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Substance data selection for acids

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hannaholmquist's picture
Substance data selection for acids


in USEtox 2 pKa values are included in the substance data, indciating that the model now takes into account the two species generated for dissociating substances in water. But, remaining substance data is still only one row, i.e. one value per parameter such as vapour pressure etc. Acids may however behave rather differently if they are in their protonated or disscociated form. Take perfluorooctanoic acid (PFOA) for example for which the acid (PFOA) has some vapour pressure while the PFO- anion has negigible vapour pressure. Under environmental conditions PFOA is almost completely disscoiated, which means that we are mostly interested in the further fate of PFO- (it is the PFO moiety that is causing chronic toxicity).

Should then substance data for PFO- rather than PFOA be added in USEtox? The inventory data does not allow for differentiation between PFOA and PFO-.

As far as I understand the draft documentation the fate calculations focus on the original species and the dissociated species is considered removed from the system?


Thanks in advance!


USEtox Team
USEtox Team's picture
pKa data in USEtox

The two pKa values required in USEtox 2 are to indicate that the model takes three species into account: the (uncharged) ‘original’ species, the protonated (cationic) species, and the dissociated (anionic) species.

Generally, only two of these chemical species (most often one only!) are present in significant amounts in the environment. The dominant species are lumped into two fractions, named original (in USEtox 2, sheet "fate": ".orig") and alternate (in USEtox 2, sheet "fate": ".alt"). Different physicochemical properties and different intermedia partition coefficients are assigned to these two fractions (‘.orig’ and ‘.alt’). Partitioning of the ‘total’ substance is modeled as the mass-weighted average of the partitioning of the two dominant fractions.

The essence of the new USEtox 2 calculation is that the model can calculate the fractions of a substance in its ‘.orig’ and ‘.alt’ forms (at ambient pH), and can calculate Kp and Kh values for the mixture, based given properties of the ‘.orig’ form and model-derived properties of the ‘.alt’ form.

All in all, for each new substance USEtox needs as input into a single row per substance (i) properties of the original, unionized form (‘.orig’), (ii) the two pKa values (of ‘gain’ and ‘loss’ reactions), and (iii) the pH at which partition coefficients are needed in the mass balance calculation. Example: to do the mass balance calculation for PFOA (acid only) in fresh water, it needs the pKa of the proton loss reaction, the pH of natural water (given in USEtox), plus vapor pressure, water solubility and octanol-water partition coefficient of the original, undissociated form of PFOA. USEtox then ‘knows’ how to derive the fraction of original species in water at the corresponding pH, and ‘knows’ how to assemble the Kp of the mixture of original and alternate forms.