Effects of auxiliary complex-forming agents on the rate of metallochromic indicator colour-change-III: mechanism of the colour change of tac in nickel-edta titrations.
The rate of the ligand-substitution reaction of nickel(II)-TAC chelate (NiR(2)) with EDTA (Y) and 1,10-phenanthroline (X) has been determined spectrophotometrically in 20% v v dioxan over the pH range 5.7-6.3 at mu = 0.1 (KNO(3)) and 25 +/- 1 degrees . The substitution reaction with EDTA proceeds through the following two pathways: NiR(2) + H(+) right harpoon over left harpoon NiR(+) + HR, and NiR(2) + H(2)O right harpoon over left harpoon NiR(OH) + HR, The reaction of NiR(+) or NiR(OH) with EDTA is the rate-determining step, and k(1) = 2.1 x 10(3) l .mole(-1) .sec(-1) and k(2) = 7.9 x 10(6) l .mole(-1) .sec(-1).The substitution reaction with 1,10-phenanthroline proceeds as follows: NiR(+) + X right harpoon over left harpoon NiRX(+) At higher concentrations of 1,10-phenanthroline the release of TAC from NiR(2) by hydrogen ion is the rate-determining step, and k(3) = 2.4 x 10(5) l .mole(-1). sec(-1). At lower concentrations of 1,10-phenanthroline -d[NiR(2)]/dt is proportional both to [H(+)] and [X]. The value k(4) = 5.1 x 10(4) l. mole(-1). sec(-1) was calculated by the use of the steady-state approximation for [NiRX(+)]. The substitution with 1,10-phenanthroline proceeds much faster than that with EDTA. By the addition of a small amount of 1,10-phenanthroline, Ni can be titrated with EDTA at 50 degrees, with TAC as an indicator.
Nakagawa
G
G
Laboratory of Analytical Chemistry, Nagoya Institute of Technology, Showa-ku, Nagoya 466, Japan.
Wada
H
H
Nakazawa
O
O