Boron – The Group IIIA Element

Occurrence of Boron

Boron is not an abundant element. It constitutes about 0.001 percent by weight of Earth’s crust. It happens in traces in a lot of soils and has been found to be essential in extremely small amounts for the proper growth of numerous plants. Boron is always found in nature combined with oxygen, usually as oxyborate ions. Boron present mainly as salts of different polyboric acids.

PeculiarBehaviour of Boron

Boron is the very first member of the Group IIIA, it reveals lots of dissimilarities with the members of its own group. The difference in the properties of boron and those of the other members of the series is generally due to the big distinction in their sizes and ionization energies.

1. Boron is the only element in GroupIIIA which is non-metallic in behavior
2. It is the only element with less than 4 electrons in the outermost shell which is not a metal.
3. Boron always utilizes all three of its valence electrons for bonding purposes and its common oxidation states are + 3 and -3.
4. Among the outstanding functions of the chemistry of boron is its ability to form molecular addition substances.
5. Boron does not form ionic substances with sulphate, nitrate, or other anions because boron does not form a stable cation.

Compounds of Boron

Borax (Sodium Tetraborate Na₂B₄O₇.10H₂O)

Borax is the sodium salt of tetra boric acid. It is the most crucial of all borates.

Occurrence

Borax takes place as a natural deposit called tincal in the dried-up lakes of Tibet and California.

Manufacture

• 1.Formally borax was produced by treating a hot solution of boric acid with the appropriate amount of soda ash.

 

• 2.Now-a-days borax is almost exclusively obtained from calcium borate. Carefully powdered colemanite is boiled with Na₂CO₃ option when CaCO₃ precipitates out and a mixture of borax and sodium metaborate is formed.

The clear solution from the top is removed and is then permitted to crystallize when crystals of borax separate out. To get more borax, CO₂ is blown through the mother-liquor, the salt metaborate is decomposed into borax, which separates out in the form of fine crystals.

• 3.Borax might also be acquired from tincal (Na₂B₄O₇.10H₂O) by treating tincal with water and subsequently vaporizing the clear solution when crystals of borax separate out.

Properties

1. Borax is a white, crystalline solid. It is moderately soluble in cold water but is more soluble in hot water: 100 grams of water dissolve 3 grams of decahydrate at 10 ° C and 99.3 grams at 100 ° C. If a saturated solution be enabled to crystallize above 62 ° C, octahedral crystals of the pentahydrate, Na₂B₄O₇.5H₂Oseparate out, if the temperature is below 62 ° C, the decahydrate is formed. Its liquid solution is alkaline in nature due to hydrolysis.

The hydrolysis is prevented in the existence of glycerine.

 

2. When heated up, borax loses water and swells up into a white permeable mass due to the expulsion of water: on more heating, it melts into a clear transparent glass, which liquifies many metallic oxides forming colored beads. This reaction forms the basis of the borax bead test.

3. Its aqueous solution reacts with HCl or H₂SO₄ to form boric acid:

4. When borax is heated with ammonium chloride, boron nitride is produced:

5. Borax when dissolved in water ionizes as:

Hydrolysis of B₄O₇^-2 happens as follows:

So, a strong alkali (NaOH) is formed which is highly ionized. On the other hand, boric acid (H₃BO₃) is ionized to some degree, due to the fact that it is a weak acid. Thus, the solution of borax as a whole is alkaline in nature.

• 6.Borax Bead Test

Prepare a loop at the end of a platinum wire. Heat the wire and take a little powdered borax on the hot loop. Heat again, borax first inflates and then melts into colorless, glasslike bead on the loop. Now put a couple of grains of the compound, under examination, on the beads and re-heat it first in the oxidizing flame and then in the reducing flame.

Chemistry of the Borax-bead Test

Borax, when fused, is decomposed into sodium metaborate and boric anhydride.

The metallic oxide formed from the compound, under examination, integrates with B₂O₃giving the colored metallic borates. With cupric oxide, the beads are colored blue in the oxidizing flame because cupric borates are blue in color.

Uses of Borax

1. It is used to prepare borate glass, which is heat resistant.
2. It is used in the softening water.
3. It is utilized in borax bead test, for the detection of metallic cations.
4. It is used in metallurgical operations.
5. It is utilized as a flux in welding and in metallurgy.
6. It is used in making cleaning powders.
7. It is used in leather industry for tanning and dyeing.
8. It is used in cosmetics, soaps, fabrics, paints, medicine, match market and as a preservative.

Boric Acids

There are four important boric acids. Out of these orthoboric acid is the most important and stable one. The remaining acids are stable in solid-state and become orthoboric acid in solution:

(i) Orthoboric Acid, H₃BO₃

(ii) Metaboric Acid, HBO₂

(iii) Tetra Boric Acid, H₂B₄O₇

(iv) Pyroboric Acid, H₆B₄O₉

Orthoboric Acid or Boric acid (H₃BO₃)

It is a white crystalline chemical compound (triclinic), moderately soluble in cold water (2.6% at 40 ° C) but dissolves readily in hot water (37% at 107 ° C). This temperature variation in solubility forms the basis for its separation and filtration.

Preparation of Boric acid on Industrial Scale

• 1.From Colemanite

On an industrial scale, boric acid is prepared from a natural calcium borate called colemanite (Ca₂B₆O₁₁. 5H₂O) by suspending it in boiling water while Sulphur dioxide is passed through it. Boric acid crystallizes out from the solution while, the other product CaSO₃ stays in the solution.

• 2.From Borax:

A hot concentrated solution of borax is treated with a determined amount of conc. H₂SO₄. On cooling, crystals of boric acid formed, separate out.

Properties of Boric Acid

1. Boric acid is a white glossy crystalline solid having a soft soapy touch, very slightly soluble in cold water but fairly soluble in hot.
2. It is volatile in steam.
3. It reacts with ethyl alcohol forming ethyl borate.

4. When heated strongly, it swells to frothy mass losing water particles. It is first converted into metaboric acid, then to tetra boric acid and finally to boric anhydride.

 

 

5. It is a very weak acid and ionizes to a very restricted degree primarily as a monobasic acid.

6. Its solution has no impact on methyl orange, although it turns blue litmus red.
7. It is partly neutralized by caustic soda to provide borax.

8. When boric acid is neutralized by soda ash (Na₂CO₃), borax is obtained.

9. Boric acid being a weak acid, cannot be titrated with alkalies in the usual manner. In the existence of glycerol, however, it can be titrated against a basic alkali using phenolphthalein as an indicator.

Uses of Boric Acid

1. Boric acid is utilized in medicines as an antiseptic, e.g. cleaning powder, boric lotion, and the boric solution is used as an eye-wash.
2. It is used in pottery as a glaze because borate glazes are more fusible than silicate glazes and have a higher coefficient of expansion.
3. It is likewise used in the candle industry for the stiffening of wicks.

Multiple Choice Questions (MCQs) with Answers

 

Frequently Asked Questions (FAQs)