Use of Tin (IV) Porhyrins as Ionophores for the Construction of Phthalate‐Selective Electrodes: Influence of the Structure and Membrane Composition on their Response Properties

Tin(IV) porphyrins derivatives were used as ionophores for phthalate selective electrodes preparation. The influence of ionophore structure and membrane composition (amount of incorporated ionic sites) on the electrode response, selectivity and long‐term stability were studied. Poly(vinyl chloride) polymeric membranes plasticized with o‐NPOE (o‐nitrophenyloctylether) and containing Sn(IV)‐tetraphenylporphyrin (TPP) dichloride (Sn(IV)[TPP]Cl₂) or Sn(IV)‐octaethylporphyrin (OEP) dichloride (Sn(IV)[OEP]Cl₂), and in some cases incorporating lipophilic cationic (tetraocthylammonium bromide ‐ TOABr) and anionic (sodium tetraphenylborate – NaTPB and potassium tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate‐KTFPB) additives, were prepared and their potentiometric characteristics compared. Both ionophores are shown to operate via a neutral mechanism, and the addition of 10 mol % of lipophilic quaternary ammonium salt derivative to the membrane is required to achieve optimal electrode performance. The potentiometric units prepared, with Sn(IV)[TPP]Cl₂ (Type A) or Sn(IV)[OEP]Cl₂ (Type B) without additives, presented a slope of ?52.8 mV dec^?1 and ?58.8 mV dec^?1 and LLLR of 9.9×10^?5 mol L^?1 and 9.9×10^?6 mol L^?1, respectively. The units prepared using the same metalloporphyrins and incorporating 10% mol TOABr presented a slope of ?55.0 mV dec^?1 and ?57.8 mV dec^?1 and LLLR of 5.0×10^?7 mol L^?1 and 3.0×10^?7 mol L^?1. Their analytical usefulness was assessed by potentiometric determinations of phthalate in water and industrial products providing results that presented recoveries of about 100%.     

Mutual Separation and Determination of Th(IV) and U(VI) Using Arsenazo III as a Dye Collector Reagent by Flotation‐Spectrophotometric Method

This paper reports a simple and highly selective method for the separation, preconcentration, and determination of trace amounts of thorium and uranium in some complex samples via staircase flotation. The method is based on the initial flotation of the Th(IV)‐arsenazo III complex in the presence of U(VI) from a solution of 5 mol dm^?3 HCl, then reduction of U(VI) to U(IV) and repetition of the flotation step. In both steps, the floated complex was dissolved in a 5‐mL portion of methanol and its absorbance was measured at 655 nm, spectrophotometrically. For a 30‐mL portion of the sample, Beer's law was obeyed over the concentration ranges of 3.40 × 10^?7to 3.06 × 10^?6 mol dm^?3 for Th(IV) and3.40 × 10^?7 to 3.40 × 10^?6 mol dm^?3 for U(IV) with the apparent molar absorptivity of 4.20 × 10⁵ dm³ mol^?1 cm^?1 and 3.59 × 10⁵ dm³ mol^?1 cm^?1, respectively. The RSDs (n = 7) corresponding to 1.7 × 10^?6 mol dm^?3 of Th(IV) and U(IV) were obtained as 1.7% and 1.87%. The detection limits (7 blanks) for both the metal ions were found to be 1.7 × 10^?7 mol dm^?3. The important benefit of the method is that the determinations are free from the interference of almost all cations and anions found in the complex matrixes, such as seawater samples. The proposed method was also applied to reference materials, and the determinations were shown to have good agreement with the certified values.     

Effect of six triorganotin(IV) compounds on nitrification and ammonification in soil

The effects of six triorganotin(IV) compounds and of Thiram on nitrification and ammonification in soil were investigated. Low concentrations of up to 50 μg g^?1 of the triorganotin(IV) compounds enhanced nitrate-nitrogen (NO₃^?-N) production in soil. Except for diphenylbutyltin bromide, which inhibited nitrification at 250 μg g^?1, the other triorganotin(IV) compounds were inhibitory at concentrations of 100 μg g^?1 to less than 250 μg g^?1. At 10 μg g^?1, only triphenyltin acetate was less inhibitory towards nitrification compared with Thiram. At 250 μg g^?1, Thiram exerted a strongly persistent inhibitory effect towards nitrification. The NO₃^?-N level recorded 28 days after application was only 0.10 μg g^?1 soil. With the triorganotin compounds NO₃^?-N levels of 7.05–12.06 μg g^?1 soil were recorded 28 days following their application. The deleterious effects of the triorganotin(IV) compounds were less persistent and recovery of nitrification was evident seven days after application. Low concentrations of Thiram and triorganotin(IV) compounds inhibited ammonification, whereas higher concentrations enhanced ammonification. Complete inhibition of ammonification was attained 21–28 days after application of Thiram at 50 μg g^?1. On the other hand, with the triorganotin(IV) compounds, except for diphenylbutyltin bromide at 10–50 μg g^?1, ammonification persisted at all concentrations 28 days after application.     

The formation of flouride complexes of titanium(IV)

The complex formation equilibria between titanium(IV) and fluoride ions have been studied at 25°C in 3 M(Na)Cl ionic medium by measuring, with an ion selective electrode for F^?, the free HF concentration in acid Ti(IV) solutions. The [H⁺] was kept within 0.25 and I M where the predominant form of uncomplexed metal is the dihydroxotitanium(IV) ion, Ti(OH)²⁺₂. The potentiometric data have been explained by assuming Ti(OH)₂F⁺, TiF₄ and HTiF^?₆, with equilibrium constants given in Table 3. Within the accuracy of the present e.m.f. study, ±0.2 mV, no evidence for intermediate complexes bearing 2, 3 and 5 F^? was found.From a special series of measurements, carried out by replacing a large part of the Cl^? with ClO^?₄, it is concluded that no appreciable amount of Ti(IV)Cl complexes is formed at the 3 M level employed as ionic medium.     

Role of solution chemistry on the trapping of radionuclide Th(IV) using titanate nanotubes as an efficient adsorbent

Titanate nanotubes (TNTs) have attracted great interest in multidisciplinary study since their discovery. The adsorption of thorium [Th(IV)] onto TNTs in the absence and presence of humic acid (HA)/fulvic acid (FA) was studied by batch technique. The influence of pH from 2.0 to 10.0, ionic strength from 0.001 to 0.1 mol L^?1 NaClO₄, and coexisting electrolyte cations (Li⁺, Na⁺, K⁺) and antions (ClO₄ ^?, NO₃ ^?, Cl^?) on the adsorption of Th(IV) onto TNTs was tested. The adsorption isotherms of Th(IV) was determined at pH 3.0 and analyzed with Langmuir and Freundlich adsorption models, respectively. The results demonstrated that the adsorption of Th(IV) onto TNTs increases steeply with increasing pH from 2.0 to 4.0. Generally, HA/FA was showed to enhance Th(IV) adsorption onto TNTs at low pH values, but to reduce Th(IV) adsorption onto TNTs at high pH values. The adsorption of Th(IV) onto TNTs was also dependent on coexisting electrolyte ions in aqueous solution under our experimental conditions. The adsorption of Th(IV) onto TNTs is exothermic and spontaneous. The findings indicating that TNTs can be used as a promising candidate for the enrichment and solidification of Th(IV) or its analogue actinides from large volume solution in real work.     

Spectrophotometric Determination of Rhenium(IV) with Thiocyanate,TX-100 and N,N′-Diphenylbenzamidine

A spectrophotometric method to determine rhenium(IV) at trace level is based on the extraction of Re(IV)-SCN^? complex in sulphuric acid media with N,N′-diphenylbenzamidine(DPBA) in presence of a non-ionic surfactant triton X-100 (TX-100) in chloroform. The complex shows maximum absorbance at 435 nm with amolar absorptivity value of 4.24 × 10⁴ L mol^?1 cm^?1 at an acidity range 3.5-6.5 M H₂SO₄. The method followed Beer's Law for the system Re(IV)-SCN^?(TX-100)-DPBA upto 4.0 μg Re(IV) mL^?1. The detection limit of the method is 5 ppb. None of the tested foreign ions, except molybdenum(VI), interfere with the determination of rhenium. The interference due to molybdenum could effectively be removed by prior precipitation with oxine. The effect of various analytical parameters on the extraction of the metal are discussed.     

Voltammetric study of the reduction process of ethylenediaminetetraacetatovanadium(IV): oxidation-state-dependent speciation

Cyclic voltammetry, differential-pulse polarography (d.p.p.), and direct-current tast polarography (d.c.t.p.) were used to study the electrochemical reduction of ethylenediaminetetraacetatovanadium(IV) [V(IV)OEDTA^2?]. The two-electron reduction wave of V(IV)OEDTA^2? is gradually split into two one-electron waves when the concentration of the supporting electrolyte is increased. A detailed study of the electrochemistry of V(III)EDTA^? allowed the conclusion that the two one-electron waves of V(IV)OEDTA^2? reflect the reduction sequence V(IV)OEDTA^2?$\text{+e}{}^{\mathrm{?}}$ V(III)EDTA^?$\text{+e}{}^{\mathrm{?}}$|$\text{?e}{}^{\mathrm{?}}$ V(II)EDTA^2?. The process responsible for the observed “chemical” peak separation was found to be ion pairing, by which the reduction of V(IV)OEDTA^2? is facilitated. A method is presented by which the parallel determination of V(IV) and V(III) (when present as the appropriate EDTA complexes) is possible with d.p.p, or d.c.t.p.     

Rapid and selective determination of osmium(IV) by UV-visible spectrophotometry using o-methylphenyl thiourea as a chromogenic chelating ligand: sequential separation of osmium(IV), rhodium(III) and platinum(IV)

The objective of this research work was to develop a simple, highly sensitive and precise method for spectrophotometric determination of osmium(IV). O-Methylphenyl thiourea (OMPT) coordinates with osmium(IV) as a 1:1 (osmium(IV)–OMPT) complex in hydrochloric acid media (0.8 mol l^?1). The novelty of the investigated method is instant complex formation at room temperature with no need of heating or standing. The complex is stable for more than 8 days. The method is applicable over a wide linearity range (up to 110 µg ml^?1). A low reagent concentration is required (2 ml, 0.009 mol l^?1 in ethanol). The complex exhibits maximum absorption in the range of wavelength 510–522 nm and 514 nm was selected for further study. The molar absorptivity was 1.864 × 10³ l mol^?1 cm^?1, Sandell’s sensitivity was 0.102 µg of osmium(IV) cm^?2. Proposed method was successfully applied for separation and determination of osmium(IV) from binary and ternary synthetic mixtures containing associated metal ions. A scheme for mutual separation of osmium(IV), rhodium(III) and platinum(IV) is developed.     

Stripping Voltammetric Determination of Zirconium with Complexing Preconcentration of Zirconium(IV) at a Morin‐Modified Carbon Paste Electrode

A sensitive, selective and economic stripping voltammetry is described for the determination of trace amounts of zirconium at a morin‐modified carbon paste electrode (morin‐MCPE). Zirconium(IV) can be preconcentrated on the surface of the morin‐MCPE due to forming the Zr(IV)–morin complex. The complex produces two second‐order derivative anodic peaks at 0.69 V (vs. SCE) and 0.75 V when linear‐scanning from 0.0 to 1.0 V. The optimum analytical conditions are: 2.2 mol L^?1 HCl, 0.0 V accummulation potential, 90 s accummulation time, 250 mV s^?1 scan rate. A linear relationships between the peak currents at 0.75 V and the Zr(IV) concentration are in the range of 2.0×10^?8 to 3.0×10^?6 mol L^?1. The detection limit is 1.0×10^?8 mol L^?1 (S/N=3) for 120 s accumulation. The RSD for determination of 4.0×10^?7 mol L^?1 Zr(IV) is 4.8% (n=8). The proposed method has been applied to determine zirconium in ore samples, unnecessarily extracted.     

Selective preconcentration of trace thorium from aqueous solutions with Th(IV)-imprinted polymers prepared by a surface-grafted technique

A novel ion-imprinted adsorbent for selective solid phase extraction of thorium(IV) based on the surface of silica gel was prepared by a surface-grafted technique with methacrylic acid (MAA) as a functional monomer. After removal of Th(IV) ions with 3?mol?L^?1 HCl solution, the obtained imprinted particles for Th(IV) exhibited specific recognition and relatively rapid kinetic process. The maximum static and total dynamic adsorption capacity of the ion-imprinted polymers (IIPs) for Th(IV) was 33.2 and 17.3?mg?g^?1, respectively. A comparison of the selectivity coefficient of the imprinted polymers with that of non-imprinted polymers showed that the imprinted matrix for Th(IV)/U(VI), Th(IV)/Ce(III), Th(IV)/La(III) and Th(IV)/Zr(IV) was 58.8, 107, 106.4 and 151.7 times greater than non-imprinted matrix, respectively. With a series of samples loading flow rate of 3?mL?min^?1 for preconcentration, an enrichment factor of 14.6 and the detection limit of 0.59?µg?L^?1 were obtained. The relative standard deviation of the method under optimum conditions was 2.1% (n?=?7). The developed method was successfully applied to the determination of trace Th(IV) in real water samples with satisfactory results.     

Catalytic Adsorptive Stripping Voltammetry of Germanium(IV) in the Presence of Gallic Acid and Vanadium(IV)‐EDTA

A highly sensitive and selective catalytic adsorptive cathodic striping procedure for the determination of trace germanium is presented. The method is based on adsorptive accumulation of the Ge(IV)‐gallic acid (GA) complex onto a hanging mercury drop electrode, followed by reduction of the adsorbed species. The reduction current is enhanced catalytically by addition of vanadium(IV)‐EDTA. The optimal experimental conditions include the use of 0.03 mol/L HClO₄ (pH1.6), 6.0×10^?3 mol/L GA, 3.0×10^?3 mol/L V(IV), 4.0×10^?3 mol/L EDTA, an accumulation potential of ?0.10 V(vs. Ag/AgCl), an accumulation time of 120 s and a differential pulse potential scan mode. The peak current is proportional to the concentration of Ge(IV) over the range of 3.0×10^?11 to 1.0×10^?8 mol/L and the detection limit is 2×10^?11 mol/L for a 120 s adsorption time. The relative standard deviation at 5.0×10^?10 mol/L level is 3.1%. No serious interferences were found. The method was applied to the determination of germanium in ore, mineral water and vegetable samples with satisfactory results.     

Electron transfer. 127 Intermediate oxidation states in the reduction of chromium(VI) with formate

When HCrO₄ ^? is reduced by formate in solutions buffered by 2-ethyl-2-hydroxybutanoic acid and its anion, chelated complexes of both Cr(IV) and Cr(V), both of them stabilized in the medium used, are formed. It appears that Cr(V) is not generated directly from the Cr(VI)-formate reaction but arises instead from oxidation of Cr(IV) by Cr(VI). When the Cr(VI)-formate reaction is allowed to go to completion in the presence of [Cl(NH₃)₅Co]²⁺, a scavenger for Cr(II), 84–86% of the Cr(VI) taken is found to be converted to Cr(II), indicating that nearly all of the reacting system proceeds through Cr(IV) and bypasses the more usual state Cr(III). Initial rates for formation of Cr(IV) lead to a rate law pointing to a transition state containing the two redox partners, two ligating carboxyl groups, and two units of H⁺. Substitution of DCO₂ ^? for HCO₂ ^? retards formation of Cr(IV) by a factor of 3.3, whereas the solvent isotope affect, rate_{D 2}O/rate_{H 2}O, favors the deuterated system by a factor of 1.4. Our observations are in accord with a sequence initiated by the ligation of HCrO₄ ^? to a chelate derived from the buffering carboxylate anion. Conversions of Cr(VI) to Cr(IV), and Cr(IV) to Cr(II) appear to entail hydride shifts from formate to the Cr(=O) function.     

Novel Membrane Potentiometric Thiocyanate Sensor Based on Tribenzyltin(IV) Dithiocarbamate

A novel PVC membrane ion‐selective electrode based on tribenzyltin(IV) dithiocarbamate [Sn(IV)–TBDTB] as neutral carrier was developed for thiocyanate (SCN^?) determination. The electrode exhibits a near‐Nernstian response for SCN^? with a slope of 62.8±2.0 mV per decade over a wide concentration range 1.0×10^?1–2.0×10^?6 mol L^?1 and a detection limit of 1.0×10^?6 mol L^?1 in MES–NaOH buffer, pH 6.0, at 25 °C. The electrode prepared with 1.5 wt.% Sn(IV)–TBDTB, 32.5 wt.% PVC and 66.0 wt.% 2‐nitrophenyloctyl ether (o‐NPOE) shows optimal response characteristics. Anti‐Hofmeister selectivity sequence for a series of anions shown by the electrode was as follows: SCN^?>Sal^?>I^?>ClO >phCOO^?>CH₃COO^?>Br^?>Cl^?>NO >NO >Citrate>SO₄^2?. The useful pH range for the electrode was found to be 3–7 with a response time 30–40 s. The electrode has been used for direct determination of thiocyanate in wastewater with satisfactory results.     

Aqueous polymerization of methyl methacrylate initiated by the redox system Ce(IV)–isopropyl alcohol. Kinetics and molecular weight

Polymerization of methyl methacrylate was carried out in aqueous nitric acid in the temperature range 26–40°C, with the redox initiator system ceric ammonium nitrate–isopropyl alcohol. A short induction period was observed, as well as the attainment of a limiting conversion, and the total ceric ion consumption with reaction time. The reaction orders were 1/2 and 3/2 with respect to the IPA and monomer concentration, respectively, within the range (3–5) × 10^?3M of Ce(IV). But at lower Ce(IV) concentration (≤ 1 × 10^?3M), the order with respect to monomer and Ce(IV) changed to 1 and 1/2, respectively. The rate of ceric ion disappearance was first order with respect to Ce(IV) concentration and (R_Ce)^?1 was proportional to [IPA]^?1. Both the rate of polymerization and the rate of ceric ion consumption increase with rise in temperature. The average-molecular weight can be controlled by variations in IPA, Ce(IV), and monomer concentrations, and in temperature. A kinetic scheme involving oxidation of IPA by Ce(IV) via complex formation, whose decomposition gives rise to a primary radical, initiation, propagation, and termination of the polymeric radicals by bimolecular interaction is proposed. An oxidative termination of primary radicals by Ce(IV) is also included.     

Halide and complex halogeno anions as salts of oxinato bis(η5-indenyl)titanium(IV)/zirconium(IV) chelates

Bis(η⁵-indenyl)titanium(IV) dichloride and bis(η⁵-indenyl)zirconium(IV) dichloride, when treated with 8-hydroxyquinoline (oxine) in aqueous medium form ionic derivatives of the type [(η⁵-C₉H₇)₂ML]⁺Cl^? (M = Ti(IV), Zr(IV), L is the conjugate base of oxine). A number of halide and complex halogeno anions present in aqueous solution were isolated as salts of these ionic complexes giving derivatives of the type, [(η⁵-C₉H₇)₂ML]⁺X^? (X = Br^?, I^?, ZnCl₃(H₂O)^?, CdCl₄^2?, HgCl₃^?). Conductivity measurements in nitrobenzene indicate that these complexes are electrolytes. Both the IR and ¹H NMR spectral studies demonstrate that the ligand L is chelating. Consequently there is tetrahedral coordination about the titanium(IV) or zirconium(IV) ion.     

Xylenol Orange,Zirconium(IV) and Fluoride Color Reaction in Mixed Micelles of N-Hexadecylpyridinium Chloride and Brij 35, and Its Analytical Application1

Abstract

The color reaction between Xylenol orange (XO), zirconium (IV) and fluoride ions in the presence of various surfactants alone or in combination was studied at various pH. The XO -zirconium)IV)-fluoride ion ternary complex in mixed micellar media containing a low concentration of N-hexadecylpyridinium chloride (HPC) as a cationic surfactant and large amounts of (poly{oxyethylene)dodecyl ether (Brij 35) as a nonionic surfactant at weakly acidic media was found to be the most stable, and showed a remarkable bathochromic shift and clear contrast against a reagent blank. The maximum absorbance was at 600 nm in the mixed micellar media at pH 3.5, and the apparent molar absorptivities at 600 nm were 7.0 × 10⁴ 1 mol^?1 cm^?1 for zirconium(IV) and 1.4 × 10⁴ 1 mol^?1 cm^?1 for fluoride ion. The calibration curves covered the ranges of 0.5 ～ 20.0 μg/10 ml zirconium! IV) and 0 ～ 20.0 μg/10 ml fluoride ion with the Sandell sensitivities being 0.0013 μg/cm² for zirconium(IV) and 0.0016 μg/cm² for fluoride ion.     

Determination of uranium using a flow system with reagent injection. Application to the determination of uranium in ore leachates

The determination of uranium by a flow system with reagent injection is based on the reaction of U(IV) with Arsenazo III in 3.6 M HCl; U(IV) is generated by reduction of uranyl ion in a lead reductor minicolumn installed in the sample channel of the manifold. The interference effect caused by several ions is studied. The calibration graph is linear up to 1.0 × 10^?5 M (2.4 mg l^?1) and the detection limit is 2.8 × 10^?8 M (6.6 μg l^?1). The modification of the manifold by including a second valve to by-pass the reducing column allows the measurement of the difference in peak heights, which makes the method specific for uranium.     

Determination of Se(IV) in On‐Line System by Cathodic Stripping Voltammetry

A sensitive and selective on‐line voltammetric procedure for determination of traces of Se(IV) is presented. The pulsed potential accumulation was proposed for minimization of interferences of surface active substances and foreign ions. The calibration plot was linear from 1×10^?9 mol L^?1 to 4×10^?8 mol L^?1 for accumulation time of 180 s. The relative standard deviation was 6.1% (n=5) for a Se(IV) concentration of 1×10^?8 mol L^?1. The detection limit estimated from (3 σ) for an accumulation time of 180 s was about 4×10^?10 mol L^?1. The validation of the procedure proposed was made by a recovery tests for tap and river water samples.     

The kinetics of complex formation between Ti(IV) and hydrogen peroxide

The kinetics of the formation of the titanium‐peroxide [TiO²⁺₂] complex from the reaction of Ti(IV)OSO₄ with hydrogen peroxide and the hydrolysis of hydroxymethyl hydroperoxide (HMHP) were examined to determine whether Ti(IV)OSO₄ could be used to distinguish between hydrogen peroxide and HMHP in mixed solutions. Stopped‐flow analysis coupled to UV‐vis spectroscopy was used to examine the reaction kinetics at various temperatures. The molar absorptivity (ε) of the [TiO²⁺₂] complex was found to be 679.5 ± 20.8 L mol^?1 cm^?1 at 405 nm. The reaction between hydrogen peroxide and Ti(IV)OSO₄ was first order with respect to both Ti(IV)OSO₄ and H₂O₂ with a rate constant of 5.70 ± 0.18 × 10⁴ M^?1 s^?1 at 25°C, and an activation energy, E_a = 40.5 ± 1.9 kJ mol^?1. The rate constant for the hydrolysis of HMHP was 4.3 × 10^?3 s^?1 at pH 8.5. Since the rate of complex formation between Ti(IV)OSO₄ and hydrogen peroxide is much faster than the rate of hydrolysis of HMHP, the Ti(IV)OSO₄ reaction coupled to time‐dependent UV‐vis spectroscopic measurements can be used to distinguish between hydrogen peroxide and HMHP in solution. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 457–461, 2007     

Kinetics and mechanism of ruthenium (III) catalyzed oxidation of ethanol by cerium (IV) in aqueous sulfuric acid media

The kinetics of oxidation of ethanol by cerium(IV) in presence of ruthenium(III) (in the order of 10^?7 mol dm^?3) in aqueous sulfuric acid media have been followed at different temperatures (25–40°C). The rate of disappearance of cerium(IV) in the title reaction increases sharply with increasing [C₂H₅OH] to a value independent of [C₂H₅OH] over a large range (0.2–1.0 mol dm^?3) in which the rate law conforms to: where [Ru]_T gives the total ruthenium (III) concentration. The values of 10^?3k_c and 10^?3k_d are 3.6 ± 0.1 dm³ mol^?1 s^?1 and 3.9 ± 0.2 s^?1, respectively, at 40°C, I = 3.0 mol dm^?3. The proposed mechanism involves the formation of ruthenium(III)^? substrate complex which undergoes oxidation at the rate determining step by cerium(IV) to form ruthenium(IV)^? substrate complex followed by the rapid red-ox decomposition giving rise to the catalyst and ethoxide radical which is oxidized by cerium(IV) rapidly. The mechanism is consistent with the existence of the complexes Ru^III · (C₂H₅OH) and Ru^III · (C₂H₅O^?) and both are kinetically active. The overall bisulphate dependence conforms to: k_obsd = A[Ru]_T/{1 + C[HSO₄^?]} where A = 2.2 × 10⁴ dm³ mol^?1 s^?1, C = 1.3 at 40°C, [H⁺] = 0.5 mol dm^?3, and I = 3.0 mol dm^?3. The observations are consistent with the Ce(SO₄)₂ as the kinetically active species. © 1995 John Wiley & Sons, Inc.