Structure Of Aqueous Rubidium Polyborate Solutions

In terms of the diverse chemical structure and wide uses of rubidium borate in many fields,the idea that macroscopic properties description associated with microscopic structure analysis was used to investigate rubidium borate compound in this paper.Firstly,their physicochemical properties related to structure,density,electrical conductivity and pH,were measured at 298.15 and 333.15 K,which revealed its change rules as the concentration and temperature varied.Secondly,the polyborate anion existing forms,content and equilibrium conversion between polyborate anions in aqueous rubidium borate solutions were studied using Newton iteration,and verified by the nuclear magnetic resonance spectroscopy.And on the basis the synchrotron radiation extended X-ray absorption fine structure spectroscopy at Beijing was used to investigate the neighboring structure of rubidium ion in aqueous rubidium borate solutions at room temperature for the first time,which the structural parameters can be obtained,such as the hydration number,hydration distance of Rb-O and Debye-Wallor factor,etc.Study is beneficial to understand clearly the formation condition,transformation rules and microscopic hydration structure,and replenish data stock of physicochemical properties and EXAFS structure about aqueous rubidium borate solutions at the same time.Moreover,designing and optimizing the best extraction technology is of great important theoretical help in both salt lake brine and ground brine.In this paper specific research contents and main results are as follow:Firstly,the preparation of rubidium pentaborate and rubidium tetraborate crystals was made from rubidium carbonate and boric acid in stoichiometric proportion.Then the volumetric method,atomic absorption spectroscopy,TG/DTG/DSC,XRD,FT-IR,Raman,NMR and EXAFS,etc.were carried out to analyse and identify synthesized compounds.At 298.15 and 333.15 K,the physicochemical properties of aqueous rubidium pentaborate and rubidium tetraborate solutions were measured,in which the density and electrical conductivity data were fitted with empirical equation for check experimental data reliability.Secondly according to measured pH data and reported chemical equilibrium constants in literature,the polyborate anion distribution of aqueous rubidium pentaborate and rubidium tetraborate solutions were calculated by Newton iteration at 298.15 and 333.15 K.The results show that these five existing forms B(OH)3,B(OH)4-,B3O3(OH)4-,B4O5(OH)42-and B5O6(OH)4-were mixed in solution for above mentioned systems.When concentration is extreme low,there only exist B(OH)3 and B(OH)4-,and their mole fractions are equal to 1.0.It states that rubidium pentaborate and rubidium tetraborate crystals dissolve and hydrolyse or depolymerize completely into B(OH)3 and B(OH)4-.As concentration increases,the mole fractions of B(OH)3 and B(OH)4-decrease,while that of B3O3(OH)4-and B4O5(OH)42-increase.Moreover,the decreasing mole fractions of the former two species are equivalent to the increasing mole fractions of the latter two species,which suggests that B(OH)3 reacts with B(OH)4-to generate B3O3(OH)4-and B4O5(OH)42-by condensation.When concentration goes up,B5O6(OH)4-starts to appear and the mole fractions of these five species change with concentration.The mole fractions of B(OH)3 and B(OH)4-decrease,that of B3O3(OH)4-,B4O5(OH)42-and B5O6(OH)4-increase.The results are in accordance with the tendency of NMR data at 297.5K.However,the dominant species is very different,which B3O3(OH)4-and B4O5(OH)42-are dominant species for aqueous rubidium pentaborate and rubidium tetraborate solutions,respectively.This phenomenon can also be found in other alkali metal borate salt solutions,such as lithium pentaborate,lithium tetraborate,potassium pentaborate,potassium tetraborate.Thirdly using the synchrotron radiation EXAFS at Beijing,the neighboring structure of rubidium ion in aqueous rubidium pentaborate and rubidium tetraborate solutions was researched at room temperature,which the average Rb-O distance and coordination number are 2.93?,7.7 and 2.92?,8.0,respectively.These conclusions are good consistent with structural parameters of other rubidium salt compounds.In addition,it is also found that hydration number and hydration distances have an increasing tendency with an increase of ionic radius for alkali metal ions.Furthermore,hydration and the ability forming the second hydration shell become increasingly weaker on passing form Li~+ to Cs~+.