THERMODYNAMIC POSSIBILITY OF THE COMPLEX FORMATION PROCESSES IN POLYTUNGSTOTERBIATE SOLUTIONS
The purpose was to investigate the formation of complex compounds in heteropolytungtoterbiate-ion solutions and to consider thermodynamic probability of these reactions.
Methods and methodology. pH-potentiometry at 25 ± 0.1 °C was used to study interactions within TbW10O369- – H+(OH-) – H2O system (CTbW10O9−36=1⋅10−3MCTbW10O369−=1⋅10−3M). A model describing equilibrium processes in acidic and alkaline areas was selected by using computer program Clinp 2.1. Pitzer equations were used to calculate of the thermodynamic constants of processes in solutions.
Results. A model that describes the formation of heteropolyanions HnTbW10O36(9-n)- and HmTbW5O18(3-m)- was provided. Diagrams of ion distribution in aqueous solutions were constructed. Heteropolytungtoterbiates hydrolysis and hydrolysis-depolymerization processes were determined in initial solutions and areas of prevailing ion presence in solution were defined. Logarithms of the concentration and thermodynamic constants, Gibbs energy values for processes with monomer ions and standard Gibbs energy values (∆G°f.) of heteropolyanions formation were calculated. A series/parallel scheme of ion changes was provided.
According to which in the beginning an anion W6O20(OH)26- is formed, which has an octahedral tungsten-oxygen surrounding that is similar to lacunar ligands in the anion TbW10O369-. Nevertheless, the W6O20(OH)26- ion does not accumulate in the solution and is not fixed in the distribution diagram. It can probably be explained by its instantaneous consumption to the aprotonic pentatungtoterbiate, which then forms H2TbW5O18-. Anions HmTbW5O18(3-m)- are initial for the formation of the decatungtoterbiates HnTbW10O36(9-n)-. The close values of Gibbs energies of the protonation reactions for decatungtoterbiates confirm the parallel nature of these processes.
Conclusions. The formation of heteropolytungtoterbiate anions in solutions was selected by computer modelling based on the results of pH-potentiometry. Thermodynamic characteristics were determined and used for providing a scheme of ion changes, the thermodynamic possibility of processes being assessed by calculating the Gibbs free energy of the ions changes.
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