Fluorosulphuric acid

Hydrolysis

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

hydrolysis

Type of information:

other: expert statement

Adequacy of study:

other information

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Reason / purpose for cross-reference:

reference to other study

Reason / purpose for cross-reference:

reference to other study

Reason / purpose for cross-reference:

reference to other study

Qualifier:

no guideline required

Guideline:

other: expert statement (collection of handbook data)

Transformation products:

yes

No.:

#1

No.:

#2

Physico-chemical properties

Fluorosulfuric acid, also known by the alternative name fluorosulfonic acid, is a tetrahedral molecule, with the molecular formula HSO₃F. Fluorosulfuric acid is considered completely miscible in water due to the known water solubilities of its hydrolysis products sulfuric acid (1 000 g/L) and hydrogen fluoride (no water solubility value but completely miscible too). No quantifiable value can be determined for fluorosulfuric acid since conducting a study is technically not feasible because of its violent reaction with water.

Hydrolytic behaviour

According to all available information provided in the registration dossier and other references, fluorosulfuric acid reacts violently with water to generate hydrogen fluoride (HF) and sulfuric acid (H₂SO₄). The extent of this initial fast hydrolysis depends on temperature, concentration (acid/water ratio) and how the water is added. No sufficient information is available to further quantified or calculate this first transformation process. In addition to the fast hydrolysis, the self-ionisation of fluorosulfuric acid also occurs. The hydrolysis subsequent to the initial fast reaction is slow presumably because part of the acid is converted to fluorosulfate ions (SO₃F^-) according to the ionisation reaction, which hydrolyse slowly even at elevated temperatures. A summary of the above transformation processes are presented below:

 

Fast hydrolysis         HSO₃F + H₂O <=> H₂SO₄ + HF

Ionization                 HSO₃F +  H₂O <=> H₃O⁺ + SO₃F^-

Slow hydrolysis       SO₃F^- + H₂O <=> HSO₄^-+ HF

 

No further information was provided in the literature on the hydrolytic behaviour and measured rate of hydrolysis of fluorosulfuric acid. However, some literature is available on a precursor of fluorosulfuric acid: Sulfuryl fluoride.Regarding the abiotic degradation/transformation processes of the substance in the environment, hydrolysis is the most important process.

ANALOGUE APPROACH, SULFURYL FLUORIDE

Sulfuryl fluoride was selected as an analogue substance to determine the hydrolytic behaviour of fluorosulfuric acid. Both substances are similar inorganic substances. Fluorosulfuric acid is formed in water via nucleophilic attack on the S atom of sulfuryl fluoride, with displacement of an F ion. Relevant literature data are available on the hydrolytic behaviour of sulfuryl fluoride and are useful to strengthen the hydrolysis endpoint of fluorosulfuric acid. Both substances share the same transformation products, as presented in the next sections.

 

Physico-chemical properties

Sulfuryl fluoride (CAS No. 2699-79-8; EC No. 220-281-5), also known by the alternative name sulfonyl fluoride, is a tetrahedral molecule, with the molecular formula SO₂F₂. Sulfuryl fluoride is a colorless, odorless gas, relatively inert in its gaseous form, stable to light, and stable to 400 °C when dry. When heated to decomposition, very toxic hydrogen fluoride and sulfur dioxide fumes are emitted. Sulfuryl fluoride is soluble in water and soluble in common organic solvents such as ethanol, toluene, and carbon tetrachloride.

 

Fate and behaviour in water

Hydrolytic degradation in surface waters is expected to be the most important loss process of sulfuryl fluoride in the aquatic compartment. The substance hydrolyses slowly in water under neutral conditions, and more rapidly under basic conditions. Sulfuryl fluoride is first hydrolysed to fluorosulfate (SO₃F^-), with release of a fluoride ion, followed by further hydrolysis to sulphate and release of the remaining fluoride ion. The half-live values (DT₅₀) measured in aqueous solutions are reported below, confirm that the rate of hydrolysis of sulfuryl fluoride is very rapid in alkaline/basic solutions but slower in neutral and acidic solutions:

 

-       At pH 2, 25°C: DT₅₀= 5.3 days;

-       At pH 7, 25°C: DT₅₀= 4.6 hours;

-       At pH 7, 12°C: DT₅₀= 12.7 hours;

-       At pH 8.07 (temperature unknown): DT₅₀< 18 minutes:

-       At pH 9, 25°C: DT₅₀= 2.8 minutes;

-       At pH 9.2 (temperature unknow): DT₅₀= 3.3 minutes.

 

Another source mention that incubation of sulfuryl fluoride in phosphate buffered saline (pH 7.4) for just 2 hours resulted in considerable hydrolysis (measured by HPLC) while fluorosulfate, the relevant transformation product, was completely intact after 24 hours.

 

Fluorosulfate, species of interest

Both fluorosulfuric acid and sulfuryl fluoride are rapidly hydrolysed in aqueous solution and/or ionised into a common hydrolysis product, fluorosulfate (SO₃F^-). The fluorosulfate ion has previously been reported to decompose rapidly in strong mineral acid, and much more slowly in bases. The stoichiometries of the respective reactions are presented below.

 

SO₃F^-+ H₂O --> HSO₄^-+ HF

SO₃F^-+ 2 OH^- --> SO₄^2-+ H₂O + F

Kinetics of decomposition in acid and basic aqueous solutions are publicly available. As pseudo first-order rate constants were measured at varying hydrochloric acid (HCl) and sodium hydroxide (NaOH) concentrations, half-lives values were calculated and are fully reported in the Appendix 1. In summary, the ionic catalysis is relatively fast in extreme acid and basic aqueous conditions (t1/2 at 50 minutes and 2.6 hours, for 4M HCl and 5M NaOH aqueous solutions, respectively), while slower in less extreme conditions (t1/2 at 15.4 hours and 16.3 days, for 0.5M HCl and 0.25M NaOH aqueous solutions, respectively). 

Executive summary:

Fluorosulfuric acid is known to react violently with water and to hydrolyse rapidly to its hydrolysis products sulfuric acid and hydrogen fluoride. The extent of this initial fast hydrolysis depends on temperature, concentration (acid/water ratio) and how the water is added. The ionising potential of fluorosulfuric acid was estimated as well as its potential to volatilise to air from liquid phases. However, no direct quantitative experimental information was available on the hydrolysis of the substance other than its rapid and violent reaction when in contact with water. However, information was available on the structurally analogue substance sulfuryl fluoride, which quickly forms fluorosulfate under aqueous conditions. The same hydrolysis product (fluorosulfate) is formed from fluorosulfuric acid during primary abiotic degradation due to its ionising properties. After the transformation to fluorosulfate, the hydrolysis pathway of sulfuryl fluoride and fluorosulfuric acid are identical. Fluorosulfate, will also be quickly hydrolysed, dependent on the environmental pH. The end hydrolysis products will be hydrogen sulfate (HSO₄^-) and hydrogen fluoride (HF). The hydrolytic behaviour of fluorosulfuric acid can be concluded based on the above approach and therefore further assessments is not required. The half-life of fluorosulfuric acid is expected to be between 50 minutes and 16.3 days dependent on pH and temperature. This assumption is considered as a worst case as it does not take into account the first and very fast hydrolysis process of fluorosulfuric acid into sulfuric acid and hydrogen fluoride, in addition to the self-ionisation process.

Reference 2

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

other information

Reliability:

4 (not assignable)

Rationale for reliability incl. deficiencies:

secondary literature

Principles of method if other than guideline:

data from handbook

GLP compliance:

not specified

Radiolabelling:

not specified

The reaction of fluorosulfuric acid and water is violent and exothermic; it proceeds as follows:

Fast hydrolysis         HSO₃F + H₂O <=> H₂SO₄ + HF

Ionization                 HSO₃F +  H₂O <=> H₃O⁺ + SO₃F^-

Slow hydrolysis       SO₃F^- + H₂O <=> HSO₄^-+ HF

The extend of the initial hydrolysis depends on temperature and how the water is added. Hydrolyses is reduced at slower addition rates and lower temperatures.

The hydrolysis subsequent to the initial fast reaction is slow presumably because part of the acid is converted to fluorosulfate ions which hydrolyze slowly even at elevated temperatures.

Reference 3

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

other information

Reliability:

4 (not assignable)

Rationale for reliability incl. deficiencies:

secondary literature

Principles of method if other than guideline:

data from handbook

GLP compliance:

not specified

Radiolabelling:

not specified

Violent reaction with water although it is incompletely and reversibly hydrolyzed.

Reference 4

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

other information

Reliability:

4 (not assignable)

Rationale for reliability incl. deficiencies:

secondary literature

Principles of method if other than guideline:

data from handbook

GLP compliance:

not specified

Radiolabelling:

not specified

The halogen is more firmly bonded than that of chlorosulfuric acid so that fluorosulfuric acid is hydrolyzed more slowly than chlorosulfuric acid.

Description of key information

Fluorosulfuric acid is known to react violently with water and to hydrolyse rapidly to its hydrolysis products sulfuric acid and hydrogen fluoride. The extent of this initial fast hydrolysis depends on temperature, concentration (acid/water ratio) and how the water is added.The ionising potential of fluorosulfuric acid was estimated as well as its potential to volatilise to air from liquid phases. However, no direct quantitative experimental information was available on the hydrolysis of the substance other than its rapid and violent reaction when in contact with water. However, information was available on the structurally analogue substance sulfuryl fluoride, which quickly forms fluorosulfate under aqueous conditions. The same hydrolysis product (fluorosulfate) is formed from fluorosulfuric acid during primary abiotic degradation due to its ionising properties. After the transformation to fluorosulfate, the hydrolysis pathway of sulfuryl fluoride and fluorosulfuric acid are identical. Fluorosulfate, will also be quickly hydrolysed, dependent on the environmental pH. The end hydrolysis products will be hydrogen sulfate (HSO₄^-) and hydrogen fluoride (HF).The hydrolytic behaviour of fluorosulfuric acid can be concluded based on the above approach and therefore further assessments is not required.The half-life of fluorosulfuric acid is expected to be between 50 minutes and 16.3 days dependent on pH and temperature. This assumption is considered as a worst case as it does not take into account the first and very fast hydrolysis process of fluorosulfuric acid into sulfuric acid and hydrogen fluoride, in addition to the self-ionisation process.