230 A. I. RINGER AND EMIL J. BAUMANN 



/OH CuOH Cu\ 



2 Cu minus minus O 



\OH - > CuOH -* Cu/ 



water and oxygen water 



blue yellow red 



It should be noted that in Fehling's solution both cupric hydroxid 

 and cupric tartrate exist in equilibrium. As reduction occurs, more cupric 

 hydroxid is formed from the tartrate. 



This reaction is not completed in a definite time, since many of the 

 degradation products, as giuconic acid, are slowly oxidized. So that when 

 quantitative estimations are made, very definite conditions of concentration 

 and time of heating must be observed. The cuprous oxid formed may be 

 weighed directly or oxidized to cupric oxid and this weighed. Or it may 

 be dissolved in acid and estimated electrolytically or by a number of 

 volumetric methods. 



To avoid the inconvenience of keeping two solutions, Benedict has 

 substituted sodium citrate for Rochelle salts in Fehling's solution and 

 sodium carbonate for sodium hydroxid. This solution keeps indefinitely 

 and serves very well for the qualitative detection of reducing substances. 



Reduction of Carbohydrates. While most of the reactions which carbo- 

 hydrates undergo in living matter are oxidation reactions, not an incon- 

 siderable number are reductions, such as the processes whereby micro- 

 organisms, of the group known as anaerobes, metabolize sugars and give 

 off carbon dioxid in the absence of air. 



Sugars are reduced by sodium amalgam, forming, in the case of hexoses, 

 hexahydric alcohols. 



/O 

 C CH 2 OH 



HCOH HCOH 



I I 



HOCH HOCH 



I +2 H | 

 HCOH HCOH 



I I 



HCOH HCOH 



CH 2 OH CH 2 OH 



Glucose Sorbitol 



A number of these alcohols are found in plants, such as sorbitol, which 

 is derived from glucose ; mannitol from mannose ; dulcitol from galactose. 

 Mannitol is especially widely distributed. In some fungi there is more 

 mannitol present than glucose. Like the sugars, they are sweet, 



