Sulfonation process

Sulfonic acids are produced through the sulfonation process. Sulfur trioxide is usually the sulfonating agent. The wide application of this method is the production of alkylbenzene sulfonic acids. Sulphonic acids are strong acids. They are usually listed as one million times stronger than the corresponding carboxylic acid. For example, p-Toluenesulfonic acid and methanesulfonic acid have pKa values of -2.8 and -1.9, respectively, while those of benzoic acid and acetic acid are 4.20 and 4.76, respectively. However, as a result of their strong acidity, their pKa values cannot be measured directly, and the commonly quoted values must be considered as indirect estimates with considerable uncertainty. For example, various sources have reported the pKa of methanesulfonic acid as high as -0.6 or as low as -6.5. Sulfonic acids react with solid sodium chloride (salt) to form sodium sulfonate and hydrogen chloride. This property indicates an acidity two or three times that of HCl(g), whose pKa has recently been accurately determined (pKaaq = -5.9).

Because of their polarity, sulfonic acids are usually crystalline solids or viscous, high-boiling liquids. They are also usually colorless and non-oxidizing, which makes them suitable for use as acid catalysts in organic reactions. Their polarity, along with their high acidity, makes short-chain sulfonic acids soluble in water, while longer-chain acids have detergent-like properties.

The structure of sulfonic acids is illustrated by the prototype, methanesulfonic acid. The sulfonic acid group, RSO2OH, has a tetrahedral sulfur center, meaning that the sulfur is centered on four atoms: three oxygens and one carbon. The overall geometry of the sulfur center is reminiscent of that of sulfuric acid.

Sulphonic acid purity measurement method

The percentage purity of sulfonic acid: The best method to determine its purity percentage is the titration method. Hyamine with a normality of 0.004 is used as a titrant. The working method is that first one gram of the acid is weighed in 100 cc and Its exact weight is recorded. In the next step, by adding about 30 cc of distilled water to the mixture and completely dissolving it, it is titrated with 0.1 normal sodium (NaOH) with phenol reagent to determine the acid value. Then the solution in the mixture, which Transfer the pale pink color to a 500 cc flask and make up to volume. Then, with a pipette, transfer 5 cc of the solution to a Nessler (containing 15 cc of distilled water, 15 cc of indicator mix, and 10 cc of chloroform) and Hayamin, we headline.

What are the uses of sulfonic acid?

Acid catalysts

Sulphonic acids are also used as strong acids as catalysts. The simplest examples are methanesulfonic acid, CH3SO2OH, and p-toluenesulfonic acid, which are regularly used in organic chemistry as lipophilic acids (soluble in organic solvents). Polymeric sulfonic acids are also useful. Dowex resin is a derivative of polystyrene sulfonic acid and is used as a catalyst and for ion exchange (water softening). Nafion, a fluorinated polymeric sulfonic acid, is a component of proton exchange membranes in fuel cells.

Detergents and surfactants

Detergents and surfactants are molecules that combine highly nonpolar and highly polar groups. Traditionally, soaps are popular surfactants derived from fatty acids. Since the middle of the 20th century, the use of sulfonic acids has overtaken soap in developed societies. For example, it is estimated that 2 billion kilograms of alkylbenzene sulfonate are produced annually for various purposes. Lignin sulfonates, produced from lignin sulfonation, are components of drilling fluids and additives in certain types of concrete.


Many, if not most, anthraquinone dyes are produced or processed through sulfonation. Sulphonic acids tend to bind tightly to proteins and carbohydrates. Most “washable” dyes are sulfonic acids (or have a functional sulfonyl group in them) for this reason. p-Cresidinesulfonic acid is used to make food colors.


In the sulfite process for papermaking, lignin is removed from lignocellulose by treating wood chips with solutions of sulfite and bisulfite ions. These reagents break the bond between cellulose and lignin components and especially in lignin itself. Lignin is converted to lignosulfonates, useful ionomers, which are soluble and can be separated from cellulose fibers.


Sulfa drugs, a class of antibacterials, are produced from sulfonic acids.

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