Spectrophotometric studies on the composition, stability and analytical applications of lanthanum(III). 1,2,5,8 Tetra Hydroxy Anthraquinone (Quinalizarin) chelate in solution

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Abstract
The formation of a violet colour produced by the interaction of 1, 2, 5, 8 Tetrahydroxy anthraquinone (quinalizarin) and Lanthanum has been studied to determine the composition, stability and other characteristics of the chelate formed. The absorbance curves of the complex show maxima at 530 mμ. The composition of the chelate has been determined by three methods. The molar ratio of the metal to chelating agent is 1:2. The chelate is soluble in aqueous ethanol and is stable between pH 6.5–7.2. The maximum colour formation is attained at pH 6.8, when the mixture contains greater than five fold excess of the reagent with respect to the metal solution and the colour intensity remains unaltered even after keeping the mixture for three hours at room temperature. The stability constant of the chelate has been calculated by three different methods (a – c). The values of log K in 50% ethanolic medium at 30° are respectively a) 10.1 ± 0.1, b) 10.2 ± 0.1, c) 10.1 ± 0.1 and the free energies of formation at the same temperature are a) −14.0 ± 0.1, b) −14.1 ± 0.1, c) −14.0 ± 0.1 K.cal. The range of concentration for adherence to BEER's Law and value of optimum concentration range are 1.26 to 6.32 ppm and 1.4 to 3.5 ppm. Studies have also been made to establish optimum conditions for the spectrophotometric determination of Lanthanum.