Electrolysis of Brine and the Chlori-Alkali Industry

the chlori-alkali industry uses the electrolysis of aqueous sodium chloride to...

produce hydrogen gas, chlorine gas, and aqueous sodium hydroxide

brine

concentrated sodium chloride aqueous solution

solution for: hydrogen and chlorine gases will combine explosively if they meet

drawing the gases off through separate ducts

solution for: at the anode, some oxygen may form which then reacts with a graphite electrode producing carbon dioxide

using concentrated NaCl and metal-based electrodes which do not react with oxygen

solution for: chlorine and sodium hydroxide will combine to form sodium hypochlorite (bleach) if they meet

separate the products formed and the anode and the cathode

requirements of this specialised cell:

- a supply of electricity and inert electrodes

- brine must be purified to remove other metal ions (such as calcium, strontium, and magnesium ions)

- the brine is then concentrated and enters the cell on the anode side

- only water is supplied to the cathode side

- a positive ion selectively permeable membrane is used to separate the anode and cathode compartments, and only allows the movement of Na⁺ cations (along with water) across the membrane from the anode side to the cathode side

caution: chlorine gas

is a toxic gas so must be extracted and stored carefully

caution: hydrogen gas

is explosive, so must be extracted and stored carefully

caution: sodium hydroxide

highly corrosive

test for chlorine gas (at the anode)

pale green gas turns damp blue litmus paper red (slightly acidic gas) and then bleaches it white (strong bleaching agent)

test for hydrogen gas (at the anode)

colourless gas gives a squeaky pop with a lit splint

use of chlorine from the chlori-alkali process

- directly as disinfectants and bleach

- in the preparation of pesticides, plastics, synthetic rubber and refrigerants

use of sodium hydroxide from the chlori-alkali process

- directly as drain cleaner

- in the making of wood pulp and paper, textiles, soaps and detergents

use of hydrogen from the chlori-alkali process

- directly as clean fuel

- in the process of steel manufacturing

- as a chemical building block for many compounds