Compounds of Sodium - Sodium Carbonate

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Sodium forms many industrially important compounds like halides, carbonates, sulphates and hydroxides. The compounds of sodium are ionic in nature and are readily soluble in water.

Sodium carbonate (Na

₂CO₃)

Sodium carbonate exists as anhydrous (Na₂CO₃) and also as hydrated salt. The decahydrated salt (Na₂CO₃.10H₂O) is known as washing soda while the anhydrous salt is called soda ash.

Occurrence

Large deposits of this salt occur in Owens lake in California and Lake Magadi in British East Africa. It occurs native as Na₂CO₃.NaHCO₃.H₂O in Egypt.

During hot weather, soda is also collected from a large number of alkaline lakes.

Manufacture of Sodium Carbonate

Ammonia-soda process (or Solvay process)

This process is the most popularly used method. As Ernest Solvay, the Belgian chemical engineer, devised it in 1864 it is known as Solvay process.

Raw materials

The raw materials for this process are common salt, ammonia and limestone (for supplying CO₂ and quicklime).

Principle

When carbon dioxide is passed into a concentrated solution of brine saturated with ammonia, ammonium bicarbonate is produced,

The ammonium bicarbonate then reacts with common salt forming sodium bicarbonate,

Sodium bicarbonate being slightly soluble (in presence of sodium ions) gets precipitated. The precipitated sodium bicarbonate is removed by filtration and changed into sodium carbonate by heating.

The mother liquor remaining after the precipitation of sodium bicarbonate contains ammonium chloride. This is used to regenerate ammonia (one of the raw materials) by steam heating with milk of lime.

Lime is obtained by heating limestone.

Ammonia and carbon dioxide liberated are utilized in making the whole process cyclic and continuous. The only by-product in the process is calcium chloride.

The Plant

Saturating or ammoniating tank

In this tank, impurities of calcium and magnesium salts present in brine are precipitated as carbonates or hydroxides. This occurs when a little carbon dioxide gas (from ammonia recovery tower) is bubbled through a 20% sodium chloride solution (brine).

Ammonia absorption tower fitted at the top of the saturating tank absorbs the ammonia that escapes absorption in the saturating tank.

Filter

The precipitated carbonates and hydroxides are removed after pumping the ammoniacal brine through filter press.

Cooler

The clear ammoniacal brine is passed through coolers to bring it to room temperature.

Carbonating tower

Carbonation of ammoniacal brine is carried out on the principle of counter-current. The clear ammoniacal brine solution is pumped to the top of the tower. This flows downwards and meets a current of CO₂ introduced from the bottom of the tower at a pressure of 1-2 atmosphere. As a result of the reactions therein, ammonium chloride and crystals of sodium bicarbonate are formed.

These crystals remain suspended in the mother liquor forming thick

milky liquid.

Fig: 12.4 - Solvay process plant

Rotary suction filter

This thick milky liquid is then filtered by means of the rotary suction filter. Sodium bicarbonate deposits on the filter cloth. After washing with a spray of cold water it is scrapped off, and dried. The filtrate containing ammonium carbonate, ammonium chloride and some sodium chloride is pumped to the top of the ammonia recovery tower.

Lime kiln

Here, limestone is burnt to produce carbon dioxide and lime.

Carbon dioxide is sent to the carbonating tower, while the lime is slaked in a large quantity of water to form milk of lime. This is pumped to the middle of the ammonia recovery tower.

Ammonia recovery tower

Here, the filtrate from the rotary suction filter containing ammonium bicarbonate and ammonium chloride is introduced at the top of the tower. Milk of lime enters at the middle part, and steam is introduced from the base. Ammonium bicarbonate is decomposed by heat while ammonium chloride is decomposed by milk of lime.

The gaseous ammonia along with a little carbon dioxide is sent to the saturation tank, while the liquid flowing out of the tower contains calcium chloride, which is a useful by-product.

Calcination

Sodium bicarbonate obtained from the rotary suction filter is calcined in closed tubular calcining pans fitted with scrapers. On calcination, sodium carbonate is formed and carbon dioxide is evolved.

The soda ash obtained from the pans is dissolved in water and crystals of hydrated sodium carbonate Na₂CO₃.10H₂O, are recovered from the solution by crystallization.

Merits of the Solvay's process

• This process gives soda of much higher purity (99.5%).

• It is less expensive as the raw materials are cheap and CO₂ and NH₃ are recovered to be used again.

• Calcium chloride is the only waste is the solution and so no pollution problems.

• Formation of sodium bicarbonate as an intermediate.

Physical properties

Sodium carbonate is a white crystalline solid, which can exists as anhydrous salt (Na₂CO₃), monohydrate salt (Na₂CO₃.H₂O),

heptahydrate salt (Na₂CO₃.7H₂O) and decahydrate (Na₂CO₃.10H₂O - washing soda).

Sodium carbonate is readily soluble in water.

On heating, decahydrate salt gradually loses water to finally give anhydrous salt (Na₂CO₃ - soda ash).

Chemical Properties

Action of water

Sodium carbonate dissolves in water with the evolution of heat and

the aqueous solution is alkaline in character due to hydrolysis.

Na₂CO_3(aq) g 2Na⁺ + CO₃^2-

CO₃^2-+ 2H₂O g H₂CO₃ + 2OH^-

Action of acids

Sodium carbonate is readily decomposed by acids when carbon dioxide

is evolved.

Action of carbon dioxide

When carbon dioxide gas is bubbled into the aqueous solution of sodium carbonate, sparingly soluble sodium bicarbonate is precipitated.

Action of lime

Sodium carbonate is causticised by lime, giving caustic soda.

Action of sulphur dioxide

When SO₂ gas is passed into the aqueous solution of sodium carbonate, sodium sulphate or bisulphate is produced.

With metal salt solutions

With aqueous solutions of certain metal salts, sodium carbonate gives carbonates, basic carbonates or hydroxides.

Reaction with sand (SiO

₂)

When sodium carbonate is fused with sand, water glass or sodium silicate is formed.

Uses

• For the manufacture of glass.

• For washing purposes in laundries.

• For the manufacture of other sodium compounds like sodium silicates, sodium hydroxide, borax, hypo etc.

• As a household cleansing agent.

• In paper and soap/detergent industries.

• For the softening of water.

• A mixture of NaCO₃ and KCO₃ is used as a fusion mixture.

• In textile industry and petroleum refining.

Problem

6. What is the role of limestone in the preparation of soda ash by the Solvay process?

Solution

Limestone is burnt to produce carbon dioxide and lime.

The lime is slaked in a large quantity of water to form milk of lime (Ca(OH)₂). This is pumped to the middle of the ammonia recovery tower. Here, the milk of lime reacts with the filtrate containing ammonium bicarbonate and ammonium chloride. Ammonium chloride reacts with milk of lime to produce calcium chloride, a by-product and ammonia gas, a recovered raw material.