In order to find the configuration of the metaborate group the crystal structure of potassium metaborate was determined. The crystals are rhombohedral ar = 7.76A, = 110° 36 with six molecules KBO2. The density is 2.348. The space group is R c(D3d6). All atoms are lying on twofold axes at positions: ±(u, ½-u, ¼) . The parameters were determined by means of two-dimensional Fourier analyses:
Compounds have different physical and chemical properties from their constituent elements. This is the one principal criterion for distinguishing a compound from a mixture of elements or substances: a mixture's properties are generally similar or related to the properties of its constituents. Another criterion is that the constituents of a mixture can usually be separated by simple, mechanical means such as filtering, those of a compound are often very hard to separate. Furthermore, when a compound is formed from its constituents, a chemical change takes place through chemical reactions. Mixtures can be made by mechanical means alone.
An example of a mixture that is often times confused to be a compound is an alloy. It is made mechanically, most commonly by heating up all of the constituent(s) and then cooling it quickly so that the constituents are then "caught" in the base metal.
The metaborate radical is (B3O6)-3, a ring of three BO3 triangles, rather than the endless chain of BO3 triangles found in CaB2O4. The BO3 groups are slightly distorted, the B O distances being 1.33A, 1.38A and 1.38A, the O O distances 2.30A, 2.38A and 2.38A. The nine particles of the radical lie in one plane. Potassium is surrounded by seven oxygen atoms at a distance 2.82A. The Journal of Chemical Physics is copyrighted by The American Institute of Physics.
Borates in chemistry are chemical compounds containing boron bonded to three oxygen atoms written as B(OR)3. In B(OR)4- anions, this number increases to four. The borate ion is BO33-. it forms salts with metallic elements. Boron found in nature is commonly as a borate mineral. Boron is also found combined with silicate to form complex borosilicate minerals such as the tourmalines.
Borate exists in many forms. In acid and near-neutral conditions, it is boric acid, commonly written as H3BO3 but more correctly B(OH)3. The pKa of boric acid is 9.14 at 25C. Boric acid does not dissociate in aqueous solution, but is acidic due to its interaction with water molecules, forming tetra hydroxyborate:
B(OH)3 + H2O ? B(OH)4- + H+ Ka = 5.8x10-10 mol/l; pKa = 9.24.
Polyborate anions are formed at pH 7-10 if the boron concentration is higher than about 0.025 mol/L. The best known of these is the tetraborate ion, found in the mineral borax:
4B(OH)4- + 2H+ ? B4O72- + 9H2O
Even though boric acid adds hydroxide to form B(OH)4-, you may find for pKa values and other calculations, the fictitious ions are easier to use. Thus for a typical polyprotic acid, the deprotonation series dihydrogen borate [H2BO3-;], hydrogen borate [HBO32-] and borate [BO33-] may be written as pH increases. [4]
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