[Vol. 10 



320 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



subsequent series simply to ascertain the absence of the reducing 

 sugar. Applied to the determination of dextrose in such media 

 the " micro" method was found to give such variable results at 

 different dilutions that it was entirely unsatisfactory for general 

 use. Many of the following, very helpful suggestions were ob- 

 tained from members of the staff at the Washington University 

 Biochemical Department and are here given because they are not 

 included in the paper. The " iodate-iodide " solution should be 

 made slightly alkaline by the addition per liter of 0.4 ml. saturated 

 NaOH solution; this prevents the formation of hydriodic acid. 

 The standard thiosulphate solution is made permanent in the 

 same way. 



The thiosulphate solution is readily standardized by titration 

 against a 0.1 N potassium biniodate solution (93.24958 gm. 

 KH(I0 3 ) 7 per liter make 0.1 normality). To 50 ml. of the binio- 

 date are added 3 gm. KI (dissolved in 25 ml. H 2 0), and 10 ml. 

 of an approximately 5 N H2SO4 or HC1. The excess acid and 

 iodide with the biniodate liberate iodine equivalent to exactly 

 50 ml. of a 0.1 N solution, according to the following equation. 



KH(IO,), + 10KI + 11HC1 — 61, + 11KC1 + 6H,0 

 Titrate against the thiosulphate, using 2-3 ml. of starch solution 

 as an indicator when the iodine color becomes faint. 



Starch indicator solution made from arrowroot starch is pre- 

 ferable to a solution made from soluble starch because the latter 

 deteriorates more rapidly under septic conditions and then fails 

 to give the starch-iodine blue. Arrowroot starch solution kept 

 for several weeks still retained its usefulness as an indicator. Of 

 course any starch may be used, but the arrowroot is preferable 

 because it is a standard, easily obtainable product. Two grams 

 of the starch were shaken in 100 ml. cold water and poured into 

 100 ml. of boiling water; one ml. of 5 N H a S0 4 was added and 

 boiled about a minute. 



The reduction is most uniformly effected by heating over a 

 direct flame; an asbestos mat with a 2-inch circular hole is ser- 

 viceable. A carrier, very handy for removing the hot flasks, is 

 easily made by cutting away the side of a small, cylindrical wire 

 test-tube basket, making an opening large enough to admit a 

 300-ml. Erlenmeyer. To cool, flasks are set in a shallow dish, as 

 an evaporation dish, under running water. 



