title: Spectrophotometric determination of the protolytic dissociation constants of the new chromogenic reagent "PALLADIAZO"-II Study of the protonation processequals, single dot aboves undergone by the azo-groups of the reagent in sulphuric acid media.
abstract: The protonation of the reagent 1,8-dihydroxynaphthalene-3,6-disulphonic-2,7-bis(azophenyl-p-arsonic) acid ("palladiazo") has been investigated by a spectrophotometric method in 0.25-18M sulphuric acid media. Graphical assessment of the experimental results points to the protonation of only one azo-group, although a two-stage protonation process cannot be conclusively ruled out at very high acidity. The first protonation instability constant is pK(9) = -(2.4 +/- 0.1) and the second is tentatively estimated as -7.4. Most of the current views on the complexation and protonation reactions of bis(azophenyl)chromotropic acid derivatives.with metal cations and protons are reviewed and critically discussed in some detail in order to interpret the experimental findings. It is concluded that the fully protonated palladiazo molecule exists in very concentrated acid media predominantly in the form of a symmetrical positively charged tautomeric quinonehydrazone proton complex species which is responsible for the appearance of a very strong single absorption band with a maximum at 665 run which gives the protonated reagent solutions a characteristic deep-emerald-green colour.
authors.affiliation: Consejo Superior de Investigaciones Cientificas, Departamento de Quimica Analítica, Universidad de Madrid, Facultad de Ciencias, Ciudad Universitaria, Madrid-3, Spain.
authors.forename: J A
authors.initials: JA
authors.lastname: Pérez-Bustamante
authors.forename: F
authors.initials: F
authors.lastname: Burriel-Martí