Borate anomaly

As a consequence, there is a network collapse and better volume utilization. Therefore, rapid reversals occur in the variation of molar volume, refractive indices, Tg, thermal expansivity etc., as a function of alkali composition. All of these variations are directly or indirectly related to energy density and the observed variations are often referred as borate anomaly. An example of the variation of some of these properties is shown in Figure 12.12. 

Eventually, a number of new observations raised questions regarding the original explanation for the borate anomaly. First, and most importantly, nuclear magnetic resonance was used for the direct measurement of the concentrations of 3- and 4-fold borons in a large number of alkali borate glasses. The results of these studies clearly demonstrate that the coordination conversion process does occur as proposed, but also show that the maximum in the concentration of tetrahedrally coordinated borons, designated as N4, does not occur until the composition contains = 35 to 40 mol% alkali oxide. Since this concentration is well past the composition where the property trend reversals occur, a direct correlation between the borate anomaly and the value of N4 seems doubtful. 

More recent property measurements also raised new questions regarding the borate anomaly. Measurements of Tg reveal that the... 

Boric acid is a relatively weak acid compared to other conunon acids, as illustrated by the acid equilibrium constants given in Table 4. Boric acid has a similar acid strength to sihcic acid. Calculated pH values based on the boric acid equihbrium constant are significantly higher than those observed experimentally. This anomaly has been attributed to secondary equilibria between B(OH)3, B(OH)4, and polyborate species. Interestingly, the aqueous solubihty of boric acid can be increased by the addition of salts such as potassium chloride and sodium sulfate, but decreased by the addition of others salts, such as the chlorides of lithium and sodium. Basic anions and other nucleophiles such as fluorides and borates significantly increase boric acid solubility. 

In the binary glass-forming systems of alkali metal borates, it is possible to observe change in the trend of a number of physico-chemical properties in the concentration range of approximately 20 mole % of alkali metal oxide. This phenomenon is known in the literature as boric acid anomaly , and it is due to the change in the structure of the B2O3 melt caused by the alkali metal oxide addition and is related to the ability of boron to change its coordination number. 

Heat capacity measurements of K3Nb306(B03)2, a borate that exhibits super-ionic conductivity and ferroelectric properties, have been performed in the 60-220 K temperature range. In this work, Maczka et al. documented the presence of two heat capacity anomalies the first anomaly occurred at 198.3 K and has... 

A further anomaly of the reactions between CpCo(Rf)(CO)I (Rf=C2Fs, "C3F7, CsFv) and KB(pz)4, is that the major products were not the respective tetrakis(pyrazolyl)borate complexes. Rather, these materials were formulated as CpCo(Rf)(pz2H) (Chart 2), on the basis of their monomeric and diamagnetic nature, and NMR spectroscopic data. 

R. R. Shaw and D. R. Uhlmann, The thermal expansion of alkali borate glasses and the boric oxide anomaly, /. Non Cryst. Solids, 1 347,1969. 

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