Isolation of Boron class 11 chemistry Notes

Isolationof Boron

 Ores of boron

            (i)  Borax or tincal :Na₂B₄O₇.10 H₂O 

            (ii) Kernit or Rasorite:Na₂B₄O₇. 4 H₂O

            (iii) Colemanite : Ca₂ B₆O₁₁ . 5 H₂O

            (iv) Orthoboric acid :H₃BO₃  (It occurs in the jets of steam called soffioni escaping from ground in the volcanic region of the Tuscany). Boron is present to a very small extent (0.001%) in earth’s crust.

(i)        Reduction of boric oxide with highly electropositive metals like Na, K and Mg in the absence of                          air.

                

            B₂O₃ (s) + 6K(s)  2B(s) + 3K₂O(s)

            B₂O₃ (s) + 3Mg(s)  2B(s) + 3MgO(s)

            The product thus obtained is boiled with HCl and filtered where K₂O and MgO dissolves, leaving            behind elemental B. The powdered B thus obtained is thoroughly washed with water to free it from      HCl and is finally dried         

(ii)       From potassium fluoroborate (KBF4) by heating it with potassium metal.

                                   

            It is then treated with dilute HCl to remove KF and B is then washed and dried.

(iii)    Modern Method

            B is obtained these days by the electrolysis of a fused mixture containing boric anhydride, MgO and MgF₂ at 1100°C. The electrolysis is done in a carbon crucible, which acts as anode and Fe rod is used as cathode. The Mg discharged at cathode reduces B₂O₃ to B.

                           2MgO ¾®2Mg  +  O₂

                           B₂O₃ + 3Mg  ¾®  2B + 3MgO

      B thus obtained is heated electrically in vacuum at 1100°C, when the impurities are volatilised off           and pure boron is obtained.

(iv)      It is difficult to obtain pure crystalline boron due to very high melting point. Small amount of crystalline boron may be obtained by the following reactions.

            (a)                    (X = Cl or Br)

            (b)                        (Van Arkel method).

            (c)       

         (d)