Question

Preparation of borax

Answer 1

COMPOUNDS FORMED BY BORON

Borax (Na2B4O7.10H2O)

Borax is the sodium salt of boric acid. B orax occurs naturally as tincal (suhaga) in certain dried up lakes of india, Tibet, Srilanka and USA. It contains about 55% borax.


PREPERATION :-

From tincal: Tincal is boiled with water and the so;ution is then filtered to remove impurities of sand, clay etc. The filterate is concentrated till crystallization begins and then cooled. The crystals of borax are separated by filteration.


From colemanite (Ca2B6O11):


Ca2B6O11 + 2Na2CO3 -----Heat-----» Na2B4O7 + 2NaBO2 + 2CaCO3↓


4NaBO2 + CO2 -----------» Na2B4O7 + Na2CO3

The insoluble calcium carbonate is filtered off .The remaing solution containg of borax and       sod.metaborate is then treated with CO2 to convert sod.metaborate to borax.The solution is now concentrated and cooled to crystallize borax.


From boric acid : Borax is prepared from boric acid by boiling with soda ash. On cooling the solution, borax separates out.



4H3BO3 + Na2CO3 ------------» Na2B4O7 + 6H2O +CO2 ↑

PROPERTIES

It is a white crystalline solid, less soluble in cold water but more soluble in hot water.The aqueous solution of borax is alkaline due to hydrolysis. Borax is therefore used for water softener and cleaning agent.

Na2B4O7 + 2H2O ------------» 2NaOH + H2B4O7

H2B4O7 + 5H2O -----------------» 4H3BO3

On heating, borax loses its water of crystallization and swells up to form a puffy mass. On further heating, it melts into a clear liquid which solidifies to a transparent glass like bead which consists of sodium metaborate and boric anhydride .This glass bead is commonly called as borax bead (it is commonly employed in qualitative analysis for detection of certain coloured basic radicals like Ni+2, Co+2, Cr+3, Cu+2, Mn+2 etc.On adding calculated quantity of sod.hydroxide to borax, sodium metaborate is formed.

Na2B4O7 + 2NaOH ---------------» 4NaBO2 + H2O

On adding calculated quantity of Conc.sulphuric acid to hot conc. Solution of borax, boric acid is formed.

Na2B4O7 + H2SO4 ---------------» Na2SO4 + H2B4O7

                                                                                                                                            H2B4O7 + 5H2O ------------------» 4H3BO3

On heating borax with ethyl alcohol and conc sulphuric acid, vapours of triethylborate are produced .When ignited these vapours burn with green edged flame.

Na2B4O7 + H2SO4 + 5H2O -------------» Na2SO4 + 4H3BO3

H3BO3 + 3C2H5OH -------------» B(OC2H5)3 + 3H2O

USES:

Manufacture of enamels and glazes for earthen wares i,e tiles etc.As flux for solderingPreparation of medicinal soaps due to its antiseptic properties.To produce good finish at laundaryIn softening waterIn laboratory for borax bead test

Answer 2

Borax:—

• It is the most important compound of boron.
• It is a white crystalline solid of formula $\sf{Na_{2}B_{4}O_{7}.10H_{2}O}$
• It contains tetranuclear units and correct correct formula; therefore, is $\sf{Na_{2}(B_{4}O_{5}(OH)_{4}).8H_{2}O}$
• On heating borax first loses water molecules and swells up. On further heating it turns into transparent liquid, which solidifies into glass like material know as borax bead.

Preparation of Borax:—

• Borax is prepared from boric acid by boiling with soda ash. On cooling the solution, borax separates out.

$\sf{4H_{3}BO_{3}+ Na_{2}CO_{3}\longrightarrow Na_{2}B_{4}O_{7} + 6H_{2}O +CO_{2}}$

More Information:—

• The metaborates of many transition metals have characteristics colours and for that borax beads test can be used to identify them in the laboratory.
• For eg, when borax is heated in a bunsen burner flame with CoO on a loop of platinum wire, a blue coloured Co(BO2)2 bead is formed.