Catalytic hydrogen evolution in the presence of methylthiohydantoin-glycine and cobalt(II) ion: a study by cathodic stripping voltammetry at a hanging mercury drop electrode

The carbon-sulfur bond in methylthiohydantoin-glycine (MTH-Gly) is broken during the deposition on the mercury electrode at potentials around -0.1 V versus SCE, giving mercury sulfide which is detected by the characteristic cathodic peak at -0.7 V. If cobalt (II) is also present, the product of the deposition step is a mixture of mercury and cobalt sulfides. During the cathodic scan, the last one is reduced to a transient Co(0) species that catalyses the reduction of hydrogen ion to the hydrogen molecule by a mechanism alike to that emphasized for the Co(II)-sulfide ion system (F.G. B?nic?, N. Sp?taru, T. Sp?taru, Electroanalysis 9 (1997) 1341). This electrode process induces a cathodic peak at -1.4 V that enables the determination of MTH-Gly down to 10(-7) M in the borax buffer at pH 8.5. The possible extension of this method to other classes of organic sulfur compounds is briefly discussed.     

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.     

Mechanism of electroreduction of cobalt(II) in thiocyanate solutions at mercury electrodes

The process of electroreduction of cobalt(II) in thiocyanate solutions at mercury electrodes has been investigated by cyclic voltammetric, chronoamperometric and polarographic methods. The influences of pH, the concentrations of Co(II) and SCN^?, and the reduction products of SCN^?, CN^? and S^2? on the reduction waves are described. The polarographic pre-wave is an autocatalytic in nature. A mechanism involving an initial reduction of Co(II)—SCN^? at a mercury electrode followed by the chemical reduction of thiocyanate ion with the electroreduced metallic cobalt, and taking into account cyanide, sulfide, and hydroxide ions, the latter being produced by the hydrolysis of cyanide ion, is presented. Cobalt sulfide adsorbed at the electrode surface stimulates further reduction of Co(II)—CN^? and —SCN^? complexes, and depresses the interfering influence of Co(OH)₂, which is reductively desorbed from the electrode surface with giving rise to an additional peak near ?1.08 V vs. SCE.     

Study on the polarographic catalytic wave of the system cobalt(II)-dimethylglyoxime

In the cobalt (II)-dimethylglyoxime-NH₃-NH₄Cl (pH 9) system, tne complex Co(II)A₂ exhibits a sensitive polarographic wave. The mechanism of this catalytic wave has been investigated by linear potential sweep voltammetry, cyclic voltammerty and anedic stripping voltammetry. The experimental evidences showed that a zero-valence “active cobalt” or its complex formed during the irreversible reduction of Co(II)A₂, which is adsorbed on the mercury electrode surface, and simultaneously DMG is catalytically reduced by this “active cobalt”. The mechanism of this system with the conflicting explanations of a catalytic hydrogen wave or only adsorptive complex wave is discussed.     

Determination of cobalt by catalytic-adsorptive differential pulse voltammetry in the presence of 2-aminocyclopentene-1-dithiocarboxylic acid and nitrite

A novel method for the determination of cobalt(II) by stripping voltammetry is described. It involves an adsorptive accumulation of the cobalt(II)-2-aminocyclopentene-1-dithiocarboxylic acid complex on a hanging mercury drop electrode, followed by a stripping voltammetric measurement of the catalytic reduction current of the complex at –1.4 V at pH = 9 (vs. Ag/AgCl). The effects of various experimental parameters on the catalytic current were investigated. An accumulation time of 60 s results in a low experimental limit of detection of 0.1 ng/mL of Co(II), and 0.50 to 40.0 ng/mL of cobalt can be determined. The relative standard deviation at 0.50 ng/mL is 2.8%. Possible interferences from co-existing ions were also investigated. Received: 17 August 1998 / Revised: 16 November 1998 / Accepted: 20 November 1998     

Determination of cobalt by catalytic-adsorptive differential pulse voltammetry in the presence of 2-aminocyclopentene-1-dithiocarboxylic acid and nitrite

A novel method for the determination of cobalt(II) by stripping voltammetry is described. It involves an adsorptive accumulation of the cobalt(II)-2-aminocyclopentene-1-dithiocarboxylic acid complex on a hanging mercury drop electrode, followed by a stripping voltammetric measurement of the catalytic reduction current of the complex at –1.4 V at pH = 9 (vs. Ag/AgCl). The effects of various experimental parameters on the catalytic current were investigated. An accumulation time of 60 s results in a low experimental limit of detection of 0.1 ng/mL of Co(II), and 0.50 to 40.0 ng/mL of cobalt can be determined. The relative standard deviation at 0.50 ng/mL is 2.8%. Possible interferences from co-existing ions were also investigated. Received: 17 August 1998 / Revised: 16 November 1998 / Accepted: 20 November 1998     

钴(II)-丁二酮肟体系极谱催化波的机理研究

本文研究了钴(II)-丁二酮肟(DMG)在氨性底液(pH9)中极谱催化波的机理.用线性扫描伏安法,循环伏安法和阳极溶出法等方法证明,这催化波的形成是由于吸附在汞电极上的钴(II)-丁二酮肟螯合物不可逆地还原到零价的"活性钴",同时在电极表面的"活性钴"又催化了丁二酮肟的还原.     

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.     

Voltammetric determination of mercury(II) at a carbon paste electrode in aqueous solutions containing tetraphenylborate ion

The determination of mercury(II) ions can be achieved by monitoring the decrease in the oxidation peak of the tetraphenylborate ion in the presence of this metal ion at a carbon paste electrode. The reaction between mercury(II) and the tetraphenylborate ion results in the formation of diphenylmercury, thus providing the method with good selectivity over other metal ions. Using anodic stripping voltammetry in a neutral electrolyte, a linear dependence of the decrease of peak height was observed on increasing the mercury(II) concentration in the range 1 x 10(-6)-8 x 10(-9)M mercury(II). Zinc(II), cadmium(II), lead(II), nickel(II), cobalt(II), tin(II), potassium(I) and ammonium(I) ions did not interfere at a 1000-fold concentration excess. Iron(III) and chromium(III) did not interfere at a 250-fold and 50-fold concentration excess, respectively. Following masking procedures, copper(II), bismuth(III) and silver(I) did not interfere at a 100-fold concentration excess. The method can be used to determine the concentration of mercury(II) in natural waters contaminated by this metal.     

Oligonucleotide Functionalized Microporous Gold Electrode for the Selective and Sensitive Determination of Mercury by Differential Pulse Adsorptive Stripping Voltammetry (DPAdSV)

Abstract

A novel electrode modified with oligonucleotide and microporous gold was fabricated for the determination of mercury by differential pulse adsorptive stripping voltammetry (DPAdSV). Microporous gold was synthesized by electrochemical reduction using dynamic hydrogen bubble template. The oligonucleotide was immobilized on microporous gold by self-assembly. The prepared electrode exhibited an improved electrochemical response for mercury(II) ion because of the large surface area and excellent electron transfer capacity provided by microporous gold and the specific coordination between mercury ion and thymine bases in oligonucleotides. Under the optimal experiment conditions, the oligonucleotide functionalized microporous gold electrode had a linear relationship between the stripping current and mercury ion concentration in the range from 0.5 to 30?µg/L with a detection limit of 0.021?µg/L. Moreover, the prepared electrode exhibited good selectivity, reproducibility, repeatability and stability. Furthermore, the prepared electrode was applied to detect mercury in tap water with satisfactory results.     

Square-wave adsorptive stripping voltammetric determination of antihypertensive agent telmisartan in tablets and its application to human plasma

The adsorptive stripping voltammetry of telmisartan was investigated with a hanging mercury drop electrode. This compound produced a catalytic hydrogen wave at ?1.5 V in Britton Robinson buffer of pH 10.38, and the peak current increased with adsorptive accumulation at the electrode. Adsorptive stripping voltammetry with the catalytic hydrogen wave could provide a sensitive novel method for the determination of telmisartan. Various chemical and instrumental parameters affecting the monitored electroanalytical response were investigated and optimized for telmisartan determination. Under these optimized conditions the square-wave adsorptive stripping voltammetric (SW-AdSV) peak current showed a linear dependence on drug concentration over the range 0.05–3.00 μg/mL (1 × 10^?7?6 × 10^?6 M) (r = 0.999) with accumulation for 120 s at ?1.0 V vs. Ag/AgCl. The proposed electrochemical procedure was successfully applied for the determination of telmisartan in pharmaceutical tablets and human plasma. The results of the developed SW-AdSV method were comparable with those obtained by reported analytical procedures.     

Catalytic signal of rabbit liver metallothionein on a mercury electrode: a combination of derivative chronopotentiometry with adsorptive transfer stripping

Constant current chronopotentiometric stripping analysis (CPSA) in combination with adsorptive transfer stripping (AdTS) technique was used to study the rabbit liver metallothionein (MT) on a hanging mercury drop electrode (HMDE). Metallothionein yielded a distinct, sharp chronopotentiometric signal at very negative potentials (about -1.7 V), also known as the "peak H". The height and potential of this peak were dependent on experimental conditions, such as buffer composition, pH, and the presence oxygen in solutions. The peak H was highest in borate buffer with pH close to the isoelectric point (pI) of MT. The chronopotentiometric results contribute to a deeper understanding of the nature of catalytic hydrogen evolution and demonstrate the usefulness of the peak H in peptide and protein research.     

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%.

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

Voltammetric Study of Reduction Processes in Rhodium(+3)–Selenium(+4) Acidic Mixtures

Using square‐wave voltammetry and linear scan voltammetry, some reduction processes, that take place on the hanging mercury drop electrode (HMDE) in rhodium(+3)–selenium(+4) mixtures, were studied in highly acidic sulfate, chloride and perchlorate solutions. The special attention was paid to the peak located at ?1.1 V, that reflects catalytic hydrogen evolution on the rhodium islets, and its changes under the influence of added selenium(+4), or selenium(?2) produced at the electrode. It can be concluded that addition of selenite to a dilute rhodium solution has the similar effect as the increase of rhodium(+3) concentration, making the catalytic hydrogen evolution more pronounced, probably by changing the properties of metal clusters on the mercury drop. If so, the effect could be of some practical importance in production of rhodium deposits on other surfaces, too.     

Increased sensitivity of anodic stripping voltammetry at the hanging mercury drop electrode by ultracathodic deposition

An improved approach to the anodic stripping voltammetric (ASV) determination of heavy metals, using the hanging mercury drop electrode (HMDE), is reported. It was discovered that using very cathodic accumulation potentials, at which the solvent reduction occurs (overpotential deposition), the voltammetric signals of zinc(II), cadmium(II), lead(II) and copper(II) increase. When compared with the classical methodology a 5 to 10-fold signal increase is obtained. This effect is likely due to both mercury drop oscillation at such cathodic potentials and added local convection at the mercury drop surface caused by the evolution of hydrogen bubbles.     

Differential-pulse anodic stripping voltammetry of mercury with gold-film micro-electrodes

Cylindrical gold film micro-electrodes are easily produced by plasma-sputtering of gold onto carbon fiber electrodes. The micro-electrodes produced were found to maintain their cylindrical geometry indefinitely, unlike gold wire electrodes of similar dimensions. Application of these electrodes in differential-pulse anodic stripping voltammetry provides a method for quantifying trace levels of mercury(II). Up to 100 μg l^?1 Hg(II) the area of the mercury stripping peak varied linearly with mercury concentration; the detection limit was 3.7 μg l^?1. With more than 100 μg l^?1 Hg(II) a new mercury stripping peak grows in at less positive potentials; its peak height is linear with Hg(II) concentration.     

Differential pulse cathodic stripping voltammetry of the copper complexes of glycyl-L-histidyl-glycine at a hanging mercury drop electrode

The behaviour of the copper complexes of glycyl-L-histidyl-glycine (GHG) was investigated using cyclic voltammetry and differential pulse voltammetry after their adsorptive accumulation on the surface of a hanging mercury drop electrode (HMDE). The nature of the observed cathodic and anodic peaks was established and optimum conditions were found for the differential pulse cathodic stripping voltammetric detemination of GHG at the 1 x 10(-8)M concentration level using adsorptive accumulation at -0.20 V vs. Ag/AgCl reference electrode and the cathodic stripping peak around -0.4 V (pH 8.3). This peak corresponds to the reduction of the Cu(I)-GHG complex formed at the HMDE surface as an intermediate in the reduction of Cu(II)-GHG to Cu(O)amalgam.     

Determination of Co(II) by chemiluminescence after in situ electrochemical pre-separation on a flow-through mercury film electrode

A novel method of electrochemical pre-separation of Co(II) before detection by chemiluminescence is reported together with the associated instrumentation. The Co(II) ions were selectively pre-separated on a mercury film electrode (MFE) by on-line reduction, then the accumulated metal was oxidised and selectively stripped back into the flowing solution as Co(II). These secondary ions were quantified as a result of their catalytic activity on the chemiluminescent reaction between luminol and hydrogen peroxide that was also induced on-line. The whole sequence was carried out in an automated flow-through system, in which the electrochemical pre-separation of metals was performed in either continuous flow or flow injection analysis (FIA) regimes. The scope of the method, both in terms of selectivity and sensitivity, has been demonstrated and the quantitative determination of Co(II) by the proposed method has been investigated. For a period of continuous flow pre-separation of 4 min, the calibration curve for Co(II) was linear up to a concentration of 100 micrograms l-1, the relative standard deviation was 4% at the 20 micrograms l-1 level and the limit of detection was 0.5 microgram l-1 (at the 3 sigma level). The method was applied to the determination of the cobalt content in a high purity iron sample.     

Cathodic stripping voltammetric determination of 2-mercaptobenzothiazole using the catalytic cobalt peak

Previously, thiols have been determined indirectly by cathodic stripping voltammetry (CSV) after accumulation as their mercury and copper(I) salts. Following a previous report of the first use of the catalytic nickel peak (for the determination of cysteine), this paper reports the first use of the catalytic cobalt peak in CSV (for the determination of 2-mercaptobenzothiazole (MBT)): only a very ill-defined catalytic cobalt peak had been observed previously with cysteine, and was unreported. MBT is accumulated at pH 4 (Britton-Robinson buffer) as its cobalt(II) complex at -0.1 V, and is then determined indirectly by observing the reduction of the cobalt(II) in the complex at -0.95 V, i.e., with a much lowered overpotential: hydrated cobalt(II) is reduced at -1.2 V. The peak is catalytic because the thiol released on reduction of the complex complexes further cobalt ions and causes their reduction. The detection limit for the determination of MBT was calculated to be 2.5 x 10(-9) M (3sigma) using an accumulation time of 1 min. The sensitivity is about three times that obtained with the corresponding catalytic nickel peak.     

镍(II)-丁二酮肟体系极谱催化波的机理研究

研究了镍(II)-丁二酮肟(DMG)体系极谱催化波的行为, 这一体系的极谱催化波可用于生物及岩矿中测定痕量镍和同时测定痕量镍、钴, 并对照研究了Co(II)-DMG体系和Ni(II)-DMG体系的机理.