Stability constants of thorium(IV) complexes with aryl-bis-(5-hydroxy-3-methyl-1-phenyl-4-pyrazolyl) methane ligands

Summary Stability constants of complexes of aryl-bis-(5-hydroxy-3-methyl-1-phenyl-4-pyrazolyl) methane [ArBPyM] derivatives with thorium(IV) ions were determined by the potentiometric method at 30°C and an ionic strength of 0.1 mol·dm^–3 (KNO₃) in 75% (v/v) dioxane-water. The evaluation of the titration data indicated that four kinds of complexes ([ThL]²⁺, [ThLOH]⁺, [ThL ₂], and [ThL(OH)₂]^2–) were formed. The formation constants for all [ThL]²⁺ and [ThL ₂] complexes have been calculated to compare these values with those previously reported [1, 2] with Ln³⁺ and UO ₂ ²⁺ metal ions [2, 3]. The probable ligand-bonding sites of the complexes are proposed. In addition, the applicability of theHammett equation for the correlation of the stability constants of [Th(IV)-ArBPyM] complexes are discussed.

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Stabilitätskonstanten von Thorium(IV)-Komplexen mit Aryl-bis-(5-hydroxy-3-methyl-1-phenyl-4-pyrazolyl)-methan-Liganden

Zusammenfassung Stabilitätskonstanten von Komplexen von Aryl-bis-(5-hydroxy-3-methyl-1-phenyl-4-pyrazolyl)-methan — Derivaten [ArBPyM] mit Thorium(IV) — Ionen wurden bei 30°C und einer Ionenstärke von 0.1 mol-dm^–3 (KNO₃) in 75% (v/v) Dioxan-Wasser potentiometrisch bestimmt. Die Auswertung der Titrationskurven zeigte, daß vier verschiedene Komplexe vorlagen ([ThL]²⁺, [ThLOH]⁺, [ThL ₂] und [ThL(OH)₂]²⁺). Die Bildungskonstanten aller [ThL]²⁺- und [ThL ₂]-Komplexe wurden berechnet, um sie mit den früher für Ln³⁺- und UO ₂ ²⁺ -Ionen publizierten zu vergleichen. Potentielle Bindungsstellen der Komplexe für Liganden werden vorgeschlagen. Zusätzlich wird die Anwendbarkeit derHammet-Beziehung auf die Korrelation der Stabilitätskonstanten von [Th(IV)-ArBPyM] — Komplexen diskutiert.

     

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.     

Studies on the Complexation Behavior of Thorium(IV). 1. Hydrolysis Equilibria

The stability constants of thorium(IV) hydrolysis species have been measured at15, 25, and 35°C (in 1.0 mol dm^–3 NaClO₄) using both potentiometry and solventextraction. The results indicate the presence of the monomeric speciesTh(OH)³⁺, Th(OH)²⁺ ₂, Th(OH)⁺ ₃, and Th(OH)₄, in addition to the polymericspecies Th₄(OH)^{8+ 8} and Th₆(OH)⁹⁺ ₁₅. The polymeric species were found to beimportant, although the total thorium concentration was limited to 0.01–0.1mmol-dm^–3. The solvent extraction measurements required the use of acetylacetone.As such, the stability constants of thorium(IV) with acetylacetone were alsomeasured using both potentiometry and solvent extraction. All logarithms of thestability constants were found to be linear functions of the reciprocal absolutetemperature indicating that H ^o and S^o of reaction are both independent oftemperature (over the temperature range examined in the study).     

XAS study of murataite-based ceramics and crystalline film of ThO2

The local environment of thorium in murataite ceramics (Al,Ca,Ti,Mn,Fe,Zr,Th)O_x and ThO₂(001) crystalline film on Si(100) substrate as a reference was explored by X-ray absorption spectroscopy (XAS) for the first time. It was found that Th⁴⁺ is located in the center of a cube formed by 8 oxygen atoms [r(Th–O) = 2.37 ± 0.03 Å] in murataite ceramics and ThO₂ film. The Th⁴⁺ second coordination sphere [r(Th–M) ≈ 3.5 Å] in murataite is represented by 3d metals: titanium, iron or manganese     

On the hydrolysis of the titanium(IV) ion in chloride media

Determinations of the [Ti(IV)]/[Ti(III) ratio in solutions of titanium(IV) chloride equilibrated with H₂(g), at 25°C in 3 M (Na)Cl ionic medium, have indicated the predominance of the Ti(OH)₂²⁺ species in the concentration ranges 0.5 ? [H⁺] ? 2 M and 1.5 x 10^?3 ? [Ti(IV)] ? 0.05 M. From the equilibrium data the reduction potential has been evaluated Ti(OH)₂²⁺ + 2 H⁺ + e ? Ti³⁺ + 2H₂O, E_oH = (7.7 ± 0.6) x 10^?3 V. The acidification reactions of Ti(OH)₂²⁺ were also studied in 12 M(Li)Cl medium at 25°C by measuring the redox potential of the Ti(IV)/Ti(III) couple as a function of [H⁺]. The potentiometric data in the acidity range 0.3 ? [H⁺] ? 12 M have been explained by assuming Ti⁴⁺ + e ? Ti³⁺, E_o = 0.202 ± 0.002 V Ti⁴⁺ + H₂O ? TiOH³⁺ + H⁺, log K_a1 = 0.3 ± 0.01 Ti⁴⁺ + 2H₂O ? Ti(OH)₂²⁺ + 2H⁺, log K_a1K_a2 = 1.38 ± 0.05.     

Sterically Less‐Hindered Half‐Titanocene(IV) Phenoxides: Ancillary‐Ligand Effect on Mono‐, Bis‐, and Tris(2‐Alkyl‐/arylphenoxy) Titanium(IV) Chloride Complexes

A series of mono‐, bis‐, and tris(phenoxy)–titanium(IV) chlorides of the type [Cp*Ti(2‐R? PhO)_nCl_3?n] (n=1–3; Cp*=pentamethylcyclopentadienyl) was prepared, in which R=Me, iPr, tBu, and Ph. The formation of each mono‐, bis‐, and tris(2‐alkyl‐/arylphenoxy) series was authenticated by structural studies on representative examples of the phenyl series including [Cp*Ti(2‐Ph? PhO)Cl₂] ( 1 PhCl2 ), [Cp*Ti(2‐Ph? PhO)₂Cl] ( 2 PhCl ), and [Cp*Ti(2‐Ph? PhO)₃] ( 3 Ph ). The metal‐coordination geometry of each compound is best described as pseudotetrahedral with the Cp* ring and the 2‐Ph? PhO and chloride ligands occupying three leg positions in a piano‐stool geometry. The mean Ti? O distances, observed with an increasing number of 2‐Ph? PhO groups, are 1.784(3), 1.802(4), and 1.799(3) Å for 1 PhCl2 , 2 PhCl , and 3 Ph , respectively. All four alkyl/aryl series with Me, iPr, tBu, and Ph substituents were tested for ethylene homopolymerization after activation with Ph₃C⁺[B(C₆F₅)₄]^? and modified methyaluminoxane (7% aluminum in isopar E; mMAO‐7) at 140 °C. The phenyl series showed much higher catalytic activity, which ranged from 43.2 and 65.4 kg (mmol of Ti?h)^?1, than the Me, iPr, and tBu series (19.2 and 36.6 kg (mmol of Ti?h)^?1). Among the phenyl series, the bis(phenoxide) complex of 2 PhCl showed the highest activity of 65.4 kg (mmol of Ti?h)^?1. Therefore, the catalyst precursors of the phenyl series were examined by treating them with a variety of alkylating reagents, such as trimethylaluminum (TMA), triisobutylaluminum (TIBA), and methylaluminoxane (MAO). In all cases, 2 PhCl produced the most catalytically active alkylated species, [Cp*Ti(2‐Ph? PhO)MeCl]. This enhancement was further supported by DFT calculations based on the simplified model with TMA.     

Theoretical study of beryllium (II) complexes using CATIVIC: New parametric method

Calculations of several beryllium complexes {[Be(H₂O)_n]²⁺ (n = 1–4), [BeOH(H₂O)_n]⁺ (n = 1–3), and [Be(OH)₂(H₂O)_n] (n = 1, 2)} were carried out to compare different ab initio (density functional theory, MP2) and parametric (PM3(tm), CATIVIC) methods. Results show that the parametric method CATIVIC gives geometries and energies closer to the ab initio geometries than the PM3(tm) method due to the inclusion of the atomic excitation energies of the neutral atoms as well as the ions and to the dependence of the molecular parameters on the system charge. The molecular electronic density analysis of the Be? O bonds shows that the Be–water interaction in the [Be(H₂O)_n]²⁺ complexes can be considered as a closed‐shell interaction with a σ character in the bond while in the [Be(OH)₂(H₂O)_n] complexes the Be? water bond have π character. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Coordination of Alkaline Earth Metal Ions in the Inverted Cucurbit[7]uril Supramolecular Assemblies Formed in the Presence of [ZnCl4]2− and [CdCl4]2−

A convenient method to isolate inverted cucurbit[7]uril (iQ[7]) from a mixture of water‐soluble Q[n]s was established by eluting the soluble mixture of Q[n]s on a Dowex (H⁺ form) column so that iQ[7] could be selected as a ligand for coordination and supramolecular assembly with alkaline earth cations (AE²⁺) in aqueous HCl solutions in the presence of [ZnCl₄]^2? and [CdCl₄]^2? anions as structure‐directing agents. Single‐crystal X‐ray diffraction analysis revealed that both iQ[7]–AE²⁺–[ZnCl₄]^2?–HCl and iQ[7]–AE²⁺–[CdCl₄]^2?–HCl interaction systems yielded supramolecular assemblies, in which the [ZnCl₄]^2? and [CdCl₄]^2? anions presented a honeycomb effect, and this resulted in the formation of linear iQ[7]/AE²⁺ coordination polymers through outer‐surface interactions of Q[n]s.     

Prediction of 195Pt NMR chemical shifts of dissolution products of H2[Pt(OH)6] in nitric acid solutions by DFT methods: how important are the counter‐ion effects?

¹⁹⁵Pt NMR chemical shifts of octahedral Pt(IV) complexes with general formula [Pt(NO₃)_n(OH)_{6 ? n}]^2?, [Pt(NO₃)_n(OH₂)_{6 ? n}]^{4 ? n} (n = 1–6), and [Pt(NO₃)_{6 ? n ? m}(OH)_m(OH₂)_n]^{?2 + n ? m} formed by dissolution of platinic acid, H₂[Pt(OH)₆], in aqueous nitric acid solutions are calculated employing density functional theory methods. Particularly, the gauge‐including atomic orbitals (GIAO)‐PBE0/segmented all‐electron relativistically contracted–zeroth‐order regular approximation (SARC–ZORA)(Pt) ∪ 6–31G(d,p)(E)/Polarizable Continuum Model computational protocol performs the best. Excellent second‐order polynomial plots of δ_calcd(¹⁹⁵Pt) versus δ_exptl(¹⁹⁵Pt) chemical shifts and δ_calcd(¹⁹⁵Pt) versus the natural atomic charge Q_Pt are obtained. Despite of neglecting relativistic and spin orbit effects the good agreement of the calculated δ ¹⁹⁵Pt chemical shifts with experimental values is probably because of the fact that the contribution of relativistic and spin orbit effects to computed σ^{iso 195}Pt magnetic shielding of Pt(IV) coordination compounds is effectively cancelled in the computed δ ¹⁹⁵Pt chemical shifts, because the relativistic corrections are expected to be similar in the complexes and the proper reference standard used. To probe the counter‐ion effects on the ¹⁹⁵Pt NMR chemical shifts of the anionic [Pt(NO₃)_n(OH)_{6 ? n}]^2? and cationic [Pt(NO₃)_n(OH₂)_{6 ? n}]^{4 ? n} (n = 0–3) complexes we calculated the ¹⁹⁵Pt NMR chemical shifts of the neutral (PyH)₂[Pt(NO₃)_n(OH)_{6 ? n}] (n = 1–6; PyH = pyridinium cation, C₅H₅NH⁺) and [Pt(NO₃)_n(H₂O)_{6 ? n}](NO₃)_{4 ? n} (n = 0–3) complexes. Counter‐anion effects are very important for the accurate prediction of the ¹⁹⁵Pt NMR chemical shifts of the cationic [Pt(NO₃)_n(OH₂)_{6 ? n}]^{4 ? n} complexes, while counter‐cation effects are less important for the anionic [Pt(NO₃)_n(OH)_{6 ? n}]^2? complexes. The simple computational protocol is easily implemented even by synthetic chemists in platinum coordination chemistry that dispose limited software availability, or locally existing routines and knowhow. Copyright © 2016 John Wiley & Sons, Ltd.     

Adsorption recovery of thorium(IV) by Myrica rubra tannin and larch tannin immobilized onto collagen fibres

Novel adsorbents which can concentrate Th(IV) in aqueous solution were prepared by immobilizingMyrica rubra tannin and larch tannin onto collagen fibre matrices. The adsorption capacities of the immobilized tannins to Th(IV) are related to temperature and pH value of the adsorption process. For example, when the initial concentration of Th(IV) was 116.0 mg·l^-1 and the immobilized tannin was 100 mg, the adsorption capacities of immobilized Myrica rubra tannin and larch tannin were 55.98 mg Th(IV)·g^-1 and 13.19 mg Th(IV)·g^-1, respectively at 303 K, and 73.67 mg Th(IV)·g^-1 and 18.19 mg Th(IV)·g^-1 at 323 K. It was also found that the higher adsorption capacity was obtained at higher pH value. The adsorption equilibrium data of the immobilized tannins for Th(IV) can be well fitted by the Langmuir model and the mechanism of the adsorption was found to be a chemical adsorption. In general, the adsorption capacity of immobilized Myrica rubra tannin to Th(IV) is significantly higher than that of immobilized larch tannin, probably due to the fact that the B ring of Myrica rubra tannin has a pyrogallol structure which has higher reaction activity with metal ions. The breakthrough point of the adsorption column of immobilized Myrica rubra tannin was at 33 bed volumes for the experimental system. The mass transfer coefficient of adsorption column determined by Adams-Bohart equation was 1.61·10^-4 l·mg^-1.min^-1. The adsorption column can be easily regenerated by 0.1 mol·l^-1 HNO₃ solution, showing outstanding ability of concentrating Th(IV). This revised version was published online in July 2006 with corrections to the Cover Date.     

Aqueous Long-Term Solubility of Titania Nanoparticles and Titanium(IV) Hydrolysis in a Sodium Chloride System Studied by Adsorptive Stripping Voltammetry

The solubility of industrially produced titanium dioxide nanoparticles has been studied in aqueous sodium chloride media in the pH range 1 to 13 at 25 °C by using adsorptive stripping voltammetry (AdSV). Kinetic dissolution curves have been obtained as well as long-term solubilities that provide an approximation of the equilibrium solubilities. The titania nanoparticles used in the dissolution experiments have been characterized by nitrogen sorption measurements, XRD and colloid titration. The equilibrium solubilities and titanium(IV) speciation and their dependences on pH have been modelled by assuming the formation of the mononuclear titanium hydroxo complexes [Ti(OH) _n ]⁽⁴⁻ⁿ⁾⁺ (n=2 to 5) to be the only titanium species present. The solubility product of titanium dioxide and equilibrium constants for titanium(IV) hydrolysis, calculated from the AdSV solubility data, are presented.     

catena‐Poly[thorium(IV)‐tetrakis(μ2‐3‐carboxyadamantane‐1‐carboxylato)]: a quadruple helical strand driven by a synergy of coordination and hydrogen bonding

The title compound, [Th(C₁₂H₁₅O₄)₄]_n, is the first homoleptic thorium–carboxylate coordination polymer. It has a one‐dimensional structure supported by the bidentate bridging coordination of the singly charged 3‐carboxyadamantane‐1‐carboxylate (HADC⁻) anions. The metal ion is situated on a fourfold axis (site symmetry 4) and possesses a square‐antiprismatic ThO₈ coordination, including four bonds to anionic carboxylate groups [Th—O = 2.359 (2) Å] and four to neutral carboxyl groups [Th—O = 2.426 (2) Å], while a strong hydrogen bond between these two kinds of O‐atom donor [O...O = 2.494 (3) Å] affords planar pseudo‐chelated Th{CO₂...HO₂C} cycles. This combination of coordination and hydrogen bonding is responsible for the generation of quadruple helical strands of HADC⁻ ligands, which are wrapped around a linear chain of Th^IV ions [Th...Th = 7.5240 (4) Å] defining the helical axis.     

Thermodynamic Model for ThO<Subscript>2</Subscript>(am) Solubility in Isosaccharinate Solutions

Extensive studies on ThO₂(am) solubility were carried out as functions of a wide range of isosaccharinate concentrations (0.0002 to 0.2 mol⋅kg⁻¹) at fixed pH values of about 6 and 12, and varying pH (ranging from 4.5 to 12) at fixed aqueous isosaccharinate concentrations of 0.008 mol⋅kg⁻¹ or 0.08 mol⋅kg⁻¹, to determine the aqueous complexes of isosaccharinate with Th(IV). The samples were equilibrated over periods ranging up to 69 days, and the data showed that, in most cases, steady-state concentrations were reached in <15 days. The data were interpreted using the SIT model, and required the inclusion of mixed hydroxy-ISA complexes of Th(IV) [Th(OH)ISA²⁺, Th(OH)₃(ISA)₂^-_{2}^{-}, and Th(OH)₄(ISA)₂^2-]_{2}^{2-}] with log ₁₀ K ⁰=12.5±0.5,4.4±0.5 and −3.2±0.5 for the reactions:

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.     

Thio- und AmidothioDerivate der Diphosphor(IV)-säure

Thio and Amidothio Derivatives of Diphosphorus(IV) Acid Oxothiodiphosphates(IV) with anions [P₂O_nS_6?n]^4? (n = 1–5) are formed by steps wise substitution of thio by oxo ligands in hexathiodiphosphate(4–). Oxidative ammonolysis of thianion leads to amidothio derivatives, [P₂(NH₂)S₅]^3? and [P₂(NH₂)₂S₄]^2?.     

A DFT study of uranyl hydroxyl complexes: structure and stability of trimers and tetramers

A DFT study of U(VI) hydroxy complexes was performed with special attention paid to the [(UO₂)₃(OH)₅(H₂O)_4–7]⁺ and [(UO₂)₄(OH)₇(H₂O)_5–8]⁺ species. It was established that the ionicity of the U=O bond increased when moving from [(UO₂)(H₂O)₅]²⁺, [(UO₂)₂(OH)(H₂O)₈]³⁺, [(UO₂)₂(OH)₂(H₂O)₆]²⁺, [(UO₂)₃(OH)₅(H₂O)_4–6]⁺ to [(UO₂)₄(OH)₇(H₂O)_5–8]⁺ species. In both [(UO₂)₃(OH)₅(H₂O)_4–6]⁺ and [(UO₂)₄(OH)₇(H₂O)_5–8]⁺ complexes, the U=O bond was observed to have a range of different lengths which depended on the composition of the first coordination sphere of UO₂ ²⁺. The cyclic structures of trimeric complexes were somewhat more stable than their linear structures, which was probably due to the steric effect.

     

In the footsteps of August Michaelis: Syntheses and Thermodynamics of Extremely Low-Volatile Ionic Liquids

A series of nine different known ionic liquids or low melting salts was synthesised and purified. They are composed of the [NTf₂]^– (bis(trifluoromethane)sulfonimide), [OTf]^– (trifluoro-methane-sulfonate), or [B(CN)₄]^– (tetracyanidoborate) anion and [Ph₄P]⁺ (tetraphenylphosphonium), [Ph₃BzP]⁺ (triphenylbenzyl phosphonium), [ⁿBu₄P]⁺ (tetra-ⁿbutylphosphonium), [ⁿBuPh₃P]⁺ (tri-phenyl-ⁿbutylphosphonium), [ⁿBu₄N]⁺ (tetra-ⁿbutylammonium), or the [PPN]⁺ (bis(triphenylphosphine)iminium) cation. Precise vapour pressure data and enthalpies of vaporisation were measured using the Quartz Crystal Microbalance (QCM) method and evaluated. Structure-property relations are established using the obtained data as well as literature known data of ILs with alkyl-substituted imidazolium cations. It turns out that ILs with the tetracyanidoborate anion have even higher values of the enthalpy of vaporisation than those with the common [NTf₂]⁻ or [OTf]⁻ anion and therefore are even less volatile.     

Experimental and Computational Studies on the Formation of Thorium–Copper Heterobimetallics

The formation of actinide–transition metal heterobimetallics mediated by a terminal actinide imido complex was comprehensively studied. The reaction of the thorium imido complex [(η⁵‐C₅Me₅)₂Th=N(mesityl)(DMAP)] ( 3 ), prepared from [(η⁵‐C₅Me₅)₂ThMe₂] ( 1 ) and mesitylNH₂ or [(η⁵‐C₅Me₅)₂Th(NHmesityl)₂] ( 2 ) in the presence of 4‐(dimethylamino)pyridine (DMAP), with copper(I) halides gave the first thorium–copper heterobimetallic compounds [(η⁵‐C₅Me₅)₂Th(X){N(mesityl)Cu(DMAP)}] (X=Cl ( 4 ), Br ( 5 ), I ( 6 )). Complexes 4 – 6 feature an unusual geometry with a short Th?Cu distance, which DFT studies attribute to a weak donor–acceptor bond from the Cu⁺ atom to the electropositive Th⁴⁺ atom. They are reactive species, as was shown by their reaction with the dimethyl complex [(η⁵‐C₅Me₅)₂ThMe₂] ( 1 ). Furthermore, a comparison between Th and early transition metals confirmed that Th⁴⁺ exhibits distinctively different reactivity from d‐transition metals.     

A titration procedure for the determination of oxide in basic n-butylpyridinium chloride: Aluminum chloride melts

A procedure for the determination of oxide in n-butylpyridinium chloride-aluminum chloride, a basic chloroaluminate, room temperature molten salt, is reported. The procedure is based on the fact that Ti(IV) exists in two forms, a chlorocomplex, and an oxychlorocomplex, which are in slow equilibrium, and may be indicated asTiCl₆^2?+AlOCl₂^?TiOCl₄^2?+AlCl₄^? The reduction of each Ti(IV) species to Ti(III) is diffusion controlled as determined by normal pulse voltammetry. The normal pulse voltammetric determination of both Ti(IV) species, followed by appropriate graphical analysis, permits the determination of oxide and the equilibrium constant for the reaction written above. The determination of oxide added as Li₂CO₃, as well as the oxide impurity present in the melt, is described.