Arylsulfonic acids are among the strongest organic acids commonly used in industry. p‑Toluenesulfonic acid (PTSA) and xylenesulfonic acid (XSA) act as acid catalysts and activators: they initiate and control curing or chemical reactions in a customer’s process.
For process engineers and procurement teams, this means they cannot be selected on the chemical name or price per kilogram alone. Performance depends on concentration, free sulfuric acid content, the carrier — the liquid in which the acid is dissolved, such as water, ethanol or a mixture — and consistency from batch to batch. This guide explains what arylsulfonic acids are, how PTSA and XSA differ, and what to consider when choosing them.
What are arylsulfonic acids?
Arylsulfonic acids are organic acids in which a sulfonic acid group, –SO3H, is attached directly to an aromatic ring. This group gives them acidity comparable to that of mineral acids. Unlike many mineral acids, however, they also dissolve readily in organic solvents, making them suitable for both aqueous and organic systems.
They are made by sulfonation, a reaction that introduces the sulfonic acid group into an aromatic molecule: toluene in the case of PTSA and xylene in the case of XSA. The manufacturing process also gives rise to a parameter customers monitor in every batch: free sulfuric acid content, meaning the amount of unreacted H2SO4 remaining after sulfonation.
An arylsulfonic acid should not be confused with its neutralised salt, a sulfonate. They differ in reactivity and pH and therefore serve different purposes in a formulation. An enquiry should always specify the exact substance name, CAS number and required solution concentration.
PTSA and XSA – identification and key specifications
p‑Toluenesulfonic acid has CAS 104-15-4 and EC 203-180-0. HABRINOL manufactures and supplies it as an aqueous PTSA solution, typically at a concentration of 64–66%. Concentrations of 40–66% are available on request. Concentrations above 66% are generally not recommended because of the increased risk of crystallisation during storage and transport.
Xylenesulfonic acid is a mixture of dimethylbenzenesulfonic acid isomers with CAS 25321-41-9 and EC 701-247-3. PTSA and XSA are registered under EU REACH Regulation (EC) No 1907/2006. Separate UK REACH requirements apply to substances placed on the market in Great Britain.
| Parameter | PTSA (aqueous solution) | XSA (aqueous solution) |
|---|---|---|
| Active substance content | 40–66% by weight | 65–90% by weight |
| Free sulfuric acid content | 0.4–15% by weight | 1–15% by weight |
| Density at 20°C | 1.13–1.29 g/cm³ | 1.22–1.29 g/cm³ |
| Recommended storage temperature | 5–25°C | 5–25°C |
The technical data sheets show the range of grades manufactured; the agreed quality specification is definitive. Below 5°C, the solution may become cloudy or form a fine sediment. Gentle warming to the recommended storage temperature and mixing will usually restore homogeneity. Further details are available in the product sheets for p‑toluenesulfonic acid and xylenesulfonic acid.
What determines how the acid performs in a process?
The first consideration is grade. PTSA concentration alone does not determine reactivity and working time; free sulfuric acid content matters too. Grades are therefore expressed as concentration / free H2SO4, for example 65/0.5, 65/1 or 65/2. Both values can be adjusted to suit the application and achieve the required reactivity.
The second consideration is the carrier. An aqueous, alcoholic or water–alcohol solution introduces different amounts of water or solvent into the system, affecting both dosage and how the mixture behaves.
The third is batch‑to‑batch consistency. If concentration or free sulfuric acid content varies between deliveries, the customer may need to adjust the dosage, making the process less reproducible.
These parameters can also be tailored through custom formulations: adjusting PTSA concentration and free sulfuric acid content, or blending PTSA or XSA with other organic or inorganic substances, such as monoethylene glycol, phosphoric acid or lactic acid. The custom manufacturing page outlines the options.
Where are PTSA and XSA used?
Activators for binder‑system manufacturers. In the foundry supply chain, PTSA, XSA and their blends act as activators for furan and phenolic no‑bake systems. The activator is supplied to the end user alongside the resin as one component of a two‑part system. HABRINOL supplies this component to manufacturers of binder and hardener systems, not the complete system.
Catalysts for resins, coatings and adhesives. PTSA and XSA act as acid curing catalysts in resin manufacture and modification, coatings and adhesives. Formulators assess suitability within their own formulations.
Organic synthesis and intermediates. PTSA is used as a catalyst in acid‑catalysed organic reactions. XSA also serves as an intermediate in the manufacture of dyes, pigments and speciality chemicals.
pH adjustment and other applications. Both acids are used to adjust pH in industrial formulations and in selected textile‑finishing and electroplating processes.
For an overview of sectors in which these products can be used, see the applications by industry page.
Manufacturing and quality control in Děčín
HABRINOL Děčín s.r.o. has manufactured arylsulfonic acids in Děčín, Czech Republic, since 1996, building on the manufacturing tradition of the German company HABRINOL Halle. The site now has 10 reactors, an in‑house laboratory, a nitrogen supply system for reactor inerting and a water treatment plant. Every batch is tested in the company’s laboratory and supplied with a certificate of analysis.
The company operates an integrated management system to ISO 9001, ISO 14001, ISO 45001 and ISO 50001; it has held ISO 9001 certification continuously since 2017. It manufactures in accordance with IPPC/BAT requirements and holds an EcoVadis Silver rating. As an EU manufacturer, HABRINOL meets its applicable EU REACH obligations. For supplies to Great Britain, customers should separately confirm who will fulfil any applicable UK REACH importer duties; EU REACH registration alone does not remove them. Certificates can be downloaded from the about us page.
How to prepare an enquiry and approve a supplier
Qualification — approving a raw material or supplier for use in production — is usually quicker when an enquiry covers more than annual volume and price. It helps to include:
- the application and the acid’s function in the process (activator, catalyst or pH adjustment),
- the required concentration, free sulfuric acid content and carrier,
- the parameters that most affect the process and their acceptable tolerances,
- annual demand, packaging type and delivery location,
- the documentation required: technical data sheet, safety data sheet and information relevant to UK REACH and GB CLP for supplies to Great Britain.
The next steps are typically a sample, laboratory testing in the customer’s formulation and a pilot delivery. HABRINOL also supplies standard products in smaller quantities, such as a single IBC container. For custom blends and speciality products, the minimum production quantity is 3 tonnes. Packaging ranges from 10-litre jerrycans through drums and IBCs to road tankers.
Supplier evaluation should consider total cost of ownership, not just price per kilogram: transport, the safety stock required, capital tied up in inventory and the potential cost of a quality claim. For UK customers, supply from Děčín may offer shorter transit times than sourcing from overseas, although lead times will also depend on customs and delivery arrangements. HABRINOL currently supplies more than 15 countries. Communication and documentation are available in Czech, German and English.