Michael G. Burnett

Aspects of the chemistry of calcium sulfate in coal ash

Abstract Equimolar mixtures of iron sulfide, calcium carbonate and other selected additives were ignited in air at 1200 K, and it was shown that about 70% of the sulfur was retained in the residue as calcium sulfate. These reaction conditions were selected to represent those applicable when calcium carbonate is used in fluidized bed combustion. Calcium oxide is efficient in trapping sulfur oxides. It was further shown, however, that under the same well defined reaction conditions alumino-silicates react with CaSO 4 , to some extent, to release a proportion of the sulfur (~ 30%). This result is consistent with a previous conclusion that acidic oxides in coal reduce the efficiency of calcite for sulfur oxide removal from flue gases.

5 T 2 ⇌1A1 and 6A1⇌2T2 spin transitions in iron(II) and iron(III) complexes of 2,2′-bi-2-imidazoline and related ligands

A series of six-co-ordinate iron(II) complexes [FeIIL3]X2(X = ClO4 or BPh4) of the bidentate ligands 2,2′-bi-2-imidazoline (L3), 2,2′-bi-1,4,5,6-tetrahydropyrimidine (L4), 2,2′-bi-2-oxazoline (L5), and 5,5′-dimethyl-2,2′-bi-2-oxazoline (L6) have been prepared and characterised by their physical properties. In all cases co-ordination is via the α-di-imine group. Magnetic susceptibility and Mossbauer effect measurements as a function of temperature show that the complexes of L5 and L6 are fully high-spin (S= 2) over the temperature range 90–360 K, while for complexes of L3(X = ClO4 but not BPh4) and L4(X = ClO4 and BPh4) a 5T2⇌1A1 spin transition is observed in this temperature range. Electronic spectra of the corresponding nickel(II) complexes indicate a correlation between Dq(Ni) and the spin ground state of the iron(II) complexes. The iron(III) complex [FeIIIL33][ClO4]3 is high-spin (S= 5/2), while [FeL43][ClO4]3 has a 2T2 ground state but with a thermally accessible 6A1 excited state. Iron(III) complexes of L5 and L6 are unstable and could not be isolated. Reaction of the iron(II) complexes of L3 and L4 with dioxygen gave the iron(III) complexes [FeIIIL2(L – H)][ClO4]2 is good yield [(L – H)= monodeprotonated ligand] which may be reversibly converted to the [FeIIIL3]3+ species on treatment with one equivalent of HClO4. The temperature-dependent magnetic moments of the complexes [FeL2(L – H)][ClO4]2 are interpreted in terms of 2T2⇌6A1 spin equilibria.

The enhanced adsorption of cadmium on hydrous aluminium(III) hydroxide by ethylenediaminetetraacetate

The adsorption of ca. millimolar concentrations of cadmium(II) on freshly precipitated aluminium(III) hydroxide is enhanced, at pH values of below 8, in the presence of ethylenediaminetetraacetate, EDTA, via stabilisation of the adsorbed cation. At levels where the hydroxide phase is in large excess to the cadmium(II), the addition of stoichiometrically equivalent concentrations of EDTA will enhance its adsorption to approximately 95%. In the presence of EDTA, cadmium adsorption increases with pH despite the fact that 99.9% of the dissolved cadmium is present as an anionic cadmium(II) EDTA complex. The adsorption observed has been modelled using the measured values of the solubility of aluminium hydroxide and its adsorption of EDTA, the normal and the EDTA-enhanced cationic binding of cadmium(II) and the accepted equilibrium constants for EDTA complexation of cadmium(II) and aluminium(III). The structures of the dissolved and adsorbed complexes have been inferred from XAFS and Al27 NMR spectroscopy.

Ideality of water vapour and its adsorption on the glass surfaces of a conventional glass vacuum apparatus at 295 K between 0 and 10 Torr

The properties of water vapour have been investigated quantitatively between 0 and 10 Torr and at ambient temperature (295 K). Provided a satisfactory correction for surface adsorption is made, the deviation from ideal gas behaviour is not detectable within the accuracy of the corrected pressures (±3%). This is consistent with theoretical modelling calculations and also with experiments made at higher temperatures when they are extrapolated to ambient temperature. There was considerable adsorption of water vapour onto the glass of the apparatus which had previously been in constant use, up to ca. 20 monolayers and taking place in two distinct rate processes that were completed in ca. 5 and ca. 30 min. Very much less adsorption occurred on new Pyrex glass (ca. 1 monolayer). It was also shown that the measured equilibrium vapour pressures in the range 0–10 Torr were directly proportional (±8%) to the amount of water admitted.Both results are important in confirming the validity of the vacuum apparatus measurements when used to study rates of water release such as the dehydrations of solids which depend on this relationship. However as a consequence of water vapour adsorption, apparent yields were less by 32% than the values calculated using the ideal gas equation. Yield determinations therefore cannot be based on pressure and apparatus volume but must be obtained by direct calibration.

The kinetics and mechanism of lignite dehydration

The dehydration of a homogeneous cubic sample of ‘woody’ lignite at ca. 290 K follows a modified contracting cube equation, dp/dt=k(1 –α)2/3(1 –β), in which k is a constant, α is the fraction of the sample dehydrated and β is the ratio of the vapour pressure of water at time t compared to its saturated value. The rate depends on the competitive evaporation and condensation of water via the dehydration interface. The system provides a satisfactory kinetic model for the rate of release of non-bonded water across a solid/gas interface.

A kinetic study of the dehydration of Northern Ireland lignite

The kinetics of dehydration of lignite have been characterized by isothermal measurements of the water evolved under vacuum at 273–313 K. The rate of water release at low relative pressures is deceleratory and data are well expressed by the contracting volume rate equation. When the water vapour pressure, p, approaches the saturated value, po, the rate becomes controlled by the relative vapour pressure, β = ppo. These controls are combined in the kinetic analysis of the isothermal yield-time data. It is shown that the evaporation of water from lignite is satisfactorily expressed by the equation, dpdt = k(1 − α)23(1 − β), where α is the fractional dehydration at time t. The activation energy was 35 ± 5 kJ mol−1 between 273 and 313 K. It was shown that the dehydration rates of different lignite samples were identical, and that they were the same as the rate of dehydration of a rehydrated reactant sample which had previously been dried. It is concluded that the low temperature drying of lignite occurs by the evaporation of liquid water previously immobilized within the continuous coherent carbonaceous matrix. The drying rate depends on the particle size, the rate of migration of the drying interface within the particles and on the ambient pressure of water vapour which tends to rehydrate the matrix.

The application of high performance liquid chromatography to analytical problems in the synthesis, substitution and linkage isomerism of cyanocobaltate(III) complexes

Ion pair reversed phase chromatography has been applied to the analysis of mixtures of [Co(CN)5(X)]n−, n = 2, 3 or 4, at ionic strengths from zero to unity using sodium sulphate or perchlorate as background electrolytes. The general principles for optimising these separations using a tertiary ammonium ion-pairing agent have been established and the limitations of the method have been identified. Anomalous competition results in experiments with [Co(CN)5Cl]3− have been explained by the presence of traces of [Co(CN)5(OH2)]2− formed in this and related cyanocomplexes during conventional synthesis. The nucleophile efficiency of the hydroxide ion has been shown to be no greater than is normal for a univalent anion in [Co(CN)5X]3− substitutions. Trace ion analysis has been used to measure the formation constants for [Co(CN)5(S(O)CH3)2]2− with dimethyl sulphoxide concentrations from 0.1–6.0 mol dm−3. Separation of the linkage isomers of SCN− and S2O32− has been used as an indication of mechanism.

The relative nucleophilic efficiency of water and the thiocyanate ion in the acid-catalysed substitution of azidopentacyanocobaltate(III)

The acid-catalysed substitution of [Co(CN)5(N3)]3– by NCS– in water yields [Co(CN)5(OH2)]2–, [Co(CN)5(SCN)]3–, and [Co(CN)5(NCS)]3–. Spectroscopic and high-performance liquid chromatographic analytical data are quantitatively consistent with an acid-catalysed dissociative mechanism [equations (1)–(5)], in which K1= 4.47 ± 0.22 dm3 mol–1, k2=(3.46 ± 0.17)× 103 s1, [Co(CN)5(N3)]3–+ H3O+ [graphic omitted] [Co(CN)5(N3H)]2–+ H2O (1)[Co(CN)5(N3H)]2– [graphic omitted] [Co(CN)5]2–+ HN3(2)[Co(CN)5(OH2)]2– [graphic omitted] [Co(CN)5]2–+ H2O (3)[Co(CN)5]2–+ H2O [graphic omitted] [Co(CN)5(OH2)]2–(4)[Co(CN)5]2–+ NCS [graphic omitted] [Co(CN)5(SCN)]3–(5a)[Co(CN)5]2–+ NCS [graphic omitted] [Co(CN)5(NCS)]3–(5b)K3= 6.07 × 104 s1, (K5a+K5b)/K4[H2O]= 0.14 ± 0.02 dm3 mol–1, and K5/K5b≈ 4 at 40 °C and unit ionic strength. Equilibrium spectroscopic measurements of K1(4.67 ± 0.09 dm3 mol1) agree with the fitted kinetic result.

Kinetic and competition studies of substitution reactions of the trans-aquosulphitotetracyanocobalt complex, Co(CN)4(SO)3(OH2)3−

Abstract The relative nucleophile efficiencies predicted for NH3, CH3NH2 and CN− from their rates of reaction with trans-Co(CN)4(SO)3(OH2)3− are confirmed by competition experiments with similar nucleophiles (NH3-CH3NH2) but not by experiments with contrasting nucleophiles (NH3-CN−). This system, though substantially dissociative in mechanism, (D), possesses some interchange properties, (Id).

The relative nucleophilic efficiency of water and the azide and thiocyanate ions in the aqueous substitution of chloropentacyanocobaltate(III)

The kinetics of the substitution of Cl– in [Co(CN)5Cl]3– by solvent H2O and by X–= NCS– or N3– may be quantitatively interpreted by a stepwise mechanism, equations (i) and (ii). High-performance liquid chromatographic analysis confirms the absence of direct chloride substitution by reaction (iii). These results support the view that the substitution occurs by an Id interchange mechanism. [Co(CN)5Cl]3–+ H2O →[Co(CN)5(OH2)]2–+ Cl–(i)[Co(CN)5(OH2)]2–+ X–→[Co(CN)5X]3–+ H2O (ii)[Co(CN)5Cl]3–+ X–→[Co(CN)5X]3–+ Cl–(iii)