An organosulfur compound, sulfonic acid can be thought of as sulfuric acid with a hydroxyl group replaced by an organic substituent, meaning you could be forgiven for mixing them up due to their similar names and chemical structures. Powerful and polarizing, sulfonic acid can most often be found as a crystalline solid or a high-boiling liquid that is thick, viscous, colorless, and non-oxidizing—perfect as a catalyst in organic reactions. With this in mind, let’s take a look at what sulfonic acid is used for.
A catalyst is often applied as a coating and is a substance that initiates a chemical reaction. Aromatic sulfonic acid, in particular, is used in applications such as automotive or floor coatings to cure them for a more durable and protective finish against wear and tear or weathering. Sulfonic acids are also valued for being soluble within organic solvents. They are also a catalyst for ion exchange, or “water softening,” which creates soft water that requires less soap for the same amount of cleaning.
Detergent and Surfactants
Detergents and similar products like soap use sulfonic acid to break down the surface tension between water and grease, dirt, and other grime for a more effective clean. It’s a powerful cleanser that can remove stains from clothing and other fabrics, yet it is also one of the cheapest surfactants in the washing industry. This is what sulfonic acid is used for most commonly.
Sulfonic acid can even be used in papermaking. With it, lignin is removed from the lignocellulose by treating wood chips with solutions of sulfite and bisulfite ions. The acid can break the bonds between the cellulose and lignin components to make the lignin into lignosulfonates that are soluble and can be separated from cellulose fibers.
If you have allergies or have ever had a nasty cough, you might have used medicine with sulfonic acid in it. Antibacterial sulfonamides inhibit and contain the growth of bacteria by acting as a competitive inhibitor of an enzyme the bacteria needs to facilitate folate synthesis or, as it’s most commonly known, vitamin B.