Aqueous Perchloric Acid Solutions Research Articles

Hydride Abstraction from 1,2-Diols by the Pentaaqua(oxo)chromium(IV) Ion

Kinetic studies of the oxidation of 1,2-diols by the pentaaqua(oxo)chromium(IV) ion, (H{sub 2}O){sub 5}Cr{sup IV}=O{sup 2+}, were carried out in dilute aqueous solutions of perchloric acid. These reactions yield the superoxochromium(III) ion CrOO{sup 2+}, providing evidence that the two-electron reduction of Cr{sup IV}O{sup 2+} to Cr{sup 2+} occurred in a single step. The rate constants for the primary and secondary diols are comparable to each other and approximately 10 times larger than that for pinacol (k = 3.7 L mol{sup {minus}1}s{sup {minus}1}). The kinetic isotope effect of the O-H hydrogens is negligible, k{sub H}/k{sub D} = 0.95-1.3, whereas that for the C-H hydrogens is substantial, k{sub H}/k{sub D} = 3.7-4.8. A 2-equiv mechanism is proposed, in which CrO{sup 2+} abstracts a hydride ion from an {alpha}-position in the primary and secondary diols and from a {beta}-position in pinacol.

The temperature coefficient of the potential of zero charge of stepped faces of gold in contact with aqueous perchloric acid solutions

Potentials of zero charge, E σ=0, and the temperature coefficients of E σ=0 for stepped surfaces of gold single crystals (511), (533), (331) and (311) were obtained from the minimum of the differential capacity curse in dilute HClO 4 solutions. The results are compared with those obtaine for the low index faces and discussed in terms of the effect of the surface density of steps and the step orientation. The possible relevance of the effect of steps on the interaction of water with gold surfaces is discussed.

Equilibria and kinetics of the reactions between hydrogen peroxide and methyltrioxorhenium in aqueous perchloric acid solutions

In aqueous solutions the colorless compounds CH[sub 3]ReO[sub 3] (=MTO) and H[sub 2]O[sub 2] form 1:1 and 1:2 adducts. The latter is yellow, with [epsilon][sub 360] = 1.1 x 10[sup 3] L mol[sup [minus]1] cm[sup [minus]1]. Peroxide binding to MTO shows cooperativity, as shown by the inversion of the usual order of binding constants. The stepwise equilibrium constants are K[sub 1] = 7.7 L mol[sup [minus]1] and K[sub 2] = 145 L mol[sup [minus]1] at 25[degrees]C. The buildup of product, which occurs on the stopped-flow time scale at 9-680 mM H[sub 2]O[sub 2], is fit by biexponential kinetics. The equilibria and rates are independent of [H[sub 3]O[sup +]] in the range 10[sup [minus]1]-10[sup [minus]3] M. The peroxide complexes decompose more rapidly at lower [H[sub 3]O[sup +]], particularly at pH > 3. Possible structures for 1:1 and 1:2 MOT-H[sub 2]O[sub 2] adducts are presented and discussed.

Methyltrioxorhenium-catalyzed oxidation of a (thiolato)cobalt(III) complex by hydrogen peroxide

The oxidation of (en)[sub 2]Co(SCH[sub 2]CH[sub 2]NH[sub 2])[sup 2+] by H[sub 2]O[sub 2] is catalyzed by CH[sub 3]ReO[sub 3]. Studies of the kinetics and mechanism were carried out in aqueous solutions of dilute perchloric acid. The (thiolato)cobalt(III) complex is oxidized first to a sulfenato complex, (en)[sub 2]Co(S(O)CH[sub 2]CH[sub 2]NH[sub 2])[sup 2+], which is in turn more slowly oxidized to the sulfinato complex, (en)[sub 2]Co(S(O)[sub 2]CH[sub 2]CH[sub 2]NH[sub 2])[sup 2+]. The two steps are well resolved in time, the second being some 1500 times slower than the first. Both steps fit the same kinetic pattern, which is consistent with a Michaelis-Menten scheme in which there are two substrates. This scheme involves the reversible formation of a 1:1 H[sub 2]O[sub 2]/CH[sub 3]ReO[sub 3] adduct. The reversible formation of a 2:1 H[sub 2]O[sub 2]/CH[sub 3]ReO[sub 3] adduct also occurs, but it appears to be a dead-end process, in that B, if involved at all, is much less reactive than A. Rate constants were determined at 25[degrees]C, [mu] = 0.10 M (HClO[sub 4]), for the formation of the 1:1 H[sub 2]O[sub 2]/CH[sub 3]ReO[sub 3] adduct A (k[sub 1] = 77 [+-] 1 L mol[sup [minus]1] s[sup [minus]1]) and its dissociation (k[sub [minus]1] = 9.0more » [+-] 0.5 s[sup [minus]1]) and for the oxidation by A of the thiolato complex, (4.2 [+-] 0.3) [times] 10[sup 5] L mol[sup [minus]1] s[sup [minus]1], and of the sulfenato complex, 265 [+-] 7 L mol[sup [minus]1] s[sup [minus]1].« less

The Extraction Transfer of H+-18-Crown-6 Species across the Water-Nitrobenzene Phase Boundary in the Extraction of Aqueous Solutions of Perchloric Acid by Nitrobenzene in the Presence of 18-Crown-6

A study was made of the extraction of perchloric acid in water-nitrobenzene-18-crown-6 (hereafter L) systems. The experimental data was interpreted on the basis of the assumption that the extraction occurs according the equations H+aq + ClO4,aq- ↔ H+org + ClO4,org- and H+aq + Laq + ClO4,aq- ↔ HL+org + ClO4,org-. Crown is also protonated in the aqueous phase according to the equation H+aq + Laq ↔ HL+aq. The values of the corresponding extraction constants, the protonation constants of 18-crown-6 in nitrobenzene and water and the individual extraction constants of the HL+ species in the water-nitrobenzene system were determined, along with the magnitude of the molar free enthalpy for the transfer of these species across the given phase boundary.

The flash photolysis of di-μ-oxo-bis(oxo-molybdate(V)) acid aqueous solutions in the presence of excess chloride

Abstract The flash photolysis of di-μ-oxo bis(oxo-molybdenum(V)) complexes in aqueous solutions of perchloric acid in the presence of excess chloride induced both photolabilization and photo-oxidation of Cl − and/or H 2 O ligands. Photolabilization produces the formation of reaction intermediates identified as μ-oxo-bis (oxo-molybdenum(V)) species: Mo 2 O 2 (μ-O)(H 2 O) 8 4+ ( A ) and Mo 2 O 2 (μ-O)(Cl)(H 2 O) 7 3+ ( B ). A rapid equilibria between the transient species is established before they decay through a first order reaction. Intermediate A is the only species effective in promoting the observed decay leading to Mo(VI) and H 2 as end products. The relative absorption spectrum of species B is reported, as well as its stability constant in the temperature range 25–45 °C. The effect of ligands on the overall mechanism is discussed.

Kinetics and mechanism of the acid-catalyzed hydrolysis of 1,1-dimethoxyethene (ketene dimethyl acetal) and trimethoxyethene in aqueous solution

Rates of acid-catalyzed hydrolysis of 1,1-dimethoxyethene and trimethoxyethene were measured in wholly aqueous solutions of perchloric acid, sodium hydroxide, and a series of carboxylic acid buffer solutions at 25 o C. The reactions show general acid catalysis and give substantial solvent isotope effects in the normal direction (k H /k D >1)

A kinetic study of the reaction between soluble (colloidal) manganese dioxide and formic acid

The kinetics of the reaction between a soluble form of colloidal manganese dioxide (obtained by reduction of potassium permanganate with sodium thiosulfate) and formic acid in aqueous perchloric acid solution has been investigated by UV—Vis spectrophotometry. The rate law is r = (k+k ′[H +] 1.67)[MnO 2][HCOOH] 0.45 The apparent values found for the activation parameters are E a = 53.6 ± 0.8 kJ mole −1, Δ H ≠ = 51.1 ± 0.8 kJ mole −1, and Δ S ≠ = −140 ± 3 J K −1 mole −1. The reaction seems not to be very sensitive toward addition of sodium perchlorate to the solution, but is noticeably retarded by addition of gum arabic acting as a protective colloid. A mechanism in accordance with the experimental results is proposed for the reaction taking place at the colloid surface.

The flash photolysis of di-μ-thio-bis(oxo-molybdate(V)) complexes in acidic aqueous solutions

The flash photolysis of di-μ-thio-bis(oxo-molybdenum(V)) complexes was studied in aqueous solutions of perchloric acid in the pH range 0.1–2.5 and the temperature range 25–45 °C. Irradiation with light in the range 200–220 nm induced the formation of transient species which decayed through common first-order reaction kinetics. The experimental decay rate constant was shown to be dependent on the hydrogen ion concentration ▪ The end products were H 2 and monomeric Mo VI present as Mo(OH) 6, MoO 3S 2− and MoO 2S 2 2−. The overall mechanism is discussed taking into consideration the experimental observations and evidence found in the literature.

Kinetics of the silver(I)-catalysed oxidation of H3PO2 with [ethylenebis(biguanide)]silver(III) cation in aqueous perchloric acid solution

The silver(I)-catalysed oxidation of H3PO2 by [ethylenebis(biguanide)]silver(III) produces phosphite, Ag+ and the free ligand quantitatively. A coloured intermediate III is formed within the time of mixing which is probably composed of the silver(III) complex, Ag+ and hypophosphite in its active form. The first-order rate constant for the oxidation is k0=kK1K2[Ag+][PI]/([H+]+Kd), where K1K2 is the formation constant of III and Kd is the acid dissociation constant of H3PO2. Phosphorous acid does not affect the kinetics and no free radicals could be detected during the course of the reaction. The second-order rate constant KK1K2= 6.73 dm3 mol–1 s–1 at 30 °C and I= 1.0 mol dm–3 is very much larger than the second-order rate constant reported for the oxidation of hypophosphite by silver(I). A plausible mechanism involves a two-step electron transfer from PI to AgIIIvia AgI within III. The reaction has been utilised in developing a kinetic method for the estimation and detection of millimolar concentrations of hypophosphite; other oxoacids of phosphorus do not interfere.

Kinetics and mechanism of oxidation of DL-α-alanine by permanganate ion in acid perchlorate media

The kinetics of permanganate oxidation of DL-α-alanine in aqueous perchloric acid solution at a constant ionic strength of 2.0 mol dm−3 has been investigated spectrophotometrically. The reaction was found to show second-order kinetics overall with respect to each of the reactants in the slow initial stage; the second-order kinetics are not, however, maintained throughout the relatively fast final stage of reaction. The added salts lead to the prediction that Mn(III) and (or) Mn(IV) play a very important role in the reaction kinetics. A tentative mechanism consistent with the kinetics is discussed. Key words: kinetics, oxidation, reduction, amino acids, permanganate.

Reaction of carbon monoxide with dirhodium(II) aqua-cation. New pathway for Rh 4(CO) 12 synthesis

Reaction of carbon monoxide with an aqueous perchloric acid solution of Rh 2(H 2O) 10 4+ produces initially a “Rh I(CO)” 2 species, which undergoes further reaction to give Rh 4(CO) 12 in 70–80% yield as well as some Rh 6(CO) 16. The reaction was studied at room temperature and atmospheric pressure of CO.

Salting-Out and Salting-in Effects in the Reversed-Phase Thin-Layer Chromatography of Dansylated Amino Acids. Effect of Acids

Abstract The retention of 15 dansylated amino acid derivatives was determined in aqueous solutions of formic, acetic, propionic and perchloric acid in reversed-phase thin-layer chromatography. The acids increased the retention of each derivatives at the low concentration range. This effect has been tentatively explained by the suppression of dissociation of polar groups in the solute molecules resulting in increased apparent lipophilicity (salting-out phenomenon). The higher concentrations of acid solutions decreased the retention (salting-in effect), the undissociated acid molecules probably act as an organic mobile phase. Both salting-in and salting-out phenomena can be simultaneously described by bilinear function. The polarity parameters of the amino acids, their hydrophobicity and the strength of the acid in the eluent simultaneously influence the retention.

Flash photolytic generation and study of ketenes: acid-catalysed hydration in aqueous solution

Flash photolysis was used to generate CH 2CO by fragmentation of cyclobutanone and (CH 3) 3CCHCO, (CH 2) 5CCO and PhC(CH 3)CO by the photo-Wolff reaction of the corresponding diazoketones; the rates of hydration of these ketenes were measured in wholly aqueous solutions of sodium perchlorate, perchloric acid and phosphoric and acetic acid buffers. Uncatalysed and acid-catalysed reactions were observed for the aliphatic ketenes, but only an uncatalysed reaction was observed for the aromatic derivative. A mechanism for the acid-catalysed process, involving rate-determining proton transfer from the catalyst to the terminal carbon atom of the ketene functional group, is indicated by the occurrence of general acid catalysis and hydronium ion isotope effects in the range k H + / k D + = 2.7–3.3. The absence of acid catalysis in the hydration of PhC(CH 3)CO is explained in terms of a phenyl-substituent effect that retards the acid-catalysed process but accelerates the uncatalysed reaction.

Solution properties of the cuboidal mixed-metal cluster [Mo3NiS4(H2O)10]4+ including complexing (at Ni) and redox properties

The green-blue complex [Mo3NiS4(H2O)10]4+ has been prepared by NaBH4 reduction of a 1:10 solution of [Mo3S4(H2O)9]4+ and NiCl2·6H2O in 0.5 M HCl, and purified by Dowex 50W-X2 cation-exchange chromatography. Solutions (0.1–2.0 mM) in 2 M HClO4 obey Beer's law, consistent with the single cube formulation. Complexation equilibration reactions, Mo3NiS44++ X–⇌ Mo3NiS4X3+, studied by stopped-flow spectrophotometry (25°C), give forward and back rate constants for X–= Cl– of kf= 9.4 M–1 s–1, kb= 0.097 s–1, and for X–= NCS– of kf= 45 M–1 s–1, kb= 0.062 s–1. Complexation, assigned to substitution at the tetrahedrally ligated (and metal–metal bonded) nickel centre, is unusually slow, and slower than substitution on [Ni(H2O)6]2+. Redox reactions, e.g. Mo3NiS44++ 2CoIII→ Mo3S44++ Ni2++ 2CoII, with [Co(dipic)2]–(dipic = pyridine-2,6-dicarboxylate) and [Fe(H2O)6]3+ as oxidants, have also been studied. Rate laws are in both cases first order in the concentration of each reactant. Whereas at 25°C the [Co(dipic)2]– reaction is [H+]-independent and outer sphere (0.35 M–1 s–1), the reaction with [Fe(H2O)6]3+ gives a dependence ka+kb[H+]–1, with ka= 5.4 × 10–4 M–1 s–1 and kb= 5.3 × 10–4 s–1, assigned to outer- and inner-sphere mechanisms respectively. All studies were in aqueous perchloric acid solutions, I= 2.00 M (LiClO4).

In situ scanning tunneling microscopy of electrochemical oxidation of single crystal Au(100) surface in aqueous solution

In situ electrochemical scanning tunneling microscopy (ESTM) was applied to gold surfaces (Au(100)) in an aqueous perchloric acid solution. Images observed on fully oxidized surfaces seemed to be similar to those in the double layer region. This indicated that oxide layers such as AuOH and AuO were uniformly formed on the surface. New islands of a height of ca. 0.15 nm were observed on partially oxidized surfaces, suggesting that the AuOH layer was formed by an island-growth mechanism. Pits and islands of monatomic height were simultaneously created by rather mild oxidation-reduction cycles. The surface diffusion of Au was also investigated.

Potentiometric studies on aqueous fluoride complexes of actinides: Stability constants of Th(IV)-, U(IV)-, Np(IV)- and Pu(IV)-fluorides

Complex formation of Th(IV), U(IV), Np(IV), and Pu(IV) with fluoride ions in aqueous perchloric acid solution of 1M ionic strength has been studied potentiometrically, using a fluoride ion selective electrode. A highly acidic medium was essential to prevent hydrolysis and disproportionation of the tetravalent actinide ions. A procedure developed earlier in this laboratory for studying the fluoride complexes of hexavalent actinides was successfully extended to this work. The β1 values follow the trend Th(IV) Np(IV)>Pu(IV), and β2 values the trend Th(IV) Np(IV)<Pu(IV), while β3 and β4 values followed an increasing order.

The ketonization of acetophenone enol in concentrated aqueous sulphuric and perchloric acid solutions. Implication on X acidity function correlations of the enolization reaction and determination of the keto–enol equilibrium constant as a function of acidity

Rates of ketonization of the enol of acetophenone, generated by flash photolytic photohydration of phenylacetylene, were measured in aqueous sulfuric and perchloric acid solutions over the concentration range 1–50 wt.% acid; rates of enolization of acetophenone, monitored by bromine scavenging, were also measured in aqueous perchloric acid solutions over the same concentration range. The results suggest that the curvature observed in a previous X acidity function correlation of the rate of enolization in sulfuric acid solutions was an artifact produced by insufficiently efficient scavenging, and that introduction of the activity of water in the correlating expression, used previously to eliminate the curvature and believed to reflect covalent involvement of water in the enolization reaction, is unnecessary. The present results also show that the keto–enol equilibrium constant for acetophenone decreases with increasing acidity in these concentrated sulfuric and perchloric acid solutions. Key words: acetophenone, enolization, ketonization, keto–enol equilibrium, concentrated acid solutions.

Acid-catalyzed hydrolysis of cis- and trans-1,2-dimethoxyethene in aqueous solution

Rates of hydrolysis of cis- and trans-1,2-dimethoxyethene were measured in concentrated aqueous perchloric acid solution, and extrapolation of these data to dilute acid gave the catalytic coefficients k H+ =1.16×10 −4 M −1 s −1 for the cis isomer and k H+ =3.98×10 −5 M −1 s −1 for the trans isomer. These results, plus solvent isotope effects measured in 0.5 M hydrochloric acid (k H+ /k D+ =2.29 for the cis isomer and k H+ /k D+ =2.21 for the trans isomer), indicate that these reactions occur by rate-determining protonation of the substrates on carbon. The reactivities of the two substrates are discussed

ChemInform Abstract: Kinetics of Oxidation of Ethanol, Isopropyl Alcohol, and Benzyl Alcohol by (Ethylenebis(biguanide))silver(III) Cation in Aqueous Perchloric Acid Solutions.

AbstractThe title reaction proceeds via two concurrent paths, both first order with respect to the concentration of the reagent and the alcohol.