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Recent Progress on Structure of Aqueous Polyborates in Solution
關鍵詞：solution structure polyborate scattering dihydrogen
摘要：The crystal structure of metal borates has been extensively investigated by X-ray and neutron diffraction,but,the structure of aqueous polyborate solutions are still largely unknown.Over the last decade,our group has focused on studying the structure of complex aqueous polyborate solutions of Li,Na,K,Rb,Cs,and Mg using synchrotron radiation X-ray scattering (XRS),EXAFS,Raman,NMR,and DFT,as well as determining the density,conductivity and pH of such solutions.Polyborate species distributions were calculated using pH measurements,and the main species in the solution have been confirmed by NMR and Raman spectra.For alkali-metal metaborates,the dominant species is always B(OH)4^- in a wide range of concentration,while the presence of others species is negligible.For alkali metal tetraborates,when concentration is in the extreme low range,only B(OH)3 and B(OH)4^- are present in these solutions.As the total boron concentration increases,B(OH)3 and B(OH)4^- polycondensated to form more complex oligomers.Of them,while B4O5(OH)4^2- in the tetraborate solutions is the main species,B(OH)3 ,B(OH)4^-,and B3O3(OH)4^- are minor species,and B3O3 (OH)5^2- and B5O6 OH)4^- are present only in negligible amounts.As solution continues to concentrate,B4O5(OH)4^2- eventually becomes the dominant species,which is consistent with the congruent compound M2B4O7·nH2O (M=Li,Na,K,Rb,Cs) in the system M2O-B2O3-H2O (M=Li,Na,K,Rb,Cs).For alkali metal pentaborates,B(OH)3 and B(OH)4^- are the main species at low concentrations.The species distribution,Raman and NMR spectroscopy results verified that the dominant species in concentrated pentaborate solutions with Li and Na is pentaborate B5O6 OH)4^-,but it is surprising that the main species with K,Rb,and Cs is always the triborate monoanion B3O3(OH)4^-.Although all M[B5O6 OH)4 ]·nH2O (M=Li,Na,K,Rb,Cs) are all congruent compounds in the system M2O-B2O3 -H2O (M=Li,Na,K,Rb,Cs),the main species in aqueous solutions are quite different because of various cation hydration distance (d),hydration number
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