Cathodic stripping voltammetric determination at a hanging mercury drop electrode of the environmental heavy metal precipitant trimercapto-s-triazine (TMT).
Trimercapto-s-triazine (TMT) is available commercially for precipitating heavy metals in effluents prior to discharge and for recovering silver and copper. The TMT content of an effluent for discharge is normally monitored down to about 2 ppm by means of its UV absorption at 285 nm. Indirect cathodic-stripping voltammetric methods of determining TMT at sub-ppb levels in standard solutions are reported here. These methods might prove suitable for the determination of TMT in effluent at levels lower than is currently possible. TMT can be accumulated and determined indirectly at pH 9.0 as its mercury salt down to sub-ppb levels. Accumulation is made at 0 V and the mercury TMT reduction peak is at -0.47 V. Alternatively, by adding nickel(II), TMT can be determined optimally at pH 7.8, using the catalytic nickel peak at -0.73 V and accumulating between -0.10 and -0.60 V: at this pH the HgTMT peak at -0.47 V is small. At slightly higher pH (pH 8.6) the nickel TMT complex can be accumulated directly at -0.40 V, but at this pH, however, a slightly increased sensitivity can be achieved by accumulating TMT as its mercury salt, at -0.1 V in the presence of nickel(II), the nickel TMT complex being formed during the potential sweep on the release of the TMT when the mercury salt is reduced. Unlike many other thiols TMT is not accumulated as its copper(I) salt on addition of copper(II) to the solution.
Electrochemical reduction of oxygen and hydrogen peroxide catalyzed by a surface copper(II)–2,4,6-tris(2-piridil)-1,3,5-triazine complex adsorbed on a graphite electrode
Abstract A graphite electrode irreversibly adsorbed by 2,4,6-tris(2-piridil)-1,3,5-triazine (abbreviated as TPT) was examined by cyclic voltammetry. The adsorbed TPT exhibited two irreversible reduction waves in the potential range of −0.7 and −1.0 V (versus SCE). Upon strong adsorption, TPT can serve as a coordination ligand for copper ions to form a surface complex. Its three adjacent nitrogen positions provide strong affinity to the metal ions and bond copper(II) to an electrode surface. A 1:1 coordination between Cu(II) or Cu(I) and the TPT ligand to form [Cu(II)(TPT)]2+ or [Cu(I)(TPT)]+ is the predominant process, evidenced by spectrophotometry, surface cyclic voltammetry, and coordinated structural feasibility of Cu(II)/Cu(I)–TPT complexes. The predominant copper(II)–TPT surface complex shows a reversible redox wave, which is identified as one-electron process of [Cu(II)(TPT)]2+ ↔ [Cu(I)(TPT)]+. The electrode adsorbed by [Cu(II)(TPT)]2+ complex showed electrocatalytic activity towards oxygen and/or hydrogen peroxide reductions. The catalyzed reduction of oxygen and hydrogen peroxide were identified as four-electron and two-electron process to form water. It is suggested that the possible electrocatalytic reductions were due to an inner-sphere mechanism, which involved a coordination between substrate (O2 or H2O2) and [Cu(I)(TPT)]+. The reduction kinetics were also investigated by a rotating disk electrode method.
Ligand‐catalyzed metal ion reduction: Kinetics and analytical applications of nickel reduction catalyzed by nicotinamide
The catalytic reduction of Ni (II) in the presence of nicotihamide (NA) at pH < 7 was studied by direct-current and differential-pulse polarography (DPP). The prewave (with E1/2 around –0.78 V (versus saturated calomel electrode (SCE)) shows the typical behavior of an electrode process with regeneration of the reactant by a second-order, rate-determining chemical reaction. This reaction consists in the formation of the reducible nickel complex. The kinetics of the electrode process was studied by means of an adapted form of the equation derived by Koutecký for electrochemical reactions with a parallel chemical step. The rate constant for the formation of nickel-nicotinamide complex is slightly dependent en pH and buffer composition and ranges between 4.5 and 6.7 × 103 mole−1 L s−1. This electrode process enables the selective determination of Ni (II) in presence of a large excess of cobalt. For example, nickel impurity in cobalt salts can be determined by DPP, in acetate buffer (pH 4.6) containing 0.01 M nicotinamide and about 0.1 M cobalt salt sample.