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The feature of oxidizing method on carbon black surface was examined mainly in view of the change of the surface polarity which was concerned with a wettability.The black was treated with oxidizing agents such as nitric acid and hydrogen peroxide aqueous solutions in thecase of liquid phase oxidation, while in the case of gas phase oxidation, with oxidizing agents such as dried air and ozone, under various conditions. The density of the surface active hydrogen, which was assigned to hydroxyls, carboxyls, lactols and so on, more or less increased through the different oxidizing methods, and the average electrostatic field strength also became larger in accordance with an increase of the density.Those increase ratio, however, was the most in the oxidation with ozone and the least with air. It was found that the functional groups introduced on the black surface by oxidation with nitric acid, hydrogen peroxide and ozone were mainly oxides containing an active hydrogen such as hydroxyls, carboxyls, lactols and so on. On the other hand, ones by air-oxidation were certain anhydrous oxides in addition tosuch oxides. These anhydrous oxides, however, seem to have a chemical structure such as carboxylic anhydride because they contribute to the pH value of water-slurry by hydration with water molecule. In the case of liquid phase, the surface oxidation by using hydrogen peroxide is more advantageous than that by nitric acid with due regard to the effects of oxidation on the surface properties and the relative difficulty of the oxidizing procedure. In the case of gas phase, air-oxidation is advantageous in regard to the enhancement of the surface roughness, while ozone-oxidation is advantageous in regard to the increase of the surface polarity.

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