Recovery and pre-concentration of gold onto poly((N-(Hydroxymethyl)methacrylamide)–1-allyl-2-thiourea) hydrogels synthesized by gamma radiation

Poly((N-(Hydroxymethyl)methacrylamide)–1-allyl-2-thiourea) hydrogels, Poly(NHMMA–ATU), were synthesized by gamma radiation using ⁶⁰Co γ source at different irradiation doses and different ATU content in the irradiated monomer mixture. The swellability of the synthesized hydrogels was changed by irradiation doses and by the content of ATU in the irradiated mixture. These hydrogels were used for the specific gold recovery and pre-concentration from single gold ion solutions and from different natural samples. It has been observed that gold adsorption capacity onto the hydrogels was high at low pHs and reached maximum value at pH = 0.5. Adsorption capacity of the hydrogels for gold at pH = 0.5 was found to be about 698 mg g^− 1 dry hydrogels. Adsorption equilibrium time of gold ions onto the hydrogels was found to be very short and also these hydrogels were showed extremely high selectivity to the gold ions in acidic media when the concentration of the other metal ions were extremely higher than gold. Because of the high specificity of these hydrogels to gold beside the other metal ions at low pHs, all matrix effects were easily eliminated adsorbing gold onto the hydrogels and desorbing into 3 M HCl solution containing 0.8 M thiourea. These hydrogels were found to be highly open pore size. This property of the hydrogels make them attractive due to high adsorption capacity of gold ions on/in the hydrogels penetrating inside and react to all functional groups in the interior surface of the hydrogels.     

Pyrolysis–GC–MS used to study the thermal degradation of polymers containing chlorine III. Kinetics and mechanisms of polychloroprene pyrolysis. Selected ion current plots used to evaluate rate constants for the evolution of HCl and other degradation products

The thermal degradation of polychloroprene [poly(2-chlorobutadiene)/Neoprene] has been studied at 387°C by pyrolysis–GC–MS techniques. First, the overall rate of production of volatile products was measured utilising total ion current (TIC) curves obtained from sequence pyrolysis experiments in which the products were not chromatographically resolved. This overall rate decreases abruptly during the course of the pyrolysis. The early stage (Stage 1) was calculated to have a rate constant k₁=0.065 s⁻¹, and in Stage 2 this falls to k₂=0.013 s⁻¹. Kinetic analyses of these results show that these stages are the consequence of fast and slow independent parallel degradation processes, in which the fast process is limited in some way so that after about 40 s it is virtually complete, and then only the slow process remains. The kinetics also indicate that the limiting yield (number fraction) of molecules produced by the fast process is approx. 60% of the total molecules produced by the end of the pyrolysis. Selected Ion Current (SIC) curves corresponding to the mass numbers in the MS cracking pattern of hydrogen chloride (i.e. m/z=35, 36, 37, and 38) were then extracted from the above data, and these confirmed that HCl is a pyrolysis product. Despite the fact that the HCl is unresolved in the sequence pyrograms, its yields can be calculated from the areas of its SIC peaks, and on this basis the first order rate constant for HCl evolution (k_HCl) was calculated as 0.095 s⁻¹ at 387°C. This high value for HCl formation shows that this process must be contributing significantly to the fast overall rate. SIC curves for a wide range of other possible degradation products were examined, and these revealed that monomeric and linear dimeric-type products from polychloroprene are also produced with fast rates (k_mon=0.090 s⁻¹ and k_dim=0.113 s⁻¹ respectively). The formation of such products with specific rates which are identical (within experimental uncertainty) with that for HCl evolution, suggests that the loss of HCl molecules is associated with the loss of monomer and dimer molecules. The cumulative yield curves for HCl, monomeric, and linear dimeric products indicated that their total limiting yield was approx. 50% of the total products, which is close to the estimate of 60% for fast products based on the kinetic analysis of the overall yield. The overall rate constant for the formation of products by slow processes was then estimated from data obtained by subtracting the yields for the fast products from the total yield. (These slow products include a wide range of chlorohydrocarbons and some hydrocarbons.) The value was found to be k_slow=0.025 s⁻¹, which may be compared with the overall Phase 2 (slow) value k₂=0.013 s⁻¹, above. New overall rate data were then obtained from sequential pyrolyses performed in similar pyrolysis–GC equipment, but using an FID detector. On kinetic analysis a value for the rate constant was obtained (0.016 s⁻¹), which compares well with the low value for Stage 2 of the overall rate as measured by MS. However, the FID results also showed that there are also some products formed by a fast rate. Although the magnitude of this could not be measured, it was possible to estimate from the FID results that the total yield of products formed by the fast rate was ca. 10%, which is of the same order of magnitude as the yield of monomeric and dimeric products detected by MS analysis. The differences between the FID kinetic results and those from MS can be understood if it is borne in mind that the FID detector is expected to be insensitive to HCl, an important fast product, but it can detect monomer and dimeric products. Resolved pyrolysis chromatograms allowed approximate values of the ratios of the yields of the different oligomeric species to be estimated. This information allowed the calculation of the ratio of the total number of molecules of HCl evolved to the total number of units of chloroprene in the sample. This ratio was found to be much greater than the fraction of 1,2 chloroprene units in the polymer, showing that although all of these sites could be yielding HCl, an important part of the dehydochlorination is occurring by an alternative mechanism. One possibility is a process involving a concerted, or in some way associated, loss of a hydrogen chloride and a monomer molecule. On the basis of the above evidence it is concluded that the fast process could be a depropagation reaction in which there are intramolecular transfer reactions producing dimer during the unzip. Associated assumptions are that there are a limited number of initiation sites for this depropagation process, and that about half of the monomer molecules release HCl at their moment of unzip. Possible evidence for this was obtained from the observation of substantial yields of dehydrochlorinated monomer. It is also proposed that the slow process could involve random scissions, which would be expected to produce a distribution of oligomers, including some monomer. Superimposed on all of this are secondary reactions, possible examples of which are the combination of monomer with itself to form cyclic dimers, and the combination of various molecules with HCl.     

Interactions of HCl with 1-Butanol and Group Additivity Analysis for the HCl–Alcohol Interactions

An electromotive force (emf) method was used to determine thermodynamic parameters of interaction of HCl with normal 1-butanol (n-BuOH) in water between 5 and 45°C. A comparison of pair interaction parameters for HCl–n-BuOH with those for HCl–t-BuOH (tert butyl alcohol) showed that there is an obvious difference between them. This was interpreted in terms of steric structure of their carbon chains. The group Gibbs free energy parameters ( and ) for the HCl–alcohol–water systems were derived according to the Savage–Wood group additivity principle. These group parameters can be used to evaluate pair interaction parameters of HCl with alcohol molecules.     

Oxidation of a carbon/glass reinforced cyanate ester composite

The decomposition/oxidation behaviour of a carbon/glass fibre reinforced thermosetting pre-impregnated material based on polymerized aromatic cyanate ester compounds (commercial name GURIT-PN900-C582-43) has been investigated. Experiments were carried out for thermally thick samples exposed in a preheated reaction chamber (temperatures between 650–950 K) and a continuous gas flow (inlet velocity between 0.6 and 4.2 cm/s). Temperature measurements indicate that, in air, the process takes place under a mixed kinetic-diffusive control with three main dynamic stages corresponding to oxidative decomposition, heterogeneous ignition delay and oxidation. The oxidative decomposition stage is always much faster than the total duration of the ignition delay and oxidation stages (factors of 60–85). SEM pictures show dramatic changes in the material structure with amounts of solid residue, at the conclusion of the conversion process, varying from about 62% (resin char, glass and carbon fibres) to 29% (essentially the glass fibres) (against 85–75% for thermal degradation). Also, with the aid of thermogravimetric curves measured in air for the composite material and a charred residue, a three-step oxidation mechanism is proposed consisting of the oxidative decomposition of the resin, the oxidation of the resin char and the oxidation of the carbon fibres with estimated activation energies of 95, 136 and 185 kJ/mol, respectively.     

Study of Am and Pu behavior in alkaline media

The redox chemical behavior of americium and plutonium species in alkaline media has been studied, mainly two dis/reproportionation reactions: 2 Pu(VI) = Pu(V) + Pu(VII) and Am(III) + Am(V) = 2Am(IV). It was found that Am(VI) is readily reduced by the simple action of water, and that radiolysis has no effect. This is a marked departure from the behavior of Am in other media, where alpha particle self-irradiation generates redox active species that affect the Am oxidation state. An Am(VII) compound has been synthesized. All results are interesting for the treatment of alkaline high-level radioactive wastes. To cite this article: A.P. Nikonov et al., C. R. Chimie 7 (2004).

Résumé

Étude du comportement de Am et Pu en milieu alcalin. Le comportement redox de Am et Pu a été étudié en milieu alcalin, notamment les dismutations 2 Pu(VI) = Pu(V) + Pu(VII) et 2 Am(IV) = Am(III) + Am(V). Am(VI) est rapidement réduit par l’eau et la radiolyse n’a pas d’effet, contrairement à ce qui se produit dans d’autres milieux. Des indications ont été obtenues sur un composé de Am(VII). Les résultats obtenus sont intéressants pour le traitement des déchets nucléaires encore entreposés en milieu basique. Pour citer cet article : A.P. Nikonov et al., C. R. Chimie 7 (2004).     

Photooxidative and pyrolytic degradation of methyl methacrylate-alkyl acrylate copolymers

Different compositions of poly(methyl methacrylate-co-methyl acrylate) (PMMAMA), poly(methyl methacrylate-co-ethyl acrylate) (PMMAEA) and poly(methyl methacrylate-co-butyl acrylate) (PMMABA) copolymers were synthesized and characterized. The photocatalytic oxidative degradation of all these copolymers were studied in presence of two different catalysts namely Degussa P-25 and combustion synthesized titania using azobis-iso-butyronitrile and benzoyl peroxide as oxidizers. Gel permeation chromatography (GPC) was used to determine the molecular weight distribution of the samples as a function of time. The GPC chromatogram indicated that the photocatalytic oxidative degradation of all these copolymers proceeds by both random and chain end scission. Continuous distribution kinetics was used to develop a model for photocatalytic oxidative degradation considering both random and specific end scission. The degradation rate coefficients were determined by fitting the experimental data with the model. The degradation rate coefficients of the copolymers decreased with increase in the percentage of alkyl acrylate in the copolymer. This indicates that the photocatalytic oxidative stability of the copolymers increased with increasing percentage of alkyl acrylate. From the degradation rate coefficients, it was observed that the photocatalytic oxidative stability follows the order PMMABA > PMMAEA > PMMAMA. The thermal degradation of the copolymers was studied by using thermogravimetric analysis (TGA). The normalized weight loss and differential fractional weight loss profiles indicated that the thermal stability of the copolymer increases with an increase in the percentage of alkyl acrylate and the thermal stability of poly(methyl methacrylate-co-alkyl acrylate)s follows the order PMMAMA > PMMAEA > PMMABA. The observed contrast in the order of photostability and thermal stability of the copolymers was attributed to different mechanisms involved for the scission of polymer chain and formation of different products in both the processes.     

Contribution of Na counterions to H-AOT&Na-AOT-based W/O microemulsion formation using aqueous NaOH solutions as estimated by pyranine absorbance

The objective of this study is to estimate the contribution of Na⁺ as a counterion in the formation of H-AOT&Na-AOT-based W/O microemulsions using aqueous NaOH solution by pyranine absorbance measurements. A mixture of an aqueous NaOH solution containing pyranine/H-AOT&Na-AOT/isooctane was emulsified by changing the mixing ratio of Na-AOT (X_Na-AOT = 0–1) and the mole fraction of NaOH (X_NaOH = [NaOH]/the AOT⁻ concentration in the water pool = 0–1). The phase behavior of the emulsified mixture was evaluated from the absorbance of pyranine at the isosbestic point and by visual observations. W/O microelumsions are formed at the mid-range of X_Na-AOT, whereas the emulsified mixture separates into two phases at lower X_Na-AOT and higher X_Na-AOT. The two phase boundaries shift toward lower X_Na-AOT as with increasing X_NaOH. The phase behavior depends on the degree of screening of electrostatic repulsions between the polar headgroups of AOT⁻ by the Na⁺ counterion. Interestingly, nano-sized W/O microemulsions are formed without phase separation using a highly concentrated NaOH aqueous solution when the Na-AOT mixing ratio is appropriately adjusted. The phase behavior was plotted as X_NaOH versus X_Na-AOT, and the correlation equations for the two phase boundaries were obtained by fitting the points. The contribution of the Na⁺ counterion from NaOH to W/O microemulsion formation was estimated by the correlation equations. The absorbance of pyranine and the size of W/O microemulsions, as measured by DLS, were plotted as a function of X_Na⁺=(x[Na⁺   from   NaOH]+[Na⁺   from   Na-AOT])/[AOT⁻], in which x is the ratio contributed by NaOH. The absorbance and size correlates well with X_Na⁺, indicating that X_Na⁺ is a meaningful parameter for quantitatively estimating phase behavior and size variation.     

Development of soft X-ray multilayer laminar-type plane gratings and varied-line-spacing spherical grating for flat-field spectrograph in the 1–8 keV region

W/C and Co/SiO₂ multilayer laminar-type holographic plane gratings (groove density 1/σ = 1200 lines/mm) in the 1–8 keV region are developed. For the Co/SiO₂ grating the diffraction efficiencies of 0.41 and 0.47 at 4 and 6 keV, respectively, and for the W/C grating 0.38 at 8 keV are observed. Taking advantage of the outstanding high diffraction efficiencies into practical soft X-ray spectrographs a Mo/SiO₂ multilayer varied-line-spacing (VLS) laminar-type spherical grating (1/σ = 2400 lines/mm) is also developed for use with a flat field spectrograph in the region of 1.7 keV. For the Mo/SiO₂ multilayer grating the diffraction efficiencies of 0.05–0.20 at 0.9–1.8 keV are observed. The FWHMs of the measured line profiles of Hf-Mα₁(1644.6 eV), Si-Kα₁(1740.0 eV), and W-Mα₁ (1775.4 eV) are 13.7 eV, 8.0 eV, and 8.7 eV, respectively.     

Voltammetric determination of quercetin at a multi-walled carbon nanotubes paste electrode

Quercetin can effectively accumulate at multi-walled carbon nanotubes-paraffin oil paste electrodes (CNTPE) and cause a sensitive anodic peak at around 0.32 V (vs. SCE) in a 0.10 M phosphate buffer solution (pH = 4.0). Under optimized conditions, the anodic peak current is linear to quercetin concentration in the ranges of 2.0 × 10^− 9−1.0 × 10^− 7 M and 1.0 × 10^− 7−2.0 × 10^− 5 M, and the regression equations are i_p (μA) = 0.0017 + 0.928c (μM, r = 0.999) and i_p (μA) = 0.183 + 0.0731c (μM, r = 0.995), respectively. This paste electrode can be regenerated by repetitively cycling in a blank solution for about 2 min. A 1.0 × 10^− 6 M quercetin solution is measured for 10 times using the same electrode regenerated after every determination, and the relative standard deviation of the peak current is 1.7%. The method has been applied to the determination of quercetin in hydrolysate product of rutin and the recovery is 99.2–102.6%. In comparison with graphite paste electrode, carbon nanotubes-nujol paste electrode and carbon nanotubes casting film modified glassy carbon electrode, the CNTPE gives higher ratio of signal to background current and better defined voltammetric peak.     

Ion-molecular interactions in the HCl—isopropanol system

Multiple Attenuated Total Reflectance (MATR) IR spectra of solutions of HCl in isopropanol, containing 0 to 43 mol. % HCl, were studied in the 900–4000 cm^–1 range. The addition of HCl to PrⁱOH yields proton disolvates with strong symmetrical H-bonds. At high concentrations of HCl (C _HCl ⁰ > 5.7 mol L^–1, /C⁰ _HCl < 2), when the number of alcohol molecules is not enough to form disolvates with all of the protons present in the solution, (Cl...H...Cl)^– ions are formed, in addition to (C₃H₇OH)₂H⁺. The spectra of these ions have been assigned.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1753–1756, September, 1995.This work was carried out with financial support of the Russian Foundation for Basic Research (Project No. 93-03-18356).     

Occurrence of ochratoxin A in Turkish wines

A total of 95 wine samples including 34 white, 10 rosé and 51 red wines originating from four different Turkish areas were analysed for ochratoxin A (OTA). An analytical method based on immunoaffinity column (IAC) for clean-up and high performance liquid chromatography with fluorescence detection (HPLC-FD) was used to determine OTA in wines. The limit of detection (LOD) was estimated as 0.006 ng ml^− 1 for white wine and 0.010 ng ml^− 1 for rosé and red wines. The limit of quantification (LOQ) was estimated as 0.020 ng ml^− 1 in white wine and 0.030 ng ml^− 1 in rosé and red wines. Recovery experiments were carried out with spiked samples in the range 0.1–1 ng ml^− 1 of OTA. The average OTA recoveries from spiked white wine samples varied from 79.43% to 85.07%; while the mean recoveries for rosé and red wine samples were in the range of 77.48–83.96% and 76.61–83.55%, respectively. OTA was detected in 82 (86%) wine samples at levels of < 0.006–0.815 ng ml^− 1, which were below the maximum allowable limit established by the European Community. The mean OTA concentration in red wines was slightly higher than in white and rosé wines. Furthermore, our data indicate that the geographic region of origin has strong influence on OTA level for white, rosé and red wines: wines originating from Thrace (n = 44, mean = 0.158 ng ml^− 1) and Aegean (n = 28, mean = 0.060 ng ml^− 1) regions of Turkey were more contaminated with OTA compared with wines originating from central (n = 15, mean = 0.027 ng ml⁻¹) and east Anatolia (n = 8, mean = 0.027 ng ml^− 1) areas. This study showed that the occurrence of OTA in Turkish wines is high, but at levels that probably leads to a non-significant human exposure to OTA by consumption of wines.     

Enhancement in the stability of alginate gels prepared with mixed solution of divalent ions using a diffusion through dialysis tube (DTDT) approach

In this work, calcium alginate hydrogels have been prepared by a new technique, called ‘diffusion through dialysis tube’ (DTDT) which involved transfer of crosslinking ions into alginate solution via a dialysis tube with a molecular weight cutoff of 8 k Da. The various hydrogels, prepared with varying degree of crosslinking, were characterized by FTIR, XRD and TG analysis. We also investigated their dynamic water absorbency in phosphate buffer saline (PBS) of pH 7.4 and the data were interpreted in terms of various kinetic models. The equilibrium water uptake data was used to determine network parameters of hydrogels. Finally, the stability and water absorbency of these hydrogels were enhanced to a desired level by using crosslinker solutions, composed of Ba⁺⁺/Ca ⁺⁺and Zn⁺⁺ / Ca⁺⁺ ions.     

High solubility and optical transparency of novel polyimides containing 3,3′,5,5′-tetramethyl pendant groups and 4-tert-butyltoluene moiety

A series of novel polyimides (3a–d) were prepared from 3,3′,5,5′-tetramethyl-4,4′-diaminodiphenyl-4”-tert-butyltoluene (1) with four aromatic dianhydrides via a one-step high-temperature polycondensation procedure. The obtained polyimides showed excellent solubility, with the dissolvability at a concentration of 10 wt% in most amide polar solvents and chlorinated solvents. Their films were nearly colorless and exhibited high optical transparency, with the UV cutoff wavelength in the range of 322–350 nm and the wavelength of 80% transparency in the range of 395–414 nm. They also showed low dielectric constant (2.72–2.91 at 1 MHz) and low water absorptions (0.37–0.62%). Moreover, these polyimides possessed high glass transition temperatures (T_g) (above 321 °C) and good thermal stability with 10% weight loss temperatures in the range of 526–547 °C in nitrogen atmosphere. In comparison with the analogous polyimides non-containing 3,3′,5,5′ -tetramethyl pendant groups, the resultant polyimides 3a–d showed better solubility, higher optical transparency and lower dielectric constant.     

Synthesis of 3,5-ditert-butyl-4-hydroxybenzoates and their thermal antioxidation behavior for polypropylene

Butyl 3,5-ditert-butyl-4-hydroxybenzoate (BB), octyl 3,5-ditert-butyl-4-hydroxybenzoate (OB), and dodecyl 3,5-ditert-butyl-4-hydroxybenzoate (DB) were synthesized and characterized, and their performance as antioxidant for polypropylene (PP) were also investigated. The thermooxidative stability of PP with different antioxidants were assessed by the measurements of long-term accelerated aging, differential scanning calorimetery, and thermogravimetric analysis. The results showed that 3,5-ditert-butyl-4-hydroxybenzoates could effectively inhibit the oxidation degradation of PP. And it was also found that the antioxidant efficiency of BB, OB, and DB for PP was in the order of DB > OB > BB.     

Hydrolytic polycondensation of vinyltrimethoxysilane and formation of vinylpolysiloxane films

Hydrolytic polycondensation of vinyltrimethoxysilane (VTM) was investigated in detail to prepare vinylpolysiloxanes (VPS) having spinnability and tough gel films. VTM was hydrolyzed in various molar ratios r₁ (H₂O/VTM) = 0.5–1.6 (HCl/VTM = 0.105, EtOH/VTM = 1.44) at 70°C under a N₂ stream with stirring at 150 rpm to give vinylpolysiloxanes with =510–2100 ( =1.1–10.4). The controlled hydrolysis in r₁ = 1.40–1.64 formed VPSs having the various degrees of spinnability at N₂ flow rates of 100–500 ml/min. Transparent films of thickness 0.1–0.05 mm and tensile strength 5–16 MPa were prepared when a 20 wt% acetonemethanol (1:1, V/V) solution of VPS (r₁=1.64) was cast at 80°C for several ten days. The reaction was followed by ²⁹Si-NMR spectroscopy to investigate the distribution of unit structures T¹, T² and T³ in VPS depending on r₁.     

Studies of the hydrolytic stability of poly(urethane-urea) elastomers synthesized from oligocarbonate diols

Polyurethane and poly(urethane-urea) elastomers synthesized from oligocarbonate diols are characterized by very good mechanical properties, biocompatibility and excellent resistance to both oxidation and hydrolysis and therefore are widely used in medical applications. In this paper the results of studies on hydrolytic stability of poly(urethane-urea) elastomers (PURC) obtained by moisture-curing of corresponding urethane prepolymers synthesized from isophorone diisocyanate (IPDI) and four different oligocarbonate diols (OCD) are presented. OCD were synthesized from dimethyl carbonate and 1,6-hexanediol, from cyclic ethylene carbonate and 1,6-hexanediol as well as from trimethylene carbonate. The changes of the sample weight, mechanical properties and surface properties after immersion in a standard phosphate buffer solution (pH = 7.4) for up to 3 months at 70 °C were monitored. It was shown that neither sample weight nor mechanical properties changed significantly for PURC obtained from OCDs synthesized from 1,6-hexanediol and dimethyl carbonate or ethylene carbonate what confirms good resistance to hydrolysis of those PURC. Also SEM studies of those samples before and after immersion did not reveal any surface degradation effects. However, PURC sample obtained from OCD synthesized from trimethylene carbonate showed significant changes in mechanical properties and distinct change of appearance and surface erosion after 3 months immersion. The initial decrease and later increase of stress at break observed for PURC samples during immersion, was explained by the reaction of residual NCO groups present in PURC with water leading to molecular weight increase which proceeded during immersion period in parallel to hydrolysis of carbonate bond.     

Kinetik der Diazotierung des Anilins in methanolischer Bromwasserstoff- und Chlorwasserstofflösung, 2. Mitt.

The kinetics of the diazotization of aniline in 0.003n to 0.4n methanolic HBr-and HCl-solution, resp., were determined (HBr at 25 and 15°C, HCl at 25, 15, –10, –20, and –30°C).It was found that the nitrosation reaction is a preceeding advance-back-reaction.The velocity coefficients of the nitrosation from bromide (at 15 and 25°C) and from chloride (at 25, 15, –10, –20, and –30°C) were determined. The decomposition of I (splitting off a proton) is the rate determining reaction. The free enthalpies of activation for the nitrosation reaction above bromide and chloride at the said temperatures are calculated (table 3).     

Synthesis of amberlite XAD-2-PC resin for preconcentration and determination of trace elements in food samples by flame atomic absorption spectrometry

A new chelating sorbent has been developed using Amberlite XAD-2 resin anchored with pyrocatechol through –N=C– group. This sorbent, characterised by elemental analysis and infrared (IR) spectra, was used as packing for the minicolumn in an on-line system preconcentration system for cadmium, cobalt, copper and nickel determination. Metal ions were sorbed in the minicolumn, from which it could be eluted directly to the nebulizer–burner system of the flame atomic absorption spectrometer (FAAS). Elution of all metals from minicolumn can be made with 0.50 mol L^− 1 HCl or HNO₃. The enrichment factors obtained were 16 (Cd), 24 (Co), 15 (Cu) and 19 (Ni), for 60 s preconcentration time, and 39 (Cd), 69 (Co), 36 (Cu) and 41 (Ni), if used 180 s preconcentration time. Under the optimum conditions, the proposed procedure allowed the determination of cadmium, cobalt, copper and nickel with detection limits of 0.31, 0.32, 0.39 and 1.64 μg L^− 1, respectively, when used preconcentration periods of 180 s. The accuracy of the developed procedure was sufficient and evaluated by the analysis of the certified reference materials NIST 1515 apple leaves and NIST 1570a spinach leaves. The method was applied to the analysis of food samples (spinach, black tea and rice flour).     

Use of a graphite–polyurethane composite electrode for electroanalytical determination of indole-3-acetic acid in soil samples

A new electroanalytical methodology was developed for the quantification of the phytohormone indole-3-acetic acid (IAA), using a graphite–polyurethane composite electrode (GPU) and the square wave voltammetry (SWV), in 0.1 mol L^− 1 phosphoric acid solution (pH 1.6). Analytical curves were constructed under optimized conditions (f = 100 s^− 1, a = 50 mV, E_i = 5 mV) and the reached detection and quantification limits were 26 μg L^− 1 and 0.2 mg L^− 1, respectively. The developed methodology is simple and accurate for the routine determination of IAA. In order to verify the application of the electroanalytical methodology in fortified soil samples without previous treatment, an IAA assay was performed without serious interferences of the soil constituents.     

Effects of pH on dynamics and rheology during association and gelation via the Ugi reaction of aqueous alginate

Effects of solution pH on the physical properties of a semidilute aqueous alginate solution without cross-linker agent and during gelation via the Ugi multicomponent condensation reaction at a fixed cross-linker concentration have been investigated. Both rheology and turbidity results on the alginate solution without cross-linker revealed enhanced associations at low pH. In the course of the cross-linker reaction, the time of gelation is shortest at pH = 3.5 and at pH values above 3.8 no gel is formed but only a viscosification of the solution is observed. The turbidity during the cross-linking reaction rises as the pH increases from 3.5 to 5. Furthermore, the initial change of the turbidity in the course of the cross-linking process is more pronounced at higher pH. The dynamic light scattering (DLS) results of the reaction mixture at pH = 4.0 (ergodic features at this condition) show that the chain relaxation is slowed down as the reaction proceeds. The effect of pH on the kinetics of the Ugi reaction is discussed.