Zinc (II)–Boron (III) Aqueous Complex Formation Between 25 and 70 °C

The zinc boron complex formation was studied as a function of temperature (25, 50 and 70 °C) in boric acid solutions of various concentration (0.25, 0.50 and 0.68 mol·kg⁻¹). pH was monitored during zinc ion addition by galvanostatic dissolution of a zinc metal electrode, in a solution of boric acid. The determination of the complex formation showed the importance of an accurate model of the polyborate speciation, recalculated for this work based on the previous literature data mainly potentiometric measurements completed by Raman spectroscopy and Ab Initio calculations. Modelling of our experimental results, considering various scenarios of boric acid speciation, was performed using R and PhreeqC, suggesting the formation of an aqueous triborate-zinc (II) complex, \({{\text{ZnB}}}_{3}{{\text{O}}}_{3}{({\text{OH}})}_{4({\text{aq}})}^{+},\) according to the reaction: \({{\text{Zn}}}^{2+}+3{{\text{B}}({\text{OH}})}_{3} \rightleftharpoons {{\text{ZnB}}}_{3}{{\text{O}}}_{3}{({\text{OH}})}_{4({\text{aq}})}^{+}+2{{\text{H}}}_{2}{\text{O}}+{{\text{H}}}^{+}\). The nature and structure of this aqueous complex disagrees with the results reported previously in the literature. Three formation constants of the triborate-zinc (II) complex were determined at 25, 50 and 70 °C as \({{\text{log}}}_{10}{K}_{\text{ZnB}}\) = − 4.73 ± 0.10, − 4.21 ± 0.16 and − 4.94 ± 0.12, respectively. The evolution of zinc boron complex formation as a function of temperature (between 25 and 70 °C) provides information on the effect of the polyborate predominance in the solution on the complexation of zinc.