User Guide to Thresholds and Classification - Environmental ...

375User Guide for Thresholds and ClassificationsLog n-octanol-water partition coefficientDefinition and general considerationsThe log K OW is a measure of the lipophilicity of a substance. As such, log K OW is a key parameter in theassessment of environmental fate. Many distribution processes are driven by log K OW , for example, sorptionto soil and sediment and bioconcentration in organisms.The basis for the relationship between bioconcentration and log K OW is the analogy for the partition processbetween the lipid phase of fish and water and the partition process between n-octanol and water. The reasonfor using KOW arises from the ability of octanol to act as a satisfactory surrogate for lipids in fish tissue.Highly significant relationships between log KOW and the solubility of substances in cod liver oil and triolinexist (Niimi, 1991). Triolin is one of the most abundant triacylglycerols found in freshwater fish lipids(Henderson and Tocher, 1987).The determination of the n-octanol-water partition coefficient (KOW) is a requirement of the base data set tobe submitted for notified new and priority existing substances within the European Union (EU). As theexperimental determination of the KOW is not always possible, for example, for very water-soluble and forvery lipophilic substances, a Quantitative Structure Activity Relationship (QSAR) derived KOW may be used.However, extreme caution should be exercised when using QSARs for substances where the experimentaldetermination is not possible (as for, for example, surfactants).Appropriate methods for experimental determination of KOW valuesFor experimental determination of KOW values, two different methods, Shake-flask and High PerformanceLiquid Chromatography (HPLC), have been described in standard guidelines for example, OECD 107 andOECD 117. For highly lipophilic substances, which are slowly soluble in water, data obtained by employing aslow-stirring method are generally more reliable (De Bruijn et al, 1989; Tolls and Sijm, 1993; OECD TestGuideline 123, 2006).Shake-flask methodThe basic principle of the method is to measure the dissolution of the substance in two different phases,water and n-octanol. In order to determine the partition coefficient, equilibrium between all interactingcomponents of the system must be achieved after which the concentration of the substances dissolved in thetwo phases is determined. The shake-flask method is applicable when the log KOW value falls within therange from -2 to 4 (OECD 107, 1995). The shake-flask method applies only to essential pure substancessoluble in water and n-octanol and should be performed at a constant temperature (±1ºC) in the range 20–25ºC.High Performance Liquid Chromatography methodHPLC is performed on analytical columns packed with a commercially available solid phase containing longhydrocarbon chains (for example, C8, C18) chemically bound onto silica. Chemicals injected onto such acolumn move along at different rates because of the different degrees of partitioning between the mobileaqueous phase and the stationary hydrocarbon phase. The HPLC method is not applicable to strong acidsJanuary 2012 EPA0109