Making a caustic soda solution from flakes looks simple: flakes plus water. But sodium hydroxide gives off a lot of heat as it dissolves, and doing the steps in the wrong order, in the wrong tank, can boil the liquid and spray hot caustic. The method below is what we suggest to customers making solutions for cleaning, pH correction, soap making and effluent treatment.
Rule 1: flakes go into water, never water onto flakes
When water is poured onto a heap of flakes, the first few litres meet a large amount of solid NaOH and heat up very fast, at one spot. That spot can boil and throw out caustic. Adding the flakes a little at a time to a full volume of water spreads the heat through the whole batch. Start with cold water, add flakes in small portions, and keep stirring the whole time.
How hot does it get?
Dissolving sodium hydroxide releases roughly 44 kJ of heat per mole, which is about 1,100 kJ per kilogram of flakes. In practice, a 10% solution made from cold water warms by around 20–25°C, while strong solutions of 30% or more can approach boiling point if the flakes go in quickly. Make strong solutions in stages, let the batch cool between additions, and allow for the heat when choosing the tank.
Choosing the tank and fittings
- Good: mild steel and stainless steel (304/316) for most strengths; HDPE and polypropylene for dilute and warm solutions within the plastic's temperature rating.
- Avoid: aluminium, zinc, galvanised steel, tin and brass. Caustic attacks these metals and gives off hydrogen gas, which is flammable.
- Avoid: thin PVC and PET containers for hot, freshly made solutions.
- Use a vented lid, not a sealed cap, while the solution is still warm.
Step by step
- Put on PPE: chemical splash goggles plus a face shield, nitrile or neoprene gloves with long cuffs, a chemical-resistant apron and closed shoes. See our PPE guide.
- Fill the tank with the full amount of cold water you need, or most of it if you will top up to a volume mark later.
- Start the stirrer, or have a long stainless or plastic paddle ready.
- Add the flakes slowly, a scoop at a time, onto the moving water. Do not dump a whole bag.
- Watch the temperature. If it climbs above about 60°C, stop and let it cool before adding more.
- When all the flakes have dissolved, let the solution cool to room temperature, then top up to final volume and check the strength.
- Label the tank with the contents, strength, date and hazard pictograms.
Worked example: 100 kg of 10% w/w solution
Caustic soda flakes are usually 98% NaOH or better. To get 10 kg of NaOH, weigh 10 ÷ 0.98 = 10.2 kg of flakes, and add them to 89.8 kg of water, making 100 kg of solution in total. If your COA shows a different assay, divide by that figure instead. Strengths given "by volume" are not the same as w/w, so check which one your process uses.
If you would rather not handle flakes at all, caustic soda lye (48% liquid) can be diluted in the same way. It still releases heat, but much less than flakes.
If caustic gets on skin or in the eyes
Rinse straight away with plenty of running water for at least 15 minutes, while removing contaminated clothing. For eyes, keep the eyelids open while rinsing and get medical help at once; caustic causes serious eye damage quickly. Keep an eyewash station or bottle near where solutions are made. The product SDS gives full first-aid measures.
Frequently asked questions
Why does caustic soda get hot when dissolved in water?
Dissolving sodium hydroxide is strongly exothermic: about 44 kJ of heat is released per mole of NaOH. Strong solutions made quickly can reach boiling point, which is why flakes must be added slowly to cold water with stirring.
Can I dissolve caustic soda in an aluminium or galvanised container?
No. Caustic soda attacks aluminium, zinc, galvanised steel and tin, producing flammable hydrogen gas and damaging the container. Use mild steel, stainless steel, or HDPE/PP within its temperature limit.
How much caustic soda flakes do I need for a 10% solution?
For 100 kg of 10% w/w solution using 98% flakes, dissolve 10.2 kg of flakes in 89.8 kg of water. Adjust using the assay on your certificate of analysis.
This article is general guidance. Always follow the Safety Data Sheet and your own site procedures for the specific product and grade you use.