Preparation and Properties of Metal Borates

This section briefly outlines the preparation and properties of the major metal borates, with particular emphasis on their hydrates. Recent and comprehensive reviews of the various group metal borates are found in Mellor (60, 307), from which additional data and references covering the entire field can be obtained. For brevity, the borates are referred to in terms of the mole ratio M20 B203 H20 or M20 B203 of their composition. 

These have been reviewed by Nies (307), and the commercially important borates have been reviewed by Doonan and Lower (113). 

Numerous studies have been made into the phase equilibria in the Na20-B203-H20 system at 0-100°C (e.g., 135, 216, 217, 308), and more recently at 150° (96). A partial phase diagram for this system covering the range of mole ratio Na20/B203 to 0.5 in solution is shown in Fig. 6. Over 25 borates of sodium in the absence of other cations have been identified (307). 

The pentahydrate Na20 2B2O3 5H20 has the same polyborate ion as borax. It crystallizes from aqueous solution above 60°C but is also unstable with respect to kernite in solution above 40°C. In accord with its structure, Na2[B405(OH)j3H20, the first three water molecules are lost at a temperature of 140 or 150°C, with the anhydrous product formed at over 400°C. 

In kernite Na20 2B203 4H20, the borate anion contains parallel infinite chains of [B40 (0H)2]2 . The formation of this ion by B-0 bond cleavage (Section III, A) accounts for its slow rate of crystallization and dissolution in aqueous media. Kernite is the stable phase in contact with its solutions from 58 to 95°C. 

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