Sodium hydroxide is a manufactured chemical, produced almost entirely by running electricity through saltwater. While the sodium and hydroxide ions that make it up exist naturally in soil, groundwater, and living tissue, the concentrated substance itself (the white solid or thick liquid you’d encounter in industry or cleaning products) is synthetic. Global production hit roughly 80 million tonnes in 2024, valued at nearly $47 billion.
The Raw Material: Salt and Water
The starting ingredient is ordinary salt, sodium chloride, dissolved in water to make brine. Most large-scale producers use salt mined from underground deposits or extracted from seawater. The brine is purified to remove minerals that would interfere with the electrical process, then fed into electrolysis cells. That’s really it for raw materials: salt, water, and a large amount of electricity.
Researchers have also explored less conventional sodium sources, including the brackish water produced during coal seam gas extraction and the leftover brine from seawater desalination plants. These contain sodium chloride, sodium bicarbonate, and sodium carbonate, all of which can serve as feedstock. For now, though, mined salt remains the dominant source.
How Electrolysis Turns Salt Into Caustic Soda
The industrial method is called the chlor-alkali process, and it works by passing an electric current through the brine solution. The electricity splits the dissolved salt and water into three products at once: chlorine gas forms at the positive electrode, while sodium hydroxide and hydrogen gas form at the negative electrode. Every chlor-alkali plant produces all three, which is why the supply of sodium hydroxide is closely tied to the demand for chlorine (used in plastics, disinfectants, and countless other products).
Three variations of this process have been used over the decades, each separating the electrodes differently to keep the chlorine from reacting with the sodium hydroxide:
- Diaphragm cells use a porous barrier (historically asbestos-based) between the electrodes. The sodium hydroxide produced this way contains residual salt and needs extensive evaporation to reach commercial concentration. These cells run at lower voltage and require less pure brine, which keeps costs down.
- Mercury cells use liquid mercury as the cathode. Sodium dissolves into the mercury to form an amalgam, which is then reacted with water in a separate chamber to produce very pure sodium hydroxide. The product quality is the highest of the three methods, but the energy consumption is also the highest, and trace mercury contamination poses serious environmental and health concerns.
- Membrane cells use a selective polymer membrane that allows sodium ions to pass through while blocking chlorine. This produces high-quality sodium hydroxide with lower energy use than mercury cells and without the environmental baggage. All new chlor-alkali plants built today use membrane technology because of its lower capital costs, lower operating costs, and lower power consumption compared to the older methods.
Energy Cost of Production
Making sodium hydroxide is energy-intensive. Conventional chlor-alkali electrolysis typically consumes around 2.5 to 3.5 kilowatt-hours per kilogram of sodium hydroxide produced. To put that in perspective, producing a single tonne requires roughly the same electricity an average American home uses in a month. Newer experimental methods using bipolar membrane electrodialysis have achieved consumption as low as 1.2 kilowatt-hours per kilogram under ideal lab conditions, but scaling those approaches remains a challenge. Less optimized setups can consume far more, with some experimental configurations reaching over 40 kilowatt-hours per kilogram.
The Older Chemical Route
Before electrolysis became dominant, sodium hydroxide was made by reacting soda ash (sodium carbonate) with slaked lime (calcium hydroxide). Mixing these two chemicals in water causes a straightforward swap: the calcium grabs the carbonate, settling out as calcium carbonate (chalk), while sodium and hydroxide ions remain dissolved. The liquid is then filtered and concentrated. This causticizing method is still used in some smaller operations and in the pulp and paper industry, but it produces lower-purity sodium hydroxide and can’t match the scale of electrolysis.
What Form It Takes
At room temperature, pure sodium hydroxide is a white, odorless, crystalline solid that aggressively absorbs moisture from the air. In practice, it’s most commonly shipped and sold as a 50% solution in water, sometimes called liquid caustic soda or lye. The solid form (flakes, pellets, or beads) is used when shipping water weight doesn’t make economic sense or when specific industrial processes require it. Both forms are highly corrosive and react vigorously with acids, fats, and many metals.
Sodium and Hydroxide in Nature
Sodium hydroxide as a concentrated compound doesn’t occur naturally, but the individual ions that compose it are everywhere. Sodium ions and hydroxide ions are common in soils, groundwater, plants, and animal tissues. Water supplies in limestone regions contain significant amounts of both. Because these ions are naturally present at low levels and pose little hazard in typical concentrations found in drinking water, there are no regulatory limits on them. The distinction matters: what you buy in a container is manufactured, but the building blocks are part of the natural mineral landscape.