1916] REED—OXIDASE ACTION 63 
placed in 100 cc. of a o.2M solution of potassium iodide, which 
was maintained at a constant temperature of 20°C. Portions of 
5 cc. were removed at frequent intervals and titrated with o.orM 
sodium thiosulfate (with starch indicator) for the amount of iodine 
_ liberated in the oxidation. The curves OA, AB, BC, fig. 3, 
represent the velocity of oxidation of potassium iodide after succes- 
sive chargings of the platinum with oxygen. 
VOLTS 
S 
a5; ' 
Lae os i ee oe 
T 
5 10 4 MINUTES 
Fic. 4.—Curves showing fall of oxidation potential of colloidal platinum in 
3 Successive oxidations of a potassium iodide solution after the platinum had-been 
charged with oxygen at an anode at the beginning of each oxidation; ordinates repre- 
sent volts; abscissae represent time in minutes. 
By connecting the platinum, as a cathode, to an apparatus for 
the measurement of the oxidation potential it was found that the 
tate at which the oxygen potential dropped, that is, the rate at 
which oxygen was given up by the platinum electrode in the potas- 
sium iodide solution, followed very nearly the velocity of the oxi- 
dation reaction. This may be seen by comparing the curves in 
fig. 4, showing the changes in the potential of the platinum after 
three successive chargings with oxygen, with the rates of oxidation 
produced by the platinum as plotted in fig. 3. It is evident that 
the drop in potential is somewhat more rapid than the oxidation as 
