Manufacturing of Sodium Hydroxide

talking about the process of manufacturing of (NaOH) and types of cells used in ,, also some uses of Cl2 gas which is by product in this process ..

1. Manufacturing of Sodium Hydroxide

2. Sodium Hydroxide -Chemical name : sodium hydroxide -Chemical formula : NaOH - With Mwt. 39.99 g/ mol -Commercial name : caustic soda , Lye “ It is a white solid substance which dissolves readily in water, its solution has soapy feeling and corrosive to skin .” -Commonly used as a Base Raw Material used in manufacturing: Rock salt “ NaCl ” sodium chloride & water “H 2 O”

3. Production by Electrolysis Electrolysis of concentrated NaCl solutions (brine) produces chlorine gas , hydrogen gas and aqueous sodium hydroxide. 2NaCl ( aq )  + 2H 2 O (l )    H 2(g)   + Cl 2(g)  + 2NaOH ( aq ) Uses of NaOH : - domestic oven and drain cleaner -pulp and paper production -manufacture of rayon and other synthetic fibres -soap  and  detergent production -removal of sulfur dioxide and hydrogen sulfide in petroleum refining

4. Uses of Cl 2 Gas: treat drinking water and swimming pool water by killing bacteria in . 20% of chlorine produced is used to make PVC used in organic chemistry processes-for example; as an oxidizing agent and a substitution for hydrogen make chloroform (an anesthetic) and carbon tetrachloride (a dry-cleaning solvent ).  Chlorine gas is itself very poisonous, and was used as a chemical weapon during the First World War.

5. Types of electrolytic cell  There are three types of electrolytic cell are used to produce sodium hydroxide from brine: Castner -Kellner Cell (Mercury Process) Nelson Diaphragm Cell Membrane Cell

6. Castner -Kellner Cell (Mercury Process) - Anode (+ ve electrode): titanium  * Anode reaction (oxidation):  2Cl - ( aq )    Cl 2(g)  + 2e -Cathode (- ve electrode): mercury flowing along bottom of cell  *Cathode reaction (reduction):  Na + ( aq )  + e  Na (s) -Na (s)  dissolves in the liquid mercury to form an amalgam which is removed to the decomposer. -In the decomposer the amalgam reacts with water to form sodium hydroxide, hydrogen gas and mercury.  2Na/Hg + 2H 2 O (l )  2NaOH ( aq )  + H 2(g)  + 2Hg (l)   Sodium hydroxide solution and hydrogen gas are collected.  Mercury is recycled through the electrolytic cell - Mercury is toxic so care must be taken to prevent mercury losses.

7. Nelson Diaphragm Cell Porous diaphragm of asbestos or metal oxide with polymer separates anode and cathode compartments.  Diaphragm prevents hydroxide ions entering anode compartment and prevents chloride ions entering cathode compartment. Saturated brine enters anode compartment where chlorine gas is produced.  Anode (+ ve electrode): carbon (graphite) or titanium coated with Ru- Ti oxide.  Anode reaction (oxidation): 2Cl - ( aq )  Cl 2(g )  + 2e Cathode (- ve electrode): steel mesh  Cathode reaction (reduction): 2H 2 O (l)  + 2e  H 2(g)  + 2OH - ( aq ) Na +  migrates across diaphragm to cathode compartment combining with OH - to form NaOH.  Overall cell reaction (showing Na +  spectator ions): 2H 2 O (l)  + 2Cl - ( aq )  + 2Na + ( aq )    2NaOH ( aq )  + H 2(g)  + Cl 2(g) Product contains sodium chloride and sodium hydroxide. NaOH (s)  can be crystallised out.

8. Membrane Cell Favored method for producing very pure NaOH with no hazardous waste, and with lowest energy requirements. Ion-exchange membrane selectively allows Na +  and water to flow to the cathode compartment but prevents products from moving between compartments. Saturated brine enters the anode compartment where chlorine gas is formed.  Anode (+ ve electrode ): titanium Anode reaction (oxidation): 2Cl - ( aq )    Cl 2(g)  + 2e Cathode (- ve electrode ): nickel  Cathode reaction (reduction): 2H 2 O (l)  + 2e  H 2(g )  + 2OH - ( aq ) Na +  migrates across the membrane to cathode compartment combining with OH -  to form NaOH .  Overall cell reaction (showing Na +  spectator ions): 2H 2 O (l)  + 2Cl - ( aq )  + 2Na + ( aq )    2NaOH ( aq )  + H 2(g)  + Cl 2(g ) Product is concentrated sodium hydroxide.

9. Process Advantages: - Very Pure NaOH . - No hazardous waste. - Need lowest energy. Membrane Cell Castner -Kellner Cell Amalgam is mercury alloy “Hg& metal” All most all metals can form amalgam with Hg, exception : Iron, Platinum, Tungsten and Tantalum. Process Advantages : Process very efficient. Produce very pure NaOH . Process Disadvantages : High electricity consumption . Environmental pollution due to escape of mercury vapors .

10. References : AUS-e-TUTE is a science education website http://www.ausetute.com.au / Business Science Articles website http://www.business-science-articles.com / Periodic Table website http :// www.rsc.org/periodic-table Look Chem “Look for chemicals” website http ://www.lookchem.com/

11. Prepared By : Afrah Al- Mherat Majdoleen Alyan Hanan Ghanameh