Showing a rheological effect of gelation on epoxy-amine systems

The effect of gelation was studied on the rheological curves, during hardening of a resin subjected to sinusoidal shearing between parallel plates. The epoxy-amine system was chosen for the moderate value of its maximum glass transition temperature T_{g, ∞} which means that it is possible to avoid vitrification by operating at T > T_{g, ∞}. The shape of the variation curve of the storage modulus G' does not depend on temperature; G' obeys a single percolation law as a function of the reduced variable t/t_gel. On the other hand, gelation leads to a fall in the growth rate of the loss modulus G” after the gel point. This effect increases with temperature and can become considerable at very high temperatures: in this case a minimum of G” between the gel point and the rubbery plateau can be observed. We have verified that this effect is indeed a rheological effect of gelation as it begins practically at the gel point determined on the storage modulus curves. Several qualitative conclusions are drawn, and the nature of this effect is discussed.     

pHh-dependent permselectivity of the fibrinogen-modified carbon paste electrode

The modification of carbon paste matrices with fibrinogen is reported. The effect of the pH of the solution on the CV peak currents of positively or negatively charged redox analytes was examined at the fibrinogen-modified carbon paste electrode. In the presence of the coating, pH-dependent selectivity in electrochemical detection of charged species was demonstrated depending on the sign of the supported charge. Above the isoelectric pH attributed to the immobilized protein (5.5), the current response of anionic redox probes [Fe(CN)/Fe(CN)] was impeded while the response was almost totally restored below this pH. Opposite trends were observed with the Ru(NH₃)/Ru(NH₃) cationic redox analytes.     

Design and Calibration of a Large Open-Ended Coaxial Probe for the Measurement of the Dielectric Properties of Concrete

The subject of this paper is the design and calibration of an open-ended coaxial probe for the nondestructive measurement of the dielectric properties of concrete. Measurements are made between 100-900 MHz, frequencies which are often used in geophysics and civil engineering for ground penetrating radar inspection. The probe is calibrated using measurements on saline solutions in conjunction with three different mathematical techniques for comparative study. Measurements of mortar and concrete specimens having different water/cement ratios were made in order to observe the standard deviations due to their heterogeneous nature. Similar to the case of relatively homogeneous rock specimens (limestone and granite), the standard deviation for heterogeneous concrete samples do not exceed 5%. In addition, the effect of the concrete's porosity on its dielectric properties was clearly observed: measured permittivity between 4-4.5 at 900 MHz for porous concrete, and between 6.5-7.5 at 900 MHz for dense concrete.     

Screening of Fish Tissue for Methyl Mercury Using the Enzyme Invertase in a Solvent Interface

This paper describes a simple and rapid screening system for the extraction and determination of methyl mercury in tissue samples from fish. A novel clean-up procedure based on the use of two immiscible phases, an organic one containing methyl mercury and an aqueous one containing invertase, was developed. Methyl mercury was selectively extracted from the organic into the aqueous phase by its irreversible reaction with thiol groups of invertase, and the resulting inhibition of enzymatic activity served as a measure of methyl mercury concentration. Enzyme activity was measured with a spectrophotometric method using 3,5-dinitrosalicylic acid as a reagent. After parameter optimisation including pH, substrate concentration, and enzyme reaction time, it was possible to determine methyl mercury in the ppb range with this technique. Concentrations as low as 10ppb of methyl mercury in the extract, corresponding to 0.2ppm of methyl mercury in fish, were detected. The proposed procedure was successfully demonstrated as a simple screening method for methyl mercury in fish samples.     

Determination of traces of copper by anodic stripping voltammetry at a rotating carbon paste disk electrode modified with poly(1,8-diaminonaphtalene)

In the present work, carbon paste electrodes (CPE) modified with conducting polymers 1,8-diaminonaphtalene (DAN) was developed for the voltammetric determination of Cu(II). The modified electrode exhibited a significantly increased sensitivity and selectivity for Cu²⁺ compared with a bare carbon paste electrode. Copper(II) was preconcentrated at open circuit on the modified electrode. The measurements were carried out using a rotating disk electrode (RDE) as working electrode. The experimental parameters for the determination of Cu(II) were optimized. The Cu(II) ions were chemically deposited onto the surface of poly DAN-CPE in an acidic medium. Under the optimum conditions, a linear calibration graph was obtained in the concentration range of 0.1 to 250 ppb with a correlation coefficient of 0.9998. The relative standard deviation for 6 successive determinations was 4.7%. A study of interfering substances was also performed, and the method was applied to the direct determination of copper in real samples like tap water and orange juice samples. The article is published in the original.