The titrimetric determination of beryllium(II) with 1,2-phenylenediamine-N,N,N′,N′-tetraacetic acid

Beryllium (ca. 10^?2?10^?4 M) is determined by adding excess of 1,2-phenylenediamine-N,N,N′, N′-tetraacetic acid (PhDTA, H₄L) and back-titrating with copper(II); arsenazo-I serves as indicator. Formation constants of BeL and BeHL were determined by potentiometry: log K_BeL=6.48±0.02 and log K^H_BeHL=3.48±0.03 (25°C, I=1 M in NaClO₄). Expressions for the titration curve are given together with theoretical errors.     

KINETICS OF FORMATION AND STABILITY CONSTANTS OF MONO AND BIS l-TYROSINE COMPLEXES OF Cu(II), Co(II), AND Ni(II)

Abstract

The kinetics and stability constants of l-tyrosine complexation with copper(II), cobalt(II) and nickel(II) have been studied in aqueous solution at 25° and ionic strength 0.1 M. The reactions are of the type M(HL)^(3-n)+ _n-1 + HL- ? M(HL)^(2-n)+_n(k_n, forward rate constant; k_-n, reverse rate constant); where M=Cu, Co or Ni, HL^? refers to the anionic form of the ligand in which the hydroxyl group is protonated, and n=1 or 2. The stability constants (K_n=k_n/k_-n) of the mono and bis complexes of Cu²⁺, Co²⁺ and Ni²⁺ with l-tyrosine, determined by potentiometric pH titration are: Cu²⁺, log K₁=7.90 ± 0.02, log K₂=7.27 ± 0.03; Co²⁺, log K₁=4.05 ± 0.02, log K₂=3.78 ± 0.04; Ni²⁺, log K₁=5.14 ± 0.02, log K₂=4.41 ± 0.01. Kinetic measurements were made using the temperature-jump relaxation technique. The rate constants are: Cu²⁺, k₁=(1.1 ± 0.1) × 10⁹ M ^?1 sec^?1, k_-1=(14 ± 3) sec^?1, k₂=(3.1 ± 0.6) × 10⁸ M ^?1 sec^?1, k^?2=(16 ± 4) sec^?1; Co²⁺, k₁=(1.3 ± 0.2) × 10⁶ M ^?1 sec^?1, k_-1=(1.1 ± 0.2) × 10² sec^?1, k₂=(1.5 ± 0.2) × 10⁶ M ^?1 sec^?1, k_-2=(2.5 ± 0.6) × 10² sec^?1; Ni²⁺, k₁=(1.4 ± 0.2) × 10⁴ M ^?1 sec^?1, k_-1=(0.10 ± 0.02) sec^?1, k₂=(2.4 ± 0.3) × 10⁴ M ^?1 sec^?1, k_-2=(0.94 ± 0.17) sec^?1. It is concluded that l-tyrosine substitution reactions are normal. The presence of the phenyl hydroxyl group in l-tyrosine has no primary detectable influence on the forward rate constant, while its influence on the reverse rate constant is partially attributed to substituent effects on the basicity of the amine terminus.     

Stability of gold(I) glycinate complexes in aqueous solution

The stepwise substitution equilibrium AuCl ₂ ^? +iX^?=AuCl_2?i X _i ^? +iCl^?, β_i, where X^? is the glycinate ion (H₂N-CH₂-COO^?), i = 1 or 2, at 25°C in an aqueous solution with I = 1.0 mol/L (NaCl) has been studied pH-metrically. The corresponding constants are logβ₁ = 3.60 ± 0.10, and logβ₂ = 6.2 ± 0.2.     

Composition and stability constants of copper(II) complexes with succinic acid determined by capillary electrophoresis

The advantage of capillary electrophoresis was demonstrated for studying a complicated system owing to the dependence of direction and velocity of the electrophoretic movement on the charge of complex species. The stability constants of copper(II) complexes with ions of succinic acid were determined by capillary electrophoresis, including the 1?:?2 metal to ligand complexes which are rarely mentioned. The measurements were carried out at 25 °C and ionic strength of 0.1, obtained by mixing the solutions of succinic acid and lithium hydroxide up to pH 4.2–6.2. It was shown that while pH was more than 4.5 the zone of copper(II) complexes with succinate moves as an anion. It is impossible to treat this fact using only the complexes with a metal-ligand ratio of 1?:?1 (CuL⁰, CuHL⁺). The following values of stability constants were obtained: log β(CuL) = 2.89 ± 0.02, log β(CuHL⁺) = 5.4 ± 0.5, log β(CuL₂^2?) = 3.88 ± 0.05, log β(CuHL₂^?) = 7.2 ± 0.3.     

Stability constants for aqueous silver/bromide/thiocyanate complexes

Stability constants for aqueous Ag⁺/Br^?, Ag⁺/SCN^?, and mixed Ag⁺/Br^?/SCN^? complexes are determined at 25° C by using data generated potentiometrically in solutions having ionic strengths of 0.4, 1.0, and 2.0 m. Monte Carlo numerical methods which yield apparent stability constants for these complexes as well as confidence limits are described in detail. Explicit consideration of speciation shows that under useful precipitation conditions (high bromide and low thiocyanate), a significant fraction of soluble silver is present as AgBr_n (SCN)^1?n?m_m complexes. The most prevalent mixed complexes under these conditions are AgBr (SCN)^? (log β₁₁=8.0 ± 0.5) and AgBr₂(SCN)^2? (log β₂₁=9.2 ± 0.3). The free energies of formation of the other tri- and tetra-coordinate mixed complexes are nearly indistinguishable (log β₁₂=9.3 ± 0.5; log β₃₁=9.0 ± 0.6; log β₂₂=9.6 ± 0.9; log β₁₃=10.3 ±0.5).     

Oxoacidobasic properties of water in the molten LiCl−KCl eutectic

The quantitative study of the oxoacidobasic properties of water, in the LiCl?KCl eutectic, was carried out by means of a galvanic cell consisting of a pO^2? indicator electrode (made of a calcia-stabilized zirconia tube filled with a Ni+NiO mixture and an Ag/AgCl reference electrode). The equilibrium constants of the following reactions:2OH^? agO^2?+H₂O (K₁)H₂O+2Cl^? agO^2?+2HCl (K₂) have been determined in the temperature range 642–742°C and are given by:log K₁=7.86?7.68×10³/Tlog K₂=2.29?10.03×10³/T where T is the thermodynamic temperature, K₁ and K₂ being expressed in the atm and molar fraction scale.     

Thermodynamics of the lanthanum-hydrogen system at 917°K

The binary system lanthanum-hydrogen has been studied at pressures up to 1 atm at 917°K by a calorimetric-equilibrium method. From the calorimetric measurements we found the enthalpy of formation of LaH₂ at 917°K to be ?45.7 kcal mole^?1 with an estimated uncertainty at ±0.3 kcal mole^?1. This result is about 4 kcal mole^?1 less negative than the values derived indirectly from plateau pressure equilibrium measurements by Mulford and Halley and by Korst and Warf. A comparison between the calorimetric and equilibrium measurements at 917°K provides information on the partial entropy of hydrogen in lanthanum and in the dihydride LaH_2±δ. The excess entropy of hydrogen in lanthanum is about 6 cal K^?1 mole^?1 at 917°K: this value is essentially fully accounted for by the estimated vibrational entropy contribution of the hydrogen atoms. In LaH_2±δ the partial entropy of hydrogen changes from small negative values at X ≈ 1.95 to positive values for X > 2. This entropy change is explained by an assumed intrinsic disorder of hydrogen in LaH₂ of about 0.02.     

Complexation equilibria and spectrophotometric determination of zinc (II) with 2-(4′-methyl-2′-thiazolylazo)-4,6-dimethylphenol

2-(4′-methyl-2′-thiazolylazo)-4,6-dimethylphenol has been synthesized and its acid—base behaviour (pK_a1=0.03 ± 0.01, pK_a2=9.70±0.09) and complexation equilibria with zinc (logβ₁₀₁=6.70±0.04, logβ₁₀₂ = 13.70±0.02) studied in a 40% (v / v) ethanol—water medium at I=0.25 M NaClO₄. A spectrophotometric method for the determination of 0.2–1.5 ppm of zinc has been developed (ε=1.83 × 10⁴ 1. mol⁻¹ cm⁻¹ at 590 nm) and applied to its determination in lubricating oils.     

Solubility of B-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> and the Hydrolysis of Niobium(V) in Aqueous Solution as a Function of Temperature and Ionic Strength

B-Nb₂O₅ was recrystallized from commercially available oxide, and XRD analyses indicated that it is stable in contact with solutions over the pH range 0 to 9, whereas solid polyniobates such as Na₈Nb₆O₁₉?13H₂O(s) appear to predominate at pH>9. Solubilities of the crystalline B-Nb₂O₅ were determined in five NaClO₄ solutions (0.1≤I _m /mol?kg^?1≤1.0) over a wide pH range at (25.0±0.1)?°C and at 0.1 MPa. A limited number of measurements were also made at I _m =6.0 mol?kg^?1, whereas at I _m =1.0 mol?kg^?1 the full range of pH was also covered at (10, 50 and 70)?°C. The pH of these solutions was fixed using either HClO₄ (pH≤4) or NaOH (pH≥10) and determined by mass balance, whereas the pH on the molality scale was measured in buffer mixtures of acetic acid?+?acetate (4≤pH≤6), Bis-Tris (pH≈7), Tris (pH≈8) and boric acid?+?borate (pH≈9). Treatment of the solubility results indicated the presence of four species, \(\mathrm{Nb(OH)}_{n}^{5-n}\) (where n=4–7), so that the molal solubility quotients were determined according to:

$0\mathrm{.5Nb}_{2}\mathrm{O}_{5}\mathrm{(cr)+0}\mathrm{.5(2}n-5\mathrm{)H}_{2}\mathrm{O(l)}_{\leftarrow}^{\to}\mathrm{Nb(OH)}_{n}^{5-n}+(n-5)\mathrm{H}^{+}\quad (n=4\mbox{--}7)$

and were fitted empirically as a function of ionic strength and temperature, including the appropriate Debye-Hückel term. A Specific Interaction Theory (SIT) approach was also attempted. The former approach yielded the following values of log?₁₀ K _sn (infinite dilution) at 25?°C: ?(7.4±0.2) for n=4; ?(9.1±0.1) for n=5; ?(14.1±0.3) for n=6; and ?(23.9±0.6) for n=7. Given the experimental uncertainties (2σ), it is interesting to note that the effect of ionic strength only exceeded the combined uncertainties significantly in the case of log?₁₀ K _s6 to I _m =1.0 mol?kg^?1, such that these values may be of use by defining their magnitudes in other media. Values of Δ _f G ^o, Δ _f H ^o, S ^o and \(C_{p}^{\mathrm{o}}\) (298.15 K, 0.1 MPa) for each hydrolysis product were calculated and tabulated.

     

Buffer Standards for the Physiological pH of the Zwitterionic Compound, HEPPSO from 5 to 55 °C

The values of the thermodynamic second dissociation constant, pK ₂, and related thermodynamic quantities of N-(2-hydroxyethyl)piperazine-N′-2-hydroxypropanesulfonic acid (HEPPSO) have already been reported from 5 to 55?°C, including 37?°C, by the emf method. This paper reports the results for the pH of one chloride-free buffer solution containing the composition: (a) HEPPSO (0.08 mol?kg^?1)+NaHEPPSO (0.04 mol?kg^?1). The remaining seventeen buffer solutions contain a saline medium of ionic strength I=0.16 mol?kg^?1, matching closely that of physiological fluids. Conventional pH values, denoted as pa _H, for all eighteen buffer solutions from 5 to 55?°C have been calculated. The operational pH values, designated as pH, with residual liquid-junction corrections for five buffer solutions, one without NaCl, and four with buffer solutions in saline media of I=0.16 mol?kg^?1 are recommended as pH standards in the range of physiological application. These are based on the NBS/NIST standard scale for pH measurements.     

NMR assignments and the acid–base characterization of the pomegranate ellagitannin punicalagin in the acidic pH-range

In exploring the capability of nuclear magnetic resonance (NMR) spectroscopy for pomegranate juice analysis, the eight aromatic singlet resonances of α- and β-punicalagin were clearly identified in the ¹H NMR spectra of juice samples. The four downfield resonances were found to be sensitive to small pH changes around pH 3.50 where the NMR spectra of the juice samples were recorded. To understand this unusual behavior, the ¹H and ¹³C resonance assignments of the punicalagin anomers were determined in aqueous solution and pH titrations with UV and ¹H NMR detection carried out to characterize the acid–base properties of punicalagin over the pH range 2–8. Simultaneous fitting of all of the pH-sensitive ¹H NMR signals produced similar but significantly different pK _a values for the first two deprotonation equilibria of the gallagic acid moiety of the punicalagin α- (pK _a1?=?4.57?±?0.02, pK _a2?=?5.63?±?0.03) and β- (pK _a1?=?4.36?±?0.01, pK _a2?=?5.47?±?0.02) anomers. Equivalent pK _a values, (α?:?6.64?±?0.01, β?:?6.63±?0.01) were measured for the third deprotonation step involving the ellagic acid group, in good agreement with a prior literature report. The punicalagin anomer equilibrium readjusts in parallel with the proton dissociation steps as the pH is raised such that β-punicalagin becomes the most abundant anomer at neutral pH. The unusual upfield shifts observed for the glucose H3 and H5 resonances with increasing pH along with the shift in the α/β anomer equilibrium are likely the consequence of a conformational rearrangement.

Stability of diammine and chloroammine gold(I) complexes in aqueous solution

The substitution equilibria AuCl ₂ ^? + iNH ₄ ⁺ = Au(NH₃)_iCl_{2 ? i} + iCl^? + iH⁺, β _i ^* . were studied pH-metrically at 25°C and I = 1 mol/L (NaCl) in aqueous solution. It was found that logβ ₁ ^* = ?5.10±0.15 and logβ ₂ ^* = ?10.25±0.10. For equilibrium AuNH₃Cl_solid = AuNH₃Cl, log K _s = ?3.1±0.3. Taking into account the protonation constants of ammonia (log K _H = 9.40), the obtained results show that for equilibria AuCl ₂ ^? + iNH₃ = Au(NH₃)_iCl_{2 ? i} + iCl^?, logβ₁ = 4.3±0.2, and logβ₂ = 8.55±0.15. The standard potentials E ₀ ^1/0 of AuNH₃Cl⁰ and Au(NH₃) ₂ ⁺ species are equal to 0.90±0.02 and 0.64±0.01 V, respectively.     

A new semiempirical kinetic method for the determination of ion exchange constants for counterions of cationic micelles: Study of the rate of pH‐independent hydrolysis of phthalimide as the kinetic probe

Pseudo‐first‐order rate constants (k_obs) for pH‐independent hydrolysis of phthalimide ( 1 ), obtained at a constant total concentration of cetyltrimethylammonium bromide and hydroxide ([CTABr]_T), 2.0 × 10^?4 M 1 , 0.02 M MOH (M⁺ = Li⁺, Na⁺ and K⁺) and various concentrations of inert salt MX (= LiCl, LiBr, NaCl, NaBr, KCl and KBr), follow a relationship derived from the pseudophase micellar (PM) model coupled with an empirical equation. This relationship gives empirical constants, F_X/S and K_{X /S}, with S representing anionic 1 . The magnitude of F_X/S is the measure of the fraction of micellized anionic 1 (S^?_M) transferred to the aqueous phase by the limiting concentration of X^?. The value of K_X/S is the measure of the ability of the counterions (X^?) to expel the reactive counterions (S^?) from the cationic micellar surface to the aqueous phase. The values of F_{X/ S} are ～ 1 for MBr (M⁺ = Li⁺, Na⁺ and K⁺) and in the range ? 0.7 to ? 0.5 for MCl (M⁺ = Na⁺ and K⁺) at 0.006, 0.010 and 0.016 M CTABr. For LiCl, the values of F_X/S become ～1 at 0.006 and 0.010 M CTABr and 0.8 at 0.016 M CTABr. The values of the empirical constants, F_X/S and K_X/S, have been used to determine the usual ion exchange constant (K^Cl_Br). The mean values of K^Cl_Br are 3.9 ± 0.5, 2.7 ± 0.1, and 2.6 ± 0.3 for LiX, NaX, and KX, respectively. These values of K^Cl_Br are comparable with those obtained directly by other physicochemical techniques. Thus, this new method for the determination of ion exchange constants for various counterions of cationic micelles may be considered as a reliable one. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 43: 9–20, 2011     

Copper Catalysed Oxygenation of Bis (2-pyridyl) methane and 6-Methyl-bis (2-pyridyl)methane to the Corresponding Ketones

The ligands (L) bis (2-pyridyl) methane (BPM) and 6-methyl-bis (2-pyridyl)methane (MBPM) form the three complexes CuL²⁺, CuL, and Cu₂L₂H with Cu²⁺. Stability constants are log K₁ = 6.23 ± 0.06, log K₂ = 4.83 ± 0.01, and log K (Cu₂L₂H + 2H²⁺ ? 2 CuL²⁺) = ?10.99 ± 0.03 for BPM and 4.56 ± 0.02, 2.64 ± 0.02, and ?11.17 ± 0.03 for MBPM, respectively. In the presence of catalytic amounts of Cu²⁺, the ligands are oxygenated to the corresponding ketones at room temperature and neutral pH. With BPM and 2,4,6-trimethylpyridine (TMP) as the substrate and the buffer base, respectively, the kinetics of the oxygenation can be described by the rate law with k₁ = (5.9 ± 0.2) · 10^?13 mol l^?1 s^?1, k₂ = (4.0 ± 0.6) · 10^?4 mol^?1 ls^?1, k₃ = (1.1 ± 0.1) · 10^?12 mol l^?1 s^?1, and k₄ = (9 ± 2) · 10^?14 mol l^?1 s^?1.     

Thermodynamic Model for SnO2(cr) and SnO2(am) Solubility in the Aqueous Na+–H+–OH−–Cl−–H2O System

The solubility of SnO₂(cassiterite) was studied at 23±2?°C as a function of time (7 to 49 days) and pH (0 to 14.5). Steady state concentrations were reached in <7 days. The data were interpreted using the SIT model. The data show that SnO₂(cassiterite) is the stable phase at pH values of 10 K ⁰ value of ?64.39±0.30 for the reaction (SnO₂(cassiterite) +2H₂O?Sn⁴⁺+4OH^?) and values of 1.86±0.30, ≤?0.62, ?9.20±0.34, and ?20.28±0.34 for the reaction ( $\mathrm{Sn}^{4+} + n\mathrm{H}_{2}\mathrm{O} \rightleftarrows \mathrm{Sn}(\mathrm{OH})_{n}^{4 - n} + n\mathrm{H}^{+}$ ) with values of “n” equal to 1, 4, 5, and 6 respectively. These thermodynamic hydrolysis constants were used to reinterpret the extensive literature data for SnO₂(am) solubility, which provided a log?₁₀ K ⁰ value of ?61.80±0.29 for the reaction (SnO₂(am)+2H₂O?Sn⁴⁺+4OH^?). SnO₂(cassiterite) is unstable under highly alkaline conditions (NaOH concentrations >0.003 mol?dm^?3) and transforms to a double salt of SnO₂ and NaOH. Although additional well-focused studies will be required for confirmation, the experimental data in the highly alkaline region (0.003 to 3.5 mol?dm^?3 NaOH) can be well described with log?₁₀ K ⁰ of ?5.29±0.35 for the reaction Na₂Sn(OH)₆(s)?Na₂Sn(OH)₆(aq).     

Kinetics of the dissociation and cis-trans isomerization of o,o'-azodioxytoluene (dimeric o-nitrosotoluene)

The dimer-monomer reactions were investigated for the system cis and transo,o'-azodioxytoluene-o-nitrosotoluene in acetonitrile solvent. For the reaction cis dimer-monomer the following thermodynamic and activation parameters have been derived: ΔH°=58.5±2.5 kJ mole^?1, ΔS°=206.2±3.8 J mole^?1 K^?1, ΔH^≠=63.6±3.3 kJ mole^?1, ΔS^≠=6.3±0.3 J mole^?1 K^?1. The corresponding values for the reaction trans dimer-monomer are: ΔH°=45.6±2.1 kJ mole^?1, ΔS°=162.7±7.1 J mole^?1 K^?1, ΔH^≠=80.8±2.9 kj mole^?1, ΔS^≠=-13.4±0.8 mole^?1 K^?1. There is no evidence of a direct cis-trans isomerization (i.e. a reaction not proceeding via the monomer). NMR and various perturbation techniques monitoring the visible absorption of the monomer were employed.     

The effect of the synthesis method on the parameters of pore structure and selectivity of ferrocyanide sorbents based on natural minerals

Ferrocyanide sorbents were obtained via thin-layer and surface modification of natural clinoptilolite and marl. The effect of modification method on surface characteristics of these sorbents and their selectivity for cesium was studied. It was shown that the modification resulted in an increase of selectivity of modified ferrocyanide sorbents to cesium as compared with the natural clinoptilolite in presence of Na⁺, as well as in an increase of cesium distribution coefficients in presence of K⁺. The nickel–potassium ferrocyanide based on the clinoptilolite showed the highest selectivity for cesium at sodium concentrations of 10^?4—2 mol L^?1: cesium distribution coefficient was lg K _d = 4.5 ± 0.4 L kg^?1 and cesium/sodium separation factor was α(Cs/Na) = 250. In the presence of NH₄ ⁺, all modified sorbents showed approximately equal selectivity for ¹³⁷Cs. Probable applications of the sorbents were suggested.     

Low-temperature thermodynamics of Ln(Me2dtc)3(C12H8N2) (Me2dtc = dimethyldithiocarbamate, Ln = La, Pr, Nd, Sm)

The heat capacities of Ln(Me₂dtc)₃(C₁₂H₈N₂) (Ln = La, Pr, Nd, Sm, Me₂dtc = dimethyldithiocarbamate) have been measured by the adiabatic method within the temperature range 78–404 K. The temperature dependencies of the heat capacities, C _p,m [La(Me₂dtc)₃(C₁₂H₈N₂)] = 542.097 + 229.576 X ? 27.169 X ² + 14.596 X ³ ? 7.135 X ⁴ (J K^?1 mol^?1), C _p,m [Pr(Me₂dtc)₃(C₁₂H₈N₂)] = 500.252 + 314.114 X ? 17.596 X ² ? 0.131 X ³ + 16.627 X ⁴ (J K^?1 mol^?1), C _p,m [Nd(Me₂dtc)₃(C₁₂H₈N₂)] = 543.586 + 213.876 X ? 68.040 X ² + 1.173 X ³ + 2.563 X ⁴ (J K^?1 mol^?1) and C _p,m [Sm(Me₂dtc)₃(C₁₂H₈N₂)] = 528.650 + 216.408 X ? 16.492 X ² + 12.076 X ³ + 4.912 X ⁴ (J K^?1 mol^?1), were derived by the least-squares method from the experimental data. The heat capacities of Ce(Me₂dtc)₃(C₁₂H₈N₂) and Pm(Me₂dtc)₃(C₁₂H₈N₂) at 298.15 K were evaluated to be 617.99 and 610.09 J K^?1 mol^?1, respectively. Furthermore, the thermodynamic functions (entropy, enthalpy and Gibbs free energy) have been calculated using the obtained experimental heat capacity data.     

Microwave spectrum of N2D4, 14N quadrupole coupling constants,and the barrier to inversion of the amino group

The microwave spectrum of N₂D₄ has been observed and analyzed. Based on five low-J rotational transitions the effective rotational constants are: A = 74712.9 ± 1.9 MHz, B = 18500.42 ± 0.46 MHz, and C = 18439.91 ± 0.46 MHz. The quadrupole coupling constants of the ¹⁴N nuclei are X_aa = 4.23 ± 0.04 MHz, X_bb = 1.98 ± 0.05 MHz, and X_cc = ?2.25 ± 0.05 MHz. Using the observed ground state inversion splittings for N₂D₄ and N₂H₄ the barrier to inversion of a single amino group is computed to be 5.00 kcal mol^?1.     

Kinetics and mechanism of the oxidation of tris(2,2′-bipyridine)iron(II) and tris(1,10-phenanthroline)iron(II) complexes by nitropentacyanocobaltate(III) in acidic aqueous medium

The kinetics of the oxidation of tris(2,2′-bipyridyl)iron(II) and tris(1,10-phenanthroline)iron(II) complexes ([Fe(LL)₃]²⁺, LL = bipy, phen) by nitropentacyanocobaltate(III) complex [Co(CN)₅NO₂]^3? was investigated in acidic aqueous solutions at ionic strength of I = 0.1 mol dm^?3 (HCl/NaCl). The reactions were carried out at fixed acid concentration ([H⁺] = 0.01 mol dm^?3) and the temperature maintained at 35.0 ± 0.1 °C. Spectroscopic evidence is presented for the protonated oxidant. Protonation constants of 360.43 and 563.82 dm³ mol^?1 were obtained for the monoprotonated and diprotonated Co(III) complexes respectively. Electron transfer rates were generally faster for [Fe(bipy)₃]²⁺ than [Fe(phen)₃]²⁺. The redox complexes formed ion-pairs with the oxidant with increasing concentration of the oxidant over that of the reductant. Ion-pair constants for these reaction were 160.31 and 131.9 dm³ mol^?1 for [Fe(bipy)₃]²⁺ and [Fe(phen)₃]^2+, respectively. The activation parameters measured for these systems have values as follows: ?H ^≠ (kJ K^?1 mol^?1) = +113.4 ± 0.4 and +119 ± 0.3; ?S ^≠ (J K^?1) = +107.6 ± 1.3 and 125.0 ± 1.6; ?G ^≠ (kJ K^?1) = +81 ± 0.4 and +82.4 ± 0.4; and E _a (kJ mol^?1) = 115.9 ± 0.5 and 122.3 ± 0.6 for LL = bipy and phen, respectively. Effect of added anions (Cl^?, $ {\text{SO}}_{4}^{2 - } $ and $ {\text{ClO}}_{4}^{ - } $ ) on the systems showed decrease in the electron transfer rate constant. An outer-sphere mechanism is proposed for the reaction.