Note I — Chapter XV · OXIDATION AND REDUCTION. II. OXIDATION BY OXYGEN, PERMANGANATES, ETC.; OXIDATION OF ORGANIC COMPOUNDS
Reading edition
The potential of oxygen at 18°, 760 mm., against an alkaline solution in which [HO−] = 1. KIon refers to the concentration of HO−, with which oxygen under atmospheric pressure would be directly in equilibrium, at 18°.