44 CUPRIC OXIDE REDUCING PROPERTIES. [BOOK II. 



The cupric solution employed, which is usually termed Fehling's 

 solution, is prepared by dissolving 34-639 grammes of pure crystallised 

 cupric sulphate in about 200 c.c. of water, and in another vessel 173 

 grammes of pure potassic sodic tartrate so-called Rochelle salts in 

 480 c.c. of a solution of pure sodic hydrate of specific gravity 1-14; the 

 first solution is then added gradually to the second, and the deep-blue 

 coloured clear fluid is diluted to 1000 c.c. It must be kept in a cool, dark 

 place, in well-closed bottles, filled to the top, as the action of light or the 

 absorption of carbonic anhydride would lead to the separation of cuprous 

 oxide on mere exposure to heat. Before using the solution, mix 10 c.c. of 

 it with 40 c. c. of water, and boil the mixture for some minutes ; if this 

 produces the least change, and causes the separation of even the smallest 

 quantity of cuprous oxide, the solution is unfit for use. Of this solution 

 25 30 c.c. are poured into a beaker of 130 140 c.c. capacity, together 

 with about 50 c.c. of boiling well-boiled water; the beaker is then placed 

 in a water bath, which is kept boiling, and at the end of five or six 

 minutes, when the dilute copper solution has acquired as nearly as possible 

 the temperature of the bath, a known quantity of the solution to be tested 

 is added, and the heating continued for twelve or fourteen minutes. If 

 the blue colour completely disappears in the first three or four minutes, it 

 can be restored by adding quickly more copper solution, but if two or 

 three additions be necessary to insure an excess, the experiment must be 

 sacrificed, and a fresh one made with a smaller quantity of the carbo- 

 hydrate solution. Satisfactory results cannot be obtained unless this pre- 

 caution be adopted : the numbers generally falling too low with solutions 

 of maltose or the glucoses, and too high when dextrin is also present, if the 

 amount of cupric solution employed be not from the first in excess. After 

 thirteen or fourteen minutes heating, the precipitated cuprous oxide is 

 rapidly filtered out, washed with boiling, well-boiled water, dried, and 

 ignited in the usual way ; strong ignition in an open porcelain crucible for 

 five or six minutes completely converts the cuprous into cupric oxide, and 

 treatment with nitric acid is unnecessary. 



The time of heating above mentioned gives the true reduction for 

 dextrose and maltose, and the quantity of cuprous oxide precipitated 

 remains constant, even if the heating be continued for twenty minutes ; 

 but if the solution in addition contains dextrin, the reduction becomes 

 greater, owing doubtless to the slow conversion of the dextrin into sub- 

 stances capable of acting upon the cupric solution (O'Sullivan, Journ. Ghem. 

 Soc. 1876, n. 130). 



The cupric oxide reducing power is often determined volumetrically, 

 with the previously mentioned Fehling's solution, of which 10 c.c. corre- 

 spond to 0'05 grammes of dextrose. 10 c. c. of the cupric solution and 40 c. c. 

 of water are heated in a water bath kept boiling, and the highly dilute 

 solution of the carbohydrate is added in small quantities from a burette, 

 until the blueish-green colour of the solution entirely disappears. Although 

 concordant results may, with great care, be obtained by this process, the 

 polarimetrical method is far more reliable, and involves very little more 

 trouble in its execution." 



The products It is true of all enzymes that the substances which 

 of ferment- result from their action as they accumulate in the 



