The factors governing the electrochemical behavior of manganese dioxide in dry cells have never been adequately correlated with the usual physical and chemical properties of this material. Two tests are described, (a) determination of the pH-potential relation in zinc and ammonium chloride electrolyte and (b) determination of the "utilization factor," i.e.(the extent of cathodic reduction in a continuous stream of fresh electrolyte. In combination, these give valuable information regarding the general suitability of a sample for dry cell use and afford a partial measure of intrinsic depolarizer capacity, heretofore not obtainable by other th~n actual battery tests.

A study of the mechanism of the reduction of manganese dioxide in strongly alkaline electrolytes has shown that the rather complex process may be considered as occurring in three steps. The first is the simultaneous reduction to form a divalent manganese compound, presumably, normalMnfalse(OH)2 and an intermediate oxide tentatively identified as Mn4O7 ; the second step is the electrochemical reduction of Mn4O7 to form both Mn3O4 and normalMnfalse(OH)2 ; the final step is the electrochemical reduction of Mn3O4 to Mn3O4 and normalMnfalse(OH)2 . Both chemical and x‐ray diffraction analyses of cathodes at various stages of reduction are presented.

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The reactions at the cathode of the Leclanch~-type dry cell are considered in terms of three heterogeneous chemical reactions for which there is advanced a plausible mechanism, leading to a unified theory of the cathode process. When reviewed in terms of this theory, the numerous observations of others that formerly have appeared to be incongruous are correlated and rationalized in terms of the over-all cathode reaction.