Preparation and properties of Borax

Preparation

            Borax occurs naturally as tincal(contains about 55% borax) in certain inland lakes of India, Tibet andCalifornia (U.S.A.).

            Borax can also be prepared from certain other minerals such as boracite, colemanite and boranatrocalcite. The minerals are powdered and boiled with sodium carbonate solution.

                     Ca₂B₆O₁₁  +  2Na₂CO₃¾®    2CaCO₃  +  Na₂B₄O₇  +  2NaBO₂

            Borax is crystallised from the filtrate. Sodium metaborate, present in the mother liquor, is converted into borax by passing carbon dioxide through it

                     4NaBO₂       +     CO₂¾®  Na₂B₄O₇       +      Na₂CO₃

                     sod. metaborate                               

Borax is also obtained by the action of Na2CO3 on orthoboric acid.

                             

            Two important hydrates of borax are known. These are,

      (i)  Pentahydrate, Na₂B₄O₇.5H₂O.

      (ii) Decahydrate or monoclinic borax, Na₂B₄O₇.10H₂O.

            The pentahydrate is obtained when the solution is crystallised at above 60°C, while the monoclinic variety is obtained when crystallisation is carried out below 60°C. Both the varieties on heating form anhydrous borax.

Properties of Borax

      (i)  Borax is a colourless, crystalline solid sparingly soluble in cold water but dissolves readily in hot water.

      (ii) Basic nature

            Borax is slightly hydrolysed in solution. As boric acid is a weak acid, the solution is alkaline in nature.

                     +  7H₂O      2OH^-    +   4H₃BO₃

      (iii)         Action of acids

            Borax reacts with HCl or H₂SO₄ to form boric acid. On cooling, the white flakes of boric acid are obtained

                     Na₂B₄O₇ + 2HCl + 5H₂O  ¾®  2NaCl + 4H₃BO₃

                                                                                      boric acid

     

      (iv) Action of heat

            When powdered borax is heated strongly in a bunsen-flame, it loses water of crystallization and forms colourless, transparent glass-like bead which is made up of sodium metaborate and boric anhydride.

            Na₂B₄O₇.10H₂O    Na₂B₄O₇  2NaBO₂        +       B₂O₃

            borax                                                                    sod. Metaborate     boron oxide

 

                                                                                                         transparent bead

             (blue bead)

            Colour of metaborates : Cu(blue), Fe(green), Co(blue), Cr(green), Ni(brown)

           

Uses of Borax

            Borax is used,

            (i)  for manufacturing enamels, glazes and optical glass.

            (ii) as a flux for soldering and welding.

            (iii)      as an analytical reagent, e.g. in borax bead test.

            (iv) in borax bead test

            (v) in purifying gold