Chlorosulfuric acid: Reactions, Production and Uses

Chlorosulfuric acid

Chlorosulfuric acid, also known as chlorosulfonic acid, is an colorless and mobile liquid with exceptional reactivity. It exhibits strong fuming properties when exposed to air and has found applications as a smoke-generating agent in military operations.

In this context, it is commonly mixed with oleum, containing approximately 50-65% by weight of HSO3Cl and 50-35% by weight of SO3. When 2 liters of chlorosulfuric acid are combined with 1000 cubic meters of air, it results in the production of a dense, opaque mist.

This acid possesses high corrosiveness and hygroscopicity. In the presence of water, chlorosulfuric acid decomposes explosively, giving sulfuric acid and hydrogen chloride. The initial synthesis of chlorosulfuric acid was accomplished by A. Williamson in 1854.

Table of Contents

1. Physical Properties of Chlorosulfuric acid

The purification of chlorosulfuric acid presents challenges, often leading to its utilization in experimental determinations in an impure form. However, achieving a high degree of purity is possible through fractional crystallization.

The substantial heat of vaporization exhibited by chlorosulfuric acid can be attributed to the strong association between its molecules, primarily resulting from hydrogen bonding. This acid demonstrates solubility in various solvents such as methylene chloride, chloroform, 1,1,2,2-tetrachloroethane, 1,2-dichloroethane, acetic acidacetic anhydride, trifluoroacetic acid, trifluoroacetic anhydride, sulfuryl chloride, liquid sulfur dioxide.

However, it is sparingly soluble in carbon tetrachloride and carbon disulfide and it reacts with alcohols, ketones, ethers, and dimethyl sulfoxide.

In contrast, sulfur trioxide exhibits complete miscibility with chlorosulfuric acid, whereas the solubility limit of free hydrogen chloride is 0.5% by weight at 20°C and atmospheric pressure.

Physical Property Value Unit
Molar mass 116.53 g/mol
Freezing point (fp) -80 to -81 °C
Boiling point (bp) (decomp.) 151 - 152 °C
Density at 20 °C 1.753 g/ml
Density at -10 °C 1.80 g/ml
Density at -70 °C 1.90 g/ml
Viscosity at -31.6 °C 10 mPa s
Viscosity at -17.8 °C 6.4 mPa s
Viscosity at 15.6 °C 3.0 mPa s
Viscosity at 49 °C 1.7 mPa s
Refractive index at 14 °C 1.437 -

Caution must be exercised when handling chlorosulfuric acid in the presence of solvents, as an increase in temperature can induce violent reactions, particularly when catalysts are also present.

2. Chemical Reactions of Chlorosulfuric acid

Chlorosulfuric acid is a stable strong acid under normal conditions that consists of a tetrahedral molecule with a relatively weak S-Cl bond. The force constant of this bond is approximately one-third that of the single bond in S-O(H).

The specific reaction pathway of chlorosulfuric acid depends on factors such as the co-reactant, solvent (if present), temperature, and the quantity of excess HSO3Cl.

In some cases, the initial step involves the formation of free chlorine atoms, chlorine ions, or protons. Prolonged heating or distillation, particularly under vacuum, can result in partial decomposition to

hydrogen chloride, chlorine, sulfur dioxide, sulfuryl chloride, pyrosulfuryl chloride, and sulfuric acid.

When chlorosulfuric acid is stored with an excess of sulfur trioxide, the following reaction occurs:

2 HSO3Cl + SO3 ⇌ H2SO4 + S2O5Cl2

In freshly prepared and cooled solutions, chloropyrosulfuric acids are formed through the following equilibrium reaction:

H(SO3)nCl + SO3 ⇌ H(SO3)n+1Cl

Here, n represents 1, 2, or 3.

Chloropyrosulfates are obtained by reacting alkali metal chlorides with sulfur trioxide in liquid sulfur dioxide at a temperature of -12°C.

Many reactions involving chlorosulfuric acid can be conducted at low temperatures, facilitated by its low melting point. Sulfonic acids are produced by reacting equimolar quantities of aromatic hydrocarbons under mild conditions, typically without affecting other substituents in the molecule.

However, in some cases, amino groups need to be protected through acylation to prevent them from reacting with chlorosulfuric acid.

Side reactions may yield small quantities of sulfonyl chlorides, which often undergo further reactions with aromatic hydrocarbons to form sulfones. These sulfonyl chlorides serve as intermediates in the production of pharmaceuticals or dyes.

By using an excess of chlorosulfuric acid under specific reaction conditions, sulfonyl chlorides, sulfones, or chlorinated aromatic hydrocarbons can be obtained. Sulfonyl chloride formation occurs in two steps: the formation of a sulfonic acid followed by an equilibrium reaction:

ArH + ClSO3H ⇌ ArSO3H + HCl

ArSO3H + ClSO3H ⇌ ArSO2Cl + H2SO4

This equilibrium can be shifted to the right by reacting the resulting sulfuric acid with pyrosulfuryl chloride or thionyl chloride. The reaction with pyrosulfuryl chloride generates chlorosulfuric acid and sulfur trioxide, while thionyl chloride reacts with the nascent sulfuric acid to produce chlorosulfuric acid, hydrogen chloride, and sulfur dioxide.

The reaction of chlorosulfuric acid with aliphatic hydrocarbons is slow, provided there are no double bonds or other reactive groups present. Consequently, long-chain saturated fatty alcohols react with chlorosulfuric acid to form the corresponding sulfates without chain cleavage or discoloration of the product. This type of reaction finds extensive use in the production of detergents:


Both chlorosulfuric acid and fluorosulfuric acid have been suggested as catalysts for polymerization processes.

3. Production of chlorosulfuric acid

All industrial methods of producing chlorosulfuric acid involve the reaction between hydrogen chloride and sulfur trioxide:

HCl + SO3 ⇌ HSO3Cl

Previously, processes utilized contact process gas containing 6-7% SO3. However, nowadays, it is more common to use pure sulfur trioxide. This change allows for easier handling of waste gases and enables a more compact plant design.

The production processes differ in terms of how the raw materials are brought into contact and how the heat generated during the reaction is removed.

One approach involves a packed column reactor with a spray of chlorosulfuric acid at the top, while hydrogen chloride and sulfur trioxide enter from the bottom. In this setup, the reaction and heat removal occur in a single piece of equipment.

Alternatively, the steps can be separated by intensively mixing the components in a separate mixer, such as a mixing nozzle. The hot reaction product is then rapidly cooled with cold chlorosulfuric acid in a packed column or another suitable unit.

Cooling can also be achieved using a water-cooled condenser. In a modified version of the process, the initial product is chlorosulfuric acid containing a small excess of sulfur trioxide. It is then cooled and saturated with hydrogen chloride in a bubble column. The resulting off-gas is typically scrubbed first with 98% sulfuric acid and then with water.

Several patents describe the recycling of recovered hydrogen chloride and sulfur trioxide. The heat generated during the formation of chlorosulfuric acid can be utilized to evaporate sulfur trioxide from low-percentage oleum. Subsequently, the sulfur trioxide reacts with hydrogen chloride under sub-atmospheric pressure.

Construction materials for reactors, containers, and other equipment should ideally have minimal or no iron content. Enamel, glass, aluminum, or steel lined with polytetrafluoroethylene (PTFE) are suitable choices.

Analyzing chlorosulfuric acid typically involves hydrolyzing the acid to form SO3 and HCl, followed by determining the total acidity and chloride content. When calculating the content of HSO3Cl, H2SO4, and free SO3 or HCl, the amount of water absorbed by SO3 to form H2SO4 must be taken into account.

Special methods are required to determine the presence of sulfuryl chloride and pyrosulfuryl chloride (SO2Cl2 and S2O5Cl2). Specific color reactions, such as a cherry red color with tellurium powder or a moss green color with selenium, can help identify chlorosulfuric acid with a melting point lower than expected.

Typically, a low iron content is desired in the product, and the following analysis is required:

  • HSO3Cl : 98 – 99.5 %
  • H2SO4 : 0.2 – 2 %
  • Free SO3 : 0 – 2 %
  • Free HCl : 0 – 0.5 %
  • Fe : 5 – 30 ppm

4. Safety Precautions, Transportation

Due to the highly aggressive nature of chlorosulfuric acid, it is important to adhere to regulations governing hazardous materials. It reacts violently with water and decomposes to hydrogen chloride and sulfuric acid mist in the presence of moist air.

All containers, vessels, pipes, and metallic hoses must be kept dry, and even minor leaks should be prevented. Containers should be handled with caution, avoiding any drops as internal pressure may build up.

Opening the stopper should be done only with a wrench and not with a hammer and chisel. Emptying containers should be done through siphoning rather than compressed air. To prevent the entry of moist air, tight connections to a gas bypass should be ensured. After each use, containers must be tightly closed.

Although chlorosulfuric acid itself is not flammable, it can cause the ignition of combustible materials. It can also produce hydrogen when reacting with moist metals. Naked flames should be strictly avoided near containers or pipes.

In cases where welding is necessary, safety precautions for handling hydrogen should be followed. Proper ventilation is essential in enclosed spaces where chlorosulfuric acid is being handled. It is necessary to wear acid-proof clothing, goggles, gloves, a full-face gas mask with an acid fume-absorbing filter, and safety boots.

Suitable materials for gloves, protective clothing, and gas masks can include polychloroprene coated with a copolymer of vinylidene fluoride and hexafluoropropylene, while boots can be made of poly(vinyl chloride). Contaminated clothing should be changed promptly.

In the event of small spills, chlorosulfuric acid should be washed away with water while observing necessary safety precautions. The spilled material should be downwind, and the area should be cordoned off.

High-velocity water jets should be avoided, and instead, the spillage should be worked from the outside toward the middle using a hose with a spray nozzle attachment. The area can be neutralized with substances like sodium carbonate, sodium hydrogen carbonate, or lime.

Preventing chlorosulfuric acid from leaking into the sewerage system or soil is crucial. Spilled acid can be covered with paraffin oil or FEP film (perfluorinated ethylene-propylene copolymer). The contaminated area can be identified by fog formation, which can be contained by water shielding. Personnel involved in cleaning operations must wear full protective clothing and use compressed air breathing apparatus.

Transportation regulations differ depending on the mode of transport (road, rail, or sea). When feasible, rail transportation is recommended for chlorosulfuric acid. Tanks for rail or road trucks are typically constructed from stainless steel, although enameled steel may also be utilized.

5. Uses of chlorosulfuric acid

The breakdown of chlorosulfuric acid production based on end use is as follows:

1. Detergents: 40%

Chlorosulfuric acid is widely used in the production of detergents. It is involved in the manufacturing of sulfates used as surfactants in various cleaning products.

2. Pharmaceuticals: 20%

Chlorosulfuric acid finds applications in the pharmaceutical industry for the synthesis of intermediate compounds and active pharmaceutical ingredients (APIs).

3. Dyes: 15%

The dye industry utilizes chlorosulfuric acid in the production of dyes and pigments. It is involved in the sulfonation process, which introduces sulfonic acid groups into organic compounds to enhance their color properties.

4. Crop Protection: 10%

Chlorosulfuric acid plays a role in the production of certain crop protection chemicals, including pesticides and herbicides.

5. Ion-Exchange Resins, Plasticizers, and Others: 15%

Chlorosulfuric acid has various other applications, such as the production of ion-exchange resins used in water treatment, the synthesis of plasticizers for polymers, and in other industrial processes.

It’s important to note that these percentages are approximate and may vary based on specific market conditions and regional demand.

6. Toxicology and Occupational Hygiene

Chlorosulfuric acid poses significant safety risks as it is a strong acid and dehydrating agent, capable of causing severe burns to the skin. When exposed to moist air, it produces an irritating acid fog that can strongly irritate the respiratory tract and eyes. It is important to avoid any contact with the skin or inhalation of the vapor.

In the event of accidental contact with the skin or eyes, immediate washing with large amounts of water is essential, followed by seeking medical assistance promptly. It is important to note that no Maximum Allowable Concentration (MAK) or Threshold Limit Value (TLV) has been established specifically for chlorosulfuric acid (HSO3Cl). However, the established values for its primary decomposition products are as follows:

– Sulfuric Acid (H2SO4): MAK and TLV of 1 mg/m3 (as of 1995)
– Hydrogen Chloride (HCl): MAK and TLV of 7 mg/m3 (as of 1995)

These values serve as guidelines for exposure limits and highlight the potential health risks associated with the decomposition products of chlorosulfuric acid.


FAQ about Chlorosulfuric acid

Chlorosulfuric acid, also known as chlorosulfonic acid, is a highly reactive and corrosive liquid. It is used in various industrial processes such as the production of detergents, pharmaceuticals, dyes, and crop protection chemicals.

Chlorosulfonic acid is a strong acid. Its corrosive nature and ability to release hydrogen chloride and sulfuric acid make it highly reactive.

When handling Chlorosulfuric acid, it is crucial to wear acid-proof clothing, goggles, gloves, and safety boots. Avoid contact with the skin, eyes, and inhalation of vapors. Containers should be handled with care, and any spills must be washed away with water while following safety measures. It is important to ensure proper ventilation and avoid exposure to naked flames or combustible materials.

Chlorosulfuric acid should be stored in dry containers that are tightly closed. It is important to prevent the access of moist air to avoid decomposition. Containers should be stored in a well-ventilated area, away from heat sources and incompatible substances.

In case of skin or eye contact with Chlorosulfuric acid, the affected area should be immediately washed with large amounts of water or dilluted sodium bicarbonate solution. Seek medical assistance promptly after washing the affected area.

Chlorosulfuric acid is highly corrosive and can cause severe burns to the skin. It releases irritating fumes in moist air, which can harm the respiratory tract and eyes. It is a strong acid and dehydrating agent, and precautions should be taken to avoid contact or inhalation.

Chlorosulfuric acid is used in various industries. It is primarily employed in the production of detergents, pharmaceuticals, dyes, crop protection chemicals, ion-exchange resins, and plasticizers.

Chlorosulfonic acid is typically synthesized by the reaction of hydrogen chloride with sulfur trioxide. Earlier processes used contact process gas containing sulfur trioxide, while modern methods often employ pure sulfur trioxide for improved efficiency and plant design.

Chlorosulfonic acid is typically produced by reacting hydrogen chloride with sulfur trioxide. The reaction can take place in a packed column reactor with chlorosulfuric acid spray, or by separately mixing the components in a separate mixer and then cooling the reaction product with cold chlorosulfuric acid or a water-cooled condenser.