Y. M. Temerk

Reduction of azo-compounds—I. Polarographic behaviour of some 4-hydroxymonoazo compounds at the dropping mercury electrode

Abstract The polarographic behaviour of some monoazo compounds has been studied in aqueous solutions of varying pH. The reduction current in alkaline media is almost half that of the limiting value in acid solution, indicating a change in the mechanism of reduction. A general mechanism for the reduction process in alkaline and acid media is suggested, which includes a rate-determining step involving one electron. The different kinetic parameters for the electrode reaction support the suggested mechanism. The effect of substituents is clarified.

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.

Application of stripping voltammetry at a static mercury drop electrode for the determination of aluminium and iron in Portland cement

Abstract Adsorptive stripping voltammetry for the determination of Al(III) and Fe(III) in Portland cement has been employed. The complex of iron with acetate, and the chelate of aluminium with morin are adsorbed onto the static mercury drop electrode, and then the reduction current of the accumulated chelate is measured by scanning the potential in the cathodic direction. The adsorption and redox behaviour are explored by cyclic voltammetry. Optimum experimental conditions evaluated include: preconcentration potential, solution pH, scan mode and preconcentration time. Confirmation of the chelation is assessed using different methods of analysis on the solid complex viz. IR, NMR, Elemental Analysis and Thermal Gravimetric Analysis. The method has been applied to Portland cement and results are compared with standard samples to confirm the suitability of the proposed method.

Polarography of eosin and erythrosin in solutions of varying pH at the DME

Abstract The polarographic behaviour of eosin and erythrosin in buffer solutions of pH 4–11 is discussed. The observed waves are assigned to the reduction of furan and pyrone rings of the molecule of erythrosin at pH 4–11 and for eosin at pH ⩽7. The nature of the waves is also investigated. In alkaline media the electrode reaction is essentially diffusion controlled whereas at pH

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.

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.