Conditional Equilibrium Constants Research Articles

EXTRACTION OF COBALT(II) BY CYANEX

ABSTRACT The extraction equilibria of Co(II) from 1.0 M KC1 medium by the commercial extractant Cyanex 301 dissolved in toluene has been studied. The effects of pH, extractant concentration and metal concentration on the extraction behaviour have been examined. Experimental data have been treated graphically and numerically and can be explained by assuming the extraction of the dominant species CoL2. HL being bis(2,4,4-trimethylpentyl) dithiophosphinic acid, the major component of Cyanex 301. The numerical treatment also indicated the extraction of a minor species of probable stoichiometry CoClL(HL), but whereas the former species has been clearly identified, the latter species, probably due to the fact that its contribution to the extraction process is very little, can only be considered as an approximation as its stoichiometry is not defined precisely by this numerical treatment. Conditional equilibrium constants of the extraction process are also given.

The waste sulfuric acid lake of the TiO 2-plant at Armyansk, Crimea, Ukraine. Part II. Modelling the chemical evolution with PHRQPITZ

The past and future evolution of a salt lake near Armyansk (Crimea, Ukraine), in which waste sulfuric acid has been discharged since 1969, is modelled using the equilibrium aqueous chemistry model PHRQPITZ. The initial lake is represented by a simplified salt solution, about 1.4 times concentrated average seawater. Since the waste acid contains iron and other metals, the effluent is modelled by an Fe SO 4 solution of pH 1.04, containing minor amounts of Na and Mg. The PHRQPITZ model is adapted to describe precipitation of the trivalent Fe minerals natrojarosite and ferrihydrite. The yearly input of effluent constituents is assumed constant, and the input of water from the effluent is compensated by the yearly excess of evaporation over rainfall. According to the model, calcite dissolution from the underlying calcareous clays nearly buffers the increase in acidity of the lake by the effluent, maintaining pH at a value around 1.7, up to 16 a of operation (1986). Increase of Fe, Na, and SO 4 is buffered by precipitation of natrojarosite and gypsum. At the direct contact with the sediments a relatively impervious Fe (hydr)oxide layer develops. This hampers further reaction with the sediments, which effectively stops around 1992, when pH has reached a value of 0.85. It is predicted that with further increase in concentration, either through continued acid input or by evaporation of the acid lake, dissolution of previously precipitated natrojarosite first acts as a relative H buffer, followed by resumed precipitation of natrojarosite when the potential H increase is counterbalanced by the SO 4/HSO 4 association equilibrium. The concept of buffering in its strict sense does not apply, as conditional equilibrium constants for the extremely acid and concentrated conditions in the lake are far from constant and may differ significantly from their theoretical value.

The Separation of Zn and Cu Using Chelating Ion Exchangers and Temperature Variations

This paper reports the results of a study on the ion-exchange equilibrium of Cu 2+ and Zn 2+ at different temperatures on chelating ion-exchange resins: Amberlite IRC-718 (iminodiacetic acid based functional groups), VPC-1 (picolinic acid based) and thiourea-based resins. The separation factors and the conditional equilibrium constants of the ion-exchange reaction were determined in temperature range from 15 to 75 °C. An estimation of thermodynamic functions has also been carried out. Possibility of the separation of Cu and Zn mixture by dual temperature ion-exchange method has been demonstrated.

EXTRACTION OF COBALT(II) BY CYANEX 302

ABSTRACT The extraction equilibria of Co(II) from 1·0 M KC1 medium by the commercial extractant Cyanex 302 (bis(2,4,4-trimethylpentyl)monothiophosphinic acid) dissolved in toluene has been studied. The effects of pH, extractant concentration and metal concentration on the extraction behaviour have been examined. The influence of bis(2,4,4,-trimethylpentyl)dithiophosphinic acid, one of the minor impurities of Cyanex 302 and also the major component in Cyanex 301 on the extraction of cobalt has also been considered. Experimental data have been treated graphically and numerically by means of the computer program LETAGROP-DISTR, and can be explained by assuming the extraction of the species CoR2(HR), CoR2(HR)2, CoL2 and CoCIL(HL), HL being bis(2,4,4-tri-methylpentyl)dithiophosphinic acid and HR standing for bis(2,4,4-trimethylpentyl)-monothiophosphinic acid. Conditional equilibrium constants of the extraction process are also given.

The Dimethylhydroxysulfuranyl Radical

By use of pulse radiolysis the one-electron reduction potentials, E0(DMS•+/DMS) and E0((DMS)2•+/2DMS) (dimethyl sulfide, DMS) were determined to be 1.66 ± 0.03 and 1.40 ± 0.02 V vs NHE, respectively. DMS•+ was found to be in equilibrium with DMSOH• with a pKa = 10.2. The conditional equilibrium constant for the reaction DMSOH• + DMS ⇌ (DMS)2•+ + OH- was found strongly dependent on both ionic strength and DMS concentration. In the thermodynamic limit this equilibrium constant is ≈1.3. The dimerization reaction DMS•+ + DMS ⇌ (DMS)2•+ was shown to have its equilibrium constant between 104 and 5 × 104 M-1. DMSOH• reacts with O2 with a rate constant of 2 × 108 M-1 s-1, independent of pH (11−14). From this and other observations, we estimate the pKa for deprotonation of DMSOH• to exceed 17. From spectral and kinetic data, the maximum lifetime of the radical (DMS)2OH• was predicted to be 10 ns. The stability of DMS−X• (X = OH, I, Br, Cl) in aqueous solution was shown to correlate with the one-electron reduction potential of X•. Comparison of gaseous and aqueous behavior of DMS−OH• reveals that aqueous solvation strongly stabilizes the S−O bond against dissociation into DMS and OH•. The Gibbs free energy of solvation of DMSOH• was calculated to be −12 ± 3 kcal/mol, an unusually large value for a neutral species.

Voltammetric study of the interaction of cadmium with bacterial cells

A new method is presented for the interpretation of voltammetric titration curves for systems in which the species have different diffusion coefficients. The change in the metal-to-ligand ratio along the titration curve together with its effect on the coupled diffusion and the overall lability of the system are considered in the interpretation of experimental data. Equations for the evaluation of the free metal concentration remaining in solution are used on each part of the titration curve, according to this parameter. The validity of the method is checked by applying a regression method; the conditional equilibrium constant and complexing capacity can be calculated. The methodology is applied to the case of the titration of living and dead bacterial cells with cadmium ions. Monitoring of cadmium uptake versus time is also performed to show the applicability of voltammetric techniques to the study of the interaction of metals with micro-organisms.

Cadmium Sorption on Specimen and Soil Smectites in Sodium and Calcium Electrolytes

AbstractAlthough specimen smectities (e.g., SWy‐1) are often used as analogues of the exchanger phase in smectitic soils, few comparisons of metal ion sorption on specimen and soil smectites have been made. In this study, the sorption of Cd was measured on SWy‐1 and on clay‐sized separates from two smectitic subsoils to evaluate the selectivity of specimen and soil‐derived smectites for Cd. Sorption was measured in clay suspensions (≈1 mmolc L−1 equivalent charge concentration at pH 6.0) in Na+, Ca2+, and Na+‐Ca2+ perchlorate solutions across pH 4.5 to 8.5 and at ionic strengths (I) ranging from 0.005 to 0.1. Ionic strength and electrolyte cation valence strongly influenced Cd sorption by SWy‐1 and the soil smectites. Ion exchange dominated Cd sorption at low ionic strength in Na+ electrolyte (I = 0.005–0.014). Increasing Na+ concentrations to I = 0.1 or changing the electrolyte cation to Ca2+ at I = 0.003 to 0.006 suppressed ion exchange. When ion exchange was suppressed, Cd sorption to both specimen and soil smectites showed little dependence on ionic strength and increased with pH. Except at the lowest Na+ concentration (I = 0.005), conditional equilibrium constants (Kv) for Cd2+ exchange increased with increases in both ionic strength and pH. These increases were ascribed to Cd complexation reactions to edge sites on the layer silicates whose effects became evident only under conditions that suppressed ion exchange. At pH 6 and I = 0.05–0.01, SWy‐1 did not exhibit any preference for Na+, Ca2+, or Cd2+. The smectitic soil separates, in contrast, showed (i) sorption behavior that increased sharply with pH, (ii) preference for Cd in Na+ and Ca2+ electrolytes, and (iii) variation in Kv with ionic strength, pH, and surface coverage. The contrasting sorption behavior of the soil smectites was hypothesized to result from (i) a greater edge surface area, which increased the contribution of oxidelike complexation reactions to Cd sorption, and (ii) the presence of minor associated organic material and Fe oxides that functioned as co‐complexants for Cd.

Sorption of lead and thallium on borosilicate glass and polypropylene: Implications for analytical chemistry and soil science

Adsorption of 10-100 ppb lead and thallium by borosilicate glass and polypropylene surfaces was studied. No thallium was adsorbed by either substrate at pH 7.0. About 50-60% of the lead was adsorbed by borosilicate glass at pH 5.5 and pH 7.0, with statistically indistinguishable conditional equilibrium constants (KD* values) of 1.47±0.40 mL cm-2 and 1.80±0.11 mL cm-2, respectively. Polypropylene adsorbed significantly more lead than borosilicate glass at pH 7.0 (80.9%, KD* =5.33±0.45 mL cm-2), and substantially less at pH 5.5 (16.7%, KD* = 0.27±0.03 mL cm-2), indicating a fundamental change in the nature of the polypropylene surface. All adsorption data fitted Freundlich plots. Preliminary experiments indicate that at pH 5.5, a fraction of the lead sequestered by the borosilicate glass may be bound irreversibly by migrating into the glass matrix. The results suggest that tectosilicates and silicate glasses may represent important adsorbents of some heavy metals in soils, despite showing very low intrinsic cation exchange capacities.

Chromatographic method for the determination of conditional equilibrium constants for the carbamate formation reaction from amino acids and peptides in aqueous solution

A novel and sensitive method has been developed and evaluated for the study of carbamate formation equilibria of amino acids and peptides in aqueous solution. The method is based on reversed-phase liquid chromatography with cetyltrimethylammonium bromide. The reliability of the method was established by comparing the results determined from the present study with the few data in the literature. The relaxation rate of the carbamate reaction was shown to be faster than the chromatographic distribution relaxation rate (seconds). As a result, the retention time of amine solutes is increased in the presence of CO 2 . Carbamate formation constants and mole fractions of carbamates at physiological pH of eleven L-α-amino acids and peptides were determined. No correlation between the formation constant and the ammonium pK a was found

The surface chemistry of manganiferous silicate minerals as inferred from experiments on tephroite (Mn 2SiO 4)

The dissolution rate of tephroite in oxygen-free solutions decreases with increased pH over the interval 2 ≤ pH ≤ 8 and with decreased temperature over the range 25–45°C. The pH-dependence is similar to other orthosilicate minerals at 25°C and presumably relates to variations in the concentration of adsorbed hydrogen ions. The rate order with respect to solution pH increases with temperature and the experimental activation parameters vary strikingly with solution pH. These variations are interpreted to result from contributions of enthalpy to the experimental activation energy ( E exp ) from proton adsorption and from coulombic interactions among charged sites on the mineral surface. The dependence of dissolution rate on the logarithm of solution pH is only approximately linear with pH; the dependence is sensitive to changes in temperature via the conditional equilibrium constant that describes the concentrations of charge sites on the mineral surface. X-ray photoelectron spectroscopic (XPS) measurements of the reacted mineral surfaces indicate that Mn remains in the divalent valence state at all conditions. The near-surface region of the mineral after acid dissolution has a Mn Si ratio which is lower than the unreacted material while the opposite result is observed after reaction at basic-pH conditions.

Interaction of oxygen with molten alkali chlorides

It was shown by acidometric and iodometric titration of the reduction products of oxygen on a platinum cathode that the oxygen electrode exchanges not only oxide (O 2−) but also peroxide (O 2− 2) and superoxide (O − 2) ions with molten NaCl, KCl and CsCl. The emf of the cells with oxygen and chlorine elctrodes: O 2 (g), Pt|M 2O + M 2O + MO 2 + MCl(melt)|MCl(melt)|C, Cl 2(g), where M is Na, K or Cs, was measured as a function of temperature, gas pressure on its electrodes and the total concentration of M 2O + M 2O 2 + MO 2. It was established that the coefficient B in empirical equations ε = Const − Bln P o2 is a constant equal to 0.028 ± 003 V. It is independent of temperature and electrolyte composition, which shows that n in the theoretical expression B = RT/ nF does not depend on temperature ( n < 4). An original technique of determining mole fraction of M 2O, M 2O 2 and MO 2 in the products of cathodic reduction of oxygen was developed. It enabled the expressions of temperature dependences to be derived from experimental observations for conditional standard potentials of the oxygen electrode: ▪ and ▪ as well as for conditional formal redox potentials: ▪, ▪ and ▪ in molten NaCl, KCl and CsCl. Conditional equilibrium constants, enthalpies and conditional standard entropies were calculated from the experimental data for the reactions: ▪ The activity coefficients of M 2O, M 2O 2 and MO 2 in their dilute solutions in molten alkali chlorides were estimated on the basis of the literature and experimental thermodynamic data. The first two of them show negative deviations and the last one shows positive deviation from ideal mixtures. The possibility is indicated for oxygen transfer through molten alkali chlorides by peroxide and superoxide ions: ▪

On the temperature dependence of mineral dissolution rates

Experimental activation energies ( E exp ) for the dissolution rates of silicate and oxide minerals far from equilibrium vary by tens of kJ/mol with pH. Measured E exp range from larger than 50 kJ/mol at pH conditions much more acidic or more basic than at the Point of Zero Net Proton Charge (PZNPC) to very small values near, but not necessarily at, the PZNPC. We interpret this pH variation by combining an empirical rate law for dissolution with a conditional equilibrium constant for proton adsorption to form positive surface charge. Interactions among charged sites at the mineral surface are treated with an electrostatic potential term in the conditional equilibrium constant. Proton adsorption (desorption) to create positive (negative) surface charge is exothermic (endothermic) and contributes 15–50 kJ/mol to E exp ; this contribution depends on the mole fraction of charged surface sites and, therefore, varies slightly with pH. The electrostatic potential contribution becomes increasingly important as the mineral surface acquires significant positive or negative charge; it therefore varies considerably with pH. This model approach not only explains the pH dependence of E exp but also leads to the following important general predictions: 1. 1) E exp will vary similarly with pH for minerals which differ by homovalent cation substitutions; 2. 2) negative E exp values can conceivably occur since E exp is the sum of enthalpies of similar magnitude but opposite sign; 3. 3) E exp depends linearly on the partial reaction rate order with respect to the concentration of charged sites; 4. 4) dissolution rates will become increasingly sensitive to pH at elevated temperatures; and 5. 5) little information about the detailed energetics of bond hydrolysis in dissolution processes can be inferred from E exp without correcting it for the large enthalpies of proton adsorption.

Investigations of iron coordination and redox reactions in seawater using 59Fe radiometry and ion-pair solvent extraction of amphiphilic iron complexes

Iron coordination and redox reactions in synthetic and coastal seawater were investigated at nanomolar concentrations using 59Fe radiometry and ion-pair solvent extraction of iron chelated by sulfoxine (8-hydroxyquinoline-5-sulfonate) and BPDS (bathophenanthroline disulfonate). Using sulfoxine, we determined the rate at which the monomeric Fe(III) hydroxide species present in seawater of pH 8 are complexed by the microbial siderophore deferriferrioxamine B and the synthetic chelator EDTA (ethylenediaminetetraacetic acid). Forward rate constants of 2 × 10 6M −1s −1 and 20 M −1s −1, respectively, were obtained. The kinetics of these reactions have not been measured previously at pH values near that of seawater. Conditional equilibrium constants measured for the Fe(III)-EDTA system are consistent with published stability constants for EDTA complexes and for Fe(III) hydrolytic equilibria minus the neutral Fe(OH) 3 o species, suggesting it is not quantitatively significant near pH 8. Commercial humic acid was found to have sufficient affinity for iron to compete with Fe(III) hydrolysis in seawater, and limited evidence was obtained for an interaction with dissolved organic matter in coastal seawater. In our investigations of redox reactions using BPDS to trap Fe(II) produced in the medium, we observed enhanced photoreduction of Fe(III) by humic acid as well as reduction induced by solutes released from phytoplankton in seawater of pH 8. Although the method is sensitive enough to work at near-oceanic levels of iron, the difficulty in distinguishing Fe(II) generated by Fe(III)-BPDS interactions from Fe(II) produced by other means limits its utility. This analytical ambiguity may be generalizable to other methods which measure ferrous iron in seawater using Fe(II)-specific ligands.

ACID DEPENDENCY OF THE EXTRACTION OF SELECTED METAL IONS BY A STRONTIUM.SELECTIVE EXTRACTlON CHROMATOGRAPHIC RESIN: CALCULATEDVS.EXPERIMENTAL CURVES*

ABSTRACT The extraction of selected divalent (Ca+2, Sr+2, Ba+2, Ra+2, Pb+2, UO2 +2) and monovalent (Na+, K+, Rb+, CS+, TI+) metal cations at tracer concentration level by a recently introduced strontium-selective extraction chromatographic resin has been studied as function of the nitric acid concentration in the range − 0.1-7 M. The data have been explained by combining the metal extraction, the acid extraction, and the aqueous phase metal complication equilibria. Conditional metal extraction equilibrium constants have been estimated by using calculated activity coefficients of the metal nitrate salts in the nitric acid solutions. The extraction constants have been used to predict the effect of macroconcentrations of sample matrix constituents on the extraction of Sr+2 or Pb+2 by the strontium-selective resin.

The chemistry of uranium. Part 43. The interaction between uranium(IV), Cl −, Br − and monodentate amide ligands — a quantitative treatment

The reactions of UCl 4 with chloride, UBr 4 with bromide ions and UCl 4 with an amide ligand N,N-diethylpropionamide (DEPA) in acetone solution have been treated quantitatively, by applying spectrophotometric data and iterative computing methods. Sets of simultaneous equilibria were established for each system. Conditional equilibrium constants were obtained and species distribution curves produced.

Complexation of 1‐Naphthol by Humic and Fulvic Acids

AbstractComplexation of organic solutes with dissolved humic substances may have a significant effect on the chemistry and fate of contaminants in natural systems. The complexation of 1‐naphthol with dissolved humic and fulvic acids (HA and FA) in 0.01 M KCl was studied using fluorescence quenching. No increase in quenching at elevated temperature and an increase in quenching ratio at higher viscosity both indicated that the quenching was due to the formation of naphthol‐HA water‐soluble complexes. Conditional equilibrium constants for the association of 1‐naphthol with humic and fulvic acids were estimated from the Stern‐Volmer equation, and ranged from 9.7 to 33.4 L/g C. In general, HA showed a stronger affinity for 1‐naphthol than FA. The ionic strength of the solution, adjusted using KCl and CaCl2, did not significantly affect the complexation of 1‐naphthol with HA. The quenching ratio increased as the pH of the solution increased, suggesting that changes in the dispersion of HA polymers or additional binding mechanisms result in increased HA complexation of naphthol at higher pH. The complexation of 1‐naphthol with HA was enhanced in the presence of Cu2+ and Zn2+ at a pH higher than the dissociation constant (pKa) of 1‐naphthol (9.34). This suggests that cation bridging between functional groups of anionic organic solutes and humic or fulvic acids is an additional binding mechanism for the association of organic solutes with soluble humic substances.

Adsorption of cadmium by an aquent New Zealand soil and Its components

Cadmium adsorption by a New Zealand soil, three of its particle size fractions, the soil with organic matter removed, and the soil with iron and manganese oxides removed, was studied at the soil's natural pH (5.70) and at cadmium concentrations (0.01 to 0.23 �g mL-1) typical of those found in the environment. Under these conditions, the whole soil had a strong affinity for trace amounts of cadmium, adsorbing about nine-tenths of that added. Relative conditional equilibrium constants for cadmium adsorption by the soil's components decrease in the following order: clay (1.59) &gt; [whole soil (1.00)] &gt; soil without iron and manganese oxides (0.55) &gt; silt (0.54) &gt; sand (0.52) &gt; soil without organic material (0.08) &gt; silica sand (0.001). It is estimated that the &lt;3.9 �m particle size fraction (22% of the soil) and the 3-9-563 pm particle size fraction (78% of the soil), contributed about 45% and 55%, respectively, to the overall capacity of the soil to adsorb cadmium. Most data obtained gives good fits to both linear and Freundlich adsorption isotherms, but is not consistent with the Langmuir adsorption model. Thus it is likely that, under the conditions studied, the soil and its components supply a range of cadmium adsorption sites with varying adsorption energies.

Interactions of heavy metals with organisms and proteins

Voltammetric techniques [differential pulse polarography (DPP) and differential pulse anodic stripping voltammetry (DPASV)] have been used to determine, at various pH, conditional differential functions and average equilibrium constants of copper, zinc and lead with the bacteria Klebsiella pneumoniae and Methanosarcine, the alga Selenastrum capricornutum Printz and the proteins horse-heart cytochrome C and the soluble extract of sulphate-reducing bacteria. The buffer intensity in terms of metal ion and complexing capacity has also been determined. From a comparison of the results a better insight into the groups involved has been obtained.

A quantitative study of the complexation of cadmium in hemolyzed human erythrocytes by1H NMR spectroscopy

The stability of complexes formed by Cd2+ in hemolyzed human erythrocytes was studied by spin-echo 1H NMR spectroscopy. Changes in resonances for the carbon-bonded protons of glutathione (GSH) upon addition of the ethylenediaminetetraacetic acid complex of Cd2+ (Cd(EDTA)2-) and the appearance of resonances for Mg(EDTA)2- indicate that the Cd(EDTA)2- complex dissociates in hemolyzed erythrocytes with the formation of Cd(GSH)x and Mg(EDTA)2- complexes. A semiquantitative estimate of the overall stability constant for the complexation of Cd2+ in hemolyzed erythrocytes was obtained from spin-echo 1H NMR data. The stability constant is consistent with the majority of the Cd2+ in erythrocytes present as Cd(SG)2(2-). A conditional equilibrium constant was also determined for the complexation of Mg2+ by ligands in hemolyzed human erythrocytes.

Binding of Cu(II) to algae in a metal buffer

The interactions of Cu(II) with algal surfaces and exudates were studied in metal-NTA buffers by a combination of several analytical techniques. Suspensions of living algae in the presence of NTA were titrated at constant pH with Cu(II). The various Cu species were determined as follows: a copper ion selective electrode was used reliably in the pCu range 9–12; differential pulse polarography was used to measure separately Cu(II)-NTA complexes and labile Cu(II) species and to evaluate the complexation of copper by ligands in solution; copper bound to the algal surfaces was extracted by acid treatment and measured by AAS. Thus, we determined both the binding of Cu to the algal surfaces and to exudates excreted by the algae. The results were interpreted in terms of conditional equilibrium constants valid at a given pH; the conditional constants, both for the binding to the surfaces and with the exudates increase in the pH range 5.0–6.5. Simple equilibrium models using the experimentally determined binding capacities and equilibrium constants were able to simulate the results and to evaluate the speciation of copper. Under the experimental conditions used, the binding of Cu(II) to algal exudates has a more significant effect on copper speciation than the binding to the algal surfaces. These extracellular ligands may play an important role in decreasing the concentration of free copper ion and thus mitigating the potential toxic effects in organisms.