Sulfonic acids have the general formula RSO₃H. They can be found in nature, or synthesised for commercial use in industry as reagents for detergents, dyes, surfactants, sweeteners and drugs. More importantly, sulfonated resins have been employed as industrial catalysts [1], e.g. in the hydration of propene to iso-propyl alcohol.

Considering the diversity of applications for sulfonic acids; neat, or in the form of resins, it is surprising to find that there is a lack of published work investigating sulfonic acid behaviour in aqueous media.

This work deals with the analysis of Raman spectra of aromatic sulfonic acids especially those of benzene-sulfonic and p-toluene-sulfonic acid monohydrate, from which an understanding of the behaviour of sulfonic acids in aqueous and non-aqueous media was derived. From these results, the behaviour of sulfonated resins in industrial systems is better understood.

Raman spectroscopy was used to analyse sulfonic acid solutions, as it is insensitive to water, unlike infrared spectroscopy. This leads to clearer spectra where shifts in key marker bands due to water addition and hydrogen bonding can be clearly seen.

Saturated solutions of benzenesulfonic acid, p-toluenesulfonic acid monohydrate and anhydrous p-toluenesulfonic acid were made up in N,N-dimethylformamide and acetonitrile. Then small metered amounts of water were added to these solutions. As noted by Zundel [2] from infrared spectroscopy studies, sulfonic acids are fully dissociated when each –SO₃H group is surrounded by two water molecules. This situation was investigated by adding amounts of water greater than and less than at a 2:1 ratio, to the saturated solutions and analysing the bonding using Raman spectroscopy.

Through these experiments, a study of the effect of water on the "stability" of a system and its ability to dissociate was made by comparing the dissociation of anhydrous p-toluenesulfonic acid and its monohydrate. It has been found that the dissociation of sulfonated resins seems to be similar to the dissociation of anhydrous sulfonic acids.[3] However, in the case of hydrated sulfonic acids, the situation is more complex and other factors need to be considered.

1.Edwards H.G.M., Brown D.R., Dale J.A., Plant S., 2000: Raman Spectroscopy of Sulfonated polystyrene resins J.Vib.Spec. 24:213-224.

2. Zundel G.:1966 Angew. Chem., Int. Ed. Engl. 8:499

3.Edwards H.G.M., Smith D.N., 1990: The Raman spectrum of ethanesulphonic acid, C₂H₅SO₃H, and the ethanesulphonate ion J.Mol Struct. 238:27-41