Kinetics of Ce(IV) oxidation of α-keto acids in sulfuric-perchloric acid media

The rates of oxidation of four α-keto acids, glyoxylic, pyruvic, phenylglyoxylic, and 2-oxobutyric acids by Cerium(IV) in H₂SO₄? HaHSO₄ and H₂SO₄? HClO₄ solutions, have been studied spectrophotometrically. The rate data suggest that CeSO is the most reactive of the Cerium(IV)-sulfate complexes present in the H₂SO₄? NaHSO₄ and H₂SO₄? HClO₄ systems. The oxidation reaction is proposed to be a one-electron process with the rate-determining step being the decomposition of a α-ketoacid-Cerium(IV) complex to a free radical and carbon dioxide through a transition state.     

Electrochemical and chemical polymerization of acrylamide

The electrochemical and chemical polymerization of acrylamide (AA) has been studied. The electrolysis of the monomer in N,N-dimethylformamide (DMF) containing (C₄H₉)₄NClO₄ as the supporting electrolyte leads to polymer formation in both anode and cathode compartments. The cathodic polymer dissolves in the reaction mixture and the anodic polymer precipitates during the course of polymerization. A plausible mechanism for the anodic and cathodic initiation reaction has been given. The chemical polymerization of acrylamide that has been initiated by HClO₄ is analogous to its anodic polymerization. The polymer yield increases with an increase in concentration of the monomer and HClO₄. Raising the reaction temperature also enhances the polymerization rate. The overall apparent activation energy of the polymerization was determined to be ca. 19 kcal/mole. The copolymerization of acrylamide was carried out with methyl methacrylate (MMA) in a solution of HClO₄ in DMF. The reactivity ratios are r₁ (AA) = 0.25 and r₂ = 2.50. The polymerization with HClO₄ appears to be by a free radical mechanism. When the polymerization of acrylamide is carried out with HClO₄ in H₂O, a crosslinked water-insoluble gel formation takes place.     

Aquation of K2 TcBr6 in dilute acid solutions

The photochemical aquation of K₂ TcBr₆ in 2M HRr, 1M HClO₄ and 1M H₂SO₄ has been studied. The absorption spectra of the various Tc(IV) species were measured after the electrophoretic separation. The spectrophotometric changes and the yield of each species as a function of the UV irradiation time were determined. After 25 hours of irradiation of HBr and HClO₄ solutions the main species were the cationic ones (80%) but in H₂SO₄ solution the neutral species reached a yield of 90%. The oxidation of Tc(IV) species to TcO ₄ ^– proceeded more rapidly and extensively in HClO₄ than in HBr and H₂SO₄.     

Kinetics and mechanism of chromic acid oxidation of oxalic acid in absence and presence of different acid media. A kinetic study

Kinetics and mechanism of the reaction of Cr(VI) with oxalic acid have been studied in presence and absence of H₂SO₄, HClO₄, and CH₃COOH by monitoring the formation of Cr(III)-oxalic acid complex at 560 nm. The effect of total [oxalic acid], [Cr(VI)], [H₂SO₄], [HClO₄], and [CH₃COOH] on the reaction rate was determined at 30°C. Formation of carbon dioxide was also confirmed. The oxidation rate increases with [oxalic acid] and [CH₃COOH] while it decreases with [H₂SO₄], [HClO₄], and pH. The rate law governing the oxidation of oxalic acid over a wide range of conditions is rate=k₁ K_es1 [oxalic acid]_T [Cr(VI)]_T 1+K_es1 [oxalic acid]_T, where only undissociated oxalic acid is kinetically active. Kinetic evidence for the formation of a Cr(VI)(SINGLEBOND)oxalic acid 1:1 complex has been obtained and the equilibrium constant for their formation has been determined. The 1:1 complex exists most likely in an open chain form. The rate-limiting step of the oxidation reaction involves the breaking of the C(SINGLEBOND)C bond in the 1:2 complex. Oxidizing ability of Cr(VI) species have been discussed. Mechanism with the associated reaction kinetics is assigned. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 335–340, 1998     

The influence of metal adatoms deposited at underpotential on the kinetics of an outer-sphere redox reaction

PbO₂ anodes were used to oxidize cyclohexanone oxime to nitroso compounds in 1 M H₂SO₄ or 1 M HClO₄ in a two electron reaction:

Nu^? are nucleophiles like OH^?, NO^?₂ and Cl^?. The electrode material reacts chemically with the oxime and it is oxidized electrochemically at potentials very close to the Pb^{$\text{2+}\text{4+}$} redox potential. The chemical step has been studied separately by corrosion experiments in aqueous HClO₄. An upper limit of current density is observed, independent of convection, due to the formation of a film composed of Pb(II) products on the electrode. A reaction limitation was not attained under our conditions.Contrary to this, the PbO₂ electrode behaves inertly in the case of lower aliphatic alcohols (C₂C₄) in 1 M H₂SO₄. Potentials are about 0.5 V more positive. The inert anode generates the radical cation in the rate determining step:

Corrosion experiments disclose a rate, which is lower by four orders of magnitude (j_corr=2–3 μA cm^?2) with reference to the oxime experiment and only slightly above the corrosion rate in pure 1 M HClO₄.     

Effect of ionic composition of solutions on the methanol reaction with adsorbed oxygen on smooth polycrystalline platinum electrodes: Transients of the open-circuit potential

Transients of the open-circuit potential observed in the reaction of methanol with oxygen (O_ads) preliminarily adsorbed on smooth polycrystalline platinum (pcPt) are measured in 0.05 M HClO₄, 0.5 M HClO₄, 0.05 M H₂SO₄, 0.05 M H₂SO₄ + 0.45 M Na₂SO₄, and 0.05 M H₂SO₄ + 0.45 M Cs₂SO₄. It is shown that the solution pH has a weak effect on the transient characteristics (when the reversible hydrogen electrode potential scale is used). This confirms the chemical nature of rate-controlling stages in the reaction mechanism. The changes in the reaction rate, observed upon going from one electrolyte to another, are preferentially associated with the involvement of solution ions in the formation of activated surface complexes that include CH₃OH, O_ads, and supporting-electrolyte components.     

Effect of acrylamide concentration on the kinetics of oxidation of tartaric acid by cerium(IV) in sulfuric-perchloric acid media

The kinetics of oxidation of tartaric acid by Ce(IV) in the absence and presence of acrylamide has been investigated spectrophotometrically in aqueous H₂SO₄–HClO₄ media at a constant ionic strength 2.0M and 25°C. Oxidation of tartaric acid in both cases was first order with respect to Ce(IV). Kinetic data showed that the reaction involves the formation of an unstable complex and an intermediate free radical. The activation parameters were calculated to be E _a =91.3±0.4 kJ-mol^–1, S=20.2±1.0 J-mol^–1-K^–1, H=88.8±0.4 kJ-mol^–1. A polymerization mechanism is discussed.     

Determination of selenium in biological samples by electrochemical preconcentration and atomic absorption spectrometry

Summary For the determination of selenium in biological materials the samples are digested with an HNO₃/H₂SO₄/ V₂O₅ or an HNO₃/HClO₄ mixture. Selenium is then electrolyzed onto a platinum wire, followed by atomic absorption spectrometry with simultaneous electrothermal and flame atomization. Results for the reference materials Bovine Liver and Albacore Tuna are reported (1–3±0.2 ppm Se).

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Selenbestimmung in biologischen Proben durch elektrochemische Anreicherung und Atomabsorptionsspektrometrie

Zusammenfassung Die Proben werden mit HNO₃/H₂SO₄/V₂O₅ oder HNO₃/HClO₄ aufgeschlossen; Se wird elektrolytisch auf einem Platindraht niedergeschlagen und durch AAS mit gleichzeitiger elektrothermischer und Flammen-Atomisierung bestimmt. Ergebnisse werden mitgeteilt für die Analyse von Standardreferenzmaterialien (Bovine Liver und Albacore Tuna, 1–3±0,2 ppm Se).

     

The Mechanism and Kinetic Models of the Catalytic Oxidation of Ethylene by <Emphasis Type="Italic">p</Emphasis>-Benzoquinone in Aqueous–Acetonitrile Solutions of Pd(II) Cationic Complexes

A kinetic study of ethylene oxidation to acetaldehyde by p-benzoquinone in the Pd(OAc)₂–HClO₄?LiClO₄–CH₃CN–H₂O system has been carried out under conditions when palladium(II) cationic complexes exist at a molar fraction of water of 0.67 and 30°С. For a reaction that mostly lead to the formation ofPd(CH₃CN)(H₂O) ₃ ²⁺ two-route mechanism and a kinetic model have been proposed that describe adequately the experimental dependence of the reaction initial rate on the concentration of p-benzoquinone, HClO₄, and palladium. The model takes into account previous findings on the H₂O/D₂O and C₂H₄/C₂D₄ kinetic isotope effects and the important role of Pd(0) quinone complexes.     

Trace metal ion catalysis; kinetics and mechanism of oxidation of metol by hydrogen peroxide in the presence of iron(III) sulphate in acid perchlorate

The kinetics of oxidation of N-methyl-p-aminophenol (metol) by H₂O₂ in the presence of trace levels of Fe^III have been studied in an HClO₄ medium. The reaction mechanism is complex and the catalytic role of Fe^III in the sulphato form is indicated. The following rate law accounts for the experimental observations:

Demetallation of α,β,γ,δ-tetrakis(p-sulfophenyl) porphiniron(III) in mineral acid–alcohol–water media

Demetallation rates of α,β,γ,δ-tetrakis(p-sulfophenyl)porphiniron(III) in hydrochloric acid–ethanol–water, perchloric acid–ethanol–water, and sulfuric acid–alcohol–water media were determined. For a given acidity value H₀ the order of the rates for the three acids was HCl > H₂SO₄ > HClO₄. This is also the order for complex formation between acid anion and iron(III). Consequently ligands as well as protons are involved in the breaking of bonds between the metal and the porphyrin leading to the formation of the activated complex. The log k values for HCl and HClO₄ media were not linearly related to the Hammett acidity function as they were for sulfuric acid–ethanol–water media. The average ΔH? and ΔS?values for the HCl media were 18.4 ± 1.4 kcal/mol and ? 19 ± 3 cal K mol, respectively, in very close agreement with those for H₂SO₄ media despite the difference in H ₀ dependence. For H₂SO₄–alcohol–water media the order of the rates was butanol > propanol > ethanol with little difference between isomeric alcohols.     

Synthesis, structure, and properties of chromium(III) sulfates

Reactions between CrO₃ and 50- are studied at temperatures up to the boiling point of the acid. Depending on the H₂SO₄ concentration and synthesis temperature, Cr₂(SO₄)₃, CrH(SO₄)₂, (H₃O)[Cr(SO₄)₂], Cr₂(SO₄)₃·H₂SO₄·4H₂O (gross formula), and (H₅O₂)[Cr(H₂O)₂(SO₄)₂], are obtained as identified reaction products in addition to the incompletely characterized chromic-sulfuric acid. The Cr^III-based sulfates are characterized by X-ray powder diffraction, thermogravimetric, and magnetic susceptibility measurements. The nuclear and magnetic structures of Cr₂(SO₄)₃ at are determined, the structure type of (H₃O)[Cr(SO₄)₂] is established, and the crystal structure of (H₅O₂)[Cr(H₂O)₂(SO₄)₂] is firmly stipulated. Magnetic susceptibility data suggest that the samples of CrH(SO₄)₂ are in a micro-crystalline rather than in an amorphous state. All Cr^III-based sulfates synthesized in this study appear to undergo paramagnetic-to-antiferromagnetic transitions at around .     

Kinetics and mechanism of chloramine-T oxidation of cinnamaldehyde in two acid media

Kinetics of oxidation of cinnamaldehyde (Cinn) by chloramine-T (CAT) has been studied in solutions containing HCl and H₂SO₄ at 313 K. The respective experimental rate laws obtained in HCl and H₂SO₄ media are as follows: and Here the value of x varies from 0.9 to zero while the values of y and z are 0.83 and 0.72, respectively. Effects, on the reaction rate, of ionic strength, reaction product (p-toluenesulfonamide), dielectric constant of the solvent medium, and the anions, Cl⁻ and SO₄²⁻, have been investigated. Activation parameters have been computed, based on the rate constants determined at different temperatures. Mechanisms proposed and the rate laws derived are consistent with the observed kinetic data represented by the experimental rate laws.     

The comparative study of CO2 + Hads reaction on platinum electrode in H2O and D2O

The observed isotopic effect in CO₂ + H_ads reaction showed that at 0.05 V the rate determining step is

formation whereas at 0.2 V the reorientation of adsorbed intermediate:

is probably the slowest step of reaction. The oxidation of adsorbed product is slower in D₂SO₄ than H₂SO₄ solution like the surface oxidation of platinum. The rate determining step of COOH_ads oxidation is a reaction with OH_ads.     

Acidity of heteropoly acids with various structures and compositions studied by IR spectroscopy of the pyridinium salts

The acidity on the proton affinity scale was determined by IR spectroscopy of the pyridinium salts for nineteen heteropoly acids of nine structural types (including two with the previously unknown structure) and one isopoly acid. All heteropoly acids exhibited a high acidity at the level of CF₃SO₃H and HClO₄. H₃PW₁₂O₄₀ was the strongest acid.     

Oxidation of glutathione by diaquatetrakis(2,2′-bipyridine)-μ-oxo diruthenium(III) ion in aqueous acidic solutions

Summary The kinetics of the oxidation of glutathione by diaquatetrakis(2,2-bipyridine)--oxo diruthenium(III) ion in aqueous HClO₄ have been investigated. The reaction obeys the empirical rate law:-2d[oxidant]/dt = k[oxidant][reductant]/[H⁺] where k = 7.42 ± 0.40 × 10^-3 s^-1 at 25.5 °C, [H⁺] = 0.005–0.05 M and I = 1.0 M (LiClO₄). Free radicals are important in the reaction and a mechanism consistent with the experimental results has been postulated.     

Palladium(II) inhibition during cerium(IV) oxidation of aldoses: a kinetic study

Summary The kinetics of oxidation of aldoses, namely xylose, arabinose, galactose and glucose, by Ce^IV have been studied in HClO₄ + H₂SO₄ medium and in the presence of Pd^II. The reactions exhibit a first order rate dependence with respect to oxidant. The rate is inversely dependent on the [HSO _inf4 ^sup– ][H⁺] ratio. The order of reaction with respect to aldose decreases at higher [aldose]. Due to the formation of a complex between Ce^IV and Pd^II, a retarding effect of [Pd^II] on the rate of disappearance of [Ce^IV] has been observed. A mechanism consistent with the observed kinetic data is proposed.     

Blue Osmium(IV) Sulfite Complex: Synthesis and Study

Blue osmium(IV) sulfite complex Na₄[Os₂(-O)₂(SO₃)₄(H₂O)₄] · 4.5 H₂O was synthesized via the reaction of aqueous solutions containing OsO₄ and equimolar amount of Na₂SO₃ and H₂SO₄ at 50°C and its composition and structure were determined by elemental analysis, X-ray electronic and IR spectroscopies, and thermogravimetric analysis. The compound is X-ray amorphous, insoluble in water but soluble in hydrochloric and sulfuric acids. The electrochemical methods (cyclic voltammetry and potential-controlled coulometry) indicate the complex polymerization in solutions. Under continuous electrolysis at high negative potentials (E _r = –0.10 V), the product under study is depolymerized and the monomeric Os(II) complexes are formed. At a high positive potential (E ₀ = 1.00 V), Os(VI) is formed that disproportionates into Os(IV) and Os(VIII).     

Evidence of esterification during the oxidation of some aromatic aldehydes by permanganate

Summary The permanganate ion oxidation of PhCHO in HClO₄ medium is first order in [MnO _inf4 ^sup- ], but of complex order with respect to [aldehyde] as well as [H⁺]. The influence of substituents on the reaction rate and mechanism has also been studied in 40% (v/v) aqueous AcOH and convincing evidence favours an ester formation mechanism. Thermodynamic values associated with 11 intermediate ester formation and the activation parameters associated with the disproportionation steps involved have been evaluated.     

Ruthenium(IV) and Osmium(II) Tetraphenylporphine Complexes: Synthesis and Oxidation Reactions

The Ru(IV) and Os(II) complexes (PhO)₂RuTPP and OsTPP were synthesized from tetraphenylporphine (H₂TPP) and K₂RuO₄ or K₂OsO₄ (taken in the molar ratio of 1 : 30) in boiling phenol. The kinetics of oxidation reactions of these complexes in solutions of HOAc (acetic), H₂SO₄, and HOAc–H₂SO₄ acids was studied. It was found that in the aerated HOAc–H₂SO₄ mixture heated above 340 K, these complexes are oxidized with participation of different reaction sites: the Ru(IV) complex is oxidized at macrocycle to give the -radical-cation (PhO)₂RuPP⁺, while in the Os(II) complex, the metal atom is oxidized to form the Os(III) complex. In the first case, the reaction follows the activation mechanism, whereas in the second case, the activation energy of the reaction is zero.