Polarographic reduction of nickel(II), copper(II) and zinc(II) in the presence of 2,3-dihydroxypyridine and 2-amino-3-hydroxypyridine

Summary Nickel(ll), copper(II) and zinc(II) give diffusion-controlled irreversible single reduction waves in the presence of 2,3-dihydroxypyridine (DHP) and 2-amino-3-hydroxypyridine (AHP). Values of transfer coefficients () and formal rate constants (k _f ⁰ , h) have been calculated. The AHP complexes are more easily reduced than the corresponding DHP complexes.     

Polarographic reduction and tautomeric structure of 2-amino-5-arylazothiazoles

The polarographic behaviour of a series of 2-amino-4-phenyl-5-arylazothiazoles has been investigated at a dropping mercury electrode. Each of the compounds studied exhibits one wave which was shown to correspond to the reductive cleavage of the azo linkage by a 4e irreversible step. On the basis of the polarographic data, it was concluded that the compounds exist only in one tautomeric form, namely the aminoazo structure 1 . The results of E_1/2 — σ_X correlations and HMO calculations of bonding energies of the various possible tautomeric forms 1–3 indicate that the aminoazo form 1 is the most stable structure of the compounds examined.     

Polarographic speciation of copper(II) in the presence of 1-dimethylaminopropan-2-ol

Volatile amines are used in boiler-water treatment and their performance is judged not only from two main factors, viz., basicity and volatility, but also from their thermodynamic stability, metal complexation capacity and interaction with ion-exchange systems. The complexation of copper by 1-dimethylaminopropan-2-ol was studied. Its pK_b value was found to be 4.5 at 25°C. It formed a coloured complex in the pH range 8.5–10.5 and its log K value was found to be 18.2.     

Examination of the range of copper complexing ligands in natural waters using a combination of cathodic stripping voltammetry and computer simulation

Knowledge of the detection capabilities of speciation techniques, gained by calculation and computer simulation, can be combined with experimental measurements to arrive at an understanding of trace metal speciation which is less dependent on operational factors than other approaches. Examples of the application of this means of measuring copper speciation to samples from the Humber Estuary are given. Although concentrations of total dissolved copper can approach the estuarine Environmental Quality Standard value of 5 μg 1^?1, there is evidence for a substantial excess of complexing ligands at all locations except the outer estuary, where copper levels are much reduced by dilution. Dissolved copper is therefore present almost totally in the form of organic complexes. The range of different types of ligand is also assessed. In sea water, there appears to be a range of ligands of differing affinities for copper; the complexing capacity ranges from 20 nM [conditional stability constant of the copper complex (K′) > 10¹⁴] to 70 nM (K′) > 10⁸). For estuarine samples, ligands with a high affinity for copper seem to be predominant and the overall complexing capacity rises to over 200 nM. In freshwater samples, it is likely that the potential for varying combinations of weak and strong complexes will depend on the water quality, but a capacity to complex over 200 nM copper is not unusual.     

Extraction-liquid chromatography with electrochemical and spectrophotometric detection for the determination of copper and iron in biological and river water samples

Reversed-phase liquid chromatography using cupferron as a precolumn derivatizing agent was developed for the determination of Cu(II) and Fe(III) in biological materials and natural water samples. In the direct method, the metal cupferronates formed in acetonitrile-water (1 + 1) are injected onto an ODS column followed by separation with a mobile phase containing acetonitrile-acetate buffer (pH 3.5) (7 + 3) and other reagents. Amperometric detection with a glassy carbon electrode at ?0.40 V vs. Ag/AgCl can be used to determine both metals simultaneously. The electrochemical detection method has better sensitivity for the determination of Fe(III) than the usual spectrophotometric detection at 375 nm. If a large volume of aqueous sample is available, concentration of the two metal ions can be made by extraction with ethyl acetate prior to the chromatographic determination. In this case, liquid chromatographic separation and determination can be performed with the ODS column using a mobile phase consisting of acetonitrile-methanol-ethyl acetate-0.02 M acetate buffer (pH 3.5) (45 + 20 + 5 + 30).     

FI automatic method for the determination of copper(II) based on coproporphyrin I-Cu(II)/TCPO/H(2)O(2) chemiluminescence reaction for the screening of waters

In this paper, an automatic method for the screening of water samples containing Cu(II) was proposed, based on peryoxalate chemiluminescence reaction using coproporphyrin I as fluorophor compound to provide selectivity and a simple flow injection (FI) chemiluminescence detector (CLD). FI system conditions were chosen in order to distinguish samples over or under legislation limit established (50 μg l⁻¹) with high reliability. The detection limit found was 9 μg l⁻¹ and the linear dynamic range was 15-125 μg l⁻¹ of Cu(II). Repeatibility and reproducibility studies gave good precision and accuracy with recovery near 100%. Under these conditions, the method resulted selective and only Fe(II), Fe(III) and Pb(II) could interfere, but at a concentration level higher than their normal concentration in waters. The proposed method was found to be highly reliable for screening purposes and it was successfully applied to the screening of a variety of real water samples.     

Polarographic behaviour of salicylaldehyde-2-pyridylhydrazone and its copper(II) complex

The polarographic behaviour of salicylaldehyde-2-pyridylhydrazone (SAPH) has been studied in aqueous buffer solution containing 40% ethanol using DC and DP polarographic methods. In the pH range 1.8–7.5 the observed single irreversible reduction wave is assigned to the splitting of the N-N bond and reduction of C=N centre. In alkaline medium, a second wave appears at a more negative potential due to the reduction of the salicylaldehyde which is formed by hydrolytic decomposition of the SAPH molecule. The effect of pH on the limiting current andE _1/2 as well as the reduction mechanism are discussed and compared with similar compounds. The kinetic parameters of the electrode reaction have been calculated.The analytical properties of the copper(II)-SAPH system is described. The complex gave rise to a single irreversible well-defined wave (E _1/2=–0.58 V at pH=5.2). The reaction process is diffusion controlled. A method is suggested for the determination of Cu(II) in presence of different metal ions as the difference in theirE _1/2 values is sufficient for the purpose.This work is taken partly from the M. Sc. Thesis of Jamal S. Shalabi     

Potentiometric behaviour of the copper electrode in aqueous copper(II) perchlorate solutions containing sodium chloride

It is demonstrated that the copper metal electrode corrodes in the presence of copper(II) ions in solution. A model based on mass balance can properly describe the experimental results. In the presence of copper(II) ions the copper electrode responds to copper(I), indicating that the electrode potential corresponds to a mixed potential.     

Polarographic behaviour of phenyl-2-picolylketone-2-pyridylhydrazone and its copper (II) complexes

The polarographic behaviour of phenyl-2-picolylketone-2-pyridylhydrazone, PPKPyH, has been studied in aqueous-ethanol mixtures of different pH's using DC and DP polarographic method. In the pH range 3.7–6.2, the observed single irreversible reduction wave is assigned to the splitting of the N—N bond and reduction of C=N centre. In alkaline medium, a second wave appears at more negative potential due to the reduction of the phenyl-2-picolylketone which is formed from the hydrolysis of PPKPyH. The optimum conditions for the analytical determination of PPKPyH was reported. Differential pulse polarography have been used to elucidate the complexation reaction between PPKPyH and copper(II) in unbuffered solutions. Two complexes are formed with log ₁ = 4.42 and log ₂ = 8.87.     

Cathodic adsorptive stripping voltammetric behaviour of guanine in the presence of copper at the static mercury drop electrode

The cathodic adsorptive electrochemical behavior of guanine in the presence of some metal ions at the static mercury drop electrode was investigated. A 1.0×10⁻³ mol l⁻¹ NaOH or a 2.0×10⁻² mol l⁻¹ Hepes buffer at pH 8.0 solutions were used as supporting electrolytes. The reduction peak potential for guanine was found to be around −0.15 V, which is very close to the mercury reduction wave. A new peak appears at −0.60 V in the presence of copper or at −1.05 V in the presence of zinc. A square wave voltammetric procedure for electroanalytical determination of guanine in 2.0×10⁻² mol l⁻¹ Hepes buffer at pH 8.0 containing 1.6×10⁻⁵ mol l⁻¹of copper ions, was developed. An accumulation potential of −0.15 V during 270 s for the prior adsorption of guanine at the electrode surface was used. The response of the system was found to be linear in the range of guanine concentration from 6.62×10⁻⁸ to 1.32×10⁻⁷ mol l⁻¹ and the detection limit was 7.0×10⁻⁹ mol l⁻¹. The influence of DNA bases such as adenine, cytosine and thymine was also examined. Cyclic voltammetry was used to characterize the interfacial and redox mechanism.     

Preconcentration of lead,cadmium, copper and zinc in water at the pg g−1 level by non-boiling evaporation

An evaluation of the non-boiling evaporation technique for the preconcentration of Pb, Cd, Cu, Zn at the pg g^?1 level in water samples is presented. Various improvements were made to allow efficient control of contamination problems at these extremely low concentrations. They include the choice of FEP Teflon for the evaporation containers and the use of sophisticated cleaning, ageing and pre-conditionning procedures. Detailed calibration graphs were obtained down to the sub-pg g^?1 level by processing ultra-low concentration standards. This technique was then applied to the determination of these four metals in snow samples collected in Greenland and Antarctica.     

Polarographic and voltammetric behaviour of 2-hydroxy-3-tert-butyl-5-methylazobenzene

The polarographic and voltammetric behaviour of 2-hydroxy-3-tert-butyl-5-methylazobenzene has been studied in aqueous-ethanol mixtures of different pH using DCP and DPP polarographic and CV and SWV voltammetric methods. The reduction of the azo linkage takes place via two electrons at pH > 7, but four electrons at pH < 7 in aqueous-ethanol mixtures. It has been determined that there is a linear relationship between the current and concentration and the lowest detection limit has been found as 1 × 10^–7 M (for SWV).     

A very sensitive flow system for the direct determination of copper in natural waters based on spectrophotometric detection

A very sensitive flow injection method with spectrophotometric detection has been developed for the on-line determination of copper in natural waters. The method exhibits a limit of detection three times lower than the most sensitive direct spectrophotometric method previously described and then allows the direct and simple in situ determination of copper in most natural waters.The method was based on the measurement of the absorbance of the coloured complex formed by copper with the chromogenic reagent di-2-pyridyl ketone benzoylhydrazone (dPKBH) in an alkaline medium. This complex presents stoichiometry 1:2 (Cu:dPKBH), and exhibits maximum absorbance at 370 nm. The manifold used was very simple, and consisted of two channels. The first one contained the sample while the second one contained the colorimetric reagent (3.3×10⁻⁴ M dPKBH in 10% ethanol), in a 1.6×10⁻² M phosphate buffer solution at pH 8. The performance of the system was optimised by using both univariate and modified simplex methodologies. When modified simplex was used, the best signal was obtained for a sample injection volume of 529 μl, a reaction coil length of 1.29 m, and a reagent flow rate of 4.8 ml min⁻¹. Under optimum conditions, the response was linear up to 3 mg l⁻¹ copper, the equation of the straight line being y=0.314x+5.2×10⁻⁴ (r²=0.998). The method allowed a sampling frequency of 40 samples per hour and exhibited a precision of 2.11% (as R.S.D., n=11). The limit of detection was 4.6 μg l⁻¹ (calculated as 3s_b/m, where s_b is the standard deviation of the y-intercept and m represents the slope of the straight line), and was therefore more sensitive than all the direct continuous methods reported previously.The method was successfully applied to the analysis of real water samples, with an average relative error of 5.32%.     

Hydrogenation of cinnamaldehyde in liquid phase in the presence of copper supported catalysts

Copper supported on zirconia is rather selective to cinnamic alcohol. The addition of chromium to such catalysts appreciably increased the activity while preserving a significant cinnamic alcohol selectivity.     

Kinetics of the medium-temperature reduction of copper chromite with hydrogen

The static volumetric method was used to study the kinetics of copper chromite CuCr₂O₄ reduction with hydrogen at 50-80 kPa and 498, 523 and 580 K. The rate of copper chromite reduction is maximal initially and decreases monotonically with time. This observation suggests that the reduction is not a topochemical process. The apparent activation energy of the reaction equals initially 107 kJ/mole. The results obtained argue for a particular mechanism of the copper chromite reduction via redox substitution of hydrogen for copper in the chromite and, therefore, agree with earlier data obtained by various structural and adsorption methods. Specific features of the reduction mechanism are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.