The superadditivity of MQ developer of higher pH (pH 12) at which both Metol and hydroquinone are active has been examined in the simplified physical development system.
As a vacuum deposited gold layer was used as development speck, a linear relationship between the development time and the mass of developed silver was held in all developers. The superadditivity of MQ combination was observed to occur at pH 12.
It was found that the active species of hydroquinone is the divalent ion, but there are a considerable amount of inactive monovalent ion in the developer of pH 12. The superadditive development rate was well fitted to the equation of catalytic reaction in electrochemistry.
Thus, the superadditivity of MQ combination at higher pH can be accounted for by the same regeneration mechanism as at lower pH: Metol is regarded as a redox catalyst which promotes the development reaction of hydroquinone.
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