Reaction mass of potassium trifluoroacetate and potassium trifluoromethanesulphinate

Dissociation constant

Link to relevant study record(s)

Reference

Endpoint:

dissociation constant

Type of information:

experimental study

Adequacy of study:

key study

Study period:

From 12 Oct 2012 to 07 Dec 2012

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Remarks:

Not GLP study but following the standard recognized guidelines

Reason / purpose for cross-reference:

reference to same study

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 112 (Dissociation Constants in Water)

Deviations:

no

GLP compliance:

no

Dissociating properties:

no

pKa:

< 2

Temp.:

20 °C

All the concentrations concerning the test item are expressed as potassium trifluorosulfinate equivalent which means that:

the10 mMsolution is about 1mM in TFSK

the100 mMsolution is about10 mMin TFSK

 

First measurements

A volume of 10 mL of the100 mMtest item solution (initial pH about 3.4) was titrated with a10 mM NaOH solution. The addition of 1 mL of the NaOH solution brings the pH to 10.3 (see Figure3 inappendix 4). Any pKa can be observed for this solution.

 

A volume of 10 mL of the500 mMtest item solution (initial pH about 2.9) was titrated with a10 mM NaOH solution. The addition of 6 mL of the NaOH solution brings the pH to 11 (see Figure3 inappendix 4). Any pKa can be observed for this solution. (see Figure4 inappendix 4)

 

As any pKa have been observed from pH 3 to 11, this mean that pKa < 3

 

Second measurements

A volume of 10 mL of the100 mMtest item solution (initial pH about 3.4) was titrated with a10 mM HCl solution. The addition of the HCl solution decreases regularly the pH. After an addition of 50 mL of the HCl solution the pH was equal to about 2. (see Figure5 inappendix 4)

 

Any pKa have been observed from pH 2 to 3, this mean that pKa < 2

Discussion

The test item contains at least two substances, potassium trifluoromethanesulfinate and potassium trifluoroacetate. Trifluoroacetic acid is a strong acid (pKa is near 0.2). No value for trifluoromethanesulfinic acid could be found in the literature. By structural analogy with trifluoromethanesulfonic acid (triflic acid), a super acid (pKa is -14.9), it can be considered that trifluoromethanesulfinic acid is also a strong acid, perhaps even a super acid. In this case, the pKa can not be determined by classical titration.

 

A calculation for different concentrations of the test item with 0.01 M solutions of NaOH have been done to validate the assumption.

pKa of Trifluoroacetic acid was taken equal to 0.2, pKa of trifluoromethanesulfinic acid was taken equal to -14.9.

 

The modelled curves are consistent with the experimental curve. A difference can be observed, mainly due to the assumption on the trifluoromethanesulfinic acid.

 

Conclusion: the test item pKa is low (pKa <2). The test item is then a strong acid.

The reason come probably from the composition of the test item (mixture of at least two substances, TFAK and TFSK) and from the low basicity of the complementary bases of the acid form (TFAH and TFSH which are strong or super acids).

Conclusions:

Using the titration method, no pKa for the test item can be measured.
Based on the experimental and on the modelling work, it can be concluded that the pKa of the test item is < 2.

Executive summary:

In this study (2012), the determination of the dissociation constant of the reaction mass of potassium trifluoromethanesulphinate (TFSK) and potassium trifluoroacetate (TFAK) in aqueous solution (approximately 36%) test item was performed according to the guideline OECD 112. Using the titration method, no pKa for the test item can be measured. Based on the experimental and the modelling work, it can be concluded that the pKa of the test item is < 2.

Description of key information

The pKa of the reaction mass of TFSK/TFAK (aqueous solution, approx. 36%) is less than 2 based on experimental study.

Additional information

In a reliable study, the determination of the dissociation constant of the reaction mass of potassium trifluoromethanesulphinate (TFSK) and potassium trifluoroacetate (TFAK) in aqueous solution (approximately 36%) was performed according to the guideline OECD 112. Using the titration method, no pKa for the test item could be measured. Based on the experimental and the modelling work, it can be concluded that the pKa of the test item is < 2.