Polyborate-borate equilibrium

The effect of neutral salts (e.g., NaCl) on the composition of borates precipitated from, or in equilibrium with, aqueous solutions doubtless arises from a reduction in water activity, metal borate complexation, and a shift in polyborate equilibria (Sections IV,A, B). The "indifferent or inert component method has frequently been used for the synthesis of borates. Potassium and sodium chlorides can be used to enhance the precipitation of specific nickel (48), aluminum (51), iron (49), and magnesium (151) borates. In the K20-B203-H20 system at 25°C (248), the presence of potassium chloride results in a reduced boric acid crystallization curve, lower borate solubilities, lower pH, and an extended B203 K20 range over which the pentaborate crystallizes. 

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. 

The chemistry of borates is complex both in solution and in the melt. It is concluded from B n.m.r. and other studies that Na3B30g dissociates in solution to B(0H)4 ions and that at low concentrations tetraborates dissociate completely into B(0H)3 and B(OH)4, but that in more concentrated solutions of borates various polyborate ions coexist in equilibrium with one another. From melts extensive series of borates are obtained, the product depending on the composition of the melt, for example ... 

With increasing concentration of boric acid (> 0.025 M) the hydration-dissociation — equilibrium is shifted towards free B(OH)3 molecules, which associate progressively. These macromolecules can be regarded as polyborate anions (J4). Some polynuclear species, B303(0H)4, B303(0H)5" and the very stable tetraborate ion, which is the most predominant dinegative ion in sodium borate solutions, have been detected (Fig. 3). 

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