EXPERIMENTAL METHODS. 33 



THE PROCESS OF REDUCTION. 



The solution containing the copper sulphate (solution A) was run out 

 from a burette of 25 c. c. capacity graduated to 0.05 c. c. The alkaline 

 tartrate solution, which need not be so accurately measured, was taken from 

 a graduated pipette, and in the same amount as the copper solution. These 

 solutions were placed directly into the specially designed tube. This con- 

 sists essentially of any ordinary centrifuge or sedimentation tube with a 

 narrow neck, the volume accurately graduated thereon, and fitted with a 

 ground-glass stopper. 1 The quantities are so arranged that there is always 

 an ample excess of copper above that required to completely oxidize the 

 sugar in the solution to be determined. As the method permits of such 

 rapid work, a rough preliminary estimation can be made very quickly, if 

 necessary, although after some experience this is rarely necessary. After 

 the introduction of the copper and alkaline tartrate solutions, the sugar 

 solution is run in. A burette of 25 c. c. graduated to 0.05 c. c. was used for 

 most determinations. The contents of the tube were then thoroughly mixed 

 by shaking. It was found that for work with very small quantities of sugar 

 burettes of 3 c. c. capacity and graduated to 0.01 c. c. could be used with 

 good results. However, in the work with the cacti there was always 

 sufficient material, so that it was not necessary to resort to this apparatus. 



The length of time prescribed by various methods and workers for heat- 

 ing the reduction mixture shows a great variation. The question has been 

 carefully investigated by Peters. From this work it becomes clear that 

 there is no advantage in protracted heating of the mixture ; in fact, certain 

 new errors are thus introduced which would quite counteract any advantage 

 that might be gained from such a procedure. As the chemical reaction is 

 actually probably quite complete only after long-continued heating,* accu- 

 rate termination of the process, characterized by constant values, can be 

 attained only by very definite and exact standardization of the procedure. 

 Thus, as Peters says: 



" It matters but little whether the amount of the reduction is always 99 per 

 cent or 97.5 per cent of x if only the conditions are so sharply defined that the 

 amount of reduction obtained at a selected point is characterized by great 

 constancy." 



This is a principle which is very generally overlooked in work with 

 Fehling's solution. In the method here used the tube is immersed in a 

 vigorously boiling water-bath and allowed to remain therein for 4f minutes 

 with the 40 c, c. tubes and 4 minutes with the 10 c. c. tubes. It was found 

 experimentally that at the end of this time practically all of the easily 

 oxidizable material in the sugar mixtures from the cactus had been ex- 

 hausted. Almost all sugar solutions obtained from plant material, such as 

 the ones under consideration, contain substances other than sugars which 



1 These tubes were made by Mr. Paul Anders, glass-blower of the department of 



chemistry of the University of Illinois, Urbana, Illinois. 

 3 NEF, J. U. Liebig's Ann. d. Chem., 357, 218, 1907. 



