Trihydrogen trifluoro[phosphato(3-)-O]borate(3-)

Hydrolysis

Description of key information

The complex composed of boron trifluoride and phosphoric acid is extremely water sensitive and is even reacting with moist air. Due to the fastness of this reaction a study on hydrolysis according to OECD guideline 111 was technically not feasible. Preliminary results on degradates are available.

Additional information

Similar to all coordination complexes of boron trifluoride with organic and inorganic species (like alcohols, ethers, amines, sulfuric acid, sulfuric dioxide, etc) the complex of boron trifluoride and phosphoric acid is extremely water sensitive and reacts even with moist air. In the instantaneous reaction with water as a first step phosphoric acid and boron trifluoride dihydrates are formed. The latter undergoes further rapid hydrolysis to boric acid, fluoboric acid and tetrafluoroborate. Supporting information is available for the hydrolysis of the degradation products.

A study of the stable dihydrated form of boron trifluoride was conducted and assessment of the hydrolysis rate was made by monitoring degradate concentrations. Measurement of fluoride ion production over a range of pH values (1.2 to 9), using both ion chromatography and an ion-selective electrode, indicated a hydrolytic half-life time of less than 30 minutes for boron trifluoride. Subsequent analysis of boric acid by titration confirmed the rapidity of the reaction.

Further hydrolysis testing was performed with the degradation products, tetrafluoroborate and boric acid. With regards to boric acid, the preliminary study showed that at each of pH 1.2, 4, 7 and 9 and 50±0.5ºC, less than 10 % hydrolysis had occurred after 5 days, equivalent to a half-life of greater than 1 year under environmental conditions (25°C). No further testing was considered necessary. Boric acid was determined to be hydrolytically stable under acidic, neutral and basic conditions.

Preliminary testing (at 50°C) has been conducted and indicates that tetrafluoroborate is unstable over the range of pH values of environmental significance. The following results were obtained:

The marked change in pH is thought partly to be due to production of strongly acidic hydrofluoric acid (HF) on hydrolysis, but is also considered to be a result of the lack of buffering capacity of the aqueous matrices employed. Conventional buffer systems resulted in interferences under the ion chromatographic conditions employed and, therefore, nominal test pH values were achieved using mixtures of glacial acetic acid and sodium hydroxide solutions, neither of which produced significant interference.

The peak at approximately 7.5 minutes, consistent with the retention time of boric acid, did not grow significantly in size over the duration of the test, whilst the broad peak between approximately 5 and 7 minutes (thought to be due to intermediate fluoroborate species, BF3OH-, BF2(OH)2- and BF(OH)3- resulting from stepwise removal of HF from tetrafluoroborate) did increase in magnitude at each time point as the tetrafluoroborate peak (4.5 minutes) diminished. The peak at approximately 10 minutes is not related to the test substance (it was attributable to the acetate ion).

It was noted that, although the final tetrafluoroborate concentration (and chromatographic profile) after 5 days was similar at each test pH, initial reaction rate at pH 1.2 was significantly faster. The results indicated half-lives for tetrafluoroborate at 50°C in the region of 2.4 hours at pH 1.2 and approximately 3 days at the other pH values investigated.

To conclude, it is apparent that tetrafluoroborate hydrolyses under environmental conditions, forming boric acid and, predominantly, partially hydrolysed fluoroborate species.