Stripping voltammetry of copper and lead using gold electrodes modified with self-assembled monolayers

One problem associated with using bare solid metal electrodes, such as gold and platinum, in stripping analysis to determine heavy metal ions such as lead and copper ions in dilute solutions is that underpotential deposition (UPD) gives multiple stripping peaks in the analysis of mixtures. These peaks are often overlapped and cannot be conveniently used for analytical purposes. Bifunctional alkylthiols, such as 3-mercaptopropionic acid, with an ionizable group on the other terminal end of the thiol can form self-assembled monolayers (SAMs) on the surface of the gold electrode. It is shown that such an SAM-modified gold electrode minimizes the UPD effects for the stripping analysis of lead and copper. The anodic peak potential shifts and the peak shape changes, indicating that the SAM changes the deposition and stripping steps of these heavy metal ions. Thus, the sensitivity levels for both single species and mixtures can be significantly improved for the conventional solid electrodes. The mechanism of the deposition reaction at the SAM-modified gold electrodes is discussed. Received: 29 May 1997 / Accepted: 24 June 1997     

31P{1H}-n.m.r. as a tool for identification of ruthenium isomers containing PPh3 or 1,4-bis(diphenylphosphino)butane ligands. X-ray structures of the cis-{RuCl2(PPh3)2 [4,4′-(-X)2-2,2′-bipy]} complexes [X=-H, -Me, -SMe and (-Cl, -Me)]

A series of cis-{RuCl₂(PPh₃)₂[4,4-(X)₂-2,2-bipy]} [cis-chlorines; X=-H, -Me, -SMe, and (-Cl,-Me)] complexes have had their structures determined by single crystal X-ray diffraction. The geometry of these complexes, also determined in CH₂Cl₂ solution by ³¹P{¹H}-n.m.r. spectroscopy, showed that the chemical shifts for the phosphorus atoms are slightly dependent on the pKa of the 4,4-(-X)₂-2,2-bipy ligands.     

Chain dynamic parameters for some acrylic polymers

The dielectric relaxation data of Ishida et al. on a number of acrylic polymers are represented in terms of the relaxation function proposed by Havriliak and Negami using the multi-response techniques developed by Havriliak and Watts. Two of the parameters of this function are interpreted in terms of a temperature dependent distribution of relaxation times. In this method of interpretation the breadth of the distribution function is temperature-dependent while the skewness is not. The temperature dependence of the breadth of the distribution function is similar for most of these acrylic polymers.The parameters of the relaxation function are also interpreted in terms of Mansfield's model which represents intra- and inter-molecular interactions in terms of springs and dash pots. Briefly, increasing the side chain length for the methacrylate series increases the inter-molecular relaxation time which may be due to an increase in the entropy of activation for the orientation process. The difference between the one acrylate in this study and the four methacrylates of the series is a reduction in the intra-molecular relaxation time, apparently due to the lack of the alpha methyl group.     

Unexpected conversion of a polycyclic thiophene to a macrocyclic anhydride

Oxygenation of 2,5,9,12-tetra(tert-butyl)diacenaphtho[1,2-b:1′,2′-d]-thiophene (1, C₄₀H₄₄S) by peracids gave the cyclic sulfonic ester 4 (2,7,10,13-tetra(tert-butyl)diacenaphtho[1,2-c:1′,2′-e]oxathiin 5,5-dioxide, C₄₀H₄₄O₃S) which, when heated in nitrobenzene, is converted into a complex, macrocyclic anhydride 3 (C₈₀H₈₈O₃), which is derived from two molecules of 4. Further investigation found a likely intermediate in this reaction, 4,4′,7,7′-tetra(tert-butyl)-1,1′-biacenaphthylenylidene-2,2′-dione (5, C₄₀H₄₄O₂), apparently formed from 4 by additional oxidation. Anhydride 3 plausibly arises by Diels-Alder reaction of 4 and 5 followed by several ring fragmentations. The structures of 3, 4, and 5 were unambiguously established by X-ray crystallography.     

Optimization of acid hydrolysis of sugarcane bagasse and investigations on its fermentability for the production of xylitol by Candida guilliermondii

The dilute-acid hydrolysis of sugarcane bagasse was optimized using a statistical experimental design resulting in hydrolysates containing 57.25 g/L of xylose, which were fermented with a high inoculum concentration (10 g/L of the yeast Candida guilliermondii IM/UFRJ 50088). The addition of urea reduced the time of conversion (t _C) to 75 h (without nitrogen source addition t _C>127 h), and, consequently, improving the rates of xylitol bioproduction. Fermentator experiments, using the optimized conditions, resulted in enhanced conversion rates, reducing t _C to 30 h. The stability of the yeast in the hydrolysate was also verified in a 480-h cultivation.     

Absolute rate constants for some hydrogen atom abstraction reactions by a primary fluoroalkyl radical in water

A combination of laser flash photolysis and competitive kinetic methods have been used to measure the absolute bimolecular rate constants for hydrogen atom abstraction in water from a variety of organic substrates including alcohols, ethers, and carboxylic acids by the perfluoroalkyl radical, *CF(2)CF(2)OCF(2)CF(2)SO(3)(-) Na(+). Comparison, where possible, of these rate constants with those previously measured for analogous reactions in the non-polar organic solvent, 1,3-bis(trifluoromethyl)benzene (J. Am. Chem. Soc, 1999, 121, 7335) show that the alcohols react 2-5 times more rapidly in the water solvent and that the ethers react at the same rate in both solvents. A transition state for hydrogen abstraction that is more reminiscent of an "intimate ion pair" than a "solvent separated ion pair" is invoked to explain these modest solvent effects.     

Influence of surfactant addition on the stability of concentrated alumina dispersions in water

The goal of this study was to assess the effects of surfactant addition on the stability and viscosity of concentrated alumina dispersions. The stabilizing effects of several candidate surfactants were investigated for concentrated dispersions of two different pseudoboehmite aluminas at pH 4 and 7. The stabilities of concentrated alumina dispersions treated by pH adjustment alone and by pH adjustment combined with surfactant addition were compared to assess the degree to which the surfactant enhanced stability. The initial rate of mass removal from a sedimenting alumina dispersion was used as a measure of stability.

The anionic surfactants Surfine WNT-A and DOWFAX 3B2 were identified as effective in enhancing the stability of concentrated alumina dispersions. The optimal doses of these surfactants for stabilizing 15% by weight VERSAL™ 250 alumina dispersions at pH 4 were determined to be about 4.6 × 10⁻⁵ mol g⁻¹ for both surfactants. On the basis of the initial rate of mass removal, surfactant-stabilized 15 wt.% suspensions were found to be approximately 2.5 and 10.6 times more stable than similar dispersions stabilized electrostatically by pH adjustment alone. These more stable dispersions exhibited lower viscosities than observed for the alumina dispersions not subjected to surfactant addition. The results indicate that the stability of concentrated alumina dispersions can be enhanced by anionic surfactant addition, and that such surfactants may therefore help to control the rheology of concentrated dispersions of alumina in water.     

KINETIC STUDIES OF THE TWO LIGHT REACTIONS IN PHOTOSYNTHESIS

Abstract— The decay kinetics of the photo-induced absorbance changes in red and green algae are very sensitive to the wavelength of the actinic light. A four to tenfold increase in half-decay time is noted in going from short wavelength (550–650 mμ) to long wavelength (> 700 mμ) excitation. The slow decay rates produced by long wavelength light can be enhanced with a steady background of short wavelength light. A relationship between initial decay rates and O₂ evolution rates is described. This relationship allows a direct correspondence between these spectroscopic studies and the 'red-drop' and 'enhancement' experiments of Emerson.     

Ab initio potential energy surface for Ar 3 +

The potential energy surface (PES) of linear Ar ₃ ⁺ is calculated at the MP4/6-31G* level including all single, double, triple and quadruple excitations. The results show that the PES of the linear Ar ₃ ⁺ has a very flat valley along the asymmetric stretching vibration normal mode, ν₃. A higher level quadratic configuration interaction calculation including single, double and triple substitutions QCISD (T) along this flat valley suggests that an asymmetric geometry energy minimum reported earlier based on MP2 [1] is due to symmetry breaking in UHF. The global minimum of the PES is found to be for the symmetric geometry atR _ab =R _bc =2.66±0.01 Å, which is in good agreement with the MRD-CI calculation [2] and expectations from our earlier photodissociation experiments [3]. The calculational results are compared with other theoretical calculations, and are discussed in the context of the photodissociation and dynamics of dissociation experiments conducted on Ar ₃ ⁺ .