Cathodic Stripping Voltammetry of Sulphur-Containing Amino Acids and Peptides in the Presence of Nickel Ion. Catalytic and Inhibiting Effects

Abstract

The paper reviews recent studies on the effect of addition of Ni(II) in the cathodic stripping voltammetry of the following compounds: cysteine, penicillamine, cystine, glutathione (either reduced or oxidised) and N-acetylcysteine. With the exception of N-acetylcysteine, the above compounds give a cathodic stripping peak at -0.6 V (vs. Ag/AgCl, 3 M KCl electrode) which is due to the catalytic reduction of nickel ion. Even in the case of the disulphides the actual catalyst is the thiol produced by the cleavage of the -S-S- bond during the accumulation step. The catalytic peak enables the detection of the analyte with a better selectivity than is obtained with the stripping peak due to the reduction of mercury thiolates. In addition, Ni(II) suppresses the mercury thiolate peak of ligands such as cysteine or penicillamine, but does not modify the behaviour of thiols with low complexing properties (such as N-acetylcysteine). Consequently, compounds such as cysteine and its N-acyl derivatives can be determined simultaneously by means of the catalytic peak and the mercury thiolate peak (at -0.4 V) respectively.     

New Electrocatalytic Reactions at a Mercury Electrode in the Presence of Homocysteine or Cysteine and Cobalt or Nickel Ions

Homocysteine (Hcy) and cysteine (Cys) mercury thiolate layers were prepared by anodic polarization of a mercury electrode in amino acid containing solutions and then investigated in the cathodic regime in the presence of Ni²⁺ or Co²⁺ ions. The sulfhydryl function in the mercury thiolate undergoes a slow disintegration resulting in surface‐attached mercury sulfide. During the cathodic scan, Hg²⁺ substitution by Ni²⁺ or Co²⁺ yields minute amounts of the relevant metal sulfide. Such a species catalyzes hydrogen evolution at ?1.3 V vs. Ag|AgCl|KCl(3 M). Hcy experiences a faster decomposition and, consequently, displays a stronger catalytic effect. Each compound catalyzes the reduction of Ni²⁺ or Co²⁺, but only Cys (bound in metal complexes) induces typical catalytic hydrogen evolution processes such as the Brdi?ka reaction (with Co²⁺; pH around 9), or the catalytic hydrogen prewave (CHP) (with Ni²⁺; pH near 7). On the other hand, Hcy catalyzes the hydrogen evolution in the presence of Co²⁺ at ?1.5 V in the same way than sulfur derivatives with no amine function do. Metal sulfide formation does not interfere with CHP and Brdi?ka processes. Correlations between the physical state of the metal sulfide (adsorbed molecule or aggregate form) and its catalytic properties are discussed and possible analytical applications suggested.     

Catalytic cathodic stripping voltammetry of oxidized glutathione at a hanging mercury drop electrode in the presence of nickel ion

Oxidized glutathione (GSSG) can be determined after previous accumulation on the HMDE at E > -0.2 V (vs. the Ag AgCl reference electrode). GSH is formed during the accumulation, possibly by a mercury-ion-assisted hydrolytic disproportionation of GSSG. In the subsequent cathodic scan GSH is released and catalyses the reduction of nickel ion, giving a peak located at -0.6 V. This enables the determination of GSSG by differential-pulse cathodic stripping voltammetry at pH 7.0 in the phosphate acetate or MOPS buffer containing 0.5-1.0 mM Ni(II). The detection limit is 10 nM. The calibration graph is linear even in the presence of small amounts of human serum albumin, HSA. However, HSA increases the detection limit (20 nM for 3 x 10(-4)% HSA). Acetyl-cysteine in small excess or Cu(II) present as reagent impurity do not interfere. Glutathione, cysteine and similar compounds, which accumulate as mercury salts and form stable nickel complexes, will interfere. The method is put forward as a novel alternative stripping voltammetric method to those involving accumulation and determination as mercury or copper salts and complexes, in the knowledge that it may have advantages in particular analytical situations. In particular the method discriminates against compounds which accumulate as mercury salts but which do not form stable nickel complexes.     

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

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.     

Catalytic hydrogen evolution in cathodic stripping voltammetry on a mercury electrode in the presence of cobalt(II) ion and phenylthiourea or thiourea.

The system Co(II)-phenylthiourea (PTU)-borax buffer was investigated by cathodic stripping voltammetry (CSV) at a hanging mercury drop electrode. The results of the voltammetric measurements showed that the presence of both PTU and Co(II) gives rise to a new irreversible peak at about -1.5 V. Based upon our previous results obtained in the study of other sulfur compounds and the sulfide ion itself, the peak was ascribed to the catalytic hydrogen evolution superimposed on the reduction of the coordinated Co(II) ion. The catalyst itself is a Co(II) complex with the sulfide ion produced by the decomposition of the analyte during the deposition step. The influence of PTU and cobalt concentration, accumulation conditions and stripping parameters was investigated and complementary data on thiourea are included. The results showed that the measurement of the catalytic hydrogen evolution peak current can be used as a basis for a simple, accurate and rapid method for the determination of PTU within the concentration range 10-100 nM. The catalytic method is relatively free of interferences and could be a suitable alternative for cases in which the stripping peak due to mercury ion reduction in the accumulated mercury compound is disturbed by some interference.     

Stripping voltammetric determination of traces of peptides and proteins containing disulphide linkages

Stripping voltammetry has been investigated for the determination of traces of ribonuclease, somatostatin, oxytocin, felypressin, insulin and oxidized glutathione at concentrations down to 1.5 × 10^?9 M. Repeated cyclic potential scans with an initial cathodic scan were used after accumulation at +0.1 to –0.3 V vs. Ag/AgCl at a hanging mercury drop electrode. In presence of excess of copper(II) ion, the first two compounds yield a well-defined peak couple at ?0.5 to ?0.6 V, with cathodic and anodic peaks of equal height, the accumulated product being adsorbed in both its oxidized and reduced state. Oxytocin and felypressin first yield two unresolved cathodic peaks, one of which disappears in the second scan cycle. Oxidized glutathione yields a large cathodic peak but a small anodic peak because of desorption in the reduced state. Excess of copper(II) is reduced during the accumulation, so that the electrode is actually copper amalgam. The peaks obtained with copper(II) present are considered to be due to redox reactions of copper complexes formed with the cysteine parts of the molecules. These peaks are suitable for quantitative purposes; calibration equations are given. Without copper(II), the substances show stripping responses of different complexity and magnitude. Insulin gives usable stripping peaks only without copper ions.     

Vergleich von Potentiometrischer Stripping-Analyse und Anodischer Stripping-Voltammetrie in der Flie?injektionsanalyse. St?rungen bei der Bleibestimmung

Summary The interferences caused by inorganic ions, organic anions and Triton X-100 in the determination of lead by flow injection potentiometric and voltammetric stripping analysis at a mercury film electrode were investigated. The experiments were performed in the concentration range of 1–10 mg/l Pb²⁺. For both methods linear calibration plots were obtained in the presence of an excess of nitrate, chloride, perchlorate and sulphate. Iodide strongly interferes due to mercury complexation.Metal ion interference caused by formation of irreversible amalgams (as in the case of codeposition of Ni, Co, Fe and Cr) is avoided by proper choice of deposition potential. No effect of water soluble reduced species, i.e. Fe(II) and Cr(II), on the stripping signals was observed. The presence of acetate, citrate and tartrate does not limit the determination of lead by potentiometric stripping analysis (PSA) but has a marked effect on anodic stripping voltammetric (ASV) signals. No influence of Triton X-100 on the determination of lead by PSA was found. In ASV 10^–3% Triton X-100 diminished the peak current by 15%.

---

---

Herrn Prof. Dr. Rolf Neeb aus Anlaß seines 60. Geburtstags gewidmet     

Voltammetric Determination of Mercury(II) at Poly(3‐hexylthiophene) Film Electrode. Effect of Halide Ions

The well‐known method for the determination of mercury(II), which is based on the anodic stripping voltammetry of mercury(II), has been adapted for applications at the thin film poly(3‐hexylthiophene) polymer electrode. Halide ions have been found to increase the sensitivity of the mercury response and shift it more positive potentials. This behavior is explained by formation of mercuric halide which can be easily deposited and stripped from the polymer electrode surface. The procedure was optimized for mercury determination. For 120 s accumulation time, detection limit of 5 ng mL^?1 mercury(II) has been observed. The relative standard deviation is 1.3% at 40 ng mL^?1 mercury(II). The performance of the polymer film studied in this work was evaluated in the presence of surfactants and some potential interfering metal ions such as cadmium, lead, copper and nickel.     

Determination of the pesticide Chlorpyrifos by cathodic adsorptive stripping voltammetry

The adsorption behavior and differential pulse cathodic adsorptive stripping voltammetry of the pesticide Chlorpyrifos (CP) were investigated at the hanging mercury drop electrode (HMDE). The pesticide was accumulated at the HMDE and a well-defined stripping peak was obtained at –1.2 V vs Ag/AgCl electrode at pH 7.50. A voltammetric procedure was developed for the trace determination of Chlorpyrifos using differential pulse cathodic adsorptive stripping voltammetry (DP-CASV). The optimum working conditions for the determination of the compound were established. The peak current was linear over the concentration range 9.90 × 10^–8– 5.96 × 10^–7 mol/L of Chlorpyrifos. The influence of diverse ions and some other pesticides was investigated. The analysis of Chlorpyrifos in commercial formulations and treated waste water was carried out satisfactorily     

Determination of nickel,cobalt, copper and uranium in water by cathodic stripping chronopotentiometry with continuous flow

The potential of cathodic stripping chronopotentiometry for the determination of trace metals in a continuous-flow system is investigated. An automated analyzer with a fast rate of data acquisition (250 kHz) is described. The cathodic scans are preceded by adsorptive collection of surface-active metal complexes on the hanging mercury drop electrode. The scans are done by passing a constant current of between 0.8 (in deaerated solution) and 60 μA (in solutions saturated with air) through the working electrode. Copper, uranium, and nickel can be determined in the presence of dissolved oxygen, but the sensitivity for nickel is then much reduced. The sensitivity of stripping chronopotentiometry in the presence of dissolved oxygen is similar to that of fast linear-sweep voltammetry in the absence of dissolved oxygen. The limits of detection were 0.1 nM Ni, 0.1 nM Co, 1.8 nM Cu and 1.6 nM U, when the measurements were preceded by 60-s stirred adsorption; in the presence of dissolved oxygen the limit of detection for nickel was higher at 0.6 nM, and cobalt could not be determined, as its peak was located on top of the oxygen peak. The determinations of copper and uranium were not adversely affected by dissolved oxygen. The limits of detection can be lowered further by using a prolonged collection period (up to 300 s). The technique was successfully tested by measuring nickel with continuous flow in water pumped on board of a small vessel in the Tamar estuary.     

Determination of very low/levels of selenium(IV) in sea water by differential-pulse cathodic stripping voltammetry after extraction of the 3,3′-diaminobenzidine piazselenol

Selenium(IV) is determined by cathodic stripping voltammetry after the formation of a piazselenol with 3,3′-diaminobenzidine. The selenium is then accumulated as HgSe on a mercury electrode by deposition at ?0.45 V. The differential-pulse cathodic stripping peak allows a detection limit of 0.01 μg l^?1. For the determination of selenium in natural waters, interferences can be avoided by extraction of the piazselenol into toluene followed by a back-extraction into 0.5 M hydrochloric acid. The accuracy of the overall procedure was checked by analyses of a standard reference material. The method was applied to the determination of selenium(IV) in sea-water samples at levels as low as 20 ng l^?1 with a concentration factor of 10 during the extraction procedure.     

Determination of folic acid by adsorptive stripping voltammetry at the static mercury drop electrode

Folic acid can be determined at nanomolar concentrations by controlled adsorptive accumulation of folic acid on a static mercury drop electrode held at ?0.3 V vs. Ag/AgCl followed by reduction of the surface species. In 0.1 M sulfuric acid, a cathodic scan gives peaks at ?0.47 v and ?0.75 V vs. Ag/Agcl; the latter peak provides greater sensitivity. Differential-pulse stripping is shown to be superior to normal-pulse and d.c. stripping. After a 5-min preconcentration, the detection limit is about 1 × 10^?10 M folic acid. The adsorptive stripping response is evaluated with respect to concentration dependence, preconcentration time and potential, solution acidity and the presence of gelatin and bromide. The relative standard deviation at the 5 × 10^?8 M level is 1.2%. This method is applied to the determination of folic acid in pharmaceutical tablets.     

Use of a modified, high-sensitivity, anodic stripping voltammetry method for determination of zinc speciation in the North Atlantic Ocean

Zinc speciation is considered to be an important determinant of the biological availability of zinc. Yet in oceanic surface waters, characterization of zinc speciation is difficult due to the low concentrations of this essential micronutrient. In this study, an anodic stripping voltammetry method previously developed for the total determination of cadmium and lead was successfully adapted to the measurement of zinc speciation. The method differs from previous zinc speciation anodic stripping voltammetry methods in that a fresh mercury film is plated with each sample aliquot. The fresh film anodic stripping voltammetry method was compared to competitive ligand exchange cathodic stripping voltammetry in a profile from the North Atlantic Ocean. Results using the fresh film anodic stripping voltammetry method were similar to those determined using the cathodic stripping voltammetry method, though ligand concentrations determined by fresh film anodic stripping voltammetry were generally slightly higher than those determined by cathodic stripping voltammetry. There did not seem to be a systematic difference between methods for the estimates of conditional stability constants. The ligand concentration in the North Atlantic profile ranged from 0.9 to 1.5 nmol L⁻¹ as determined by fresh film anodic stripping voltammetry and 0.6 to 1.3 nmol L⁻¹ as determined by cathodic stripping voltammetry. The conditional stability constants determined by fresh film anodic stripping voltammetry were 10^9.8-10^10.5 and by cathodic stripping voltammetry were 10^9.8-10^11.3.     

The cathodic stripping voltammetric determination of traces of iodide with a hanging copper amalgam drop electrode

A hanging copper amalgam drop electrode (HCADE) is used for the determination of traces of iodide by cathodic stripping voltammetry. The cathodic stripping peak of copper(I) iodide from the HCADE is better defined than that of mercury(I) iodide from a hanging mercury drop electrode. Optimum conditions and interferences are reported. With a 3-min deposition time at ?0.1 V vs. SCE, the calibration plot is linear up to 2 × 10^?6 mol dm^?3 iodide. The detection limit for iodide with the HCADE under voltammetric conditions is 4 × 10^?8 mol dm^?3; this is lowered to 8 × 10^?9 mol dm^?3 by using the differential pulse stripping technique.     

Determination of the pesticide Chlorpyrifos by cathodic adsorptive stripping voltammetry

The adsorption behavior and differential pulse cathodic adsorptive stripping voltammetry of the pesticide Chlorpyrifos (CP) were investigated at the hanging mercury drop electrode (HMDE). The pesticide was accumulated at the HMDE and a well-defined stripping peak was obtained at –1.2 V vs Ag/AgCl electrode at pH 7.50. A voltammetric procedure was developed for the trace determination of Chlorpyrifos using differential pulse cathodic adsorptive stripping voltammetry (DP-CASV). The optimum working conditions for the determination of the compound were established. The peak current was linear over the concentration range 9.90 × 10^–8– 5.96 × 10^–7 mol/L of Chlorpyrifos. The influence of diverse ions and some other pesticides was investigated. The analysis of Chlorpyrifos in commercial formulations and treated waste water was carried out satisfactorily Received: 10 July 1997 / Revised: 1 April 1998 / Accepted: 6 April 1998     

Adsorptive Stripping Voltammetric Determination of Albendazole at a Hanging Mercury Drop Electrode

ABSTRACT

Albendazole is determined by differential-pulse adsorptive cathodic stripping voltammetry at a hanging mercury drop electrode using the reduction peak of its copper(II) complex at ?0.28V at an accumulation potential 0.0V vs. Ag/AgCl electrode. The optimum conditions of pH, accumulation potential and accumulation time were studied. The calibration graph for the determination of albendazole was linear in the range 3.0X10^?8 - 9X10^?7M with a relative standard deviation of 2.8%. The detection limit was 1.0X10^?8M after 180s accumulation at 0.0V. The effect of common excipients and metal ions on the peak height of albendazole was studied. The presence of Cu²⁺ ions forms a stable complex with albendazole which is strongly adsorbed at the mercury electrode surface. The method was applied to the determination of the drug in commercially available dosage forms.     

The adsorption of the HgEDTA complex at mercury electrodes

While free EDTA has no tendency to adsorption on mercury surfaces, its complex with Hg(II) is adsorbed strongly. The coverage is very small in alkaline solutions where HgYOH^3? is present, reaches 60% at moderate pH, and is high at pH = 2, where the predominant species in solution is HgYH^?. Dependence of peak potential on pH for cathodic stripping voltammetry indicated that for pH > 3, HgY^2? is adsorbed at the surface, while at pH 2 the adsorbed complex is protonated. Cyclic chronopotentiometric experiments suggest formation of a coherent film of adsorbed material at pH 2. At pH = 2 adsorption of HgEDTA can be described by a Frumkin isotherm, and at pH = 4.8 by either a virial or HFL isotherm.     

Determination of sulfur species by cathodic square wave stripping voltammetry; Compounds relevant to natural sulfur mineralization

The feasibility of rapid analysis of a number of environmentally important sulfur compounds by cathodic square wave stripping voltammetry at a mercury electrode has been investigated. For cysteine/cystine a relatively anodic peak was identified, which is ascribed to the stripping of a mercurous species. The dependence of the peak currents on pH was found to be different for cysteine and cystine. Methionine and thioproline gave similar stripping peaks to those for cysteine. It is proposed that they arise from species deposited by oxidative hydrolysis. A complex ion is proposed to cause the stripping peak of thiosulfate and tetrathionate, while polysulfides give both a HgS stripping peak and a non-adsorptive reduction peak. Limits of detection are in the 10(-8)-10(-9)M range.     

Reduction of metol on mechanically renewed metal electrodes

The electroreduction of metol on mechanically renewed metallic electrodes is studied by direct voltammetry with linear potential sweep. Reduction peaks of metol are found in a neutral supporting electrolyte (0.02–0.5 M Na₂SO₄) on nickel, silver, and mercury electrodes before the potential of hydrogen liberation from the supporting electrolyte. The shape and parameters of the cathodic peak depend on an electrode material, and also on the composition and pH of the supporting solution. A probable mechanism of the electroreduction of metol is proposed. The regeneration of nickel and silver electrodes by in situ mechanical cutting of a 0.5-μm surface layer provides good reproducibility of the value of peak current; it is proportional to the concentration of metol in the range 2 × 10⁻³–1.8 × 10⁻² M.     

A polarographic and voltammetric study of some primary,secondary and tertiary thioamides of pharmaceutical importance

The d.c. polarographic, and cyclic and cathodic stripping voltammetric behaviour of some primary, secondary and tertiary thioamides based on tetrahydroquinoline, is described. Catalytic reduction occurs in all cases; the primary and secondary thioamides undergo anodic oxidation and cathodic stripping, with the formation of mercury(II) sulphide. Tertiary thioamides are not amenable to anodic electrolysis/stripping. Mechanisms for these oxidation and reduction processes are postulated. Differential pulse polarography has a limit of detection of 5 × 10^?7 M. Cathodic stripping voltammetry can be applied with a detection limit of 2 × 10^?8 M.