34 THE CARBOHYDRATE ECONOMY OF CACTI. 



on prolonged heating may thus introduce a not inconsiderable error. By 

 aid of the curves of the rate of reduction of copper solutions elaborated by 

 Peters, it can be established that the time decided upon for these determi- 

 nations makes for greatest accuracy. As there is always an excess of copper 

 in the solution, there is danger of effects inherent to the use of too dilute 

 solutions ; this can f uthermore be easily regulated by properly preparing the 

 sugar solution. 



At the expiration of the requisite time the tube is at once removed from 

 the boiling water-bath and rapidly cooled in cold water. In order to avoid 

 cracking of the tubes it was found advisable, first, to wet the hot tube with a 

 little cold tap-water with the hand for a moment, rather than plunge the 

 tube into a bath of cold water. Thereafter the tube can be immersed safely. 

 It is desirable that the cooling be done as rapidly as possible in order to 

 avoid any resolution of cuprous oxide. The tube (the contents of which have 

 been cooled to 15) is then filled up to the graduation mark with distilled 

 water which has been recently boiled in order to expel dissolved air, and 

 the solution is thoroughly mixed, the tube being closed with a ground-glass 

 stopper. The tubes are then placed in a centrifugal machine and revolved 

 for 5 minutes. In the arrangement used the centrifugal head was fastened 

 to the chuck of an electric motor of 1,700 r. p. m. After centrifuging for 

 5 minutes, the cuprous oxide, together with any other solid or suspended 

 matter that may have been in the sugar solution, is thrown down in a com- 

 pact mass in the bottom of the tube. Occasionally when a sugar solution was 

 used containing considerable foreign matter, a small amount of cuprous 

 oxide would settle on the sides of the tube. By shaking a little and centri- 

 fuging again this was readily clarified. 



The obvious advantage of this procedure is that the cuprous oxide is in a 

 small compact mass removed from access to the air, and the danger of 

 resolution is thus avoided. A perfectly clear solution of the residual copper 

 solution is obtained without the employment of any filter or transference to 

 another vessel. The entire operation can be carried out in a very short time, 

 and it remains then simply to determine the amount of unchanged copper 

 left in solution. Sugar values derived from these data are based upon a 

 standardization of the copper solutions by means of pure d-glucose, accord- 

 ing to the data of Peters. 



THE DETERMINATION OF COPPER. 



Peters 1 has worked out the conditions essential for obtaining accurate 

 results of copper determination by means of the iodide method. The essen- 

 tial features of these findings have been applied to the present circum- 

 stances. Aliquot portions of the clear supernatant liquid in the tubes after 

 centrifuging are removed, by means of a pipette, into 150 c, c. Erlenmeyer 

 flasks. These are acidified with concentrated sulphuric acid. It is necessary 



1 PETERS, A. W. The sources of error and the electrolytic standardization of the con- 

 ditions of the iodide method of copper analyses. Jour. Amer. Chem. Soc., 

 34, 422-454, 1912. 



