Kinetics of selenium atomization in electrothermal atomization atomic absorption spectrometry (ETA-AAS). Part 3: Chemical interference of sulphate using palladium modifiers

The interference caused by sulphate (as the sodium salt) in the electrothermal atomization atomic absorption analysis of selenium was investigated for prereduced and unreduced palladium nitrate modifiers. Kinetic parameters of the selenium atomization were calculated for both types of modifier with varying amounts of sulphate added. Prereduced palladium was a better modifier since it tolerated higher amounts of interferent. For high levels of interferent, the kinetic parameters approached that of selenium without modifier. It was postulated that the interference was caused by the formation of palladium sulphate which reduces the number of active palladium sites available for selenium stabilization. The poorer performance of the unreduced modifier was explained in that the formation of stabile palladium sulphate hindered the reduction of Pd(II) to palladium metal which was needed for the selenium stabilization. Sulphate only interfered on the high temperature stabilization process; the low temperature stabilization, linked to the formation of a [Pd,Se,O] compound, was unaffected. The results support earlier literature reports that selenium loss occurs by covolatilization with the matrix and gives a reason why palladium modifiers are rendered useless by the sulphate interferent.     

Electrothermal atomization of palladium stabilized selenium in the presence of phosphate

The interference caused by phosphate (as Na₂HPO₄) in the electrothermal atomic absorption determination of selenium was investigated for reduced and unreduced palladium nitrate modifiers. An increase of the amount of phosphate in the sample was accompanied with increasing losses of selenium. Kinetic parameters of the selenium atomization were calculated for various amounts of phosphate interference. These results were compared with previous findings for palladium stabilized selenium and the sulfate interference observed for this system. The increasing chemical interference is due to phosphorous replacing the selenium bound by palladium. The phosphorus thus makes the palladium surface unavailable for the stabilization of selenium.     

Effects of interfering elements and chemical modifier on the activation energy of electrothermal atomization of selenium

In order to obtain additional insight into the mechanism of stabilization of selenium by palladium modification and to investigate the interference mechanism of Na, K, Mg and Ca on selenium atomization, the appearance temperature and the activation energy of atomization of selenium with or without the presence of the modifier and with or without the presence of the concomitant element, have been evaluated and discussed.     

Determination of kinetic parameters for atom formation at constant temperature in graphite furnace atomic absorption spectroscopy

A new method for the simultaneous determination of the kinetic order and activation energy for atom release under isothermal condition in a graphite furnace has been developed. Tungsten wire probe atomization was employed to examine the validity of the present method. By means of this model, the kinetic parameters for the atomization of Bi, Ge, Pb and Mn at constant temperatures were successfully determined. The values of the kinetic order and activation energy were found to be 0.67 ± 0.01 and 302 ± 8 kJ mol⁻¹ for Bi, 1.01 ± 0.08 and 109 ± 2 kJ mol⁻¹ for Ge, 0.46 ± 0.01 and 159 ± 2 kJ mol⁻¹ for Pb and 0.97 ± 0.03 and 372 ± 5 kJ mol⁻¹ for Mn, respectively. The atomization mechanism for these four elements from the tungsten probe surface was also discussed.     

Kinetics of indium atomization from different atomizer surfaces in electrothermal atomic absorption spectrometry (ETAAS)

The kinetic parameters of indium atomization in electrothermal atomic absorption spectrometry (ETAAS) have been determined by a newly proposed method. Effect of the atomizer surface and the palladium modifier on the kinetics of indium atomization has been investigated. The mechanisms of indium atomization seem to be identical for the pyrolytically coated graphite and the uncoated graphite tubes, i.e. the rate-limiting step for the atomization changes from a first order kinetics at lower temperatures into a nearly 1/3 order kinetics at higher temperatures, which may suggest that the analyte moves from a dispersed state to agglomates with increasing temperature. However, for the zirconium coated graphite tube, the atomization of indium is controlled by a single mechanism with the kinetic order of near 2/3 and the activation energy of 186 ± 13 kJ/mol. Relatively weak indium—zirconium carbide interactions and the release of indium from the sphere of molten indium metal on the zirconium coated surface are suggested. In the presence of palladium, a simple mechanism, i.e. the release of indium from the solid solution of the In and the Pd on the pyrolytically coated graphite surface, is proposed to account for the observed first order kinetics and the activation energy of 421 ± 27 kJ/mol.     

Kinetics and Mechanism of Electron Transfer Reactions: Oxidation of Crotyl Alcohol by Peroxomonosulfate in Aqueous Acidic Medium

The kinetics and mechanism of oxidation of crotyl alcohol by peroxomonosulfate has been studied, and the species of the peroxomonosulfate are discussed to find out the role of activated species. A plausible reaction mechanism is suggested, and a derived rate law corresponds to all experimental observations. The activation parameters such as energy and entropy of activation have been calculated as 37.21 ± 0.5 kJ mol⁻¹ and −148.91 ± 2.7 J K⁻¹ mol⁻¹, respectively, by employing the Eyring plot.     

Effect of vanadium on oxygen diffusivity in niobium

The effect on the diffusivity of oxygen of vanadium additions to niobium was investigated by a diffusion couple technique. The addition of vanadium to niobium results in an increase of the activation energy for oxygen diffusion from 107 kJ mol⁻¹ for oxygen in niobium to 176 ± 9 kJ mol⁻¹ for the Nb-0.5at.%V alloy and to 194 ± 9 kJ mol⁻¹ for the Nb-10at.%V alloy. This increase in the activation energy is attributed to the trapping of oxygen by vanadium atoms. Applying Kirchheim's trapping model and the results of internal friction measurements, trapping energies of about −64 and −49 kJ mol⁻¹ were obtained for the Nb-0.5at.%V and the Nb-10at.%V alloys respectively.     

Determination of propagation rate coefficients for an α‐substituted acrylic ester: Pulsed laser polymerization of dimethyl itaconate

Pulsed laser polymerization experiments have been performed on the bulk polymerization of dimethyl itaconate over the temperature range 20–50 °C. The activation energy and frequency factor were calculated as 24.9 kJ/mol⁻¹ and 2.15 × 10⁵ L/mol⁻¹s⁻¹, respectively. The activation energy is comparable with the methacrylate series of monomers. The frequency factor is relatively small and reflects steric hindrance in the transition state caused by the bulky 1,1, disubstitution in the monomer (and consequently the radical). The Mark–Houwink–Kuhn–Sakurada constants were also determined for poly(dimethyl itaconate) in tetrahydrofuran, these are reported as 46 × 10⁻⁵ dL/g (K) and 0.51 (α). The influence of penultimate units (γ‐substituents) on homopropagation reactions is discussed particularly for polymerizations leading to significant 1,3 interactions in the resultant polymer. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2192–2200, 2000     

Electrochemical behavior of 5-amino-1,10-phenanthroline and oxidative electropolymerization of tris[5-amino-1,10-phenanthroline]iron(II)

The electrochemical behavior of 5-amino-1,10-phenanthroline and tris[5-amino-1,10-phenanthroline]-iron(II) at carbon paste, glassy carbon, and platinum electrodes is reported. The iron complex undergoes electrochemically induced oxidative polymerization from acetonitrile solutions and the resulting polymers are very stable. Charge transport through the polymer films occurs with a charge transfer diffusion coefficient, D_ct, equal to 3.1 × 10⁻⁸ cm² s⁻¹ corresponding to an electron self-exchange rate of 5.2×10⁷M⁻¹ s⁻¹. The activation energy and the entropy change for the charge transfer diffusion process are (approximate values) 32.0 ± 0.12 kJ mol⁻¹ and −24.7 ± 0.4 J K⁻¹ mol⁻¹, respectively.     

Rate constants and activation energies for ozonolysis of isoprene methacrolein and methyl‐vinyl‐ketone in aqueous solution: Significance to the in‐cloud ozonation of isoprene

Rate constants and activation energies for the reactions of ozone with isoprene, methacrolein, and methyl‐vinyl‐ketone in aqueous solution have been determined at temperatures from 5 to 30°C, using the stopped‐flow‐technique and monitoring ozone decay. The rate constants at 25°C and the activation energies have been found to be 4.1 (±0.2) × 10⁵ M⁻¹ s⁻¹ and 19.9 (±0.5) kJ mol⁻¹ for isoprene, 2.4 (±0.1) × 10⁴ M⁻¹ s⁻¹ and 23.9 (±0.5) kJ mol⁻¹ for methacrolein, and 4.4 (±0.2) × 10⁴ M⁻¹ s⁻¹ and 18.0 (±0.5) kJ mol⁻¹ for methyl‐vinyl‐ketone. A UV spectrum of a transient intermediate with a lifetime of about 15 s formed during the ozonation of isoprene was obtained in the range 220 to 300 nm. It rises steadily toward 220 nm. It is suggested that the spectrum can be attributed to the two unsaturated Criegee‐intermediates (carbonyl oxides), which would conceivably be stabilized by resonance. Lifetime considerations indicate that the oxidation of isoprene and its first‐generation reaction products, methacrolein and methyl‐vinyl‐ketone, by ozone and OH in the aqueous phase of a cloud environment play only a minor role compared to homogeneous gas‐phase processing. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 182–190, 2001     

Matrix modification with silver for the electrothermal atomization of arsenic and selenium

Silver as a matrix modifier is shown to improve the carbon-rod atomization of both arsenic and selenium for atomic absorption spectrometry. Compared to nickel, the efficiency of silver is greater for arsenic and about the same for selenium. Silver fulfils two functions in its reaction, namely stabilization during the ashing stage and enhancement of absorbance in the final atomization.     

Nickel and strontium nitrates as modifiers for the determination of selenium in wine by Zeeman electrothermal atomic absorption spectrometry

A mixed matrix modifier of nickel and strontium nitrates was used as a chemical modifier for the determination of selenium in wines by Zeeman electrothermal atomic absorption spectrometry. Wine samples were heated on a boiling water bath with small amounts of nitric acid and hydrogen peroxide. For complete elimination of interference, especially from sulfates and phosphates, selenium is complexed with ammonium pyrolidinedithiocarbamate (APDTC), extracted into methyl isobutyl ketone (MIBK), and measured by ETAAS. The graphite furnace temperature program was optimized for both aqueous and organic solutions. Pyrolysis temperatures of 1300?°C and 800?°C were chosen for aqueous and organic solutions, respectively; 2700?°C and 2100?°C were used as optimum atomization temperatures for aqueous and organic solutions, respectively. The optimum modifier mass established is markedly lower than those presented in the literature. The platform atomization ensures pretreatment stabilization up to 1100?°C and 1600?°C, respectively, for organic and aqueous selenium solutions. The procedure was verified by the method of standard addition. The investigated wine samples originated from the different regions of the Republic of Macedonia. The selenium concentration varied from not detectable to 0.93 μg L^–1.     

Electrothermal atomization of silver in graphite furnaces. Part 2. Effects of copper,ascorbic acid and Triton X-100

The effects of copper, ascorbic acid and Triton X-100 on the atomization process of Ag are presented as a function of the initial mass of analyte and the heating rate of atomization. In general, a double pulse structure is detected, at a heating rate of 300 K s⁻¹, in the absorbance profile and its time derivative. This behavior shows up in the Arrhenius plots as two temperature regions of atomization. In the presence of Cu and Triton X-100, a low atomization energy E_a is obtained in the low temperature region and a high value of E_a, which approaches the heat of vaporization of Ag, is obtained in the high temperature region. However, in the presence of ascorbic acid, two low desorption energies are obtained in both temperature regions, suggesting a higher dispersion of particles owing to the presence of a higher number of active sites. At a heating rate of 700 K s⁻¹, a single atomization step with an atomization energy of 233 kJ mol⁻¹ and a first kinetic order of release is detected in the presence of Cu. However, in the presence of ascorbic acid and Triton X-100, two temperature regions of atomization are obtained from the Arrhenius plots, even though the absorbance profiles look continuous. In these cases, a mass dependent E_a is obtained in the low temperature region, and a low E_a with a first kinetic order of release is obtained in the high temperature region. In summary, the low value of E_a indicates vaporization from disperse particles, whereas the mass dependent, higher value of E_a indicates atomization from small clusters, the size and energy of which increase as the initial mass of Ag increases.The structure of the absorbance profiles and their time derivatives, and also the behavior of the Arrhenius plots, correlate well with those predicted by the two-precursor atomization model proposed in our previous work [1].     

The isomolecular exchange reaction Ga2S(g) + In2S(g) = 2 InGaS(g)

Equilibria involving the molecules Ga₂S(g), In₂S(g), and InGaS(g), by the reaction Ga₂S(g) + In₂S(g) = 12InGaS(g) were investigated between 1060–1350 K by the Knudsen-effusion, mass-spectrometric method. The reaction enthalpy at 298 K was calculated to be 0±1 kJ mol⁻¹. The enthalpy of formation of InGaS at 298 K and the enthalpy of atomization of InGaS at 298 K were calculated to be 80±18 kJ mol⁻¹ and 710±18 kJ mol⁻¹, respectively. The equilibrium constant and the enthalpy of reaction indicated that the three gaseous molecules have a bent triatomic structure in which S is a center atom and no bond between metals.     

Determination of selenium in biological tissue samples rich in phosphorus using electrothermal atomization with Zeeman-effect background correction and (NH4)3RhCl6+citric acid as a mixed chemical modifier

Spectral interferences from phosphorus on the determination of selenium in biological tissue materials were not observed when a Zeeman-effect background correction was used with rhodium as a chemical modifier. A suppression effect on the selenium signal resulted when the concentration of phosphorus present was greater than 1.0 mg ml⁻¹. Rhodium was found to be more effective than palladium in overcoming the phosphate interference. Analytical procedures for the direct determination of trace selenium in standard reference materials by graphite furnace atomic absorption spectrometry following sample dissolution in nitric acid and hydrogen peroxide using a microwave oven has been described. The results obtained agreed favourably with the certified values.     

Acetylation of Glycerol over Highly Stable and Active Sulfated Alumina Catalyst: Reaction Mechanism,Kinetic Modeling and Estimation of Kinetic Parameters

The Langmuir–Hinshelwood–Hougen–Watson (LHHW) kinetic model was developed for acetylation of glycerol over highly stable and active 2 M SO₄²⁻/γ‐Al₂O₃ catalyst. The apparent reaction rate constants were determined by numerically solving the differential rate equations using ode23 tool in MATLAB coupled with the genetic algorithm optimization technique. The estimated rate constants were used to obtain the activation energy and pre‐exponential factor by using the Arrhenius equation. The estimated activation energy for direct acetylation of glycerol to monoacetylglycerol and diacetylglycerol was 7.2 kJ mol⁻¹, for acetylation of monoacetylglycerol to diacetylglycerol was 37.1 kJ mol⁻¹, and for acetylation of diacetylglycerol to triacetylglycerol was 26.6 kJ mol⁻¹, respectively.     

Study of ammonium molybdate to minimize the phosphate interference in the selenium determination by electrothermal atomic absorption spectrometry with deuterium background correction

The use of ammonium molybdate to minimize the phosphate interference when measuring selenium by electrothermal atomic absorption spectrometry (ETAAS) with deuterium background correction was evaluated. Ammonium molybdate did not produce a selenium thermal stabilization; however, the presence of ammonium molybdate decreased the phosphate interference. The study was carried out with mussel acid digests and mussel slurries. Pd–Mg(NO₃)₂ was used as a chemical modifier at optimum concentrations of 300 and 250 mg l⁻¹, respectively, yielding optimum pyrolysis and atomization temperatures of 1200 and 2100 °C, respectively. A yellow solid (ammonium molybdophosphate) was obtained when adding ammonium molybdate to mussel acid digest solutions. This precipitate can be removed after centrifugation prior to ETAAS determination. Additionally, studies on the sampling of the solid ammonium molybdophosphate (AMP) together with the liquid phase, as a slurry, were also developed. The volatilization of the solid AMP was not reached at temperatures lower than 2500 °C. By this way, phosphate, as AMP, is not present in the vapor phase at the atomization temperature (2100 °C), yielding a reduction of the spectral interference by phosphate. The proposed method was validated analyzing three reference materials of marine origin (DORM-1, DOLT-1 and TORT-1). Good agreement with the certified selenium contents was reached for all cases.     

Gold Nanoparticles Formation via Au(III) Complex Ions Reduction with l‐Ascorbic Acid

In this paper, the kinetics and mechanism of gold nanoparticles formation during the redox reaction between [AuCl₄]− complex and l ‐ascorbic acid under different conditions were described. It was also shown that reagent concentration, chloride ions, and pH influence kinetics of nucleation and growth. To establish rate constants of these stages, the model of Finke and Watzky was applied. From Arrhenius and Eyring dependencies, the values of activation energy (22.5 kJ mol⁻¹ for the nucleation step and 30.3 kJ mol⁻¹ for the growth step), entropy (about −228 J K⁻¹ mol⁻¹ for the nucleation step and −128 J K⁻¹ mol⁻¹ for the growth step), and enthalpy (19.8 kJ mol⁻¹ for nucleation and 27.8 kJ mol⁻¹ for particles growth) were determined. It was also shown that the disproporationation reaction had influence on the rate of nanoparticles formation and may have impact on final particles morphology.     

Adsorption energy of the As atom on the Pd(111) surface according to the density functional theory data

Density functional theory in the plane wave basis set is used to study As adsorption on the Pd(111) surface in order to determine the action of a palladium chemical modifier used in electrothermal atomic absorption spectrometry (ETAAS). The calculated heat of desorption of the arsenic atom is 435 kJ/mole, which corresponds to the activation energy of arsenic atomization of 439 kJ/mole (in the range of high temperatures T > 1828 K), obtained by ETAAS. Based on the calculated data, the action of the palladium modifier for the determination of As is assumed to be controlled by the process of chemisorption.