The kinetics of oxidation of kappa-carrageenan polysaccharide as natural polymer by permanganate ion in aqueous perchlorate solutions at a constant ionic strength of 2.0 mol dm−3 have been investigated spectrophotometrically. The experimental observations showed that the pseudo-first-order plots were of inverted S-shape throughout the entire course of reaction. The initial rates were found to be relatively slow in the early stages, followed by an increase in the oxidation rates over longer time periods. A first-order dependence in permanganate and fractional-order kinetics with respect to carrageenan concentration for both the induction and autoacceleration periods were revealed. The results obtained at various hydrogen ion concentrations showed that the oxidation is acid-catalyzed throughout the two stages of reaction. The added salts lead to the prediction that MnIII and/or MnIV are the reactive species throughout the autoacceleration period. Kinetic evidences for the formation of 1:1 intermediate complexes are presented. The kinetic parameters have been evaluated and a tentative reaction mechanism consistent with the kinetic results is discussed.

Spectrophotometric detection of the formation of short-lived hypomanganate(V), ½KCAR–MnVO4 3, and manganate(VI), ½KCAR–MnVIO4 2, intermediates has been confirmed through the oxidation of K-carrageenan (KCAR) by potassium permanganate in alkaline solutions of pH’s P12 using a conventional spectrophotometer. The short-lived transient species were characterized and a mechanism consistent with experimental observations is suggested.

The catalytic effect of copper(II) catalyst on the oxidation of L-tryptophan (Trp) by hexacyanoferrate(III) (HCF), has been investigated spectrophotometrically in an aqueous alkaline medium at a constant ionic strength of 0.5 mol dm
3 and at 25
C. The stoichiometry for both the uncatalyzed and catalyzed reactions was 1:2 (Trp:HCF). The reactions exhibited first order kinetics with respect to [HCF] and less than unit orders with respect to [Trp] and [OH
]. The catalyzed reaction exhibited fractional-first order kinetics with respect to [CuII]. The reaction rates were found to increase as the ionic strength and dielectric constant of the reaction medium increase. The effect of temperature on the rates of reactions has also been studied, and the activation parameters associated with the rate-determining steps of the reactions have been evaluated and discussed. Addition of the reaction product HCF(II) to the reaction mixture did not affect the rates. Plausible mechanistic schemes for uncatalyzed and catalyzed reactions explaining all of the observed kinetic results have been proposed. In both cases, the final oxidation products are identified as indole-3- acetaldehyde, ammonia, and carbon dioxide. The rate laws associated with the reactions’ mechanisms are derived. The rate constants of the slow steps of the reactions along with the equilibrium constants are also calculated.

The catalytic effect of silver(I) and copper(II) ions on the oxidation of histidine by cerium(IV) in aqueous sulfuric acid solutions was studied spectrophotometrically at a constant ionic strength of 3.0 mol dm−3 and at 25°C. In both uncatalyzed and metal ions-catalyzed paths, the reactions exhibited first-order kinetics with respect to [Ce(IV)] and [catalyst], and fractional first-order dependences with respect to [His] and [H+]. The oxidation rates increased as the ionic strength and dielectric constant of the reactions media increased. The catalytic efficiency of Ag(I) was higher than that of Cu(II). Plausible mechanistic schemes for both uncatalyzed and catalyzed reactions were proposed, and the rate laws associated with the suggested mechanisms were derived. In both cases, the final oxidation products of histidine were identified as 2-imidazole acetaldehyde, ammonium ion, and carbon dioxide. The activation parameters associated with the second-order rate constants were evaluated

The oxidation kinetics of isosorbide (S) by potassium permanganate in both perchloric and sulfuric acid solutions was investigated spectrophotometrically at a constant ionic strength of 2.0 moldm-3 and at 25 C. In both acids, the oxidation reactions showed a first-order dependence on ½MnO 4 , apparent a less than unit-order dependence with respect to [S] and a fractional-second-order dependence with respect to [H?]. Variation of either the ionic strength or dielectric constant of the reactions media did not significantly affect the oxidation rates. In both acids, the final oxidation product of isosorbide was identified by both spectroscopic and chemical tools as the corresponding monoketone derivative, namely (1S,4S,5R)-4-hydroxy-2,6-dioxabicyclo[3.3.0] octan-8-one. Under comparable experimental conditions, the oxidation rate of isosorbide in perchloric acid was lower than that in sulfuric acid. The oxidation mechanism describing the kinetic results was proposed and the rate law expression was derived. The activation parameters of the second-order rate constants were computed and are discussed

The inhibition of the corrosion of mild steel in 0.5 M H2SO4 by vanillin in the presence of various amino acids, typically valine, methionine, cysteine and methylcysteine hydrochloride, are studied using potentiodynamic technique aiming at the elucidation of the mechanism of the corrosion inhibition. The corrosion inhibition of vanillin and amino acids was evaluated, both in their individual presence and in their coexistence. The largest inhibition efficiency has been found in the presence of vanillin and cysteine mixture. To have an insight into the mechanism of the inhibition and probe the adsorption sites of the inhibitor, the inhibition efficiency of a Schiff base, synthesized from vanillin and a selected amino acid, typically cysteine, has been compared with the inhibition efficiency of the vanillin and cysteine mixture. Interestingly, it has been found that the inhibition efficiency of the vanillin and cysteine mixture is much larger than the relevant Schiff base, and that the inhibition efficiency of the latter is much smaller than the inhibition efficiency in the presence of the individual components, i.e., vanillin and cysteine. This points to the blocking of the probable adsorption sites, upon the formation of Schiff base, which is the amino group of cysteine and the aldehydic group of vanillin. The adsorption of the methylcysteine, as an example, in the presence of vanillin was found to follow Temkin isotherm and that the chemical adsorption mode is controlling as revealed from the free energy of adsorption (44.4 kJ/mol).

The electrochemical behavior of nickel, Inconel600, Incoloy800 and 316 stainless steel in different concentrations of HNO3 solutions was investigated using the cyclic voltammetry technique. . All the curves in the anodic cyclic voltammetry are characterized by an anodic dissolution peak (A) followed by a passive region (B), before oxygen evolution. On the other hand incoloy800and 316 stainless steel show an additional dissolution peak (C) and trans passive region followed by oxygen evolution. In the cathodic branch of the CVS there is one cathodic peak in the case of Ni. Inconel600 and Incoloy800.On the other hand, there is an unexpected peak in case of 316 stainless steel.This peak appeared on the anodic branch.All the anodic and cathodic peaks are interpreted. The passivation of Ni was explained based on the formation of NiO. In the alloys the passive layer was mainly composed of Ni+2, Fe+3and Cr+3 in Inconel600, Incoloy800, and Mo+6 in case of 316 stainless steel. The role of Cr+3 and Mo+6ions in the reinforcement of the passive film is explained.

The oxidation of isosorbide (S) by chromic acid (CrVI) has been studied in aqueous perchlorate solution at a constant ionic strength of 3.0 mol dm-3 and temperature of 25 C. The course of the reaction was followed spectrophotometrically. The reaction exhibited first-order dependence on [CrVI], less than unit order with respect to [S], and fractional-second-order dependence with respect to [H?]. Variation of the ionic strength or dielectric constant of the medium had no significant effects on the oxidation rate. Addition of MnII inhibited the oxidation rate. The oxidation product of isosorbide was identified as the corresponding monoketone derivative, namely (1S,4S,5R)- 4-hydroxy-2,6-dioxabicyclo[3.3.0] octan-8-one. A mechanism for the oxidation is proposed, and the corresponding rate-law expression has been deduced. The activation parameters associated with the second-order rate constant are presented and discussed

The effect of increasing the concentration of chloride ions as pitting corrosion agent on the potentiodynamic anodic polarization curves of nickel(Ni), Inconel 600, Incoloy 800 and 316 Stainless steel (SS) in 0.1M HNO3 solution at a scan rate 1.0 mV sec-1 was studied. The pitting corrosion potential varied with the logarithmic of the molar concentrations of chloride ions in case of Ni and Inconel 600 according to S-shaped curve. This behavior was explained according to re-passivation, propagation and continued propagation of the pits formed. In case of Incoloy 800 and 316SS a broken lines were observed according to re-passivation and propagation of the pits formed. It is found that,316SS is more resistance to pitting attack due to the presence of Mo in their chemical structure. The effect of the increasing concentration of some inorganic anions such as the sodium salts of chromate, tungstate, molybdate, phosphate dibasic and carbonate to inhibit the pitting corrosion was investigated. The values of the pitting corrosion potential were shifted to more positive values indicating the inhibiting action of these anions toward the pitting corrosion.

Kinetics of chromium(VI) oxidation of fluorene (Fl) and its halogenated derivatives, namely, 2,7-dichlorofluorene (Fl-Cl), 2,7-dibromofluorene (Fl-Br) and 2,7-diiodofluorene (Fl-I), in perchlorate solutions was investigated at a constant ionic strength of 2.0 mol dm3 and at 25 oC. The progress of the reactions was followed spectrophotometrically. The reactions showed a first order dependence on [Cr(VI)] and fractionalfirst order dependences with respect to hydrogen ion and fluorene derivatives concentrations. Under comparable experimental conditions, the order of the oxidation rates of the fluorene derivatives was: Fl > Fl-I > Fl-Br > Fl-Cl. The mechanism of the oxidation reactions was suggested and the rate-law expression was derived. Values of the rate constants of the slow step along with the equilibrium constants were calculated.