Polarography organic compounds

Polarography is used extensively for the analysis of metal ions and inorganic anions, such as lOg and NOg. Organic compounds containing easily reducible or oxidizable functional groups also can be studied polarographically. Functional groups that have been used include carbonyls, carboxylic acids, and carbon-carbon double bonds. 

The various oxidation states of sulfur have been determined by polarography. The electrochemical oxidation of sulfide ions in aqueous solution may lead to the production of elementary sulfur, polysulfides, sulfate, dithionate, and thiosulfate, depending on the experimental conditions. Disulfides, sulfoxides, and sulfones are typical polarographically active organic compounds. It is also found that thiols (mer-captans), thioureas, and thiobarbiturates facilitate oxidation of Hg resulting thus in anodic waves. 

Like inorganic substances, organic substances can be anodicaUy oxidized and/or cathodicaUy reduced. Such reactions are widely used for the synthesis of various organic compounds (electroorganic synthesis). Moreover, they are of importance for the qualitative and qnantitative analysis of organic substances in solutions, as for instance in polarography (see Lund and Baizer, 1991). 

Principles and Characteristics Contrary to poten-tiometric methods that operate under null conditions, other electrochemical methods impose an external energy source on the sample to induce chemical reactions that would not otherwise occur spontaneously. It is thus possible to analyse ions and organic compounds that can either be reduced or oxidised electrochemi-cally. Polarography, which is a division of voltammetry, involves partial electrolysis of the analyte at the working electrode. 

M. B. Bhargava, AC. polarography of organic compounds, Ph.D. thesis, Bhopal University, 1972. 

Quantitative and qualitative determination of metals and organic compounds at trace levels (10 4 to 10 8 M) relative precision 2-3%. Amperometric titrations are more versatile and more precise than polarography. 

The dominant tendency of my studies has been not so much to obtain and describe organic compounds but... to penetrate their mechanisms.. . . For undertaking this kind of problem, the classic methods of organic chemistry are far from sufficient. Physicochemical procedures become more and more necessary. I have been led to use especially optical methods (the Raman effect and ultraviolet spectra) and electrochemical techniques (conductibility, electrode potentials, and especially polarography).. . . The notion of reaction mechanism led almost automatically to envisioning the electronic aspect of chemical phenomena. From 1927, and working in common with Charles Prevost, I have directed my attention on the electronic theory of reactions." 56... 

Since the invention of d.c. polarography [10, 11], numerous inorganic and organic compounds have been studied by means of this method in Heyrovsky s school and extensive knowledge gathered about the electrochemical properties of these compounds. Among them, many cases were discovered where the polarographic wave appeared to be influenced by the existence of chemical equilibria between the electroactive substance and other, in most cases electroinactive, species in the electrolyte solution, more particularly by the finite rate at which these equilibria relax after the electrochemical perturbation. In fact, the chemical reaction serves as either a source or a sink to deliver or to consume the electroactive reactants and products, in addition to diffusion. 

With the discovery of polarography much of the activity in potentiometric studies of organic compounds tapered off. Instead, attention was focused on the measurement of the polargraphic half-wave potentials Em, which provide similar free-energy information. In may cases, half-wave potentials are the only data readily obtained by electrochemical measurements. Polarographic and vol-tammetric techniques have provided most of the information of the involvement of hydronium ion in electrochemical processes. 

The effects of solution acidity on the polarography of organic compounds have been reviewed, principally in aqueous solution. A thorough discussion of kinetic and catalytic currents that involve hydronium ion has been presented,52 and the irreversible polarographic and voltammetric curves that involve proton transfer in unbuffered and poorly buffered solutions have been discussed.59... 

The reader will find a systematic treatment based on older literature in the book by Bfezina and Zuman [3] or in a shorter version (in German) in a review [187]. More recent reviews present later development in a brief form [188]. If one is interested in the problems of polarographic activity of organic compounds, in pru ticular in mechanisms, Perrin s review [198] should be consulted. More details about polarography of heterocyclics are to be found in a specialised review chapter [6]. A vast quantity of material about organic electrochemistry are contained in a recent book [188a]. 

---