Zenon Kublik

Silver based mercury film electrode: I. General characteristics and stability of the electrode

Summary The influence of the manner of preparation of a MFE with silver substrate and aging of these electrodes was investigated on voltammetric curves recorded for 10 different electrolyte solutions. A procedure for estimation of the moment of transformation of the fluid mercury into solid silver amalgam was proposed. It was established that after this transformation the residual currents rose and useful potential ranges diminished. The procedure of preparation of silver based mercury film electrode with prolonged “life-time” was worked out.

Determination of traces of selenium(IV) by cathodic stripping voltammetry at the hanging mercury drop electrode

Abstract Conditions convenient for the determination of traces of seIenium(IV) by cathodic stripping technique are described. Several electrolytes were tested. Three procedures are given in which the troublesome splitting of the stripping peak is eliminated. Suitable conditions include perchloric acid solution at elevated temperature, hydrochloric acid solution after preconcentration at zero current, and perchloric acid solution containing a small amount of iodide. The detection limits are 5 × 10-9, 2 × 10-9 and 5 × 10-10 mol dm-3, respectively. The time required for the entire procedure is about 30 min starting with a soluble seIenium(IV) sample.

Analytical application of the hanging mercury drop electrode: Determination of impurities in high purity zinc

Abstract A voltammetric method for the determination of Cu, Bi, Sb, Pb, Sn, In and Cd in high purity zinc is described. It consists of a preliminary 3–5 minutes electrolytic concentration of the metals on the hanging mercury drop electrode, and subsequent recording of the oxidation currents of the amalgams formed. When no chemical separation is necessary the total time of analysis does not exceed 90 minutes. The lowest concentrations found in the zinc samples analysed were: Pb, 6.4 · 10−5%; Cd, 8.2 · 10−6%; Sb, 2 · 10−5%; In, 6.8 · 10t-4%. The absolute sensitivity is about 50 times greater, but this was not proved owing to the leak of sufficiently pure zinc samples.

Cyclic and stripping voltammetry with graphite based thin mercury film electrodes prepared “in situ”

The behaviour of a graphite based thin mercury film electrode (TMFE) prepared “in situ” and a HMDE taken as a model electrode applying cyclic and stripping voltammetry was compared. In this comparison the useful potential ranges, the residual currents, the cyclic curves for hydroquinone, nitrobenzene, iodate, Fe(C2O4)33−, Cd2+, Pb2+, Cu2+ and Zn2+ ions as well as the stripping curves for Cd, Pb and Cu were studied. The experimental results obtained for amalgam forming metals were also compared with the rigorous voltammetric theory given by de Vries and van Dalen. Under conditions where the solubilities of the metals investigated in mercury were not exceeded a very good agreement between experiments and theory was obtained for the shifts of cathodic and anodic peak potentials and for the width of the anodic peak at half height (b1/2) as well as for relationship between anodic peak current and the rate of potential scan, deposition time and concentration of metal ions in the solution. With increasing excess of metal in mercury film over the value given by solubility data the agreement between experimental and theoretical results began to fail.

Determination of traces of purine by cathodic stripping voltammetry at the hanging copper amalgam drop electrode

Abstract In the presence of purine, the copper(II)/copper(Hg) couple splits into copper(II)/copper(I) and copper(I)/copper(Hg) couples, which form two well-separated systems of peaks under voltammetric conditions. The copper(I)/purine complex adsorbs on the electrode surfacer and can be deposited on the electrode surface by electroreduction of copper(II) ions at the HMDE or by electro-oxidation of the hanging copper amalgam drop electrode (HCADE). The deposit can be stripped either cathodically or anodically over the pH range 2–9. The cathodic stripping variant at the HCADE, in solution with pH 2, offers the best results, with linear response for the range 5 × 10 −9 –1.5 × 10 −7 mol dm −3 purine after an accumulation time of 3 min. The detection limit found with the HMDE in the presence of copper(II) ions is higher.

Cyclic and stripping voltammetry of Se(+4) and Se(-2) at carbon electrodes in acid solutions

Abstract The behaviour of the Se(+4)−Se(0)−Se(−2) system was studied by cyclic and stripping voltammetry using several kinds of graphite and glassy carbon electrodes in the pH range from 0 to 8. Well-defined curves of Se(+4) reduction were obtained with a very soft graphite electrode, whereas poorly defined curves were recorded with glassy carbon electrodes. The reduction of Se(+4) in acid solution led to the formation of two forms of elemental selenium. One was formed in a direct electroreduction and the other in a subsequent chemical reaction between Se(+4) and Se(−2). These two forms of Se(0) gave separate reduction and oxidation peaks. Hydrogen selenide was anodically oxidized stepwise to elemental selenium and selenous acid. With an increase of pH the extent of Se(+4) reduction decreased and the extent of Se(−2) oxidation increased. The cathodic and anodic stripping peaks of elemental selenium cannot be used for the determination of traces of Se(+4) because they appear only in solutions with Se(+4) concentrations >1×10 −5 mol 1 −1 .

Silver based mercury film electrode: Part III. Comparison of theoretical and experimental anodic stripping results obtained for lead and copper

The anodic stripping results obtained for lead and copper at an improved model of silver based mercury film electrode (SBMFE) were compared with theoretical predictions given for such a case by de Vries and van Dalen. During the study particular attention was paid to the formation of heterogenous amalgams and to influences which this effect could exert on the shape of the stripping peaks. Under conditions where homogenous amalgams were formed a good agreement was observed between theoretical and experimental results for peak currents (ip), shifts of the peak potential (Ep-E1/2) and for half peak-widths (b1/2) obtained for the stripping of lead. On the other hand, small broadening of the values of b1/2 and small decreasing of the values of ip were observed for copper compared to theoretical results. For heterogenous amalgams of both metals investigated the anodic displacement of the peak position was observed compared to the position predicted by the theory. The b1/2 values observed under these conditions could increase, decrease or even stay unchanged depending on the scan rate used. The conditions where the significant increase of concentration of metal deposited into mercury film may lead to enhanced stripping signal were also discussed.

Cyclic and stripping voltammetry of Se(+4) and Se(−2) at the HMDE in acidic media

Abstract Electroreduction of Se(+4) and electrooxidation of Se(−2) were studied at mercury electrodes in acidic media and an improved mechanism of the reduction process was proposed. This mechanism takes into account the fact that the reduction path is concentration-dependent. At lower concentrations of Se(+4), mercury selenide and hydrogen selenide are formed at various potentials. At higher Se(+4) concentrations the electrode quickly becomes covered by a rigid deposit of mercury selenide and then the reduction starts to proceed to elemental selenium. Another form of selenium was formed in the vicinity of the mercury surface due to a chemical reaction between H2SeO3 and H2Se. Oxidation of hydrogen selenide proceeds similarly, in the sense that after coverage of the electrode surface by a deposit of mercury selenide the oxidation starts to proceed to elemental selenium. The cathodic stripping peak of mercury selenide can be obtained down to 2 × 10−8M of Se(+4), but this peak is often split and therefore the determination of traces of Se(+4) by the cathodic stripping technique is cumbersome.

Determination of traces of surfactants in distilled, potable and untreated waters and in supporting electrolytes by tensammetry with accumulation on the HMDE

A simple and rapid method of determination of traces of surfactants in various waters and in pure solutions was developed. The method consists of an accumulation of surfactants on the surface of the HMDE at constant potential in stirred solutions. The ratio of the current decrease, attained by accumulation, to the maximum current is proportional to the surfactant concentration in the range 10–300 μg 1−1. Solutions with higher surfactant concentrations should be diluted prior to the determination. The total time of analysis does not exceed 15 min. The proposed method permits the efficacy of different purification procedures to be compared quantitatively. Water with the lowest content of surfactants was obtained by combining two purification procedures based on various separation principles.

The determination of traces of thiocyanate and copper(II) ions by cathodic stripping voltammetry

Cathodic stripping methods are described for the determination of traces of thiocyanate ions down to 2 × 10-8 mol l-1 and Cu(II) ions down to 1 × 10-8 mol l-1. The method involves electrolytic accumulation of copper(I) thiocyanate on the surface of a hanging mercury drop electrode followed by stripping of the deposit during the cathodic scan. For the determination of thiocyanate, a copper amalgam electrode can be used. Examples of application of the method for the determination of traces of thiocyanate in common salts, in saliva and urine as well as for the determination of copper(II) ions in tap water are described.