Hydroquinone Systems Research Articles

Thermodynamic description of the plutonium – α-d-isosaccharinic acid system I: Solubility, complexation and redox behavior

The impact of α-d-isosaccharinic acid (HISA), a degradation product of cellulose, on the solubility and redox behavior of hydrous Pu(IV) oxide was investigated from undersaturation conditions under Ar atmosphere in 0.1 m NaCl–NaOH–NaISA solutions. Redox conditions were buffered with hydroquinone (HQ) or Sn(II), resulting in (pe + pHm) ≈ 9.0 and 1.5, respectively. The influence of ISA on Pu solubility was investigated as function of pHm (8 ≤ pHm ≤ 13) and ligand concentration (10−6 m ≤ m(ISA)tot ≤ 0.1 m).In-situ X-ray diffraction and X-ray absorption spectroscopic measurements indicate that PuO2(ncr,hyd) controls the solubility of Pu in all investigated systems. Presence of ISA poses an increase in plutonium solubility by up to 2.5 log-units. In HQ systems, slope analysis of solubility data in combination with solid phase characterization and density functional theory calculations resulted in chemical and thermodynamic models including the predominance of Pu(IV)(OH)3ISA–H– and Pu(IV)(OH)3ISA–2H2– complexes below and above pHm ≈ 12, respectively. In Sn(II) systems with pHm < 11.5, a reductive dissolution occurs involving the formation of Pu(III)(OH)ISA–H(aq). Experimental observations suggest also that Pu–ISA colloids importantly contribute to the overall Pu concentration in solution, both in HQ and Sn(II) systems. Although this process is not included in the thermodynamic model derived, it cannot be neglected for estimating Pu solubility limits under these conditions. This work provides the most comprehensive thermodynamic dataset available to date for the system Pu3+–Pu4+–OH––Cl––ISA––H2O(l) valid under a wide range of conditions relevant for nuclear waste disposal.

Kinetic studies of hydroxyquinone formation from water soluble benzoquinones

The kinetics and mechanisms of the redox reactions between hydrogen peroxide and 1,4-benzoquinone, 2-methyl-1,4-benzoquinone, 2,6-dimethyl-1,4-benzoquinone, 2-chloro-1,4-benzoquinone and 2,6-dichloro-1,4-benzoquinone were studied in aqueous media using spectrophotometric monitoring. The formation and decay of a hydroxylated 1,4-benzoquinone was detected. The formation of the intermediate was first order with respect to the parent 1,4-benzoquinone and hydrogen peroxide, whereas inverse first order dependence was revealed with respect to the hydrogen ion. The decomposition reaction had two parallel pathways: one was first order with respect to the intermediate, while the other showed second-order dependence. The values of the rate constant measured for the formation step were successfully correlated with both the redox potentials of the substituted quinone–hydroquinone systems and the pKa values of the hydroxylated quinone derivatives. Therefore, electronic effects govern the reactivity of the quinones in this process. NMR and GC-MS measurements were carried out to identify the products in the system. Quantum mechanical calculations were also carried out in these systems.

Reversible chain reaction between N,N′-diphenyl-1,4-benzoquinonediimine and 2,5-dichlorohydroquinone: Kinetics, mechanism, and the rate constants of elementary steps

The kinetics of the reversible chain reaction between N,N′-diphenyl-1,4-benzoquinonediimine and 2,5-dichlorohydroquinone was studied in chlorobenzene at 298 and 343 K. Experiments in the presence of an initiator proved that the reaction proceeds via a chain mechanism with a chain length of ∼104 to 105 units, depending on the reactant and initiator concentrations and temperature. The reaction rate first increases and then decreases with increasing concentrations of the reaction products (N,N′-diphenyl-1,4-phenylenediamine and 2,5-dichloroquinone) due to the pronounced reversibility of the chain termination and propagation steps involving the reaction products. The reaction orders with respect to the components were determined. The rate constants and activation energies of most of the elementary steps of the forward and backward chain reactions were determined or reliably estimated. Induction periods were observed for the first time in the reversible chain reactions in the quinoneimine + hydroquinone systems. The induction periods are due to the long time required for the establishment of the steady-state radical concentrations in the system.

Activity Coefficient Studies in Ternary Aqueous Solutions at 298.15 K: H2O + α-Cyclodextrin + Potassium Acetate and H2O + 18-Crown-6 + Hydroquinone Systems

Activity coefficients of the components in aqueous ternary solutions containing a fixed concentration of α-cyclodextrin and 18-crown-6 at various concentrations of the third solutes potassium acetate and hydroquinone, respectively (concentration range (0.0 to 0.20) mol·kg−1), at 298.15 K were derived from vapor pressure osmometry. Also, data for the osmotic coefficients and activity coefficients for aqueous binary solutions of potassium acetate are obtained and used along with the data for aqueous binary hydroquinone solutions reported earlier to analyze the data of the ternary mixtures. It is observed that there is a lowering of activity coefficients of potassium acetate as well as of hydroquinone in the presence of α-cyclodextrin and 18-crown-6, respectively, which is being attributed to incorporation of the acetate ion and hydroquinone into the α-cyclodextrin and 18-crown-6 cavities, respectively, to form 1:1 inclusion complexes in the solution phase. The salting-in and thermodynamic equilibrium consta...

Sorption of organic compounds by hydroxy-interlayered clays through chelation and humification processes

This experiment was carried out to study the interaction of citric acid, hydroquinone and catechol on Montmorillonite and hydroxy-interlayered Montmorillonite clays. Montmorillonite (Mt) and its interlayered, viz., hydroxy-Fe (FeMt), mixed hydroxy-Fe,Al (Fe,AlMt), and hydroxy-Al (AlMt) counterparts were used for the study. Clay–organic complexes were prepared by aging clay suspensions at adjusted pH of 6, for 30 days after adding organics at the rate of 300 cmol/kg of clay. Controls without organics, namely, Mt, FeMt, Fe,AlMt, and AlMt, were aged and separated similarly. The CEC, specific surface area and total C were determined, and X-ray diffraction patterns were obtained for the separated solid phase. Final pH of each suspension was recorded and the absorbance spectra between 400 and 600 nm were obtained in the supernatants of clay–catechol and clay–hydroquinone suspensions. Final average pH of the clay suspensions in citrate (Ct), hydroquinone (HQ) and catechol (CC) systems were 6.8, 5.9 and 4.7, respectively. Specific surface area and CEC in FeMt, Fe,AlMt and AlMt clays increased significantly after aging with citric acid. Amounts of Fe and Al determined in the supernatant solutions of clay–catechol and clay–hydroquinone systems were in the range of 0 to 18 cmol(+)/kg of clay while in clay–citrate systems were 170 to 347 cmol(+)/kg of clay. The d-spacings of AlMtCt and Fe,AlMtCt were 1.26 nm and 1.24 nm, respectively, which were lower than the original values of 1.40 nm and 1.43 nm for AlMt and Fe,AlMt. Similarly, FeMtCt reported lower d-spacing of 1.26 nm in comparison to its original d-spacing of 1.33 nm in FeMt, after aging with citrate. Hydroxy-interlayered clays aged with HQ and CC did not show large changes in d-spacings. The absorbance values measured at 400 nm in catechol and hydroquinone systems were in the range of 0.16–0.95 and 0.05–0.21, respectively. The amounts of organic C retained by the solid phases in catechol systems were significantly greater than that for hydroquinone or citrate systems. Results of the study showed that the citric acid had caused the dissolution of the hydroxy-interlayer polymers in hydroxy-interlayered clay minerals, while hydroxy-interlayered clays catalyzed the formation of catechol- and hydroquinone-derived humic substances.

Kinetic analysis of reversible chain reactions in quinoneimine + hydroquinone systems

Kinetic analysis of reversible chain reactions in quinoneimine + hydroquinone systems

Reductive Dissolution of PuO2(am): The Effect of Fe(II) and Hydroquinone

PuO2(am) solubility was investigated as a function of time, for pH from 0.5 to 11, and in the presence of 0.001 M FeCl2 or 0.00052 M hydroquinone to determine the effect of environmentally important reducing agents on PuO2(am) solubilization under geological conditions. Equilibrium was reached in <4 days. The observed PuO2(am) solubilities were many orders of magnitude higher than the Pu(IV) concentrations predicted from thermodynamic data. Spectroscopic, solvent extraction, and thermodynamic analyses of data showed that Pu(III) was the dominant aqueous oxidation state. The experimental pH, pe, and Pu(III) concentrations from both the Fe(II) and hydroquinone systems provided a log K0 value of 15.5 ± 0.7 for [PuO2(am) + 4H+ + e− ⇌ Pu3+ + 2H2O]. The data show that reduction reactions involving Fe(II) and hydroquinone are relatively rapid and that reductive dissolution of PuO2(am), hitherto ignored, may play an important role in controlling Pu behavior under reducing environmental conditions.

The kinetics and thermodynamics of quinone–semiquinone–hydroquinone systems under physiological conditions

The steady-state concentration of semiquinones (Q˙–) determined by EPR in the mixtures of eleven alkyl-, methoxy- and chloro-substituted 1,4-benzoquinones as well as 1,4-naphthoquinone (Q) with corresponding hydroquinones (QH2) in aqueous buffer, pH 7.40, was used to calculate a constant for equilibrium (1) Q + QH2⇌Q˙– + Q˙– + 2H+ (k1; 2k–1; K1 = k1/2k–1). The rate constants for comproportionation between Q and QH2, k1, were calculated from the combination of K1 determined in this work and 2k–1 reported previously. The Nernst equation was applied to calculate the change in one-electron reduction potential ΔE1 = E(Q/Q˙–) – E(Q˙–/QH2) in equilibrium (1). The E(Q˙–/QH2) values were calculated from ΔE1 and the values of E(Q/Q˙–) known from the literature. The correlations between E(Q˙–/QH2) and E(Q/Q˙–) as well as between ΔE1 (k1) and E(Q/Q˙–) are discussed. The values of ΔE1 and k1 are suggested to be the key factors governing the autoxidation of QH2.

Could the acid doping of polyaniline represent the charge transfer interaction?

It is proposed to describe the acid doping of the emeraldine base (EB) of polyaniline (PANI) as a donor-acceptor interaction, followed by a charge separation (charge transfer) inside the complex formed according to the scheme: D + H-Acc= (D→H-Acc) D +-H +Acc −, where D is the imine center of EB and H-Acc the acceptor possessing the mobile H atom. The hydrogen mediator supplied from the acceptor molecule is joined to the polymer chain in the second stage of transformation. The electrochemical system, PANI electrode/hydroquinone (4F-hydroquinone) solution, has been investigated as a model of this D-H-Acc interaction. It has been found that the redox reactions of both hydroquinone systems take place on PANI practically without overpotential and, probably, the two-electron charge transfer processes are realized in such systems. Reaction currents depend on PANI quantities deposited on the Pt electrode. On the PANI/electrolyte interface the stoichiometric complex is formed; it consists of nearly four units of aniline and one molecule of quinone (hydroquinone). There exists a great electronic exchange into this surface complex involving the acid centers of PANI as hydrogen mediators in the charge transfer on an organic acceptor. D-H-Acc interaction between EB and hydroquinone/quinone under the ‘wet’ electrochemical and ‘dry’ solid-state condition has been compared. We have concluded that the electron exchange between the EB chain and surface adsorbed molecules is the necessary condition for the doping occurring in both cases. The similarity has been revealed between the reactive center in the PANI-quinone/hydroquinone redox system and the donor-acceptor interaction in porphyrin-quinone photochemical reactions and other biological systems. It is proposed to involve the quinone and other acceptor molecules in the optical processes occurring with the π-electron system of conducting polymers.

Extension of the voltammetric theory to inert, permeable thin-film coated rotating disc electrodes Part II. Behaviour of ferrocene—ferricinium ion and quinone—hydroquinone systems on polymer-coated electrodes

The voltammetric behaviour of ferrocene-ferricinium ion and quinone-hydroquinone systems is investigated on electrochemically thin film polymer coated electrodes. Ferrocene oxidation is studied in 0.1 M NBu 4 ClO 4 -nitromethane on rotating disc platinum electrodes coated with poly(2-hydroxymethyl-1,4-phenylene) oxide (I), poly-(2,6-dimethyl-1,4-phenylene) oxide (II), and poly[4-(2-aminoethyl)-1,2-phenylene] oxide (III) films. The quinone-hydroquinone system is investigated in aqueous medium (1 M HClO 4 ) with (I), (II) and poly-(2-cyano-1,4-phenylene) oxide films. Experimental results are in good agreement with the previously calculated voltammetric curves in steady-state mass transfer conditions. The charge transfer and diffusion parameters on these polymer-coated electrodes are calculated The quinone-hydroquinone system is rendered reversible by coating a platinum electrode with I.

Oxidation-reduction polymers

Abstract