Protolytic reactions of 3,3′-dimethylnaphthidine and 3,3′-dimethoxybenzidine

Abstract

The dissociation constants of diprotonated 3,3′-dimethylnaphthidine (DMN) and 3,3′-dimethoxybenzidine (DMB) have been determined spectrophotometrically. They are: p K a1 = 2·62 ± 0·03, p K a2 = 3·33 ± 0·09 for DMN: p K a1 = 2·83 ± 0·07, p K a2 = 4·05 ± 0·12 for DMB. The molar absorptivities (l.mole −1.cm −1) of all forms of the indicators have been also determined: ε B = 1·68 × 10 4, ε BH + = 9·34 × 10 3, ε BH 2+ 2 = 1·80 × 10 3 at 300 nm for DMB; ε B = 7·33 × 10 3, ε BH + = 3·73 × 10 3, ε BH 2+ 2 = 0 at 330 nm for DMN.