The Borax formula is articulated as \(({\rm{N}}{{\rm{a}}_2}{{\rm{B}}_4}{{\rm{O}}_7})\). It is one of the most important compounds of boron. Borax is also known as sodium borate, sodium tetraborate, or disodium tetraborate. It is the sodium salt of tetraboric acid \(({{\rm{H}}_2}{{\rm{B}}_4}{{\rm{O}}_7}).\) It is a soft, colourless compound.

The chemical formula of borax is \({\rm{N}}{{\rm{a}}_2}{{\rm{B}}_4}{{\rm{O}}_7}.\) The decahydrate form of borax is the most common form of borax. That is, it is most commonly found as sodium tetraborate decahydrate. Its chemical formula is \({\rm{N}}{{\rm{a}}_2}{{\rm{B}}_4}{{\rm{O}}_7}.\,10{{\rm{H}}_2}{\rm{O}}.\) In this article, we will provide detailed information on the borax formula. Scroll down to learn more!

Structure of Borax (Sodium Tetraborate Decahydrate)

Borax molecules consist of \({[{{\rm{B}}_4}{{\rm{O}}_{\rm{5}}}{({\rm{OH}})_4}]^{2 – }}\) ions, and eight water molecules are associated with it. It is made up of two tetrahedral and triangular units. According to this, the correct formula of borax \({\rm{N}}{{\rm{a}}_2}[{{\rm{B}}_4}{{\rm{O}}_5}{({\rm{OH}})_4}].8{{\rm{H}}_2}{\rm{O}}\). The structure of borax can be given as,

Occurrence of Borax

In India, borax can be found in dried-up lakes in the eastern Kashmir valley in the name of tincal. Tincal is the crude form of borax, and it contains many impurities. Borax is also found as tincal in Ceylon, Tibet and California. In Tibet, tincal is used as a cleaning product by the native tribes.

Preparation Methods of Borax

Let us discuss about the preparation methods of borax:

1. Preparation of Borax from Tincal

Borax \({\rm{(N}}{{\rm{a}}_2}{{\rm{B}}_4}{{\rm{O}}_7})\) can be prepared from tincal by dissolving it in boiling water. The solution thus obtained is filtered to remove insoluble impurities such as sand, clay, etc. The resulting solution can be concentrated to get the pure crystals of borax \({\rm{(N}}{{\rm{a}}_2}{{\rm{B}}_4}{{\rm{O}}_7})\).

2. Preparation of Borax from Colemanite \(({\rm{C}}{{\rm{a}}_2}{{\rm{B}}_6}{{\rm{O}}_{11}})\)

In this method, the colemanite mineral is finely powdered and boiled with a solution of sodium carbonate solution. As a result, borax and sodium metaborate are formed. The chemical equation for the reaction can be given as,

The white precipitate of calcium carbonate obtained during the reaction is removed by the process of filtration, and the solution upon concentration and then cooling gives the pure crystals of borax. By passing carbon dioxide gas through the sodium metaborate, which is present in the mother liquor, it can be converted to borax. The chemical equation for the reaction can be given as,

\(4{\rm{NaB}}{{\rm{O}}_2} + {\rm{C}}{{\rm{O}}_2} \to {\rm{N}}{{\rm{a}}_2}{{\rm{B}}_4}{{\rm{O}}_7} + {\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{O}}_3}\)

3. Preparation of Borax from Boric acid \(({{\rm{H}}_3}{\rm{B}}{{\rm{O}}_3})\)

Borax can be prepared in small amounts by the reaction of boric acid \(({{\rm{H}}_3}{\rm{B}}{{\rm{O}}_3})\) with sodium carbonate. This is an example of a neutralization reaction. The chemical equation for the reaction can be given as,

\({\rm{4}}{{\rm{H}}_3}{\rm{B}}{{\rm{O}}_3} + {\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{O}}_3} \to {\rm{N}}{{\rm{a}}_2}{{\rm{B}}_4}{{\rm{O}}_7} + 6{{\rm{H}}_2}{\rm{O}} + {\rm{C}}{{\rm{O}}_2}\)

Properties of Borax

Let us discuss the physical and chemical properties of borax in detail.

Physical Properties of Borax

Borax is a white crystalline solid. It is sparingly soluble in cold water, but it is readily soluble in hot water. Borax generally forms transparent crystals having ten molecules of water of crystallization when crystallized at ordinary temperature. When borax is crystallized above \(338\,{\rm{K}}\), the pentahydrate crystals \(({\rm{N}}{{\rm{a}}_2}{{\rm{B}}_4}{{\rm{O}}_7}.5{{\rm{H}}_2}{\rm{O)}}\) of borax are formed. This variety of borax is called Jeweler’s borax. It is harder than ordinary borax.

Chemical Properties of Borax

The chemical properties of Borax are explained below:

1. Hydrolysis of Borax

Borax, on hydrolysis, gives an alkaline solution of borax. This is because, on hydrolysis, it gives a weak acid \(({{\rm{H}}_3}{\rm{B}}{{\rm{O}}_3}{\rm{)}}\) and a strong base \(({\rm{NaOH)}}\) The chemical equations for the hydrolysis can be represented as,

Due to its alkaline nature, the aqueous solution of borax gives a pink colour with the phenolphthalein indicator, which gets discharged on adding glycerol to the aqueous solution. This is due to the fact that boric acid produced during the hydrolysis combines with glycerol forming a complex along with the release of a proton \(({{\rm{H}}^ + }).\) This proton neutralizes \({\rm{NaOH}}\) and hence, the solution becomes neutral again. As a result, the solution becomes colourless.

2. Action of Heat – Borax Bead Test

On heating, borax swells up due to the escape of water of crystallization (as steam) from it.

\({\rm{N}}{{\rm{a}}_2}[{{\rm{B}}_4}{{\rm{O}}_5}{({\rm{OH}})_4}].8{{\rm{H}}_2}{\rm{O}} \to {\rm{N}}{{\rm{a}}_2}{{\rm{B}}_4}{{\rm{O}}_7} + 10{{\rm{H}}_2}{\rm{O}}\)

On further heating, it finally gives a colourless glassy bead (made of \({\rm{N}}{{\rm{a}}_2}{\rm{B}}{{\rm{O}}_2}\) and \({{\rm{B}}_2}{{\rm{O}}_3}\)). This bed is called the borax bed. This bead further acquires a characteristic colour when heated along with coloured salt. The process can be shown as,

The borax bead test is used for the detection of cations of coloured salts. In this test, the borax is heated on a platinum loop till a glassy bead is obtained. This colourless glassy bead, when brought in contact with the coloured salt (to be identified) and heated again in the oxidizing flame, the glassy bead acquires a characteristic colour due to the formation of metaborate.

Similarly, for \({\rm{C}}{{\rm{r}}^{3 + }}\) and \({\rm{N}}{{\rm{i}}^{2 + }}\) ions, \({\rm{Cr}}{({\rm{B}}{{\rm{O}}_2})_3}\) (green) and \({\rm{Ni}}{({\rm{B}}{{\rm{O}}_2})_2}\) (reddish-brown) beads are respectively obtained.
When the coloured bead is heated in a reducing flame, the colour of the bead changes. That is,

3. Reaction of Borax with Ammonium Chloride

Borax, on heating with ammonium chloride \({\rm{(N}}{{\rm{H}}_4}{\rm{Cl)}},\) it forms boron nitride \({({\rm{BN}})_{\rm{X}}}\). For example,

Uses of Borax

Borax has got a lot of uses in different fields. They are:

1. Borax is used as a flux in soldering, welding, and certain metallurgical operations.
2. It is used in enamelling and in making glazes for pottery and tiles.
3. Borax is used in the leather industry for cleaning hides and skins and in leather dyeing.
4. Borax is used as a good cleansing agent since it hydrolyses to form a mildly alkaline solution.
5. It is used as a water-softening agent.
6. The borax bead test is used to detect the cations of coloured salts.
7. It is used for impregnating match sticks to prevent after-glow.
8. It is used as a mild antiseptic.
9. Borax is used in fire retardants.
10. It is used for stiffening candle wicks.
11. Borax is used in making optical glasses and borosilicate glass (Pyrex glass), which are resistant to heat and shock.
12. It is used as a booster for laundry detergent.
13. In biochemistry, borax is used to make buffer solutions.

Summary

Borax \(({\rm{N}}{{\rm{a}}_2}{{\rm{B}}_4}{{\rm{O}}_7})\) is one of the most important compounds of boron. Sodium tetraborate decahydrate is the most common form of borax. It is a white crystalline solid. Borax is naturally found in the form of tincal. It is one of the raw materials used for the preparation of borax. In this article, we have learnt more about borax. The formula, structure, preparation methods, the physical and chemical properties. Borax has got a greater commercial value. Therefore, it is used in different fields. We have also discussed the uses of borax in detail.

FAQs on Borax

Q.1. What is the formula of borax?
Ans: Borax is sodium metaborate. Its chemical formula is \({\rm{N}}{{\rm{a}}_2}{{\rm{B}}_4}{{\rm{O}}_7}.\) The decahydrate form of borax is the most common form of borax. That is sodium tetraborate decahydrate. Its chemical formula is \({\rm{N}}{{\rm{a}}_2}{{\rm{B}}_4}{{\rm{O}}_7}.10{{\rm{H}}_2}{\rm{O}}.\)

Q.2. What is a borax bead?
Ans: Borax, when heated, gives a colourless glassy bead. This glassy bed is called as the borax bed. It is made of sodium metaborate \({\rm{(NaB}}{{\rm{O}}_2})\) and boric oxide \({\rm{(}}{{\rm{B}}_2}{{\rm{O}}_3}).\)

Q.3. What is borax used for?
Ans: Borax has got many uses. That is, it is used as a flux in soldering, welding, and certain metallurgical operations, used in enamelling and in making glazes for pottery and tiles, it is used in the leather industry, etc.

Q.4. What is the action of borax with water?
Ans: Borax, on hydrolysis, gives an alkaline solution of borax. This is because, on hydrolysis, it gives a weak acid \(({{\rm{H}}_3}{\rm{B}}{{\rm{O}}_3})\) and a strong base \(({\rm{NaOH}}).\)

Q.5. Is borax soluble in water?
Ans: Borax is sparingly soluble in cold water, but it is readily soluble in hot water.

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