Spectrophotometric determination of ruthenium with 3,4-diaminobenzoic acid

Ruthenium(III) chloride and 3,4-diaminobenzoic acid, in aqueous solution at pH 4.0-4.5, react slowly at room temperature, but more rapidly when the mixture is heated, giving solutions that go through colour transitions from amber to purple-red; maximum absorbance of fully developed solutions occurs at 550 nm. The effects of heating temperature and time, pH, reagent concentration, and other variables have been studied. The system conforms to Beer's law; optimum concentration range, for measurement in 10-mm cells, is about 0.5-2 ppm of ruthenium. Interference from foreign ions, especially other platinum elements, is avoided by a distillation separation of ruthenium. The spectrophotometric mole-ratio and continuous-variation methods indicated the presence of complexes of 1:2 and 1:3 ruthenium-to-reagent stoichiometry. Elemental analysis of solid products isolated from solution confirmed the 1:2 reaction ratio. Several other o-diamines gave similar coloured solutions and reaction stoichiometry.