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APPENDIX XI
Appendix XI Environmental risk assessment for ionising substances
Introduction
The degree of ionisation of an organic acid or base greatly affects both the fate and the toxicity
of the compound. The water solubility, the adsorption and bioconcentration, as well as the
toxicity of the ionised form of a substance may be markedly different from the corresponding
neutral molecule.
When the dissociation constant (pKa/pKb) of a substance is known, the percentage of the
dissociated and the neutral form of the compound can be determined. For example, for an acid
with a pKa of 5.5, the pH dependency of the behaviour of the substance can be described as
follows:
• 1% dissociated at pH 3.5;
• 10% dissociated at pH 4.5;
• 50% dissociated at pH 5.5;
• 90% dissociated at pH 6.5;
• 99% dissociated at pH 7.5.
Thus, even slight changes in the pH of the environment considerably affect the form in which the
substance is present in the environment. This is the case especially for substances with pKa/pKb
values around the pH values of the environment (i.e. pH 4-9 for surface water). In the assessment
of ionised substances, due attention has to be paid as to how much fate and effects of the
substance are affected by the pH of the environment.
Exposure assessment
The water solubility of organic acids and bases are very much dependent on the pH. The water
solubility of the dissociated compound can be orders of magnitude higher than the neutral
species. Therefore, the pH dependence of the water solubility should be known. At least the pH
of the test water needs to be identified. This also applies to log Kow.
The basic parameters used in the exposure assessment (log Kow, Henry's law constant,
adsorption/desorption coefficients) are only applicable to the non-ionised form of the substance.
Therefore, every time when partitioning of a substance between water and air or solids is
concerned, a correction needs to be made in order to take only the undissociated fraction of the
compound into account at a given pH. In practice, this implies that Henry's law constant and Kp
in soil, sediment, and suspended solids need to be corrected. This can be done by using the
following correction factor:
320
CORR =
where:
A 1 for acids, -1 for bases
pH pH-value of the environment
pKa acid/base dissociation constant
1
A 1+
10
(pH - pKa)
The above correction can only be used for partitioning coefficients which refer to the unionised
form of the substance. This means that for estimated partitioning coefficients, water solubility