Sodium Carbonate (Washing soda) (Na₂CO₃)

Sodium carbonate is generally prepared by a process called the ammonia – soda process or solvay process as described below:

When carbon dioxide gas is bubbled through a brine solution saturated with ammonia, it results in the formation of sodium hydrogen carbonate.

 NaCI + NH₄HCO₃ → NaHCO₃ + NH₄CI         

Sodium hydrogen carbonate so formed precipitates out because of the common ion effect caused due to the presence of excess of NaCl. The precipitated NaHCO₃ is filtered off and then ignited to get Na₂CO₃.

The various parts for the manufacture of Na₂CO₃ by solvay process have been illustrated in figure below.

Alt text: Plant used for the manufacture of washing soda

A 30% solution of brine is saturated with ammonia (from recovery tower) is introduced into absorber tower. Various impurities like calcium and magnesium salts present in the commercial NaCl precipitate out as corresponding insoluble carbonates. (which comes along with ammonia from the ammonia recovery plant)

2NH₃ + H₂O + CO₂ → (NH4)₂ CO₃               

 CaCI₂ + (NH4)₂ CO₃ → CaCO₃ ↓ + 2NH₄CI              

MgCI₂ + (NH₄)₂ CO₃ → MgCO₃ ↓ + 2NH₄CI               

The ammoniated brine is filtered to remove the precipitated calcium and magnesium carbonate.

 It is high tower fitted with perforated plates. Ammoniated brine solution is made to trickle down from the top of the tower while CO₂ gas from the lime kiln is admitted from the base of the tower. CO₂ rises through the small perforations and its interaction with ammoniated brine results in the formation of insoluble sodium hydrogen carbonate.

NH₃ + H₂O + CO₂ → NH₄HCO₃               

 NaCI + NH₄HCO₃ → NaHCO₃ ↓ + NH₄CI              

The solution containing crystals of NaHCO₃ is drawn off from the base of the carbonation tower and filtered to get NaHCO₃.

(iv)   The NaHCO₃ obtained from the above step is heated strongly in kiln to covert it into sodium carbonate (Na₂CO₃)

2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O               

The carbon dioxide produced here is sent to carbonation tower.

The filtrate, after removal of NaHCO₃ contains ammonium salts such as NH₄HCO₃ and NH₄Cl.

The filtrate is mixed with Ca(OH)₂ and is heated with steam in ammonia recovery tower.

NH₄HCO₃ → NH₃ + H₂O + CO₂               

2NH₄CI + Ca (OH)₂ → 2NH₃ + 2H₂O + CaCI₂              

The mixture of ammonia and CO₂ gases is obtained which is used for saturation of brine while calcium chloride is obtained as a by – product.

Here limestone is heated at about 1300 K to obtained CO₂ and calcium oxide

CaCO₃ → CaO + CO₂               

The CO₂ gas goes to the carbonation tower while lime is slaked with water in tank known as slakes to form ca(OH)₂.

2NaCI + CaCO₃ → NaCO₃ + CaCi₂               

Flow sheet diagram of Solvay process

(i) The aqueous solution absorbs CO₂ yielding sparingly soluble sodium bicarbonate.

(ii) It dissolves in acids with an effervescence of carbondioxide and is causticised by lime to give caustic soda.

Na₂CO₃ + SiO₂ → Na₂SiO₃ + CO_­2

(iv)   Hydrolysis – being a salt of a strong base (NaOH) and weak acid (H₂CO₃), when dissolved in water sodium carbonate. Undergoes hydrolysis to form an alkaline solution

Na₂CO₃ + 2H₂O→ H₂CO₃ + 2NaOH

(ii)    A mixture of sodium carbonate and potassium carbonate is used as fusion mixture.

(iii)    As an important laboratory reagent both in qualitative and quantitative analysis.

(vi)   It is used in the manufacture of glass, borax, soap and caustic soda.

It is also known as pearl ash. It is made by passing CO₂ into a conc. solution of the chloride, containing hydrated mangesium carbonate in suspension at 20°C when an insoluble potassium hydrogen magnesium carbonate is precipitated.

2KCl + 3(MgCO₃. 3H₂O) + CO₂ → 2(MgCO₃. KHCO₃×4H₂O) + MgCl₂

The precipitate is separated by filtration, and then decomposed either by heating with water under pressure at 140°C or by the action of magnesium oxide below 20°C. 

2(MgCO₃. KHCO₃. 4H₂O) → 2MgCO₃ + K₂CO­₃ + 9H₂O + CO₂

2(MgCO₃. KHCO₃. 4H₂O) + MgO → 3(MgCO₃. 3H₂O) + K₂CO₃

(ii)    K₂CO₃ resembles Na₂CO₃ in properties, but is more alkaline and more soluble than Na₂CO₃.

Sodium carbonate, which is one of the most important products of the chemical industry, is prepared by the Solvay process based on the interaction of sodium chloride with ammonia and carbon dioxide. The reaction yields