M. E. Ahmed

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.

Adsorptive stripping voltammetric behaviour of hypoxanthine

The effective adsorption of hypoxanthine at a hanging mercury drop electrode (HMDE) has been studied by using ac and cyclic voltammetry (CV). It was found that the complexation of hypoxanthine molecules enhances the stacking interactions which might facilitate the formation of a perpendicularly stacked layer of Cu(II)-hypoxanthine complex on the electrode surface. A rapid and sensitive controlled adsorptive accumulation of copper complexes with hypoxanthine on the HMDE provides the basis for the direct stripping measurement of this compound on the subnanomolar concentration level. The height of the chelate peak is shown to be proportional to the hypoxanthine concentration. Experimental and instrumental parameters for the quantitative determination are optimized. Under optimum conditions the calibration curve was linear and the detection limit was 1 × 10−8M (1.36 μgl−1), where in the presence of Cu(II) the method is developed and the detection limit is lowered to 4 × 10−10M (0.05 μgl−1). The interferences of several organic compounds are described.

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.

Adsorption and association of xanthine in absence and presence of some divalent metal ions at the mercury/solution interface

Abstract The adsorption and interfacial orientations of xanthine were studied by out-of-phase ac voltammetry at a hanging mercury drop electrode. The adsorption equilibrium and its attainment have been investigated as a function of various parameters such as pH, adsorption potential, temperature, adsorption time, the nature of anions of the supporting electrolyte and the bulk concentration of xanthine. The changes in the stacking interactions of xanthine molecules in the presence of Cd(II), Cu(II), Co(II), Ni(II), Ca(II) and Mg(II) were studied by ac voltammetry as changes in “pits” or “wells” on double layer capacitance curves due to adsorption and association of M(II)-xanthine complexes on the charged interface. The results indicate that the complexation of xanthine molecules enhances the stacking interactions and hence would be expected to facilitate the formation of perpendicularly stacked layer of M(II)-xanthine complex on the electrode surface. The time dependence of the electrode impedance indicates that the formation of a compact film in absence and presence of these metal ions controlled by a fixed number of nuclei and the data were analysed according to the Avrami equation. The adsorption parameters of xanthine have been computed at different pH values in the absence and the increased presence of metal ions.

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.

The interfacial behaviour of guanosine mono-, di- and triphosphate in solutions of varying pH at charged interfaces

Abstract A systematic study of the adsorption and interfacial behaviour of guanosine mono-, di- and triphosphate (5′-GMP, 5′-GDP and 5′-GTP) at the h.m.d.e., has been carried out in different buffer solutions by phase-sensitive a.c. voltammetry. At low bulk concentrations, molecules of guanosine phosphate adsorbed at the maximum adsorption potential in a dilute adsorption layer are oriented planar to the electrode surface, where the interaction of π electrons with the interface favours adsorption. At bulk concentrations above a threshold value, the stacking interactions between vertically oriented molecules lead to association and formation of a compact layer. The adsorption can be described quantitatively for both types of adsorption layers by single- and double-step Frumkin isotherms, respectively. The resulting adsorption parameters are evaluated and conclusions on the respective interfacial behaviour, orientations and interactions of these substances are discussed. The influence of the phosphate group on the surface reorientation of guanosine at the charged interface was also considered.

Adsorption and association of 6-thiopurine and 6-thiopurine riboside at charged interfaces

Abstract A systematic study on the adsorption and association of 6-thiopurine (6-TP) and 6-thiopurine riboside (6-TPR) has been carried out at various pH values and the adsorption parameters were determined quantitatively. The adsorption was followed by out-of-phase alternating current voltammetry and cyclic voltammetry at a hanging mercury drop electrode. A comparative study was undertaken on the adsorption and association of the investigated thiopurines and the similar type of nucleic acid components containing purine bases. The base-containing thio group enhances stacking interaction and facilitates formation of the perpendicularly stacked layer on the electrode surface.

Comparative studies of the interfacial behaviour of some nucleic acid bases with and without thio group

Abstract A comparative study on the adsorption and association of nucleic acid bases with and without thio group such as guanine (Gua), guanosine (Guo), 6-thioguanine (6SGua), 6-thioguanosine (6SGuo) and 8-thioguanosine (8SGuo) was conducted using phase sensitive a.c. voltammetry. The reduction of the adsorbed species at the charged interface was studied using cyclic voltammetry and controlled potential coulometry. The influence of the thio group on the stacking interaction between vertically oriented molecules is discussed. The limiting value of the surface concentration Γm and the average surface area Sm per adsorbed molecule indicates the rather more compact interfacial structure for thioguanine and thioguasine than for guanine and guanosine. The extent of intermolecular and intramolecular association is most pronounced for the nucleic acid bases containing thio group.