An evaluation of voltammetric titration procedures for the determination of trace metal complexation in natural waters by use of computers simulation

The impact of random analytical errors on the determination of metal complexation parameters of natural waters by metal titration procedures based on cathodic stripping (CSV) or anodic stripping (ASV) voltammetry is investigated by means of computer simulation. The results indicate that random analytical errors are of overriding importance in establishing the range of ligand concentrations and conditional stability constants that can be accurately determined by these techniques. Simulations incorporating realistic estimates of random analytical error show that only stability constants lying within a relatively narrow range, typically three orders of magnitude, can be determined accurately by the ASV procedure. The CSV procedure suffers from the same limitations, but is potentially more flexible in that the available detection window can be moved (but not widened) by adjustments to the method. Both techniques are capable of accurately determining ligand concentrations provided that the corresponding stability constant, K′, is greater than a threshold value which corresponds to the lower end of the available detection window for the stability constant. Realistically attainable improvements in analytical precision did not greatly improve the performance of either technique. Two graphical treatments for the evaluation of metal complexation parameters from titration data are compared: the Scatchard and Van den Berg/Ruzic plots. Simulations indicate that at least for the single-ligand model of complexation, the Van den Berg/Ruzic method is superior. The importance of the simulation results with respect to determining metal complexation parameters in natural waters is discussed. This study illustrates the value of computer simulation when complex, time-consuming analytical techniques are applied and the need for rigorous analysis of errors in producing data of environmental relevance.     

Determination of chromium(III) and total chromium in marine waters

The development of an analytical technique is described which may be used to determine chromium, chromium(III) and chromium(VI) in estuarine and coastal waters. The method is based on selective micro-solvent extraction with subsequent GFAAS. The technique has been applied in a major North Sea estuary. The results obtained confirm that thermodynamic factors alone cannot be relied upon to describe the form of chromium in estuaries. Kinetic factors appear to have a strong influence over speciation and lead to the persistence of Cr(III) species in environments where Cr(VI) would be expected to be present.     

Accuracy of determination of trace metals in soil and sewage sludge

Errors in the analysis of liquid samples are often large enough to be important. For solid materials, additional potential complications arise in separating the determinand from the sample matrix and in controlling possible interferences from co-extracted species. These considerations have led to the development of a stepwise, co-ordinated approach by which a group of laboratories have achieved predefined standards of accuracy. Sample digestion has been examined. The analysis of digests has then been addressed; the control of analytical precision has been emphasised. Finally, a demonstration of overall comparability has been undertaken. The results are presented.     

Dediazoniations of arenediazonium ions part 25 Influence of Substituents on the Exchange of the Diazonio Group for External Molecular Nitrogen and on the N(α), N(β)-Rearrangement in the Diazonio Group

The N(α), N(β)-rearrangement of the two N-atoms which can be observed in solutions of [β-¹⁵N]-labelled p-substituted benzenediazonium ions follows dual substituent parameter treatments. The reaction yields a negative field and a positive resonance reaction constant (p_F =?3.35, p_R = 2.47). The magnitude of these constants is, within experimental error, the same as the respective reaction constants for solvolytic dediazoniation. The exchange of the diazonio group of ¹⁵N-labelled p-substituted benzenediazonium ion yields, however, field and resonance reaction constants which are close to zero. This result is attributed to cancellation of the reaction constants for the forward and reverse steps in the complex mechanism of the exchange reaction.     

Determination of chromium(III) and total chromium in marine waters

The development of an analytical technique is described which may be used to determine chromium, chromium(III) and chromium(VI) in estuarine and coastal waters. The method is based on selective micro-solvent extraction with subsequent GFAAS. The technique has been applied in a major North Sea estuary. The results obtained confirm that thermodynamic factors alone cannot be relied upon to describe the form of chromium in estuaries. Kinetic factors appear to have a strong influence over speciation and lead to the persistence of Cr(III) species in environments where Cr(VI) would be expected to be present.     

Control samples for the determination of pH in low ionic strength waters: consensus values from interlaboratory tests

The results of a series of interlaboratory tests using low ionic strength buffer solutions are reported. The test data and other background information are presented as the basis for more realistic quality control materials for pH determination in low ionic strength waters. The effect of atmospheric carbon dioxide on the pH of low ionic strength reference samples is discussed. Several control samples are recommended for routine use.