Chlor-Alkali Process

Part of Electrochemistry

How electrolysis of salt water produces chlorine gas, hydrogen gas, and sodium hydroxide — three industrially critical chemicals from a single process.

Why This Matters

The chlor-alkali process is one of the highest-value electrolytic processes because it produces three essential chemicals simultaneously from the cheapest possible feedstock — salt (NaCl) dissolved in water. Chlorine enables water treatment and sanitation at scale. Sodium hydroxide (lye, caustic soda) is essential for soap making, paper production, aluminum processing, and dozens of other industrial processes. Hydrogen is a clean fuel and chemical feedstock.

A society with access to electricity and salt can produce all three from a single electrolyzer. This transforms a modest electrical installation into a chemical production facility. Understanding this process is therefore among the highest-leverage knowledge for rebuilding industrial chemistry capability.

The Electrochemical Reactions

Electrolyte: Saturated sodium chloride solution (brine), approximately 300 g/L NaCl.

At the anode (+) — chloride ions are oxidized: 2 Cl⁻ → Cl₂ + 2 e⁻

Chlorine gas (Cl₂) evolves at the anode.

At the cathode (−) — water is reduced: 2 H₂O + 2 e⁻ → H₂ + 2 OH⁻

Hydrogen gas (H₂) evolves at the cathode, and hydroxide ions accumulate.

Net reaction in the cathode chamber: NaCl + H₂O → ½ Cl₂ + ½ H₂ + NaOH

Why separation is critical: If Cl₂ and NaOH mix, they react: Cl₂ + 2 NaOH → NaCl + NaClO + H₂O — producing sodium hypochlorite (bleach) instead of pure caustic soda and chlorine. For applications requiring either pure product, anode and cathode compartments must be separated.

Cell Designs

Diaphragm Cell

• An asbestos or synthetic fiber diaphragm separates the anode and cathode compartments.
• Brine flows from anode to cathode compartment through the diaphragm, driven by pressure differential.
• Cathode liquor is dilute NaOH (10–12%) contaminated with NaCl. Requires evaporation to concentrate.
• Diaphragm gradually clogs and requires replacement.

Membrane Cell (Modern Standard)

• Ion exchange membrane (Nafion or equivalent) separates compartments.
• Membrane allows Na⁺ ions to pass but blocks Cl⁻ and OH⁻ ions.
• Produces concentrated NaOH (30–35%) with low NaCl contamination.
• Most energy-efficient design.
• Membrane requires clean brine (low calcium, magnesium — these precipitate in the membrane).

Mercury Cell (Historical — Do Not Use)

• Mercury cathode prevents Cl₂ and NaOH mixing. Sodium forms amalgam with mercury, then reacts with water in a separate vessel.
• Produces high-purity NaOH.
• Mercury contamination of effluent and products — severe environmental and health hazard. Banned in most countries. Do not construct.

Building a Small-Scale Chlor-Alkali Cell

For laboratory or small industrial use, a simple diaphragm cell can be constructed:

Materials

Item	Specification
Cell body	Two PVC chambers, each ~5 L
Membrane/diaphragm	Unglazed terracotta tile (improvised) or glass fiber cloth
Anode	Graphite or titanium/RuO₂ coated (DSA)
Cathode	Stainless steel or nickel mesh
Brine feed	300 g/L NaCl, deionized water preferred
Power supply	3.5–5 V per cell, high current

Construction

1. Fabricate cell body — two PVC chambers separated by a terracotta tile or glass fiber cloth diaphragm. The diaphragm must fit tightly to prevent gas mixing while allowing electrolyte permeation.

2. Mount electrodes — anode in the anode compartment, cathode in the cathode compartment. Both must have low-resistance connections to the bus bar.

3. Fill anode compartment with saturated brine.

4. Fill cathode compartment with dilute NaOH or distilled water initially. This compartment will accumulate NaOH as the process runs.

5. Energize — apply 3.5–5 V. Chlorine bubbles from anode, hydrogen from cathode.

6. Maintain brine level in anode compartment by continuous or periodic brine addition.

7. Collect and concentrate cathode liquor — withdraw periodically and evaporate to concentrate NaOH.

Operating Parameters

Parameter	Value
Cell voltage	3.5–4.5 V
Current density	1,000–3,000 A/m²
Brine concentration	280–310 g/L NaCl
Temperature	60–90°C (higher = lower resistance, better conductivity)
NaOH concentration produced	10–15% (diaphragm), 30–35% (membrane)

Chlorine Gas Hazards

Chlorine is Toxic

Chlorine gas is a war chemical — it was the first chemical weapon used in WWI. Never inhale it. At 0.5 ppm, it causes eye and throat irritation. At 10 ppm, severe respiratory damage in minutes. At 50 ppm, lethal within 30–60 minutes.

Safety measures:

• Conduct anode-side operations in well-ventilated areas or under a fume hood
• Always have a downwind escape route
• Use only in low concentrations for water treatment (add to water immediately as generated)
• Neutralize any excess with sodium bisulfite solution if needed

Applications of Products

Product	Primary Uses
Chlorine	Water disinfection, bleach production (with NaOH), PVC precursor, pharmaceutical synthesis
Sodium hydroxide	Soap making, paper (pulping), aluminum processing (Bayer), textile processing, drain cleaning
Hydrogen	Fuel, ammonia synthesis (Haber process), hydrogenation of oils

These three products together enable entire industrial sectors — soap and sanitation, paper and publishing, aluminum and materials, agriculture (via ammonia), and water treatment. Few single electrochemical processes unlock as much downstream capability as chlor-alkali.