Application of the Method of Amalgam or Film Stripping Polarography for Determining Low Concentrations of a Number of Organic Substances
Abstract Generated abstract
This paper examines the use of amalgam and film stripping polarography to determine very low concentrations of organic compounds that form sparingly soluble salts or complexes with metals at an electrode surface. The authors propose expanding earlier mercury electrode approaches by first concentrating metals such as copper on stationary mercury or solid electrodes, then oxidizing them anodically in the presence of the organic analyte to form a thin insoluble film whose cathodic dissolution can be measured. Experiments with the copper, thiourea system showed a cathodic peak attributable to film dissolution and a linear dependence of peak height on thiourea concentration from 6 x 10^-7 to 1 x 10^-5 M. The results suggest that the method can support trace determination of selected organic substances and investigation of thin film formation and dissolution kinetics.
Full Text
APPLICATION OF THE METHOD OF AMALGAM OR FILM STRIPPING POLAROGRAPHY FOR THE DETERMINATION OF LOW CONCENTRATIONS OF A RANGE OF ORGANIC SUBSTANCES
A. A. KAPLIN, T. F. RYASHENTSEVA, N. P. SOROKINA
(Presented by the scientific seminar of the Department of Physical Chemistry)
At the present time, the problem of determining low concentrations of individual organic substances in various objects is becoming very urgent. A broad range of organic compounds is determined by the now rapidly developing polarography of organic compounds. However, by this method it is possible to determine concentrations on the order of \(10^{-4}\)—\(10^{-5}\) mol/l. In the literature there are isolated works on the indirect determination of lower concentrations of a limited range of organic compounds [1, 2].
The basis of this method is the formation of a sparingly soluble compound on the electrode surface by the organic substance being determined, present in the solution, and by the product of oxidation of the electrode material. Cathodic dissolution of the precipitate makes it possible to estimate the amount of the organic substances being determined that are present in the solution. Owing to a number of advantages, in this method a mercury electrode in the form of a hanging drop is used. This considerably reduces the possibilities of the method because of the limited range of organic substances that form sparingly soluble compounds with mercury.
We have begun studies to clarify the question associated with expanding the range of determinable organic compounds through the use of the method of ASP and FSP.
In the present case, one of the components of the sparingly soluble compound formed on the electrode surface is a metal, previously concentrated in the mercury drop or on the surface of a solid electrode, which in the process of anodic oxidation forms with the organic substance being determined in the solution a thin film on the working electrode.
This makes it possible to use a fairly broad range of metals concentrated on stationary mercury and solid electrodes (Cd, Pb, Zn, Cu; alkali metals; Na, K, Se, Fe, Ni, Co, etc.). Consequently, the number of organic compounds determinable by this method increases—compounds that form with ions of these metals sparingly soluble salts or sparingly soluble complexes on the electrode (in this case the concentration of the metal in the bulk of the solution is insufficient for formation of a precipitate in the solution).
In addition, there arises the possibility of broadly varying the concentration of the metal on the electrode surface by changing the factors of the accumulation stage and anodic oxidation (the time of metal accumulation
on the electrode, electrode dimensions, rate of potential change, temperature, etc.).
We have demonstrated the possibility of applying the ASP and FSP methods to solve the stated problem in the copper–thiourea system.
All measurements were carried out on a ZP-60 polarograph in a conventional U-shaped cell. Bottom mercury served as the reference electrode. Reagents of “special-purity” grade were used in the work. The stage of anodic oxidation of copper was carried out at a potential-change rate of 400 mV/min, after which the cathodic polarogram was recorded immediately. The influence of thiourea on the anodic oxidation current of copper was investigated on stationary mercury and platinum cylindrical electrodes. The observed decrease in the anodic copper-oxidation current in the presence of thiourea at the moment when copper passes from the electrode surface from the amalgam into the ionized state confirms the formation on the electrode surface of a film of an insoluble compound. The cathodic polarogram is characterized by the appearance of a peak caused by dissolution of the film from the electrode surface; this peak is absent when only copper ions or thiourea are present in the solution. The dependence of the height of this peak on the thiourea concentration is linear in the concentration range studied \((6 \cdot 10^{-7} M—1 \cdot 10^{-5} M)\).
From the influence of the indifferent electrolyte \((\mathrm{KNO_3}, \mathrm{HNO_3}, \mathrm{HClO_4}, \mathrm{KCl})\) on the range of determinable thiourea concentrations, one may assume different solubilities of the poorly soluble compound under study in these supporting solutions.
The change in the character of the anodic oxidation and cathodic reduction currents of copper on a stationary electrode in the presence of thiourea confirms the possibility of applying this method both for studying the kinetics of formation and dissolution of thin films on the electrode surface and for carrying out numerical calculations of a number of parameters.
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