Effects of auxiliary complex-forming agents on the rate of metallochromic indicator colour change-IV Mechanism of the colour change of xylenol orange in copper(II)-edta titrations

The rate of ligand substitution of copper(II)-Xylenol Orange (XO) with EDTA (Y) has been determined spectrophotometrically over the pH range 4.8-6.0 at mu = 0.1 (KNO(3)) and at 25 degrees . In 2-(N-morpholino)ethane sulphonic acid buffer, copper forms a 2:1 chelate (Cu(2)XO(2-)) with XO, and the rate-law is expressed as -dCu(2)XO(2-)]/dt = 10(2.89)Cu(2)XO(2)-]Y']- The release of the first copper ion from Cu(2)XO(2-) is the rate-determining step. The resulting CuHXO(3-) or CuH(2)XO(2-) may undergo fast substitution with EDTA. In the presence of hexamine, the copper(II)-XO chelate forms a mixed-ligand complex with hexamine (L). The formation constant K(Cu(2)XOL)(L) = Cu(2)XOL(2-)]/Cu(2)XO(2-)] L] = 10(2.14) (mu = 0.1, 25 degrees ). At 3 x 10(-2)-2 x 10(-1)M hexamine the rate-law is expressed as -dCu(2)XOL(2-)]/dt = 5.39Cu(2)XOL(2-)]Y']/L']. The dissociation of hexamine from Cu(23)XOL(2-) has to precede the substitution reaction of Cu(2)XO(2-) with EDTA. Hence, hexamine at higher concentrations than 10(-3)M slows down the rate of colour change of XO in the copper-EDTA titration.