The pH of aqueous K₂B₄O₇ solutions at 298.15 and 323.15 K at a wide range of concentration has been determined and the chemical species distribution of the solutions calculated from equilibrium constants by Newton iterative algorithm. The chemical species of typical samples were confirmed by Raman spectrum. The results show that the major species B₄O₅(OH)₄^2? and the minor chemical species B₃O₃(OH)₄^? and B(OH)₄^? exist in highly concentrated aqueous solutions. The structure of highly concentrated aqueous K₂B₄O₇ solutions at 298 and 323 K was studied by X-ray scattering further. The interactions of intra-anion, interionic and intermolecular of B₄O₅(OH)₄^2?, B₃O₃(OH)₄^? and B(OH)₄^? were descripted quantitatively by geometrical model and calculated by least square method. The model calculation and refinement provided distance and coordination number between atoms in boroxol six-member ring, which are in good agreement with the crystal structure. The order degree in intra-anion is high in concentrated aqueous solutions. At the same time, we gained information about the hydration number and hydration distance of K⁺, borate ions and contact ion-pair. We also briefly discussed the effect of temperature and concentration on solvent structure.

Key words: potassium tetraborate, solution structure, x-ray scattering, contact ion-pair, chemical species distribution