Page 28 - Volume 3, Issue 2, July 2010

© Benaki Phytopathological Institute
Vlastaras et al. 58
(a) Polarity
It is usually reported as a logarithm, most
commonly as log Kow or log Pow. It is de-
fined as the ratio of the equilibrium concen-
trations of the two-phase system consisting
of water and n-octanol. Polarity refers to the
extent to which charge is unevenly distrib-
uted within the molecule and to the occur-
rence of polar functional groups in it. This
parameter is characteristic of the liphophili-
ty of the molecule and gives an indication of
the compound’s tendency to accumulate in
biological membranes and living organisms.
It’s determination gives data required for
the registration of new organic chemicals.
It is generally considered that substances
with a log Kow value higher than 3 can show
accumulation. This risk is measured experi-
mentally by the bioconcentration factors in
aquatic organisms, and some correlations
have been found between these two param-
eters for very hydrophobic pesticides, such
as the organochlorines. Some persistent or-
ganochlorines withdrawn from the market
were all characterised by log Kow > 4 (1).
As a rough rule, non-polar analytes are
characterised by log Kow values above 4-5,
whereas polar analytes have log Kow values
below 1 or 1.5. Between these two values,
compounds are classified as moderately po-
lar (Figure 1).
(b) Water solubility
It is a fundamental, chemical-specific
property defined as the concentration of a
chemical dissolved in water when that wa-
ter is both in contact and at equilibrium
with the pure chemical. As a general rule,
a very soluble ingredient (water solubility
above several g/l) cannot be extracted from
water with the available extraction proce-
dures. Very insoluble ones (water solubility
<0.5-1 mg/1) are difficult to analyse at trace
levels because they have a tendency to ad-
sorb everywhere, especially on glassware;
this leads to low extraction recoveries, un-
less some organic solvent is added to the
samples prior to extraction (1). Water solu-
bility indicates the tendency of a pesticide
to be removed from soil by run-off or irriga-
tion water and to reach the surface water. It
also indicates the tendency to precipitate
at the surface soil. However, this parameter
alone cannot be used for predicting leach-
ing through soil, although the distribution
of pesticides in the environment is condi-
tioned by a variety of partition coefficients
into water, and several authors have shown
correlations between these partition coeffi-
cients and the water solubility (Figure 2) (1).
(c) Vapour pressure
It is another chemical-specific property,
defined as the partial pressure of a chemi-
cal, in the gas phase, in equilibrium with the
pure solid or liquid chemical. Vapour pres-
sures are very temperature-dependent. This
parameter governs the distributionbetween
liquid and gas phase or between solid and
gas phase. Experimental measurements are
Figure 1. Distribution of the analytes studied according to their log Pow.