DEPARTMENT OF BOTANICAL RESEARCH. 55 



lication No. 287 of the Institution, has continued to yield very satisfactory 

 results for the determination of reducing sugars. 



It now was necessary to determine quantitatively small amounts of glucose, 

 levulose, sucrose, and maltose. It soon became evident that any method 

 dependent upon the use of the polariscope was not sufficiently accurate. The 

 cause of this was that the various sugars are present in only very small amounts 

 and a very slight error in the reading of the angle of rotation resulted in serious 

 errors in the final results. This was further complicated by the facts that 

 most of the solutions used are more or less highly colored, increasing the 

 difficulty just mentioned; and that in leaf extracts there may be other op- 

 tically active substances, through slight errors caused by varying amounts 

 of inorganic salts. It was therefore imperative that some other method be 

 developed. 



Accordingly, Dr. F. A. Cajori of Stanford University spent three months 

 at the Laboratory and worked out the principle of a method which is applica- 

 ble to the present needs. This method is based upon the fact that aldoses, 

 glucose, mannose, galactose, and maltose are oxidized to the corresponding 

 monobasic acids, while levulose and sucrose are not affected. The use of this 

 principle, together with the copper method already referred to, answers the 

 requirements of determining the small amounts of the various sugars found 

 in leaves. The many conflicting reports regarding the accuracy of the iodine 

 method, it was found, were due to the fact that only under precisely controlled 

 conditions does the reaction proceed according to the formula 



RCHO+2I+H20 = RCHOOH+2HL 



Dr. Cajori has worked out the proper conditions for each of the sugars 

 encountered in the leaf. He has established the degree of alkalinity, the 

 length of time required for complete oxidation of each sugar, and the most 

 favorable temperature for the reaction. Since each sugar behaves differently, 

 it was necessary to work out conditions which would cover all cases without 

 allowing the reaction to go too far. The oxidation of the sugar is carried out 

 in alkaline solution with an excess of iodine at 25°. The mixture is then 

 neutralized with sulphuric acid and the excess iodine titrated back with 

 standard sodium thiosulphate. 



It was first established that levulose is not oxidized by iodine; this is the 

 case even when the solution contains 15 per cent of sodium carbonate and is 

 allowed to stand for 90 minutes at 25°. In oxidizing the aldoses it is desirable 

 to avoid the effect of the alkali on the sugar. Sodium carbonate has no 

 enolization action on the ordinary hexoses and it was found that the concen- 

 tration of this salt can vary within wide limits without changing the rate of 

 oxidation. In the final method oxidations are carried out in a solution of 

 0.8 to 1.5 per cent sodium carbonate. 



It was impossible to get the oxidation of glucose to proceed in neutral 

 solution even in the presence of iron salts. Glucose is oxidized by iodine in 

 the presence of disodium hydrogen phosphate, but the rate of oxidation is much 

 slower than with sodium carbonate. There is no indication of any specific 

 action of the phosphate and the rate of oxidation seems to be a matter of the 

 hydroxyl-ion concentration. 



