Journal of the Iranian Chemical Society
Volume:5 Issue: 4, Dec 2008

This contribution describes the practical and computational aspects relevant for the quantitative analysis of chemical equilibria. First a systematic nomenclature based on stability constants is introduced. Next, practical aspects of potentiometric pH titrations and spectrophotometric titrations are discussed. The description of the computational aspects includes the Newton-Raphson algorithm that allows the computation of all species concentrations for a given model, set of formation constants and total component concentrations. The second computational part introduces the Newton-Gauss algorithm for non-linear data fitting. Three practical examples illustrate all the above. They include the spectrophotometric investigations of the interaction of Bi(III) with Cl-, of Cu(II) with EDTA and the potentiometric investigation of the Zn(II) ethylenediamine system. All Matlab softwares for the data generation and analysis are available from the authors.

The kinetics of the permanganic oxidation process of glycine, L-alanine and L-leucine in strong acid media were investigated using a spectrophotometric technique. Conclusive evidence has proven that the autocatalytic activity of Mn(II) in these reactions in strong acidic media is analogous to that of weak acid media, but in the former, Mn(II) ions should acquire a critical concentration for them to show autocatalytic characteristics. This critical concentration depends on the nature of the amino acid used. Considering the delayed autocatalytic behavior of Mn(II) ions, we herein present the rate equations and mechanisms satisfying observations for both catalytic and noncatalytic routes. The correspondence of the pseudo-order rate constants of the catalytic and noncatalytic pathways to Eyring law verify both the critical concentration as well as the delayed autocatalytic behavior concepts. In general, the onset of delayed behavior can be attributed to the concentration ratio of Mn(II) to amino acid which can be of a certain value for any particular amino acid.

The H-point standard addition method (HPSAM) was applied to the simultaneous determination of zinc(II) and cobalt(II). This method is based on the difference in the absorbance of methylthymol blue complexes of Zn(II) and Co(II) at pH 6 buffered solution in different wavelength pairs. The results showed that Zn(II) and Co(II) can be determined simultaneously with concentration ratios of 20:1 and 1:7.5. Under working conditions, the proposed method was successfully applied to the simultaneous determination of zinc and cobalt in synthetic and real samples.

A method for capillary electrophoretic enantiomeric separation of a racemic clenbuterol has been established with hydroxypropyl-β-cyclodextrin as the chiral selector. General equations and data analysis are presented to relate mobility to the equilibrium constants in simple binding equilibria and used to determine binding constants and thermodynamic parameters for host-guest complexation of clenbuterol enantiomers with hydroxypropyl-β-cyclodextrin as a selector. The effects of β-cyclodextrin type and concentration, buffer type, concentration and pH, as well as separation voltage and capillary temperature were investigated in detail. A maximal resolution of 6.78 was obtained. The binding constants of the host-guest complex of clenbuterol enantiomers with hydroxypropyl-β-cyclodextrin, KR-CD and KS-CD are 22.50 and 43.09 l mol-1, respectively.

In this study, the electropolymerization of 3-methylthiophene (3-MT) was preformed in the presence of a catalytic amount 1-(2-pyrrolyl)-2-(2-thienyl) ethylene (PTE) by cyclic voltammetry in acetonitrile as a solvent and lithium perchlorate as the electrolyte on a glassy carbon (GC) electrode. First, PTE was synthesized via the Wittig reaction. The addition of a catalytic amount of PTE during the electropolymerization of 3-MT changes the cyclic voltammograms such that the analysis of cyclic voltammograms of poly(3-MT) shows a considerable increase in the electroactivity and redoxability. Furthermore, the presence of PTE during the electropolymerization of 3-MT increases the polymerization rate. The CV measurement of the electron transfer ferro/ferricyanide redox system using different modified GC electrodes shows that the rate of charge transfer for poly(3-MT) in the presence of PTE is greater than that of poly(3-MT) alone. The conductivity of the obtained polymers was determined by electrochemical impedance spectroscopy (EIS) in 3.5% NaCl (w/v) solutions. With the application of Zview(II) software to the EIS, we estimate the parameters of the proposed equivalent circuit, based on a physical model for the electrochemical behavior of coatings on the GC electrode, to be 15739 ohm cm2 for the charge transfer resistance (Rct) for poly(3-MT) alone and 9700 ohm cm2 for poly(3-MT) in the presence of PTE. Thus, the film of poly(3-MT) formed in the presence of PTE is more conductive.

We report a first-principle study of electrical transport and switching behavior in a single molecular conductor consisting of a dithiole-benzene (DTB) sandwiched between two Au(100) electrodes. Ab initio total energy calculations reveal DTB molecules on a gold surface, contacted by a monoatomic gold scanning tunneling microscope (STM) tip to have two classes of low energy conformations with differing symmetries. Lateral motion of the tip or excitation of the molecule cause it to change from one conformation class to the other and to switch between a strongly and a weakly conducting state. Thus, surprisingly, despite their apparent simplicity, these Au-DTB-Au nanowires are shown to be electrically bi-stable switches, the smallest two-terminal molecular switches to date. The projected density of states (PDOS) and transmission coefficients are analyzed, and it suggests that the variation of the coupling between the molecule and the electrodes with external bias leads to switching behavior.

A new mercury(II) complex [Hg(Hpyterpy)(SCN)2]2(MeSO4)2 was prepared from the reaction of 4''-(4-pyridyl)-2,2'':6'',2''-terpyridine (pyterpy), as a polypyridyl ligand, with mercury(II) thiocyanate. The compound was characterized by elementalanalysis, IR, 1H NMR and 13C NMR spectroscopy and its structure was determined by X-ray single-crystal diffraction. Thethermal stability of compound was studied by thermogravimetric (TG) and differential thermal analyses (DTA).

Herein we describe the synthesis and characterization of compounds having the formulae R2SnL2 and R3SnL, where R = Me, n-Bu, Ph and n-Oct and L = 2-[N-(2,4,6-tribromophenylamido]propanoic acid. All the complexes have been characterized by various spectroscopic methods (IR and 1H, 13C, 119Sn NMR), elemental analysis, mass spectrometry and physical data. These compounds were also screened for their biological activity and found some encouraging results.

12-Tungstophosphate-based organic/inorganic hybrid, [HNMPA]3[O40PW12].0.5NMPA.2H2O (NMPA = N-methylpropion-amide) was synthesized and characterized by IR, UV-Vis, 1H, 13C and 31P NMR spectroscopies, elemental analysis, TGA, DTA and single crystal X-ray determination. Full decomposition of the hybrid leads to WO3 powder was studied by IR, SEM and XRD.

Amorphous zirconium phosphate (ZP), an inorganic ion exchange material of tetravalent metal acid (tma) salt, is synthesized by the sol-gel method and characterized by elemental analysis (ICP-AES), thermal analysis (TGA, DSC), FT-IR and X-ray diffraction studies. The resistivity of the material to acids, bases and organic solvents is assessed. The sorption behavior of the dyes acriflavin (AF) and brilliant green (BG) toward ZP was studied at 313, 323 and 333 K and the kinetic and thermodynamic parameters evaluated. Adsorption isotherms [Langmuir and Fruendlich], breakthrough capacity and elution behavior of these dyes are also studied. The sorption affinity of dyes towards ZP is BG > AF.

In this study, we use dipping and spinning methods to coat glass slides with sol-gel ZnO thin films, composed of zinc acetate dihydrate, monoethanolamine (MEA), de-ionized water and isopropanol. The effect of the annealing temperature on the structural morphology and optical properties of these films is investigated. These ZnO films were preheated at 275 °C for 10 min and annealed either at 350, 450 or 550 °C for 60 min. As-deposited films, formed by amorphous zinc oxide-acetate submicron particles, are transformed into a highly-oriented ZnO after thermal treatment. The surface morphology, phase structure and optical properties of the thin films were investigated by scanning electron microscopy, X-ray diffraction (XRD) and optical transmittance. Both techniques produced nanostructured ZnO thin films with well-defined orientation. The annealed films were transparent in the visible range with an absorption edge at about 375 nm and a transmittance of ca 85-90% with an average diameter of 40 nm. XRD results show the film was composed of polycrystalline wurtzite, with a preferential c-axis orientation of (002) and a single sharp XRD peak at 34.40, corresponding to the hexagonal ZnO. The grain size is increased by the annealing temperature. Both coating techniques create sol-gel ZnO films with the potential for application as transparent electrodes in optic and electronic devices.

Charge-transfer complexation of iodine with a new benzo-substituted macrocyclic diamide 5,6,7,8,9,10-hexahydro-2H-1,13,4,7,10-benzodioxatriazacyclopentadecine-3,11(4H,12H)-dione (L) with iodine was studied spectrophotometrically in chloroform, dichloromethane and their 1:1 (v/v) mixture. The observed time dependence of the charge-transfer band and subsequent formation of I3- ion are related to the slow formation of the initially formed 1:1 L.I2 outer complex to an inner electron donor-acceptor (EDA) complex, followed by fast reaction of the inner complex with iodine to form a triiodide ion, as follows: L + I2 ->L.I2 (outer complex), fast L.I2 (outer complex)-> (L.I+)I- (inner complex), slow(L.I+)I- (inner complex) + I2 -> (L.I+)I3-, fastThe pseudo-first-order rate constants for the transformation process were evaluated in different solvent systems. The stability constants of the resulting EDAr complexes were also evaluated and the solvent effect on their stability is discussed. The resulting complexes were isolated and characterized by FTIR and 1H NMR spectroscopy.

Aza-Michael addition of sulfonamides to α±,β-unsaturated esters efficiently proceeds in the presence of catalytic amount of sodium hydroxide (NaOH) and tetrabutylammonium bromide (TBAB) as a phase-transfer catalyst (PTC) under microwave irradiation to afford N,N-dialkyl sulfonamides as biologically interesting compounds in good to excellent yields and short reaction times.

The kinetics of oxidation of glucose, galactose, fructose, maltose and sucrose by alkaline permanganate anion has been studied. The reactions studied spectrophotometrically over a wide range of experimental conditions show that the rate of the reactions is enhanced by increase in pH, ionic strength, and temperature as well as the reactant concentrations. The mechanism has been proposed to proceed via the formation of enediol intermediate complexes and the order of reactivities of the sugars is fructose > glucose ≈ galactose > maltose > sucrose. The activation parameters were evaluated and lend further support to the proposed mechanism.

Kinetics of the oxidation of tris(2,2′-bipyridine)iron(II) sulfate by ceric sulfate was spectrophotometrically studied in an aqueous sulfuric acid medium. Different methods, including isolation, integration and half-life, were employed to determine the reaction order. The redox reaction was found to be first-order with respect to the reductant, tris(2,2′-bipyridine)iron(II) sulfate, and the oxidant, ceric sulfate. Complex kinetics was observed with an increase in the initial concentration of the oxidant. The influence of the dielectric constant, [H+] and [SO42-] on the rate was also investigated. The increase in the dielectric constant and H+ ion concentration of the medium retard the rate, while an increase in the SO42- ion concentration first accelerates the rate, and then retards the reaction. The effect of each factor, i.e., the dielectric constant, H+ ions and SO42- ions, suggests that Ce(SO4)32- is the active species of cerium(IV).

We have studied the effect of impurity on electronic properties of single-walled carbon nanotubes using Density Functional Theory. Electronic band structures and density of states of (4, 4) and (7, 0) carbon nanotubes in the presence of different amount of B and N impurities were calculated. It was found that these impurities have significant effect on the conductivity of carbon nanotubes. The metallic (4, 4) nanotube remains to be metallic after doping with B and N. The electronic properties of small gap semiconducting (7, 0) tube can extensively change in the presence of impurity. Our results indicate that B-doped and N-doped (7, 0) carbon nanotubes can be p-type and n-type semiconductors, respectively.

A new octa functionalized calix[4]resorcinarene bearing eight hydroxamic acid groups (OFCHA) has been synthesized and its analytical properties have been investigated. To elucidate the structure of the compound, elemental analysis, FT-IR and 1H NMR spectral data have been used. The compound showed high affinity and selectivity for vanadium(V) in presence of large quantities of associated metal ions. The complexation of vanadium(V) with OFCHA has a 4:1 metal: ligand stoichiometry as evaluated by Job’s plot. A spectrophotometric method is proposed for the extractive determination of vanadium(V) in an acidic medium in the presence of diversified matrix, and verified by ICP-AES. Under the optimum condition of acidity, solvent, interfering ions and OFCHA concentration, the molar absorptivity of the complex is 5630 l mol-1 cm-1 at 495 nm. The system obeys Beer’s law over the range 0.125-8.75 µg ml-1 of vanadium(V) with Sandell sensitivity 0.009 µg cm-2. The preconcentration factor and overall stability constant evaluated at 25 °C were 142 and 14.18, respectively. The complexation is characterized by favorable enthalpy and entropy changes. Liquid membrane transport studies of vanadium(V) were carried out from source to the receiving phase under controlled conditions and a mechanism for transport is suggested. To check the validity of the proposed method, vanadium is determined in environmental, biological samples and some standard reference materials from NIST and BCS

In this research polypyrrole (PPy) and polyaniline (PAni) conducting polymers which were synthesized chemically on the surface of wood sawdust (SD). These electroactive polymers as coated form on sawdust then were used for sorption or removal of silver ion from aqueous solutions under simple open circuit conditions. The effects of some important parameters such as pH, initial concentration, sorbent dosage, and contact time on uptake of silver ion were also investigated in this research. The experiments were carried out using both batch and column systems at room temperature. The treatments of the data were carried out using both Freundlich and Langmuir adsorption isotherms. In order to find out the possibility of the regeneration and reuse of the exhausted adsorbent, desorption studies were also performed. It was found that conducting electroactive polymers such as PPy or PAni are efficient sorbents for removal of the silver ions from aqueous solutions. Sorption/desorption processes can be simply carried out with pH control with satisfactory performance. Recovery percentage observed for PAni/SD in column study was more than PPy/SD and SD, under the same operational conditions.

The Feed-Forward Artificial Neural Networks (FFANNs) were used to predict the corrosion behavior of lead. A 3-9-2 network was adopted to train the networks and predict the lead corrosion behavior. The descriptors (input) were obtained using experimental methods. Solution concentration, pH and passive time were selected as the ANN input to predict the corrosion current and potential. To this end 80 samples were selected. The criterion of TSE was 0.004. It was found that the FFANNs could be used to predict the corrosion of lead.

A very simple, ultra-sensitive and fairly selective spectrophotometric method is presented for the determination of cobalt at trace levels using bis(5-bromosalicylaldehyde)orthophenylenediamine (BBSOPD). The method is based on the reaction of non-absorbent BBSOPD in a slightly acidic (0.001-0.0025 M H2SO4 or pH 3.4-4.0) and 50% N,N-dimethylformamide media with cobalt(II) to produce a highly absorbent orange colored chelate-product with an absorption maximum at 473 nm. The reaction is instantaneous and the absorption remains stable for 24 h. The average molar absorption coefficient and Sandell’s sensitivity were found to be 5.84 × 104 l mol-1 cm-1 and 9.0 ng cm-2 of cobalt(II), respectively. Linear calibration graphs were obtained for 0.02-4.0 mg l-1 of Co(II). The stoichiometric composition of the chelate is 1:1 (Co(II):BBSOPD). A large excess of over 50 cations, anions and complexing agents do not interfere in the determination. The method was successfully used for the determination of cobalt in several standard reference materials (steels and alloys), environmental waters (potable and polluted), biological samples (human blood and urine), pharmaceutical and soil samples and solutions containing both cobalt(II) and cobalt(III) as well as some complex synthetic mixtures. The method has high precision and accuracy.

The rhenium(I) compound fac-[Re(CO)3(daa)].Hpab.H2O (Hpab = N,N''-(1,2-phenylene)bis(2''-aminobenzamide); Hdaa = 2-amino-N-(2-aminophenyl)benzamide) was synthesized from the reaction of [Re(CO)5Br] with two equivalent of Hpab in toluene. The monoanionic tridentate ligand daa was formed by the rhenium-mediated cleavage of an amido N-C bond of the potentially tetradentate ligand Hpab. The compound was characterized by IR spectroscopy and X-ray crystallography, and daa is coordinated as a diaminoamide via three nitrogen-donor atoms.

Under solvent free conditions between 40-50 �°C, BF3.SiO2, a mild solid acid catalyst, is applied to regio-chemo and stereoselective Claisen-Schmidt condensation. The procedure is very simple and the products are isolated with an easy workup in good to excellent yields.

LiAlH4/silica chloride system was used as an efficient and chemoselective reductant for organic substrates. The reduction of phosphine oxides was achieved with this system. Good chemoselectivity in reduction of aldehydes and ketones in the presence of other carbonyl compounds was observed.

A low-cost، a high-yielding of vanadium oxide nanobelts 2 was simply synthesized by hydrothermal treatment of our (NH4) 2V3O8، 1، with deionized water at 200 �°C for 3 days. The SEM observations show that ribbon-like nanobelts are about 150-300 nm wide and thousands of nanometers long. The XRD pattern reveals a set of the paper form reflections، characteristic of (00l) reflections for layered phases. Based on FT-IR results and the EDX spectra، the chemical formula of the as-obtained ribbon-like nanobelts could be described as VxOy. nH2O with a mixed valence V (V) and V (IV).

Electrochemical oxidation of catechol 1a and 4-tert-butylcatechol 1b has been studied in the presence of 1-methyl-1Himidazole-2-thiol 3 as nucleophile in aqueous solution, using cyclic voltammetry and controlled-potential coulometry. The resultsindicate the participation of catechol 1a and 4-tert-butylcatechol 1b in Michael reaction with 3 to form the corresponding catecholthioethers 6a and 4b. Based on the observed EC mechanism, the homogeneous rate constants were estimated by comparing theexperimental cyclic voltammetric responses with digital simulated results.