Sodium Hydroxide (Caustic Soda)

What is sodium hydroxide and what does it do?

Sodium hydroxide is a white solid that forms a very high-pH (alkaline) solution when dissolved in water. It breaks down fats and proteins: this is why it can clear drains clogged with hair, grease and food scraps, and strip burnt-on grease from ovens.

The same chemical reaction is also the basis of soap making. Fats react with sodium hydroxide and turn into soap and glycerol (saponification). In properly made bar soap, no free caustic soda remains. Traditional liquid soft soap, on the other hand, is usually made with potassium hydroxide instead of sodium hydroxide.

Which products contain it?

• High-risk group: granular and gel drain unblockers, oven and grill cleaners, industrial degreasers
• Low-risk group: bar soap, washing-up liquid, laundry detergent and surface cleaners (to adjust pH or neutralise surfactants)

On the label it appears as "sodium hydroxide" or "caustic". In detergents, seeing it towards the end of the list usually means a small pH adjustment; if the packaging carries the corrosive pictogram, the product contains free caustic soda.

Is it harmful? The scientific assessment

Under EU CLP classification, sodium hydroxide causes severe skin burns and eye damage (H314) and may be corrosive to metals (H290). Alkali burns can be more insidious than acid burns: although the pain may feel mild at first, tissue damage can progress deeper. Because a splash in the eye can lead to permanent loss of sight, it is among the most serious household accidents.

Granular drain unblockers release heat when mixed with water and can spatter; some contain aluminium particles, and gas is produced during the reaction. Some gel drain unblockers also contain sodium hypochlorite in addition to caustic soda; mixing these products with acids can release chlorine gas.

In soaps and detergents the situation is different: the substance has either been neutralised with acids or turned into soap; the product's pH is kept within a range that is safe for use. Sodium hydroxide does not accumulate in the body and is not classified as a carcinogen.

Safe use and warnings about mixing

• When using a drain unblocker, wear rubber gloves and eye protection, and do not lean over the drain.
• Never use a drain unblocker together with hydrochloric acid (spirits of salt), a descaler or another drain unblocker; intense heat, spattering and toxic gas can result.
• Do not use a plunger after the product; corrosive water can splash up.
• If it gets into the eyes or on the skin, rinse with plenty of water for at least 15 minutes; if swallowed, do not induce vomiting and call your local poison control centre (in Türkiye: UZEM, 114).

What to do when you see it on a label

Seeing sodium hydroxide in the ingredient list of a soap or detergent is no cause for concern. With products such as drain unblockers and oven cleaners, however, follow the label warnings to the letter and keep them away from children. Using a drain strainer, not pouring fat down the sink and running hot water regularly reduce the need for chemical unblockers. For minor blockages, it is safer to try mechanical methods (plunger, drain snake) first.