A spectrophotometric study on equilibria of p-(4-hydroxy-1-naphthylazo)nitrobenzene with proton and of p-(2,4-dihydroxyphenylazo)nitrobenzene with proton and copper(II) in aqueous solution

Abstract

The protonation constants of p-(2,4-dihydroxyphenylazo)nitrobenzene (‘Magneson I’) and p-(4-hydroxy-1-naphthylazo)nitrobenzene (‘Magneson II’) have been determined by spectrophotometric methods at 25 °C and I= 1.0 mol dm^–3(KCl). The equilibria between Cu^II and p-(2,4-dihydroxyphenylazo)nitrobenzene have also been investigated in acidic medium under the same conditions. The mixed cumulative stability constants of the protonated species have been refined with two different computer programs: a modified version of the program SQUAD and the program SPEFO (an application of LETAGROP to spectrophotometric data). The spectrophotometric characteristics and equilibrium constants for the various species are reported. The least-squares values of the protonated species and the formation constants of the complexes are log β₀₁₁= 11.816(4), log β₀₂₁= 21.590(17), log β₁₁₁= 23.327(33), and log β₁₀₂= 35.706(64) for Magneson I and log β₀₁₁= 12.474(37) for Magneson II. The experimental spectra are in good agreement with the calculated ones and the constants are discussed in connection with those of similar compounds reported in the literature.