155
c
Trifuoroacetic Acid from CFC Replacements
TABLE 9.1
pK a of the Fluorinated Acetic Acid Compounds
Chemical
pK a
Acetic acid
4.80 a , 4.76 b
Fluoroacetic acid
2.66 a , 2.59 b
Difuoroacetic acid
1.24 a
TFA
0.23 a , 0.52 b , 0.1 to 1.0 (modeled) c
0.19 d
a Data summarized in Richard and Hunter (1996).
b Data summarized in Lide (2003).
Reference Namazian et al. (2008).
d Calculated from dissociation constant at 25°C presented in Milne and Parker (1981).
subsequently removed by wet deposition. Partitioning into aerosols was a
more complex issue that was highly dependent on aerosol pH, where TFA
would not signifcantly partition into highly acidic aerosols (Bowden et al.,
1996). More recent measurements of the Henry’s law constant gave a similar
value of 5,800 ± 100 mol/dm 3 /atm at 298 K (Kutsuna and Hori, 2008), which
confrmed the propensity of TFA to partition out of the gas phase into the
liquid phases. However, feld measurements that divided the atmospheric
TFA into gas-phase TFA and aerosol-phase TFA have repeatedly shown
higher concentrations in the gas phase (Guo et al., 2017; Martin et al., 2003;
Wu et al., 2014). Guo et al. (2017) showed that adsorption of TFA to particles
was energetically favored but appears to be limited by diffusion processes.
The partitioning of TFA to aerosols is a mechanism by which dry deposition of TFA can occur and this deposition may be as important as wet deposition (Martin et al., 2003).
The strong propensity of TFA to be an ion has implications for its behavior
in terrestrial and aquatic systems. In general, terrestrial and aquatic systems
have more moderate pH conditions (pH 5–8), so TFA will effectively be present as an ion. At pH 7.0, there would be approximately 3 million molecules in
the ionic form for each molecule in the acid form. As an ion, TFA is effectively
nonvolatile; therefore, it does not revolatilize to the atmosphere. TFA’s high
water solubility differentiates it from the longer chain perfuorinated acids,
such as PFOS and PFOA. Once TFA is dissolved in water, it will predominately stay in the aqueous phase and be moved by the water to the oceans.
The one notable exception to this is TFA inputs into terminal waterbodies,
which are lakes, playas, and wetlands, that lack an outfow. In these cases, the
ultimate fate of the water in this scenario is to be lost by evaporation while
the TFA ion is left behind. This could cause the concentrations of TFA to
increase over time in these systems as rain, dry deposition, and surface water
inputs continually add more TFA to the system that cannot be lost by volatilization or degradation (Tromp et al., 1995). In many ways, TFA behaves in a
similar fashion as some small inorganic ions such as chloride and bromide; it
c
Trifuoroacetic Acid from CFC Replacements
TABLE 9.1
pK a of the Fluorinated Acetic Acid Compounds
Chemical
pK a
Acetic acid
4.80 a , 4.76 b
Fluoroacetic acid
2.66 a , 2.59 b
Difuoroacetic acid
1.24 a
TFA
0.23 a , 0.52 b , 0.1 to 1.0 (modeled) c
0.19 d
a Data summarized in Richard and Hunter (1996).
b Data summarized in Lide (2003).
Reference Namazian et al. (2008).
d Calculated from dissociation constant at 25°C presented in Milne and Parker (1981).
subsequently removed by wet deposition. Partitioning into aerosols was a
more complex issue that was highly dependent on aerosol pH, where TFA
would not signifcantly partition into highly acidic aerosols (Bowden et al.,
1996). More recent measurements of the Henry’s law constant gave a similar
value of 5,800 ± 100 mol/dm 3 /atm at 298 K (Kutsuna and Hori, 2008), which
confrmed the propensity of TFA to partition out of the gas phase into the
liquid phases. However, feld measurements that divided the atmospheric
TFA into gas-phase TFA and aerosol-phase TFA have repeatedly shown
higher concentrations in the gas phase (Guo et al., 2017; Martin et al., 2003;
Wu et al., 2014). Guo et al. (2017) showed that adsorption of TFA to particles
was energetically favored but appears to be limited by diffusion processes.
The partitioning of TFA to aerosols is a mechanism by which dry deposition of TFA can occur and this deposition may be as important as wet deposition (Martin et al., 2003).
The strong propensity of TFA to be an ion has implications for its behavior
in terrestrial and aquatic systems. In general, terrestrial and aquatic systems
have more moderate pH conditions (pH 5–8), so TFA will effectively be present as an ion. At pH 7.0, there would be approximately 3 million molecules in
the ionic form for each molecule in the acid form. As an ion, TFA is effectively
nonvolatile; therefore, it does not revolatilize to the atmosphere. TFA’s high
water solubility differentiates it from the longer chain perfuorinated acids,
such as PFOS and PFOA. Once TFA is dissolved in water, it will predominately stay in the aqueous phase and be moved by the water to the oceans.
The one notable exception to this is TFA inputs into terminal waterbodies,
which are lakes, playas, and wetlands, that lack an outfow. In these cases, the
ultimate fate of the water in this scenario is to be lost by evaporation while
the TFA ion is left behind. This could cause the concentrations of TFA to
increase over time in these systems as rain, dry deposition, and surface water
inputs continually add more TFA to the system that cannot be lost by volatilization or degradation (Tromp et al., 1995). In many ways, TFA behaves in a
similar fashion as some small inorganic ions such as chloride and bromide; it