LABSA, sold as acid slurry or linear alkyl benzene sulphonic acid, is the main active ingredient in many detergent powders, cakes and liquids. As supplied, it is a strong acid. Before it can clean, it must be neutralised to its sodium salt, sodium linear alkylbenzene sulphonate. Getting the amount of alkali right decides the pH, stability and feel of the finished product.
The chemistry in one line
Each molecule of LABSA has one acidic sulphonic acid group. Caustic soda neutralises it one-for-one:
R–SO₃H + NaOH → R–SO₃Na + H₂O
Soda ash (sodium carbonate) can do the same job and releases carbon dioxide while doing it. That fizzing is why powder makers who use soda ash in dry neutralisation see the mix foam and rise.
Use the acid value, not a rule of thumb
LABSA varies in strength between makers and batches. It also carries a little free sulphuric acid, which uses up extra alkali. The most reliable figure is the acid value (mg KOH per g) on the certificate of analysis, because it counts all the acid present. To convert it:
- Caustic soda (100% NaOH) needed, kg per kg of LABSA = acid value × 40 ÷ 56,100
- Soda ash (100% Na₂CO₃) needed, kg per kg of LABSA = acid value × 53 ÷ 56,100 (when the carbonate is fully converted to CO₂)
Example: a LABSA batch with an acid value of 175 mg KOH/g needs about 175 × 40 ÷ 56,100 = 0.125 kg of NaOH per kg. Using 48% caustic lye, that is 0.125 ÷ 0.48 = 0.26 kg of lye per kg of LABSA. With soda ash, the same batch needs about 0.165 kg of 100% Na₂CO₃ per kg. Treat these as starting points and finish by pH.
Caustic or soda ash: which to use
| Caustic soda | Soda ash | |
|---|---|---|
| Typical product | Liquid detergents, dishwash liquids, pastes | Detergent powders and cakes (dry neutralisation) |
| Speed | Fast, easy to control by pH | Slower; gives off CO₂ gas |
| Excess | Small excess raises pH sharply | Excess acts as a builder in powders |
| Heat | Significant; add slowly | Lower, but watch foaming |
A sensible method for liquid detergent
- Add most of the formula water to the mixer, then add the LABSA with gentle stirring. Diluting first spreads the heat.
- Add caustic solution slowly, in portions, keeping the stirring steady. The batch warms as it neutralises.
- Check the pH of a 1% solution as you get close. Most liquid detergents target about pH 7 to 8 after neutralisation, before other ingredients go in.
- Only then add SLES, CAPB, salt, perfume and colour. Many of these are sensitive to high or low pH.
Common mistakes
- Overshooting with caustic. A high pH makes the product harsh on hands, and it can drift on storage. Add the last part slowly.
- Using a fixed recipe ratio for every batch. If the acid value changes, the alkali needed changes too.
- Thickening with salt before neutralisation is finished. The viscosity response changes with pH, so finish neutralising first.
- Skipping PPE. LABSA and caustic are both corrosive. Use goggles, gloves and an apron.
For the full raw-material picture, see our guide to chemicals used in soap and detergent manufacturing. We supply acid slurry (LABSA), caustic soda and soda ash from Chennai stock.
Frequently asked questions
How much caustic soda is needed to neutralise 1 kg of LABSA?
Calculate it from the acid value on the COA: kg NaOH per kg LABSA = acid value × 40 ÷ 56,100. For a typical acid value of about 170–180 mg KOH/g, that is roughly 0.12–0.13 kg of 100% NaOH, or about 0.25–0.27 kg of 48% caustic lye. Finish the adjustment by pH.
What pH should neutralised LABSA be?
Most liquid detergent makers neutralise to about pH 7–8 (1% solution) before adding other ingredients, then adjust the final product pH to their specification.
Can soda ash be used instead of caustic soda for LABSA?
Yes, especially in detergent powders and cakes. It reacts more slowly, gives off carbon dioxide, and any excess stays in the powder as a builder. For liquid detergents, caustic soda is easier to control.
This article is general guidance. Always follow the Safety Data Sheet and your own site procedures for the specific product and grade you use.