Anodic stripping voltammetry with mercury electrode potential-step and current-step methods

Abstract

Prof. N. Tanaka (Sendai): (1) What is the difference of this method and the method proposed by Dr. Hickling? (2) How can this method be applied when two substances are present in the solution? Prof.P. Delahay: (1) If I recall correctly hickling in his work recently published [Anal. Chim. Acta, 14 (1956) 297] carries out the anodic oxidation with a stirred amalgam 1 would say that method is quite different from that of hickling. (2) The situation is similar to that in polarography. Both metals are deposited, and the potential first switched at a value at which only the nobler metal is oxidized. A second current-time curve is recorded at a potential at which both metals are oxidized. The concentration of the less noble metal is obtained by difference. Prof. H.A. Laitinen (Urbana): (1) What are the relative sensitivity limits of the potential step and current step methods? (2) Would Prof. Delahay care to comment on the application of these methods with solid microelectrodes? Here no diffusion can occur within the electrode and diffusion in the solution is ill-defined. Prof. P. Delahay: (i) Sensitivities are quite comparable although the potential-step method is more sensitive than the current-step method. (2) These methods can be applied to solid electrodes but a coulometric determination must then be made. Direct evaluation of the concentration from the current-time caracterstics (potential-time curves in the current-step method) is only possible when less than a monolayer is involved (or perhaps a few layers for a non-uniform deposit). See the recent work of nicholson published in J.Am.Chem.Soc. Dr. H. Gerischer (Stuttgart): (t) Wenn ich Sie recht verstanden habe, beruht die grosse Empfindlichkeit der Methode darauf, days eine elektrolitische Trennung und Anerkennung der Untersuchungssubstanz in dem kleinen Hg-Tropfen Erfolgt. (2) Wie wird die Auswertung des anodischen Stromstosses vorgenommen; durch einfache graphische Integration? (3) Glauben Sie nicht, dass die starke Konvektion in der Lösung auch eine Konvektion im Amalgam zur Folge hat, was zu einem weitgehenden Konzentrationsausgleich im Amalgam führen sollte wenn t relativ gross ist? Prof. P. Delahay: (1) The gain in sensitivity is indeed due to the extraction of the metal from the solution into a thin layer of mercury. The metal is deposited into mercury at a rate larger than that at which it can diffuse away. (2) There was no current integration but only direct measurement on the recorded current-time curve. (3) Convection obviously interferes but reproducible conditions can be achieved in analytical applications. Dr. Mechelynck (Mol, Belgique): Dans le cas ou les temps d'électrolyse sont longs, le gain expérimental est supérieur an gain théorique. Je pense qu'au point de vue analytique c'est un avantage, puisque ce phénomène, qui est dû à la sph ' ́ ericité; de 1'électrode, est parfaitement reproductible si les électrolyses sont effectuées pendant (des temps identiques. Est-ce effectivement le cas? Prof. P. Delahay; Tout à fait d'accord. On est toutefois limité par la solubilité du métal dans le mercure. Prof. W. Kemula (Warsaw): Which substances have you studied? Have you met difficulties with purification of basic electrolytes? Prof. P. Delahay : Our study was limited to the development of some fundamental ideas and their verification and not with the details of application. We studied primarily cadmium. I might add that other metals were studied by nikei.ly and cooke in the work they reported at the Lisbon IUPAC meeting (see recent publication in Anal. Chem.). The purification of the supporting electrolyte is a serious problem. Another problem is the necessity of equilibrating the solution with its container. However, these difficulties are often common to many trace analysis methods.