) . . 200 grams. 



(or anyhdrous sod. carb. 75 grams.) 

 Potassium thiocyanate (sulphocyanide) 125 grams. 



in enough distilled water to make about 800 cc. of the mixture and 

 filter, and cool to room temperature. 



Dissolve 1 8 grams, of the purest, air-dried crystalline copper 

 sulphate in about 100 cc. of distilled water, and pour it slowly into 

 the other liquid with constant stirring. Add 5 cc. of a 5 per cent, 

 solution of potassium ferrocyanide and then distilled water to make 



