3/O PHYSIOLOGICAL CHEMISTRY. 



3. Purdy's Method. Purdy's solution 1 is a modification of 

 Fehling's solution and is said to possess greater stability than the 

 latter. One of the most satisfactory points about the method as 

 suggested by Purdy is the ease with which the exact end-reaction 

 may be determined. In determining the percentage of dextrose by 

 this method proceed as follows : Place 35 c.c. of Purdy's solution 

 in a 200 c.c. Erlenmeyer flask and dilute the fluid with approxi- 

 mately two volumes of distilled water. Fit a cork, provided with 

 two perforations, to the neck of the flask and through one perfora- 

 tion introduce the tip of a burette and through the second perfora- 

 tion introduce a tube bent at right angles in such a manner as to 

 allow the steam to escape and keep the fumes of ammonia away 

 from the face of the operator as completely as possible. 2 Now 

 bring the solution to the boiling-point and add the urine, drop by 

 drop, until the intensity of the blue color begins to diminish. When 

 this point is reached add the urine somewhat more slowly until 

 the blue color is entirely dissipated and an absolutely decolorized 

 solution remains. Take the burette reading and calculate the per- 

 centage of dextrose in the urine examined according to the method 

 given on p. 371. 



Care should be taken not to boil the solution for too long a per- 

 iod, since, under these conditions, sufficient ammonia might be 

 lost to allow the cuprous hydroxide to precipitate. 



Some investigators consider it to be advisable to dilute the urine 

 before applying the above manipulation, but ordinarily this is not 

 necessary unless the urine has a high content of dextrose (5 per 



1 Purdy's solution has the following composition : 



Cupric sulphate 4-752 grams. 



Potassium hydroxide 23.5 grams. 



Ammonia (U. S. P., sp. gr. 0.9) 35O.O c.c. 



Glycerol 38.0 c.c. 



Distilled water, to make total volume i liter. 



In preparing the solution bring the cupric sulphate and potassium hydroxide 

 into solution in separate vessels, mix the two solutions, cool the mixture and 

 add the ammonia and glycerol. After this has been done the total volume 

 should be made up to I liter with distilled water. 



Thirty-five cubic centimeters of Purdy's solution is exactly reduced 'by 0.02 

 gram of dextrose. 



2 This side tube may also be equipped with a simple air-valve, thus insuring the 

 exclusion of air and thereby contributing to the accuracy of the determination, 

 inasmuch as the cuprous salts would be reoxidized upon coming in contact with 

 the air. If one is careful to maintain the solution continuously at the boiling- 

 point throughout the entire process, however, there is no opportunity for air to 

 enter and therefore no need of an air-valve. 



