Abstract
4-(2-Thiazolylazo)-2-methylresorcinol (TAR-2 Me), 4-(2-thiazolylazo)-5-methylresorcinol(TAO) and 4-(2-thiazolylazo)-resorcinol(TAR-6Et) were synthesized. The acid dissociation constants of these reagents were determined spectrophotometrically in 20%(v/v) dioxane at μ=0.1 and at 25°C: the values of pkNH were 0.59, 0.93 and 0.73, pkoH(p) 6.54, 6.12 and 6.13 and pkoH(o) 11.8, 12.8 and 11.1 for TAR-2Me, TAO and TAR-6Et, respectively. TAR-2Me and TAR-6Et react with a number of metal ions to form the red or purple complexes as TAR does, while TAO does appreciably with a few metals. The formation constants of copper chelates in 20%(v/v) dioxane at μ=0.1 and at 25°C were log KcuA=16.0, 15.8, 15.8, and log KHCuHA=4.25, 4.75, 3.72 for TAR-2Me, TAO and TAR-6Et, respectively. TAO forms less stable metal chelates, because the methyl group atο-position of azo group in TAO may hinder the rotation of resorcinol on the formation of metal chelates.
TAR-2Me and TAR-6Et are much similar to TAR rather than TAC[2-(2-thiazolylazo)-4-methylphenol] in the behaviors of chelate formation, and very good metal indicator for Cu-EDTA titration. TAO can be used for colorimetry of copper, or mercury without serious interference with other metal ions.
