Oxoacids of boron III

However, as in the cases of the other oxoanions considered above, the formed metaborate, BO2, shows appreciable acidic properties. After the pO drop at the equivalence point the oxo-anions continue to fix oxide ions, and the value of the ligand number rises to 5. This means that the second stage of the neutralization process takes place. Neutral orthoborate ion is the product resulting from such an interaction, and it is a strong Lux base. The buffer solutions metaborate/orthoborate exist, mainly, within a 1-2 pO range, and possess buffer numbers of the order of /3 -0.4 to - 0.6. The pK value describing the following equilibrium  

Oxides of germanium(IV) and niobium(V) are stable at temperatures of the order of 700 °C, and under these conditions they are solids depending on its crystalline modification Ge02 has its melting point in the temperature range 1086-1116 °C, whereas niobium oxide exists as a single crystalline phase with the melting point at 1512 °C [121]. 

Potentiometric investigation of the oxoacidic properties of the oxides mentioned above was reported in Refs. [160,161]. A priori, before performing this study it was assumed that niobium(V) oxide would be the complete analogue of V2O5 and that its acidic properties would be even stronger. The basis for such an assumption may be found in Group VIb, where there is an appreciable increase of the acidic properties in the sequence Cr-Mo-W. 

As for germanium(IV) oxide, this was considered to be slightly soluble in the molten chloride—similar to Si02—and its acidic properties were expected to be too weak. However, one feature was not mentioned. While considering aqueous solutions we can see that Ge02 possesses appreciable solubility in water. For example, at 100 °C approximately 1.0 g of Ge02 is dissolved in 100 g of water, and this allows us to assume that under the experimental conditions conventional for molten chlorides this oxide will be sufficiently soluble. Indeed, this is the case for the molten equimolar KCl-NaCl mixture. 

In contrast, germanium oxide was found to be appreciably soluble in the molten equimolar KCl-NaCl mixture and its solutions show weak acidic properties. Its acid-base titration using NaOH as a titrant results in the appearance of a small pO drop at the equivalence point ( 1 pO unit), which corresponds to the ligand number value —0.5. This allows us to conclude that the acid-base titration runs in one step, and the final product is di-germanate ion, Gc202 

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