Hermes Carrero

Electrochemically active films of negatively charged molecules, surfactant and synthetic clays

Abstract The electrochemical behavior of anionic molecules such as hexacyanoferrate and vitamin B 12 hexacarboxilate intercalated into clay–surfactant films is investigated. The results indicate that composites of surfactant cetyltrymethylammonium bromide (CTAB) and synthetic clay facilitate the permeation of the charged molecules through the film. By heating the modified electrode at 120°C for 1 h, the charged molecules, depending on hydrophobic and coulombic forces, are able to accommodate inside the film. Coulombic forces retain the anionic molecules in the clay–CTAB composite. The films are stable with good electroactivity.

Organic compounds and heavy metals in the atmosphere of the city of Caracas, Venezuela — I: Atmospheric particles

Aliphatic and aromatic hydrocarbons, fatty acids and heavy metals (Pb, Cu, Ni, Zn, Fe and Cd) were analyzed in atmospheric particulate matter in the city of Caracas, Venezuela. Samples were taken from 6 stations within the metropolitan area of Caracas, characterized as industrial (one), urban (two), suburban (two) and rural (one). In addition, the concentration and composition of the organic compounds was monitored over a 5 month period at an urban site to determine seasonal and temporal variabilities.In general terms, the concentrations of pollutants decreased from industrial and urban sites to suburban to rural. A similar trend was observed for preliminary toxicity tests carried out on the particulate extracts. The concentration levels of most of the pollutants were high for the industrial and urban sites, and comparable with those of other major cities worldwide.

THE GENERALIZED STANDARD ADDITION METHOD IN A CLOSED-LOOP FLOW SYSTEM

A new approach using flow-injection configuration is proposed as an alternative to apply the generalized standard addition method. The flow system contains a calibrated closed loop in which a spectrophotometric flow-cell is placed. The sample is entrapped into the closed loop by a suitable valve and it is allowed repetitive passages through the flow-cell; the addition of the standard solutions is made through an additional sample valve with a relatively small sample volume. After the addition, transient signals are obtained due to the repetitive passage of the added standard through the detector until it is completely dispersed into the sample by a mixing chamber. The proposed method resembles the manual, conventional procedure; a very small volume of standard solution is added to a relatively large volume of sample in such a way that the matrix of the latter is not significantly modified. Mathematical derivations that relate the concentration of the sample with that of the standard and the number of additions are performed with the analysis of the pH indicator methylorange. The analysis of iron in milk shows that good reproducibility is obtained using only one standard and one addition.

MECHANISTIC ASPECTS OF PHOTOCATALYTIC ACTIVITY OF METALLOPORPHYRIN-TITANIUM MIXTURES IN MICROEMULSIONS

Photocatalytic activity of titanium citrate and its water-in-oil microemulsions mix with zinc (II) tetraphenylporphyrin have been studied by means of adding 4-chlorophenol and following its degradation using artificial ultraviolet and visible light. 4-Chlorophenol is degraded in 71 and 86%, respectively, in 30 min with the generation of various intermediaries, whose possible formation mechanisms have been proposed based in the results of this research. The generation of benzene derivatives, different to those usually reported for this type of treatment, suggests the possibility of controlling the generation of intermediaries by simply changing the type of surfactant. In the case discussed here, we have derived a simple method for isomer generation of trimethylbenzene using a model organohalogenated compound as water contaminant and cetyltrimethylammonium bromide as surfactant.