M. M. Kamal

Electrochemistry of adsorbed flavin adenine dinucleotide in acidic solutions

Abstract The adsorption of flavin adenine dinucleotide (FAD) at a mercury electrode surface has been investigated in dilute FAD solutions from pH ca. 2–7. FAD is found to adsorb initially in an orientation in which the isoalloxazine and adenine moieties lie parallel to the electrode surface (Stage I) at all acidic pH values and potentials. At low coverage in Stage I, a persistent separation of the anodic and cathodic peaks is present, independent of sweep rate, perhaps indicative of the expected planar (oxidized) and nonplanar (reduced) conformations of the isoalloxazine ring system. As the surface coverage increases, this difference diminishes and a sudden reorientation occurs to a structure in which it is suggested that alternating isoalloxazine and adenine moieties are adsorbed perpendicularly to the electrode surface (Stage II). At pH ≲ 4.5 this structure is stable over a very wide potential range. Also, a second vertical highly condensed orientation is observed at positive potentials in these more acidic solutions. These observations may reflect the effects of adenine protonation and the consequent coulombic interactions between it and the charged electrode surface. A third FAD orientation is observed (Stage III) after some time in acidic solutions, possibly involving a bilayer or a multilayer adsorbed on the mercury electrode surface.

The effect of pH and the nature of the anions of the supporting electrolyte on the adsorption and the association of uracil and uridine at the charged interface

Abstract The effect of pH and the nature of the anions of the supporting electrolyte on the adsorption and association of uracil and uridine was studied by ac polarography and ac voltammetry. The adsorption equilibrium and its attainment have been investigated as a function of various parameters such as pH, adsorption potential Es, adsorption time ts and the bulk concentration of uracil and uridine. Moreover, the role of the anions of the supporting electrolyte in the formation of a condensed film of the adsorbed molecules has been elucidated. The stacking interactions between adsorbed molecules are affected by the nature of the anions of the supporting electrolyte and are hindered by their specific adsorption. Characteristic properties and adsorption parameters of dilute and compact layers of uracil and uridine were evaluated from the two-step Frumkin isotherm obtained and from the potential dependence of adsorption. The resulting adsorption parameters of uracil and uridine have been computed at various pHs and for various anions in the supporting electrolyte.

Adsorptive stripping voltammetric measurements of trace amounts of platinum(II) and ruthenium(III) in the presence of 1-(2-pyridylazo)-2-naphthol

An extremely sensitive stripping voltammetric procedure for low level measurements of platinum (II, IV) or ruthenium (III, IV) is reported. The method is based on the interfacial accumulation of the platinum (II) or ruthenium (III)-1-(2-pyridylazo)-2-naphthol complex on the surface of a hanging mercury drop electrode, followed by the reduction of the adsorbed complex during the cathodic scan. The peak potential was found to be −0.8 V vs. Ag/AgCl electrode and the reduction current of the adsorbed complex ions of platinum (II) or ruthenium (III) was measured by differential pulse cathodic stripping voltammetry. The optimum experimental conditions were: 1.5×10−7 mol/l of 1-(2-pyridylazo)-2-naphthol solution of pH 9.3, preconcentration potential of −0.2 V, accumulation time of 3 min and pulse amplitude of 50 mV with 4 mV s−1 scan rate in the presence of ethanol-water (30% v/v) — sodium sulphate (0.5 mol/l). Linear response up to 6.4 × 10−8 and 5.1 × 10−8 mol/l and a relative standard deviation (at 1.2×10−8 mol/l) of 2.4 and 1.6% (n=5) for platinum (II) and ruthenium (III) respectively were obtained. The detection limits of platinum and ruthenium were 3.2×10−10 and 4.1×10−10 mol/l, respectively. The electronic spectra of the Pt(II) — PAN and Ru(III) — PAN complexes were measured at pH 9.3 and the stoichiometric ratios of the complexes formed were obtained by the molar ratio method. The effects of some interfering ions on the proposed procedure were critically investigated. The method was found suitable for the sub-microdetermination of ruthenium (IV) and platinum (IV) after their reduction to ruthenium (III) and platinum (II) with sulphur dioxide in acid media. The applicability of the method for the analysis of binary mixtures of ruthenium (III) and (IV) or platinum (II) and (IV) has also been carried out successfully. The method is simple, rapid, precise, and promising for the determination of the tested metal ions at micro-molar concentration level.

The effect of pH on the adsorption and the association of cytidine, cytidine-5′-monophosphate and cytidine-5′-diphosphate at the charged interface☆

Abstract The effect of pH on the adsorption and association of cytidine, cytidine-5′-monophosphate and cytidine-5′-diphosphate was studied by phase-sensitive a.c. polarography. At pH ≤ 5.2 and at lower concentrations of N(3)-protonated cytidine or cytidine phosphate, a single-step Frumkin isotherm was obtained, whereas at elevated bulk concentration the formation of a compact film reflects a double-step Frumkin isotherm. In the neutral and slightly alkaline range (pHs 7.0, 8.0), the alternating stacking model, due to hydrophobic interactions of the bases of the substances investigated, decreases and consequently pit formation does not occur. The effect of pH on the adsorption parameters of cytidine and cytidine phosphate is discussed. The influence of the phosphate group on the extent of adsorption of cytidine is also considered.

Differential pulse polarographic determinations of iodate, bromate and periodate

ZusammenfassungOptimale Bedingungen zur Bestimmung dieser Ionen im Spurenbereich wurden ausgearbeitet (Pulsamplitude 100 mV, Scangeschwindigkeit 2 mV s−1, Tropfzeit 2 s). Die Nachweisgrenze für die drei Ionen liegt bei 2,4 · 10−6 M für pH 3. Bei pH 9,3 ergaben sich für Iodat und Periodat konzentrationsabhängige Peakhöhen bis herab zu 9,9 · 10−7 M. Da der Bromatpeak bei wesentlich negativerem Potential auftritt als der Iodat bzw. Periodatpeak konnten Iodat und Bromat bzw. Periodat und Bromat nebeneinander bestimmt werden.SummaryThe application of differential pulse polarography for quantitative trace determination of iodate, periodate and bromate has been investigated. Optimum conditions were found (pulse amplitude 100 mV, scan rate 2 mV s−1, drop time 2 s). The detection limit for IO3−, BrO3−and IO4−is ca. 2.4×10−6M at pH 3 whereas at pH 9.3 the reduction of IO3−and IO4−gave concentration sensitive peak heights down to 9.9 ×10−7M. A differential pulse polarographic method has been developed for the simultaneous determination of iodate-bromate or periodate-bromate.

Chelate adsorption for trace voltammetric measurements of 2-thiouracil and 4-thiouridine

A very sensitive electrochemical stripping method for trace measurements of 2-thiouracil and 4-thiouridine in presence of Cu(II) is described. The chelate of Cu(II) with 2-thiouracil and 4-thiouridine is adsorbed on the hanging mercury drop electrode, and the reduction current of the accumulated complex is measured by cathodic stripping voltammetry. The adsorption and redox behaviour are indicated by cyclic voltammetry. Optimum experimental conditions include a preconcentration potential of 0.0 V, solution of pH 7.2, adsorption time 5 min, pulse amplitude 100 mV, and a linear scan mode. The sharp chelate peak, associated with the effective interfacial accumulation, coupled with the flat baseline, facilitates measurements at the nanomolar and submicromolar concentration levels.

Adsorption stages and association of 1-methylguanosine and 7-methylguanosine in solutions of varying pH at charged interfaces

Abstract A systematic study on the adsorption and association of 1-methylguanosine and 7-methylguanosine has been undertaken at the mercury|solution interface. The characteristic properties and adsorption parameters of the dilute and compact adsorption layers of both compounds were evaluated from the double-step Frumkin isotherm obtained and from the potential dependence of adsorption. The adsorption parameters were computed at various pH values and at different temperatures. The results are compared with the similar type of adsorption of guanosine. Methylation of guanosine molecules in positions 1 and 7 greatly affected its adsorption behaviour and the stacking interactions of the perpendicularly oriented layer of adsorbed molecules on the electrode surface.

Cathodic adsorptive stripping voltammetry of 6-thiopurine in absence and presence of some metal ions

SummaryCathodic stripping voltammetry of an adsorbed 6-thiopurine at HMDE was investigated in solutions of varying pH. A rapid and sensitive differential pulse voltammetric method was selected for its trace determination. A method has also been developed for the determination of 6-thiopurine in presence of Cu(II), due to the strong adsorption of the Cu-6-thiopurine complex at the surface of the HMDE and subsequent reduction of the surface-bound complex. A detection limit of 9.9×10−9 mol/l was achieved in presence of Cu(II) and the slope of the straight line was seven times the slope in absence of Cu(II). Cathodic adsorptive stripping (CAS) voltammetry of 6-thiopurine in presence of Ni(II), Pb(II), Cd(II), and Fe(III) was also investigated. The influence of several operational parameters has been considered. Statistical analysis of the calibration curve data is included.

Adsorptive stripping voltammetric behavior of gold(III) at a hanging mercury drop electrode in the presence of 1-(2′-pyridylazo)−2-naphthol

Abstract The stripping voltammetric determination of gold(III) based on the adsorptive accumulation of the Au(III)—1-(2′-pyridylazo)−2-naphthol complex on a hanging mercury drop electrode is reported. The reduction current of the adsorbed gold complex ions is measured by differential-pulse cathodic stripping voltammetry. The peak potential is at −0.8 V vs. Ag/AgCl. The effects of various parameters (pH, ligand concentration, accumulation potential and collection time) on the response in the presence of ethanol-water (30%, v/v) and 0.5 M sodium sulphate are discussed. A linear response up to 5.0×10 −8 M and a relative standard deviation of 2.9% at 3.0×10 −8 M were obtained. The UV-visible spectrum of the complex formed was also measured. The applicability of the method to the determination of gold(I) and mixtures of gold(I) and (III) compounds was also successfully carried out. Possible interferences by trace metals on the proposed method were examined.

Adsorptive stripping analysis of thiocytosine at the static mercury electrode surface

SummaryThe cathodic adsorptive stripping method has been applied for the trace determination of thiocytosine (Tcyt) using differential pulse voltammetry at the HMDE. The strong interaction of Tcyt with the mercury electrode surface leads to the formation of an adsorbed film of Hg(II)-thiolate complex at the electrode surface. The formation of this film is followed by cyclic DC- and differential pulse voltammetry. It has been found that formation of such film depends on various parameters, e.g. pH, concentration, adsorption time and adsorption potential. The influence of these parameters on the shape and the peak height of the cathodic stripping peak during the negative scan of potential has been determined. The influences of anions (SO42−, Cl−, ClO4−, NO3−) and cations [Cu(II), Ni(II), Pb(II) and Cd(II)] on the method of analysis are considered. The calibration curve for the determination of Tcyt is linear and the detection limit is 1.49×10−8 mol/l, whereas in the presence of Cu(II) the method is developed and the detection limit for Tcyt determination is lowered to 4.99×10−9 mol/l. A statistical analysis of the calibration curve data is included.