Borax, boric acid, silicate and silica

Borax (Na₂B₄O₇.10H₂O)

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

Manufacture of Borax

From tincal

It is obtained from tincal by extracting it with water, concentrating the solution and leaving it for crystallization, when crystals of borax, Na₂B₄O₇.10H₂O separate out.

From other minerals

Borax can also be prepared from minerals such as boracite, colemanite, and boronatrocalcite. These minerals are powdered and boiled with sodium carbonate solution:

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

Two important hydrates of borax are:

• Pentahydrate, Na₂B₄O₇.5H₂O.

• Decahydrate or monoclinic borax, Na₂B₄O₇.10H₂O.

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

Properties of borax

Borax is a colorless, crystalline solid sparingly soluble in cold water but dissolves readily in hot water. It forms two important hydrates: octahedral borax Na₂B₄O₇.5H₂O and monoclinic borax Na₂B₄O₇.10H₂O.

Basic nature

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

Action of acids

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

Action of heat

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

Borax-bead test

Boric anhydride reacts with certain metal salts such as, Ni²⁺, Co²⁺, Cr³⁺, Cu²⁺, Mn²⁺ etc. to form colored metaborates. The color of the metaborates is used to identify the metallic ion (cation) in the salt.

Taking a small quantity of borax on a small loop of platinum wire to perform the test.When this is heated in a flame, till a transparent bead is formed. The hot transparent bead is touched with the salt when few particles stick to it. The bead (with the sticking salt) is then heated in an oxidizing flame (outer zone) and then in a reducing flame (luminous zone). From the color of the bead when hot and when cold, the basic radical present in the salt can be identified.

Certain metaborates are reduced to free metal in a reducing flame. For metals with variable valence, the metaborate changes to lower valence state. For example, with copper sulphate, the borax bead test gives,

In oxidizing flame:

In reducing flame:

Nickel salts give brown colored bead, cobalt gives blue colored bead, while chromium salt gives green bead.

Structure of borax

In borax, two boron atoms are in triangular geometry and two boron atoms are in tetrahedral geometry. The ion is [B₄O₅(OH)₄]^2- and the remaining eight water molecules are associated with the two sodium ions. Hence, borax contains tetranuclear units [B₄O₅(OH)₄]^2-, and is formulated as

Na₂(B₄O₅(OH)₄].8H₂O.

Uses of borax

• For manufacturing enamels, glazes, optical glass, soaps and drying oils.

• As a flux for soldering and welding.

• For stiffening the candle wicks.

• In the manufacture of washing powders and soaps as a fabric softener.

• As an antiseptic.

• An analytical reagent e.g., in borax bead test.

3BO₃)">

Boric acid (H₃BO₃)

Boron trioxide with different amounts of water gives several boric acids. The most important of these is orthoboric acid (boric acid), H₃BO₃.

Preparation of Boric Acid

From borax

Boric acid is obtained by the action of hydrochloric acid or sulphuric acid on borax. On cooling the reaction mixture, white flakes of boric acid are obtained.

By the hydrolysis of boron compounds

Boric acid may also be obtained by the hydrolysis of most boron compounds like nitrides, sulphides, halides etc.

Properties of boric acid

Boric acid is a white crystalline substance, soft and soapy to touch.

Boric acid is moderately soluble in cold water, but fairly soluble in hot water.

Action of heat

On heating boric acid decomposes to form metaboric acid at 375 K, tetraboric acid at 435 K and boron trioxide at red heat.

Acidic character

Boric acid is a very weak monobasic acid, (K_a = 1x10^-9). It behaves as a Lewis acid accepting a hydroxyl ion,

With alkalies

With strong alkalies, boric acid forms salts known as metaborates.

Structure of boric acid

Boric acid contains planar BO₃^3-units bound together through hydrogen bonds forming a trigonal planar layer structure. Hydrogen atoms act as bridge between two oxygen atoms of different BO₃^3- units

Structure of boric acid (dotted line represent hydrogen bonding)

Uses of boric acid

• It used in the manufacture of enamels and pottery glazes.

• As a food preservative in food industry.

• In the manufacture of pigments and borax.

• In medicines as eyewash.

• For making borosilicate glass.

Problems

2. The order of acid strength is reverse of what is normally expected in the boron halides, explain?

Solution

The order of acid strength is reverse of what may normally be expected on the basis of electronegativity of halogens. Since F is most electronegative, BF₃ should be most electron deficient and thus should be strongest acid. The anomalous behavior is explained on the basis of tendency of halogen atom to back-donate its electrons to boron atom. For example, in BF₃ one of the '2p' orbital of F atom having lone pair overlaps sidewise with the empty '2p' orbital of boron atom to form p_p - p_p back bonding. This is also known as back donation. As a result of this back donation, the electron deficiency of boron gets compensated and its Lewis acid character decreases.

p

p- pp back bonding

The tendency for back donation is maximum in the case of fluorine due to its small size and more inter-electronic repulsions. Therefore, it is the least acidic. The tendency of back bonding falls as we move from BF₃ to BCl₃ and BCl₃ to BBr₃ due to increase in the size of halogen atoms. Consequently, the acidic character increases accordingly.

3. What is the test for detecting borates and boric acid in qualitative analysis?

Solution

On heating borax with ethanol and concentrated H₂SO₄, vapors of triethylborate are produced. These vapors burn with a green edged flame on ignition.

The compound triethylborate (C₂H₅)₃BO₃ is responsible for the green edged flame in the test for borate ion.

Silicate

Silicate is the general term given to solids with silicon-oxygen bonds. A large percentage of the earth's crust consists of silicate minerals. Some of the important silicate minerals are quartz, asbestos (calcium magnesium silicate, CaMgSi₂O₆), feldspar (potassium aluminium silicate, KAlSi₃O₈) and zeolites (sodium aluminium silicate, Na₂Al₂Si₂O₈.xH₂O).

The silicates are complex network solids having silicate ion (SiO₄)^4- as the basic structural unit. The silicate ion has a tetrahedral structure. As a result of sharing of one or more oxygen atoms between such tetrahedrons, a complex structure arises. The basic classification of silicates into chain silicates, ring silicates, cyclic silicates, sheet silicates, three dimensional silicates depends on the way in which the (SiO₄)^4- tetrahedral units are linked together.

Silica

i) Quartz glass is used for manufacturing optical instruments. Under the trade name Vitreosil.

ii) Colored quartz is used for manufacturing gems.

iii) Sand is used in manufacture of glass, porcelain, sand paper and mortar etc.

iv) Sand stone is used as a building material.

v) Kieselguhr (siliceous rock composed at remains of minute sea organisms) is used as absorbent for nitroglycerine.