Polarographic reduction Subject

Two polarographic methods have been developed for the determination of cohalt(II) at concentrations ranging from approximately 1 to 80 mM in an aqueous sample. For the first method [15], which is suitable for samples containing large amounts of nickel]11), the cobalt(II) is oxidized to Co(NH3)6 in an ammoniacal medium with the aid of sodium perborate, after which the cobalt(III) species is determined. A second procedure [16] entails the use of lead dioxide in an acetic acid-acetate buffer containing oxalate to convert cobalt(II) to the 0(0204)3 ion, which can be subjected to polarographic reduction. This latter approach is well suited to the determination of cobalt in the presence of copper(II), iron(III), nickel(II), tin(IV), and zinc(II), whereas the chief interferences are cerium, chromium, manganese, and vanadium. 

The hetero atom parameters have been obtained in various ways, for example by striving for a best correlation of HOMO values with ionization energies, or of polarographic reduction potentials with LUMO values. The whole subject of SHM parameters and best heteroatom parameters is now of little practical importance, since much better quantitative molecular orbital methods are now readily available. 

Polarographic reduction of nitro compounds is a subject matter of a recent monograph by Shading [87],... 

Cohen subjected triamcinolone acetonide to polarographic reduction in dimethylformamide and found two reducing waves ... 

Polarographic reduction of the parent gave a value for the half-wave potential (E1/2 = -0.85 v) which is intermediate between the values for quinoxaline ( 1/2 =-1.09 v) and pteridine ( 1/2 =-0.52 v). The electrochemical reduction of substituted pyrido[2,3-fc]pyrazines has been the subject of a recent study. Along with other fused pyrazine systems, the... 

The syntheses of other triptycene derivatives have been carried out similarly (308-314). These compounds have been characterized, in particular, by their IR spectra. For many of them the kinetics of hydrogenation on nickel and palladium have been measured simultaneously with the measurement of the electric potential of the catalyst, and for the quinones, in addition, polarographic reduction on a mercury electrode was investigated (315-317). The compounds which were synthesized by us (many of them for the first time) are listed in Table V. The substances which were subjected to catalytic hydrogenation are marked with an asterisk. 

The ruthenium compound also has been prepared and displays similar properties, although it has not yet been the subject of an x-ray study. Bis-indenylcobalt, the first member of the series to be prepared, has a structure similar to the iron analogue on the basis of a comparison of x-ray powder photographs. This latter compound can be readily oxidized to give the quite stable Co(III) compound which, like the cobalticinium ion, may be precipitated as the triiodide, tribromide picrate, or perchlorate. The perchlorate exhibits a well-defined polarographic reduction wave at —0.6 V vs SCE. The corresponding half-wave potential for cobalticinium perchlorate is — 1.16 V. Thus the annulation of a benzene ring produces a very marked decrease in the reduction potential and, conversely, the oxidation of the new compound to the ion should become more difficult. 

The reduction of quaternary derivatives of nicotinic amide as model compounds for the phosphopyridine nucleotides has been subject to many polarographic [235, 247-255] and electrolytic investigations. At pH 4—7 a one-electron wave is found, whereas in alkaline solution two, one-electron waves are seen. Controlled potential reduction in a phosphate buffer at the potential (—1.2 V) of the first wave of l-methyl-3-carbamoyl or 1-benzyl-3-carbamoylpyridinium ions produces a mixture of dimers, including diastereomers of 6,6 - [248] and 4,4 - [253], in addition to unsymmetrical dimers [254]. 

For instance, benzene is one of the most hard-to-reduce compounds. It is not active polarographically and is not subjected to direct electrode reduction in liquid ammonia . Benzene does not react with solvated electrons obtained by dissolving alkali metals in liquid ammonia in the absence of proton donors However, in the presence of proton donors benzene is hydrogenated by solvated electrons (Table 10). 

The electrochemistry of [VCl2Cp2], like that of [TiCl2Cp2] (Section VII,A), is the subject of some controversy. In 1,2-dimethoxyethane (86) or acetone (298), two polarographic one-electron reduction waves are observed and a mechanism similar to that in Scheme 19 was proposed. However, more recent studies in thf suggest the first process ( ° = —0.29 V) to be reversible with chloride loss only after formation of [VCl2Cp2]2- ( 1/2 = —1.54 V). A third one-electron wave corresponds to the reduction of vanadocene (Section X,B) (299). 

For this example a modification of the existing analytical method which employed titrimetry, was used. After the conversion, which is carried out essentially in the same way as in the titri-metric method, the solution (instead of extraction, oxidation-reduction treatment and final titration) is directly subjected to polarographic analysis in a pH 8-8 ammonia and ammonium chloride buffer. 

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