Material Insights · Understanding the Material World

Caustic soda and Chlorine

Chlorine and caustic soda come out of the same brine electrolysis in a fixed ratio of about 1.1 t of caustic per tonne of chlorine, so neither can be made alone. Plants run for whichever product is in demand and the other's price absorbs the imbalance; electricity is the largest cost of both.

Caustic soda (chemical) and Chlorine (chemical): 4 relationships.

How they are linked

1. Fixed-ratio co-products

Joint production · Strong · One electrolysis, two products

Passing electricity through salt brine splits it into chlorine at one electrode and caustic soda and hydrogen at the other, in proportions set by chemistry.

  • 1.0 to 1.1 tonnes of chlorine to tonnes of caustic soda
  • 0.03 t hydrogen per tonne of chlorine

The industry sells and costs the pair as an electrochemical unit, or ECU. A plant cannot raise chlorine output without making more caustic, so total revenue per ECU, not either price alone, decides whether a plant runs.

Sources: Olin Corporation (US SEC filing)

2. Demand imbalance and ECU pricing

Cost and price · Asymmetric · The product in surplus takes the price cut

Chlorine goes mainly to vinyls, urethanes and epoxy while caustic goes to alumina, pulp, water treatment and soaps, so the two demands rarely move together and one product is usually in surplus.

Chlorine is hard to store or ship, so when its demand falls producers may have to cut output of both, which can tighten caustic. When PVC demand is strong, extra caustic tends to weigh on its price. Olin describes the cycle: high margins bring new capacity, followed by oversupply and falling prices.

Sources: Olin Corporation (US SEC filing)

3. Same power bill

Shared risk · Strong · Electricity costs hit both

Electricity is the largest raw material in chlor-alkali, so a rise in power prices raises the cost of the whole ECU and is passed on through whichever product the market will bear.

Because the cost is shared, regions with expensive power lose competitiveness in both products together. Plants with cheap gas-fired or hydro power have an advantage in both chlorine derivatives and caustic exports.

Sources: Olin Corporation (US SEC filing)

4. Same plants and owners

Geography and ownership · Strong · Every producer sells both

Every chlor-alkali producer must find buyers for both products, so firms build chlorine-consuming units to keep caustic plants running.

  • 63.84 lakh t India's caustic soda capacity a year, 31 March 2025
  • 82% Indian capacity utilisation, FY2023-24

In FY2023-24 India produced 46.13 lakh t of caustic soda against domestic demand of 43.56 lakh t, while domestic chlorine demand was 40.87 lakh t. Gujarat Alkalies reported challenges in chlorine off-take in FY2024-25 and commissioned a chlorotoluenes project at Dahej in March 2025 to raise its own chlorine use.

Sources: Gujarat Alkalies and Chemicals Ltd (via IndiaInfoline); Indian Chemical News (Alkali Manufacturers Association of India data)

For investors

Key learnings

  1. The economic unit is the ECU: value one product without the other and you misread a chlor-alkali producer's earnings.
  2. Chlorine demand, mainly PVC, usually sets run rates, so strong vinyls markets tend to lower caustic prices and weak ones raise them.
  3. Power is the largest cost, so electricity prices decide which regions stay competitive in both products.

What to track

  • ECU netback (caustic price times 1.1 plus chlorine or derivative value): Shows whether plants are earning their power cost (Weekly · industry price reports)
  • PVC demand and operating rates: Drive chlorine run rates and so caustic supply (Monthly · industry reports)
  • Industrial power tariffs in producing regions: Set the cost floor for both products (Quarterly · regulators, utilities)

For policymakers

Key learnings

  1. India's caustic capacity reached about 6.4 Mt a year by March 2025, and output above domestic demand depends on export markets or new chlorine uses.
  2. Because chlorine is hard to move, local chlorine-consuming industries decide how much caustic India can make.
  3. Power tariff policy for electrolysis-based industries sets India's cost position in both products together.

What to track

  • India caustic soda capacity, output, demand and exports: Show whether the industry is chlorine- or caustic-limited (Annual · AMAI)
  • Chlorine derivative projects (PVC, chlorotoluenes, CPVC): Add chlorine outlets that let caustic plants run (As announced · company filings)

India

India had about 6.4 Mt a year of caustic soda capacity at end-March 2025; in FY2023-24 it made 4.61 Mt against domestic demand of 4.36 Mt, while domestic chlorine demand was 4.09 Mt. Producers such as Gujarat Alkalies report difficulty selling chlorine and are adding units that consume it.

Sources

  1. Olin Corporation Form 10-K for fiscal year 2025, Olin Corporation (US SEC filing) (2026)
  2. Caustic soda demand grew by 5.3%, soda ash by 4% in FY 2023-24: AMAI, Indian Chemical News (Alkali Manufacturers Association of India data) (2024)
  3. Gujarat Alkalies and Chemicals: management discussion and analysis, FY2024-25, Gujarat Alkalies and Chemicals Ltd (via IndiaInfoline) (2025)