Chapter 2 provides an overview of various analytical methods to determine partitioning property data required for an ERA. Comments are provided for each of the methods regarding their applicability of each of the test protocols towards ionisable organic compounds. Specifically, it is demonstrated that a number of test methods developed for KOW tend to be inappropriate for ionisable organic compounds. Given the relative importance of the use of KOW within the risk assessment framework (i.e. as a trigger value for persistence and bioconcentration testing, and for assessing bioavailability), having methods that can reliably and reproducibly measure physical-chemical properties such as KOW is of paramount importance.
Faced with the challenge of how to prioritise large datasets of chemicals, many of which will have limited measured physical-chemical property data, many regulatory bodies utilise a suite of physical-chemical property estimation methods, or quantitative structure activity relationships (QSARs) (Cronin et al, 2003; ECETOC, 2003a). In an earlier ECETOC task force report (ECETOC, 2003b), which reviewed a number of QSARs, it was suggested that the majority of QSARs used for physical-chemical property estimations generally provide reliable estimations. Issues surrounding ionisable organic compounds, however, were raised, particularly with respect to estimations of KOC (ECETOC, 2003). Consequently, the ERA of ionisable organic compounds is likely to be complicated due to the pronounced influence of ionic interactions, which are poorly accounted for within standard tests or property estimation methods (ECETOC, 2003a). The importance for improving the need to address ‘difficult’ substances, such as ionisable organic compounds is highlighted in a recent study by Franco et al (2010), who analysed a subset of the 117 000 pre-registered substances published on the ECHA website. The subset of chemicals defined contained 1510 chemicals, 49% of which are described as being partly or totally ionisable organic compounds (27% as acids; 14% as bases; and 8% as zwitterions) (Franco et al, 2010). Consequently, analogous to the emphasis of chapter 2, there is a need to examine the use and applicability of well-established physical-chemical property estimation methods used within the regulatory framework for ionisable organic compounds.