Optical Basicity Values Research Articles

Optical Basicity: A Practical Acid-Base Theory for Oxides and Oxyanions

The optical basicity concept relies on the Lewis approach to acids and bases and was developed for dealing with chemical problems in non-aqueous, non-protonic media such as silicates, phosphates and borates which are important in glass making and (as slags) for refining steel. Basic oxides such as Na2O or CaO are ionic while SO3, P4O10 or SiO2 are covalent, and it is the magnitude of negative charge borne by the oxygen atoms or ions which governs the degree of acidity or basicity. The oxygen atoms of sulfates, phosphates or network systems such as silicates bear charges which are between those of their parent oxides. In principle, the negative charge can be estimated using the optical (ultraviolet) spectra of certain probe ions and is represented by the optical basicity value, Lambda. Optical basicity values, available for 16 oxides, increase from the acidic SO3 to the very basic Cs2O in a way which conforms with electronegativity and polarizability. The optical basicity concept also extends to fluorides and sulfides.

EPR study of vanadyl ion in CoO · PbO · B2O3 glasses

Glasses with composition x CoO · (0.4 - x )PbO · 0.6B 2 O 3 containing 1.0 mol% and 2.0 mol% V 2 O 5 have been prepared over the range O ≤ x ≤ 0.20. The electron paramagnetic resonance spectra of VO 2+ of these glasses have been measured in the X-band (≈ 9.3 GHz) at room temperature (RT ≈ 300 K). Glasses with 1.0 mol% of V 2 O 5 were prepared by melting at 1173 K and 1373 K. Spin Hamiltonian parameters g | , g ⊥ , A | , A ⊥ , dipolar hyperfine coupling parameter, P , and Fermi contact interaction parameter, K , have been calculated. It is found that these parameters are independent of the temperature of the melt. Values of g | , P decrease; value of K increases and value of g ⊥ , A | , A ⊥ are constant when V 2 O 5 is increased from 1.0 mol% to 2.0 mol% in the binary lead borate glass. g | , g ⊥ , A | , A ⊥ , P and K change from 1.922 ± 0.001 to 1.927 ± 0.001, 1.968 ± 0.001 to 1.973 ± 0.001, (166.9 ± 1.0) × 10 −4 Cm −1 to (165.1 ± 1.0) × 10 −4 Cm −1 , (58.8 ± 1.0) × 10 −4 Cm −1 to (62.6 ± 1.0) × 10 −4 Cm −1 , 117.6 × 10 −4 Cm −1 to 111.2 × 10 −4 Cm −1 and 0.7614 to 0.8270, respectively, showing that tetragonal distortion of the V 4+ O 6 complex increases with increase in the CoO:PbO ratio. It is also found that the increase in CoO:PbO ratio results in an expansion of the 3d xy orbit of unpaired electrons in the vanadium ion and that the spin Hamiltonian parameters do not depend on the value of theoretical optical basicity.

Study on phosphate and sulphide capacities of CaO‐CaCl2 slags

Experiments were carried out at 1175–1350°C to study the phosphate capacity of CaO‐CaCl2 slags and the sulphide capacity was calculated according to the correlation between the two capacities. At 1200°C the logarithm of the phosphate capacity increased from 24.5 to 26.5 with increasing CaO mole fraction from 0 to 0.22 in the slags and it decreased with increasing temperature. Sulphide capacity increased both with increasing CaO mole fraction and temperature.A good linear correlation was found between slag carbonate capacity and phosphate capacity as well as sulphide capacity. Optical basicity was used to evaluate the property of the CaO‐CaCl2 slag in the present study. The value of optical basicity of CaCl2 is established as 0.63. The correlations of optical basicity with above mentioned three capacities were found to be linear.

A review of optical basicity and its applications to oxidic systems

A description is given of the theoretical background to the optical basicity concept, which is based on orbital expansion effects reflecting the nature of the chemical bonding between a Lewis acid-base pair. Optical basicity is particularly useful for oxidic network systems, such as silicate, phosphate, or borate in crystalline, vitreous, and liquid (molten) form. The experimental measurement of optical basicity, from the ultraviolet spectra of probe ions such as Tl + and Pb 2+, is described, and it is shown how optical basicity values can be assigned to individual oxides such as Na 2O, CaO, or SiO 2. These make it possible to calculate a “theoretical” optical basicity value, Λ th , from chemical constitution, thereby allowing correlations to be made with various properties of, for example, molten silicates as a function of composition. The correlations have important applications for redox behaviour, e.g., in glass making and in the performance of metallurgical slags, and should find application in geochemistry, for example, to Fe 2+ Fe 3+ and other redox ratios in magmas. Furthermore, in the chemical reactions between metal silicate, borates, etc., the Λ th values indicate how minimisation or elimination of optical basicity differences between the reactants is a major driving force.

Limitations in the Metallurgical Application of Optical Basicity.

Basicity is a useful concept in metal processing inspite of its uncertain theoretical basis. The unlimited application of basicity in metallurgical systems, however, results in confusion of the basicity concept.After a short review on the basicity of metallurgical slags was done in the present study, optical basicities in some pure oxides and two phase glases in alkaline borate systems measured by the photoacoustic spectroscopy are reported. Two optical basicity values were obtained in the two phase regions of the borate glasses. It is suggested that basicity can not work usefully in such extreme composition regions and that limitations in basicity should be considered in its metallurgical application.

Comments on the application of optical basicity to glass

The optical basicity of a glass can be calculated from parameters assigned to constituent oxides and the stoichiometry. Such calculations, when applied to a series of compositions in a glass system, indicate the trend in basicity and this is usually close to that obtained experimentally using probe ions such as Pb2+. Deviations between predicted and experimental optical basicity can be useful for detecting critical phenomena as, for example, in the Na2O P2O5 glass system where Pb2+ indicates an optical basicity step at the 1:1 composition. The determination of optical basicity for UV opaque materials (where spectra of ions such as Pb2+ cannot be measured) using electronegativity relationships is critically assessed, and results are compared with those obtained from refractivity measurements. Assigned optical basicity values are given for 14 oxides including MnO and FeO (both of which have the value of 1.0).

Optical basicity of sulfide systems

Optical basicity expresses quantitatively the electron-donor power of oxide(-II) atoms in oxidic systems such as silicates, borates and phosphates. Recently, it has been shown that the optical basicity scale extends to fluoride systems and the present work indicates extension also to sulfide systems. Optical basicity values of metal sulfides (MgS, 1.25; CaS, 1.30; SrS, 1.35; BaS, 1.40; ZnS, 1.30) indicate a somewhat greater basicity than those of the corresponding oxides, and this is considered in the context of the performance of metallurgical slags.

A common optical basicity scale for oxide and fluoride glasses

Arguments are presented justifying the use of fluoride optical basicity values (e.g. λ ( BaF 2) = 0.50, λ ( SiF 4) = 0.21, λ ( ZrF 4) = 0.38) for calculating the optical basicity of all-fluoride and oxide/fluoride glasses. The resulting λ values agree with those obtained by probe ion spectroscopy, and provide a means for calculating trends in ideal optical basicity for glass systems such as BaF 2ZrF 4 or AlF 3Al(PO 3) 3.

Electron spin resonance of VO 2+ in Li 2OPbOB 2O 3 glasses

Electron spin resonance of VO 2+ in Li 2OPbOB 2O 3 glasses has been studied. Spin Hamiltonian parameters, Fermi contact interaction parameter and dipolar hyperfine coupling parameter have been calculated. It is found that with a decrease in B 2O 3 mol%, tetragonality of the V 4+O 6 complex decreases. Calculated values of optical basicity, Λ th, of the glasses, show that at constant mol% of Li 2 O δg ‖ Δg ⊥ , A ‖, and A ⊥ decrease with increase in Δ th whereas by varying modifier contents at constant mol% of B 2O 3 the change in spin Hamiltonian parameters is independent of the change in Λ th.

ESR of vanadyl ions in borate glasses

Electron spin resonance (ESR) of VO 2+ in Li 2 O·RO·B 2 O 3 (where R = Ba, Ca and Mg) glasses have been studied. Spin Hamiltonian parameters, the dipolar hyperfine coupling parameter, and the Fermi contact interaction parameter have been calculated. It is observed that with a decrease in B 2 O 3 mol% there is an improvement in the octahedral symmetry of the V 4+ site. The theoretical value of optical basicity, Λ th , of these glasses has also been calculated. It is found that at constant mol% of Li 2 O there is a decreasc in A ∥ , A ⊥ and Δ g ∥ /Δ g ⊥ and an increase in P with increase in the value of Λ th , whereas at constant mol% of B 2 O 3 by varying modifier contents the spin Hamiltonian parameters are independent of Λ th .

Analysis and prediction of acid–base reactions between oxides and oxysalts using the optical basicity concept

The chemical reactions of (i) oxides with each other, e.g. CaO with SiO2, (ii) oxides with oxysalts, e.g. CaO with CaSiO3, and (iii) oxysalts with each other, e.g. Ca2SiO4 with Ca3(PO4)2, have been considered from the point of view of acid–base theory. From known optical basicity values for alkali and alkaline-earth metal oxides and also for MgO, Al2O3, B2O3, SiO2, P2O5, and SO3, optical basicities were calculated for a large number of oxysalts. Examination of existing data on the reactions of these oxidic compounds at elevated temperatures, in the form of sub-solidus compatibility diagrams for ternary systems, has revealed that generally the optical basicity spanned by the products is less than that spanned by the reactants. This indicates that acid–base neutralisation is the major driving force behind the reaction; occasionally this principle is violated, as when other factors such as a stable network structure are of prime importance. It is shown how optical basicity analysis offers a method of constructing the tie-lines of sub-solidus compatibility diagrams, and it is suggested that this might be used as a guide for checking uncertainties in such diagrams.

光学的塩基度の新しい展開

A new scale of the theoretical optical basicity was established by means of the concept of the average electron density (D) expressecl in eq. (1).(This article is not displayable. Please see full text pdf.) \ oindentThe value of α in an oxide system was defined unity, and the values of α in alkaline earth fluoride and chloride systems were determined by measurments of Pbs-p spectra of CaF2 and CaCl2 using photoacoustic spectroscopy.The basicity moderating parameter values were found to have a linear relation with the values of the average electron density, as shown in eq. (2).(This article is not displayable. Please see full text pdf.) The theroretical optical basicity values of transition metal oxides were able to be calculated in the new scale, for example, ΛFeO=0.94, ΛFe2O3=0.72 and ΛTiO2=0.65. It was possible to explain the sulfide capacity values in the systems containing iron oxides using these optical basicity values. The theoretical optical basicity values of alkaline earth fluoride and chloride were also obtained in this study. The values of the optical basicity showed a good correlation to logarithmic values of the phosphate capacity at a constant temperature in the systems consisting of oxide and fluoride compounds. It is suggested that the theoretical optical basicity developed in this study was a resonable scale for a wide range basicity in slags and fluxes.

New acid sites in dilute oxide solid solutions: An optical basicity approach

Optical basicity values are obtained for groups in dilute oxide solid solutions, where protons compensate for lower oxidation state cations substituting for cations of the host oxide. These values are compared with the optical basicity of the host oxide. The results are discussed in the context of current models.

Acid-base properties of metals and glasses of Na2O.P2O5-xB2O3 system

Boron oxide in the Na2O.P2O5-x B2O3 system behaves as a Lux base. Its addition to Na2O.P2O5 brings about transformation of a Co(II) indicator from octahedral to tetrahedral configuration, increase in the optical basicity ΛPb(II), increase in the relative basicity of the melt as determined by means of a galvanic cell, and depolymerization reactions releasing PO43- ions. In the Na2O-B2O3 system free of P2O5, boron oxide behaves as a Lux acid. The amphoretic nature of B2O3 is explained in terms of Lux's acid-base theory extended in analogy with the protolysis theory. The theoretical optical basicity values do not indicate the amphoretic behaviour of B2O3 because in this approach boron oxide is a priori regarded as more acidic than Na2O.P2O5.

Ion Migration Study in a Sodium Borate Glass: Proposal of a New Oxide Transport

Electrolysis of the cellwas investigated at 50°C, using a modified moving boundary method (in the anode region) based upon sodium and lead concentration profiles (obtained by ion beam ablation) and ultraviolet spectroscopy of the Pb2+ ion. It was found that Na+ ions left the anode region for the cathode but, contrary to related experiments with silicate glasses, there was not an equivalent replacement by the Pb2+ ions generated at the anode. Instead, O2− ions migrated to the anode and most of the Pb2+ ions formed . It is suggested that a “structure‐switching ion transport” mechanism operates, migration of the O2− ions involving an interchange between threefold and fourfold coordination by the boron atoms as the oxygens are handed on from one borate group to the next. The ultraviolet spectrum of the small concentration of Pb2+ ions in the anode region registered a lower basicity than the original glass (optical basicity values 0.49 and 0.60, respectively) but the sites were more basic than for a corresponding borate glass obtained from the melt.

A molecular orbital treatment of the basicity of oxyanion units

The basicity of the oxidic groups AlO 4 5−, SiO 4 4−, PO 4 3−, SO 4 2−, ClO 4 −, can be regarded in terms of electron donor power and expressed numerically as the optical basicity, λ. The λ value depends upon the oxidation number of the central atom and its basicity moderating parameter, γ. The results of (modified) self-consistent field CNDO calculations, presented and discussed in this paper, indicate the simple relationship that the optical basicity value is directly proportional to the negative charge borne by the oxygen atoms of the oxyanion. It is apparent that the basicity moderating parameter is a measure of the ability of the central atom to withdraw charge from the oxygen atoms. The results also indicate that the electron density at the “surface” of the oxyanion originates not only from the oxygens but also from the central atom, this effect increasing with decreasing oxidation number of the central atom.