Dimethylglyoxime, oxidation

The well-known reaction of Ni(II) with dimethylglyoxime (H Dm) in alkaline medium under the influence of such oxidants as persulphate and iodine is widely used for the photometric determination of nickel. The red product (RP) of this reaction is used for this purpose. However, the nature of this red compound has not been defined yet. Using of peroxyacids makes it possible to obtain additional data concerning the conditions and mechanism of generation of RP as well as to improve the metrological pai ameters of the method. 

The base was being prepared by distilling a mixture of hydroxylamine hydrochloride and sodium hydroxide in methanol under reduced pressure, and a violent explosion occurred towards the end of distillation [1], probably owing to an increase in pressure above 53 mbar. It explodes when heated under atmospheric pressure [2], Traces of hydroxylamine remaining after reaction with acetonitrile to form acetamide oxime caused an explosion during evaporation of solvent. Traces can be removed by treatment with diacetyl monoxime and ammoniacal nickel sulfate, forming nickel dimethylglyoxime [3], An account of an extremely violent explosion towards the end of vacuum distillation had been published previously [4], Anhydrous hydroxylamine is usually stored at 10°C to prevent internal oxidation-reduction reactions which occur at ambient temperature [5], See other REDOX REACTIONS... 

In another procedure [522] the sample of seawater (0.5-3 litres) is filtered through a membrane-filter (pore size 0.7 xm) which is then wet-ashed. The nickel is separated from the resulting solution by extraction as the dimethylglyoxime complex and is then determined by its catalysis of the reaction of Tiron and diphenylcarbazone with hydrogen peroxide, with spectrophotometric measurement at 413 nm. Cobalt is first separated as the 2-nitroso-1-naphthol complex, and is determined by its catalysis of the oxidation of alizarin by hydrogen peroxide at pH 12.4. Sensitivities are 0.8 xg/l (nickel) and 0.04 xg/l (cobalt). 

Alkaline solutions of nickel(Il) salts and dimethylglyoxime (H2DMG) can be readily oxidized to form strongly red coloured solutions3110 which have been shown to contain nickel(III) and/or nickel(IV) species.3025,3111-3113 Depending on the exact experimental conditions, nickel(IIl) or nickel(IV) salts have been obtained.3050 199,3112,3114-3118... 

Most popular schemes used to collect analytes are based on coordination reactions and electrostatic attraction. Common examples include the accumulation of nickel onto dimethylglyoxime-containing surfaces [39], the uptake and voltammetry of mercury on a diphenylcarbazide-carbon paste electrode [40], the use of surface-bound crown ethers for the collection and measurements of lead [41], or of trioctylphosphine oxide for the preconcentration of uranium [42], and the utility of polyelectrolyte-coated electrodes for the electrostatic collection of counterionic reactants [43,44], Bioaccumulation through binding to surface-bound microorganisms [45] or biocatalytic processes [46] can also offer the desired sensitivity and selectivity enhancements. 

Electrography — Electrography, introduced independently by A. Glazunov and H. Fritz, is an obsolete technique for the direct electrochemical analysis of solid materials. The principle is that a solid specimen is pressed on a paper which is soaked with an electrolyte solution. By anodic oxidation of the surface of the solid specimen the reaction products (e.g., nickel(II) ions) react with a reagent in the paper (e.g., dimethylglyoxime) to give colored reaction product (red in case of nickel(II) and dimethylglyoxime). This produces a print that clearly shows the distribution of the reactive element (nickel, in our example) on the surface of the specimen. 

Kinetic studies of the oxidation of nickel(II) dimethylglyoximate by periodate, catalyzed by iridium(IV), has been published recently.629 Additionally, the kinetics and mechanism of iridiu-m(III)- and ruthenium(III)-catalyzed oxidation of isopropyl alcohol by iodosoacetate have been elucidated.630... 

In the absence of a reducing agent, [Fe(dmg)2] (dmg = dimethylglyoximate) is readily oxidized to iron(ni) species. However, in the presence of good axial ligands, stable low-spin complexes result. For example, the sfructure of [Fe(dmg)2(imidazole)2] has two planar dmg hgands with the... 

The oxidation of iron(II) clathrochelates to iron(III) complexes is a quasi-reversible process with Eirz from 775 to 580 mV for clathrochelate dimethylglyoximates, from 850 to 570 mV for nioxime compounds, from 1 250 to 1 040 mV for glyoximates, and from 940 to 760 mV for a-benzyldioxime complexes. In the dioxime series Nx > Dm > Bd > Gm, the Eyz values becomes higher. 

Two consecutive reversible steps of oxidation for all studied manganese-capped tris-dimethylglyoximates and tris-nioximates with Ei/2 from 650 to 740 and 430 - 470 mV were attributed to the following electrochemical processes ... 

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