Summary. Acid slurry (LABSA, CAS 27176-87-0) is a strong sulphonic acid. Detergent makers neutralise it with caustic soda or soda ash to form the anionic surfactant salt. The reaction releases heat, so it is done in controlled increments with stirring and cooling, and free sulphuric acid in the acid also consumes alkali. This article is general knowledge about the chemistry; it gives no dosage ratios, and your own process validation and the SDS govern.
Anyone who buys acid slurry (LABSA 90%) eventually asks the same question: how do I turn it into a usable surfactant safely, and what should I check before I start? This guide explains the process chemistry in plain terms. Product facts quoted from the SPC technical data sheet are marked as such; everything else is general chemistry. Supreme Petro Chemicals is a Chennai importer and bulk distributor of the acid, not a detergent maker, so treat this as background for your own process engineers rather than a recipe.
What neutralisation actually does
LABSA is a sulphonic acid: a strong acid with a long, oil-loving alkyl benzene tail. Neutralising it with a base swaps the acidic hydrogen for sodium and produces the sodium salt (sodium linear alkylbenzene sulphonate) plus water. The salt is the working surfactant in detergent powders, cakes, liquid soap and dishwash. The acid itself is corrosive and is not what ends up in the finished product.
Two practical consequences follow. First, the reaction is exothermic (the SDS says neutralisation with strong alkalis is exothermic), so temperature control is part of the process, not an afterthought. Second, the amount of alkali you need depends on everything acidic in the drum, not just the sulphonic acid.
Read the batch CoA before you calculate anything
The SPC technical data sheet (SPC-TDS-ACD-017) lists the limits for LABSA 90% Active. The batch Certificate of Analysis supplied with every delivery shows the actual values for the lot in front of you, and it governs. These are the numbers that matter for neutralisation:
| Parameter | TDS value | Why it matters at neutralisation (general knowledge) |
|---|---|---|
| Active matter (as LABSA) | 90.0 +/- 1.0 % mass | Sets how much sulphonic acid you are neutralising per kilogram. |
| Free sulphuric acid | 1.5 % mass max | Also takes up alkali and ends up as sodium sulphate in the product. |
| Free oil (unsulphonated matter) | 2.0 % mass max | Does not neutralise; can show as oily residue or odour. |
| Colour, 5% solution | 60 Klett max | Sets the starting colour of the surfactant, which matters for pale products. |
| Average molecular weight | 318 - 325 | Reflects the alkyl chain length distribution; a drift between lots is worth querying. |
| Appearance | Viscous transparent brown liquid | The paste can set in cold conditions; the SDS says it typically flows above about 15-20 C. |
The TDS values are the manufacturer's published figures and are indicative. If the CoA for a lot shows a different free sulphuric acid figure from the last lot, your alkali demand changes with it.
Reading the CoA against the neutralisation endpoint
The endpoint you aim for is set by your formulation, not by the acid. What the CoA tells you is how far a given lot sits from the previous one. Two lots can both pass the TDS and still need slightly different alkali, because active matter can vary within 90.0 +/- 1.0 % mass and free sulphuric acid can sit anywhere up to 1.5 % mass. A simple habit is to log active matter and free sulphuric acid for every lot next to the alkali actually used and the final pH. After a few batches you can see whether a pH miss comes from the lot or from the process.
Treat the CoA as an input to a small pilot trial, never as a substitute for one. Neutralise a small quantity first, confirm pH, viscosity, colour and odour against your own standard, and only then scale up.
Caustic soda or soda ash?
Both are used, and the choice depends on the product being made, the equipment and the process, not on one being universally better. Caustic soda is a strong base that reacts quickly and releases a lot of heat. Soda ash (sodium carbonate) is a milder base that releases carbon dioxide as it neutralises, which causes foaming and needs headspace.
| Point | Caustic soda | Soda ash |
|---|---|---|
| Strength | Strong base, fast reaction | Milder base, slower reaction |
| Heat | Releases substantial heat | Releases heat; carbon dioxide evolution causes foaming |
| Gas | None from the base itself | Carbon dioxide; foaming risk |
| Fit | Often used for liquid and paste surfactants; depends on the process | Often used in powder and cake routes; depends on the process |
| Main caution | Corrosive; can overshoot pH quickly | Needs vessel headspace and slow addition |
A careful sequence
This outline shows the order of thinking and is not a validated procedure. Quantities are deliberately absent: they come from your formulation, your equipment and the CoA of the lot.
- Prepare the area and PPE. The SDS calls for goggles, gloves and an apron. Both the acid and the alkali are corrosive.
- Check the CoA for active matter and free sulphuric acid, and note the material's temperature. Cold paste is harder to handle, as the SDS notes.
- Use acid-resistant equipment for the acid. The SDS says to keep it away from alkalis, strong oxidisers and reactive metals such as aluminium and zinc until you deliberately neutralise it.
- Set up cooling and agitation before the first addition, with a temperature probe in the mix.
- Add in controlled increments rather than in one charge, watching temperature and, with soda ash, foam level.
- Check pH at intervals against the target your own formulation sets, and adjust in small steps. Overshoot is easier to cause than to correct.
- Record the lot, the alkali used and the final pH so the next batch can be compared.
Troubleshooting walk-through
| Symptom | Likely cause (general knowledge) |
|---|---|
| Temperature rises faster than expected | Addition too fast, or too little cooling |
| Heavy foam on the soda ash route | Carbon dioxide release; vessel too full or addition too fast |
| Final pH hard to hit | Free sulphuric acid in the lot differs from the last lot |
| Product thickens unexpectedly | Concentrated surfactant systems can thicken sharply at some concentrations; adjust dilution or order of addition in trials |
| Oily film or odour | Free oil on the high side of the limit |
| Darker finished colour | Higher incoming colour, or overheating |
A practical walk-through for the most common complaint, a pH that will not settle: first confirm the pH probe is calibrated and the mix is well stirred, because a local pocket of acid or alkali gives a misleading reading. Then compare this lot's free sulphuric acid on the CoA with your log. If it differs, expect a different alkali demand and adjust in small steps. If the CoA matches your log, look at the process: addition rate, temperature at the time of reading, and whether carbonate is still reacting and releasing gas. For temperature spikes, stop the addition, restore cooling and resume more slowly. For foam on the soda ash route, pause, let the foam settle and reduce the charge rate.
Storing and handling the neutralised product
Once neutralised, the material is a surfactant salt solution or paste, no longer a strong acid, but it can still irritate skin and eyes and should be handled under its own safety data. Keep it in clean, closed containers, record the batch and final pH on the label, and avoid cross-contamination with acids or strong oxidisers. Surfactant pastes can change viscosity with temperature and concentration, so note the storage temperature when you compare batches. Any consumer product built on it needs its own qualification by the formulator.
Safety and materials
The SDS classifies acid slurry as Skin Corrosion 1C and Serious Eye Damage 1 (signal word DANGER; H302, H314, H412). It transports as UN 2586, Class 8, Packing Group III. Store it in a cool, dry, well-ventilated area in acid-resistant containers, away from alkalis, strong oxidisers and reactive metals. The SDS, not this article, is the governing document for first aid and spill response. The material is an industrial technical grade; the SDS restricts consumer and medicinal applications unless separately qualified.
What this article deliberately leaves out
You will not find alkali-to-acid ratios, percentages of caustic soda or soda ash, or target viscosities here. They depend on the lot, the formulation and the plant, and a number copied from a general article can be wrong for your batch. If you want a starting point, derive it from the CoA and validate it in a small trial before scaling up.
Related reading
The feedstock relationship is covered on the linear alkyl benzene (LAB) page, SLES is a separate ready-made anionic surfactant, and the product page for acid slurry (LABSA 90%) specifications, HSN and UN number has the identifiers and documents.
FAQ
Is neutralising acid slurry exothermic?
Yes. The SDS states that neutralisation with strong alkalis is exothermic. Control the rate of addition and provide cooling and agitation.
Does free sulphuric acid affect how much alkali I need?
Yes. Free sulphuric acid also reacts with the base, forming sodium sulphate. The TDS limits it to 1.5 % mass max, and the lot's CoA shows the actual figure.
Can I use soda ash instead of caustic soda?
Many processes do, but soda ash releases carbon dioxide and foams, so it needs headspace and slower addition. The choice belongs to your process, not to this article.