African Journals Online
South African Journal of Chemistry

Volume 53, Issue 3, December 2000
Abstracts

On the Relative Stability of Tetraoxo-bisanthrenes related to the Radical Anions of Hypericin
Gutman, I.Markovic, Z.Engelbrecht, J.P.

Abstract: In recent experimental and theoretical studies of the photochemical proton transfer from hypericin, it was conjectured that the relative stability of the various hypericin radical anions parallels the relative stability of the corresponding tetraoxo derivatives of the parent hydrocarbon (bisanthrene). In order to test this hypothesis, we determined the energies and geometries of the respective tetraoxo-bisanthrenes by quantum-chemical methods and analyzed their dependence on molecular structure. The stability order of tetraoxo-bisanthrenes was indeed found to match that of the hypericin radical anions.

Extraction of Coal-tar Pitch by Supercritical Carbon Dioxide. Dependence of Chemical Composition of the Extracts on Temperature, Pressure and Extraction Time

Markovic, Z.Markovic, S.Engelbrecht, J.P.Visser, F.D.

Abstract: Several extractions of coal-tar pitch were performed using supercritical fluid carbon dioxide. The relationships between extraction yield during supercritical fluid extraction (SFE) and the variables temperature, pressure and extraction time were investigated. For qualitative and quantitative identification of organic compounds, gas chromatography and gas chromatography-mass spectrometry were used. Each SFE experiment was performed at a specific pressure and temperature; the maximum yield was obtained at 200 bar and 125 ºC. In general, the yields of all compounds studied increased with increasing temperature, and reached their maximum values at 125 ºC; above this temperature, yields remained almost constant. Higher yields were also obtained with greater retention times or higher pressures. Partial selectivity could be achieved by extracting at different constant temperatures and constant pressures.

Determination of Total Iron as Fe(II) in Multivitamins, Haematinics and Natural Waters using a Sequential Injection (SIA) System

Van Staden, J.F.Naidoo, E.B.

Abstract: The determination of total iron in pharmaceutical products and natural waters as Fe(II) using a sequential injection system was investigated. A cadmium reductor consisting of cadmium granules was used to reduce Fe (III) to Fe (II). The Fe (II) was then determined (by its reaction with 1,10 Phenanthroline) as a [Fe(phen)₃²⁺] complex at 515 nm with a UV/Vis spectrophotometer. The linear range of the system is between 1 and 60 mg/l with a detection limit of 0.18 mg/l. The proposed system is suitable for the determination of total iron as Fe(II) in pharmaceutical products and natural waters at a rate of 24 samples/hour with a relative standard deviation of less than 2.5%. Statistical comparison between the proposed sequential injection (SIA) system, certified values and the standard methods (Inductively Coupled Plasma {ICP} and UV/Vis spectrophotometry) revealed that there is no significant difference at the 95% confidence level.

Reporting Low-level Analytical Data

Horwitz, W.

Abstract: Low level measurements - those near, at, or below limits of measurements (specification, quantification, detection) - should be reported as they appear: positive, negative, or zero, as the best estimate of the measured characteristic, usually concentration. The long-term mean of such poor estimates will often provide a surprisingly good estimate of the "true value" from the operation of the law of large numbers. This manner of reporting data near the limits of measurement can be independent of, and adaptable to, any set of agreed-upon definitions and parameters for the concept, including its uncertainty. This proposal is intended to help to standardize the method of reporting low-level analytical data, not to legitimize the data or any conclusions or actions resulting from their use or interpretation.

Application of Artificial Neural Networks for Analysis of Highly Overlapped and Disturbed Differential Pulse Polarographic Peaks in the Region of Hydrogen Evolution

Cukrowska, E.Cukrowski, I.Havel, J.

Abstract: Multivariate calibration based on a suitable experimental design (ED) and soft modelling with artificial neural networks (ANNs) is proposed for quantitative analysis of highly overlapped and disturbed differential pulse polarographic (DPP) peaks that occur in the region of a hydrogen evolution. It is demonstrated that analysis of mixtures, even if some of the constituents undergo an irreversible reduction and the background current varies significantly with a composition of a sample, can be quantified with reasonable accuracy using a combination of ED and ANNs. Examples of DPP examination of Zn^II and Cr^III mixtures and/or simultaneous determination of metal ions and a strong acid concentration are presented. The possibility of an on-line monitoring is suggested. It is demonstrated that standard hard model based refinement procedures perform much worse than ANNs combined with ED and, in principle, proved to be unsuitable for the purpose.