Sodium Hydroxide (Caustic Soda): Uses, Grades, Safety

Sodium hydroxide is a strong alkali, also called caustic soda or lye. In Hindi shops you'll hear it called kastik soda. It is the base that turns oils into soap and the active part of many heavy-duty drain and degreasing cleaners.

• Formula: NaOH, molar mass about 40.00 g/mol
• CAS number: 1310-73-2
• Appearance: white flakes, pellets or granules that pick up moisture from the air
• Melting point: about 323 °C
• Solubility: dissolves very readily in water, roughly 1 kg per litre at 25 °C, and gives off a lot of heat while dissolving

Left open, it absorbs water and carbon dioxide from the air and slowly turns into a wet, crusty mass of sodium carbonate. That is why the lid matters.

Azlok lists two forms: Caustic Soda and Sodium Hydroxide Pellets. Both sit in the cosmetic raw ingredients range, meant for soap making and small-scale formulation. The product names don't state a pack size, so check the weight on each product page.

Neither is sold as analytical or food grade. See today's price per kg for each form.

• Soap making: reacts with oils and fats (saponification) to make bar soap. Liquid soap normally uses potassium hydroxide instead.
• Cleaning: clears drains blocked by grease and hair, and removes baked-on grease from steel and iron.
• pH adjustment: small amounts of dilute solution raise pH in cosmetic and industrial batches.
• Industry: paper and pulp, textile processing (mercerising cotton), aluminium production, biodiesel and water treatment.
• Labs and schools: standard base for titrations and neutralisation experiments.

For cold process soap, the lye amount depends on the oils. Every oil has its own saponification value, so run your recipe through a lye calculator instead of guessing. Most soap makers use water at about 2 to 3 times the weight of lye and keep a 5% superfat.

1. Wear gloves, goggles and full sleeves. Work near an open window.
2. Weigh cold water into a steel or heavy plastic (PP or HDPE) jug.
3. Add the lye to the water slowly while stirring. Never pour water onto lye; it can boil up and splash.
4. Expect the solution to reach 80–90 °C. Don't breathe the fumes and let it cool to around 35–45 °C.
5. Pour into your melted oils and blend to trace.

For a blocked drain, a common approach is two to three tablespoons dissolved in a litre of cold water, poured in slowly and left for 15–30 minutes, then flushed with plenty of water. Don't use it on aluminium, chrome-plated or PVC fittings that are already damaged, and never mix it with acid-based cleaners.

Sodium hydroxide causes serious burns to skin and eyes, and the damage can continue after the first contact. Goggles are not optional. If it splashes on skin, rinse with running water for at least 15 minutes. For eyes, rinse for 15 minutes and see a doctor straight away.

• Keep it away from acids, aluminium, zinc and tin; it reacts with these metals and gives off hydrogen.
• Store in the original airtight container, in a dry place, clearly labelled.
• Don't use kitchen vessels for lye and then go back to cooking in them.
• Keep away from children and pets. Spilled pellets look like sugar or sweets.
• Small spills: sweep up dry, then wipe with plenty of water. Dilute leftover solution heavily before draining.

Pellets are easier for weighing small soap batches and make less dust. Flakes dissolve a little faster and are fine for cleaning and larger batches. If you make soap once a month, a small pack is safer because it won't sit around absorbing moisture. Once you are making soap every week, buy a larger pack and transfer what you need into a smaller airtight jar.

For lab work that needs certified assay, buy a dedicated laboratory or AR grade. For 25 kg and above, use bulk inquiry.