Like silicon, boron is found in nature in the form of oxo compounds, and never as the element. Like — Physical Chemistry — Kinetics Chemistry Question
Polyoxoanions of Boron
Like silicon, boron is found in nature in the form of oxo compounds, and never as the element. Like silicon, boron-oxygen compounds are characterized by great diversity and complexity. In these compounds boron can be bonded to three O atoms in a triangle (as in B(OH)3, BO3 3– or B3O6 3–) or to four atoms at the corners of a tetrahedron (as in [BO4] 5–). One of the most important boron-oxygen compounds is the ionic compound borax, whose formula is normally written as Na2B4O7·10 H2O. The compound is used widely in the manufacture of borosilicate glass, glass fiber, and insulation.
Hydrolysis of the borohydride ion (BH4 – ) produces hydrogen gas and a borate. Because of the possible use of borohydride salts as possible hydrogen storage devices, the aqueous chemistry of borates has again been studied thoroughly.
3.1 The species in a solution of 0.5 mol dm–3 boric acid, B(OH)3, were recently studied, and a plot of the fraction of total boron species in solution at equilibrium as a function of pH was published. The main species are boric acid as well as B(OH)4 – , B4O5(OH)4 2– (the anion found in the mineral borax), and B3O3(OH)4 – .
i. Indicate which curve in the plot below corresponds to a particular boron-oxygen species.
Model Answer
3.1 i) Circles: B(OH)3
Squares (maximum at pH 8): B3O3(OH)4 – .
Triangles: B4O5(OH)4 2–
Squares (at high pH): B(OH)4 –
The plot is taken from the paper “pH-dependent X-ray absorption spectra of aqueous boron oxides” by A. M. Duffin, C. P. Schwartz, A. H. England, J. S. Uejio, D. Prendergast, and R. J. Saykally, J. Chem. Phys, 134, 154503 (2011). See also “Identification of Polyborate … Ions in Solution by Raman Spectroscopy,” L. Maya, Inorg. Chem. 13, 2179 (1976) and references therein.
ii. Sketch the structure of each of the four boron-oxygen species above.
Model Answer
ii)
3.2 Borax is a good primary analytical standard for the titrimetric determination of acids. Others analytical standards of the same kind are anhydrous sodium carbonate and (HOCH2)3CNH2, (TRIS). Borax and TRIS react with acids according to the following balanced equations:
Borate ion:
B4O7 2–(aq) + 2 H3O +(aq) + 3 H2O(l) → 4 H3BO3(aq)
TRIS:
(HOCH2)3CNH2(aq) + H3O +(aq) → H2O(l) + (HOCH2)3CNH3 +(aq)
Which primary standard–Na2CO3, borax, or TRIS, will lead to the smallest relative error? Assume there is a weighing error of 0.1 mg in weighing the standard and that you will titrate 40.0 cm3 of HCl solution with a concentration of 0.020 mol dm–3.
Model Answer
3.2 This is solved using the information in the table below. The weighing error of 0.10 mg represents only 0.066 % of the amount of borax required in the titration, and so represents the situation that leads to the smallest error in the titration. (This question is based on an example on pages 329-330 of Analytical Chemistry-An Introduction, 7th Edition, by D. A. Skoog, D. M. West, F. J. Holler, and S. R. Crouch, Brooks-Cole, 2000.)
Compound | Molar mass (g mol–1) | Mass required by 40.0 cm3 of HCl (0.0200 mol dm-3) | (0.10/required mass)(100%)
---|---|---|---
Na2CO3 | 106 | 42.4 mg | 0.24 %
TRIS | 121 | 96.8 mg | 0.10 %
Borax | 381 | 152 mg | 0.066 %