Reduction at the dropping-mercury electrode

Like benzenoid hydrocarbons, pyridine-like heterocycles give well-developed two-electron waves on reduction at the dropping mercury electrode. The latter are polarographically much more reducible than the former. This can be explained easily in terms of the HMO theory It is assumed (cf. ref. 3) that the value of the half-wave potential is determined essentially by the energy of the lowest free 7r-molecular orbital (LFMO) of the compound to be reduced, and for models of hetero analogues this quantity is always lower than that for the parent hydrocarbons. Introduction of an additional heteroatom into the molecule leads to a further enhancement of the ease of polarographic reducibility.95 On the other hand, anodic oxidation of the heterocyclic compounds is so much more difficult in comparison with benzenoid hydrocarbons that they are not oxidizable under the usual polarographic conditions. An explanation in terms of the HMO theory is obvious. 

Solutions of certain anions such as bromate and iodate or their corresponding acids undergo reduction at the dropping mercury electrode and can be analyzed easily. Iodine and bromine can be determined after oxidation to iodate or bromate respectively. 

Somewhat of a problem arises in the removal of the excess of the second coupling component, which is polarographically active, if such excess is required. Metamphetamine can be polarographically activated[5] by treatment with picrylfluoride. Twelve electrons are consumed during reduction at the dropping mercury electrode (see Figure 7). 

The determination of acetone is made possible by measuring the decrease in the wave of sulphurous acid.( > Acetone forms in an acidic media with sulphurous acid an addition compound which does not undergo reduction at the dropping mercury electrode. The curve is recorded in acid solutions in the presence of sodium sulphite before and after additions of acetone. The decrease in wave-height after addition of acetone is proportional to the concentration of acetone in the sample. Calibration is carried out by recording a curve from a solution containing a known amount of acetone. The decrease, due to the sample, should be smaller than 50 per cent of the decrease resulting from the addition of the standard solution. The addition of the standard solution should decrease the wave of sulphurous acid by about 80 per cent. 

Copper(II) ions in the presence of chloride ions are reduced at the dropping mercury electrode (dme) in two steps, Cu(II) -> Cu(I) and Cu(I) -> Cu(0) producing a double wave at -1-0.04 and 0.22 V versus sce half-wave potentials. In the presence of peroxydisulphate , when the chloride concentration is large enough, two waves are also observed the first limiting current corresponds to the reduction of the Cu(II) to Cu(I) plus reduction of a fraction of peroxydisulphate and the total diffusion current at a more negative potential is equal to the sum of the diffusion currents of reduction of Cu(II) to Cu(0) and of the peroxydisulphate. There is evidence that peroxydisulphate is not reduced at the potential of the first wave because of the adsorption of the copper(I) chloride complex at... 

Mishra and Gode developed a direct current polarographic method for the quantitative determination of niclosamide in tablets using individually three different buffer systems, namely Mcllraine s buffers (pH 2.20 8.00), borate buffers (pH 7.80—10.00), and Britton Robinson s buffers (pH 2.00—12.00) as well as 0.2 M sodium hydroxide. The drug was extracted from the sample with methanol, appropriate buffer was added to an aliquot and the solution then polarographed at the dropping-mercury electrode versus saturated calomel electrode at 25°C [36], The resultant two-step reduction waves observed were irreversible and diffusion-controlled. For the quantitative determination, the method of standard addition was used. Niclosamide can be determined up to a level of 5—10 pg/mL. 

In a classic study, Hume and KolthofF[13] obtained polarographic evidence that, in a 1 M aqueous solution of potassium cyanide, Co(H20)(CN)s is irreversibly reduced at a dropping mercury electrode to a cobalt(I) species, the composition of which was not elucidated. furthermore, the cobalt(I) complex was reported to undergo neither oxidation nor reduction. In addition, the cobalt(III) complex, Co(H20)(CN)5 , was seen to be reducible at the dropping mercury electrode, whereas Co(CN)6 is not electroactive. In earlier work [14], cobalt(II) cyanide complexes were reduced electrolytically to cobalt(I) cyanide species. 

Interestingly, the reduction mechanism of 2-benzopyrylium salts 10 at the dropping mercury electrode involves the disproportionation of a radical intermediate 304 in acid to 10 and 107 (72MI3). 

The polarographic behavior of aromatic carbonyl compounds at the dropping mercury electrode in aqueous SSE s is rather complex 130 In acid medium two one-electron waves are observed, corresponding to reduction of the protonated ketone (protonation makes the reduction easier, shifting the first wave towards less cathodic potentials with decreasing pH) and the protonated anion radical, respectively ... 

Polarographic studies on pyrazine and methylpyrazines indicate that 1 4-dihydropyrazines are the products of reduction. The reduction of pyrazine itself at the dropping mercury electrode proceeds reversibly. The substitution of methyl groups makes the reduction more difficult with an increased number of methyl groups an increased tendency toward irreversible reduction is noted.102-104 The half-wave reduction potentials for pyrazine, methylpyrazine, 2,6-dimethyl-pyrazine, and tetramethylpyrazine are 2.17, 2.23, 2.28, and 2.50 eV, respectively. Pyrazine is thus more easily reduced than pyridine which has a half-wave potential of 2.76 eV, and less easily reduced than quinoxaline which has a half-wave potential of 1.80 eV.105... 

Following initial studies by Cavalieri Lowy 96), it was shown by Smith Elving74) that the polarographic behaviour of 2-aminopyrimidine in acid medium is similar to that for the parent pyrimidine, which exhibits three waves at the dropping mercury electrode 74). The initial le step involves formation of a free radical which dimerizes and, at the potential of the second le reduction step, 2-amino-3,4-dihydropyrimidine is formed. But, unlike pyrimidine, 2-aminopyrimidines do not undergo a second 2e reduction to tetrahydro derivatives. Wave III (pH 7-9) involves two electrons and two protons, and is due to the combined processes responsible for waves I and II at lower pH. Both Smith Elving 74), and Sugino 104>, found that reduction of 2-aminopyrimi-dine on a mercury electrode 74) and lead cathode 104) resulted in the formation of unstable products. 

The reduction of phenazine (170) to 5,10-dihydrophenazine is very nearly reversible at the dropping mercury electrode, and its semi-quinone has considerable stability.174-177... 

Sugars are only reduced at the dropping-mercury electrode when they are present at fairly high concentrations. It was originally considered that a small proportion of aldehydo form, in equilibrium with ring forms, was being reduced, and hence the step height was supposedly a measure of the concentration of open-chain form present in the bulk of the solution. It was, however, shown by Wiesner that the reduction is not diffusion-con-... 

Fig. I4J Tafel plots for the reduction of nitroalkanes at the dropping mercury electrode. pH 6.65.

Oxygen wave At the dropping mercury electrode, oxygen produces two waves, the first due to formation of peroxide, the second due to further reduction to water this can be an interference in the determination of other species but is used in the determination of dissolved oxygen. 

At the dropping mercury electrode the reduction of the cyclic azomethine bond of C = N heterocycles is well known3. Under the conditions of controlled potential electrolysis at a large-scale mercury pool electrode two partially saturated pyrido[2,l-b]quinazolinones 3 were reduced with excellent diastereoselectivity4. 

Wiesner assumed that the enediol grouping in L-ascorbic acid is primarily oxidized at the dropping-mercury electrode to a non-hydrated, electroactive, diketo grouping, thus forming a reversible system he partially proved this oscillopolarographically. This problem was solved, theoretically, by Kem and Kouteck - as an instance of a chemical reaction occurring subsequent to the electrode process. They explained what properties must be exhibited by the reduction wave of dehydro-L-ascorbic acid under the assumption that the simple depolarization scheme usually accepted for dehydro-L-ascorbic acid is valid. It follows from the relationships derived by Koutecky that, if the cathodic wave is small compared to the anodic one, the half-wave potentials of both waves should be equal, and, on the other hand, if the cathodic-wave current has diffusion character, then the value of its half-wave potential should be identical with the normal redox potential. The results of experimental work do not correspond to these theoretical conclusions. 

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