[Vol. 2 



792 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



colloidal iron (Iron dialysed, Merck) were pipetted in and the 

 tube well shaken, the iron then being flocked out with .25 gram 

 of Na 2 S04. Upon the addition of this latter the mixture was 

 again thoroughly shaken and the iron precipitate thrown 

 down by centrifuging, the resulting clear, supernatant liquid 

 then being decanted off through a small filter. This filtrate 

 was entirely free of proteins or other substances which, 

 through oxidation later, would lead to errors in the perman- 

 ganate values. Ten cc. of this filtrate were placed in a 

 50 cc. lipped centrifuge tube, and standard Fehling's solu- 

 tion added, the copper content of which was in excess of that 

 reducible by the sugar present. 1 The tube was then placed 

 in a boiling water bath for 10 minutes, at the end of which 

 time it was centrifuged at a moderate speed for 2 minutes, 

 the supernatant unreduced Fehling's carefully decanted off, 

 a like volume of distilled water added, and the cuprous oxide 

 again thrown down by a 2-minute centrifuging. All but 1 or 

 2 cc. of this wash water was carefully decanted off, and the 

 copper dissolved in the smallest amount necessary of a mix- 

 ture of equal parts of 10 per cent ammonium ferric sulphate 

 and 50 per cent sulphuric acid. It was found that if the cop- 

 per were stirred up with a glass rod just before dissolving, it 

 went into solution more readily. The dissolved copper was 

 titrated directly in the centrifuge tube against N/50 KMnO. 2 



By calculation it is found that 1 cc. of N/50 KMnC>4 is 

 equivalent to 1.27 milligrams of copper, and for the conver- 

 sion of this into glucose use was made of the table prepared 

 by Shaffer. 3 



As stated by Shaffer, care must be observed on the three 

 following points: (1) to eliminate all oxidizable substances 

 other than sugar, (2) to titrate the cuprous oxide immedi- 

 ately after dissolving, (3) to use poor conductors of heat as 

 containers of the centrifuge tubes in the water bath, else many 

 broken tubes will result. As employed here, circular wire 



1 In the determinations made, here this amount never exceeded 10 cc. 



2 It is necessary to titrate immediately after dissolving because of the danger 

 of oxidation of the cuprous oxide. If larger amounts of sugar are concerned, 

 N/10 KMnO 4 may he used. 



