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Pesticide information box

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v. 4/5/2010

Physical and Chemical Properties of Pesticides

The biological activity of a pesticide is influenced by its physical and chemical properties. These will affect various effects including the mode of dose transfer and environment issues such as pesticide run off into water courses or half life in the soil.

Molecular weight and form

References such as the Pesticide Manual give molecular weights (MW) and the physical form (appearance and odour) of the active ingredient (AI) in question. Common gas-phase pesticides have a MW of 103 or less; it can be difficult to predict of the properties of complex molecules with MWs that are substantially greater than 500.

Composition

It is important to describe the purity of of the AI: if it is technical grade ('tech.'), the composition of isomers, etc. Where a substance exists in different stereochemical forms, the material used may be a single isomer, or a mixture of stereochemical isomers. Technical grade material is usually used in practice, but with a defined minimum purity.

Solubility

Unless stated otherwise, units for solubility in water are in mg per litre (mg L-1). Measurements are influenced by the temperature, the pH and the method used.

Partition Coefficient: Kow (Log P)

The partition coefficient (Kow) is a measure for the lipophilicity of a substance. With most pesticides and other organic substances, Kow provides a useful predictor of their properties, provided the molecular weight is not too high. It is a dimensionless parameter and is the measured ratio (at equilibrium) of dissolved mass of the substance, between equal layers of n-octanol and water.

Kow is often expressed as Log P (which is log to the base 10 of the Kow) and is considered to be a good indicator of:

  • systemic action, with low values (generally of <=2) indicating likely systemic translocation of pesticides or pesticidal breakdown products
  • accumulation in organisms and food chains (bio-accumulation: with a positive correlation with log P)[1]

Other dimensionless coefficients:

  • Henry's constant: or air-water partition coefficient (sometimes Kaw) describes the concentration ratio of a substance in equilibrium between air and water - thus the tendency of a material to volatilise from aqueous solution to air. Sometimes measured, but more usually calculated, as the ratio of vapour pressure (in Pascals) molecular weight / solubility (mg L-1).
  • Adsorption Coefficient, Koc: is the ratio (at equilibrium) of the mass of a substance, adsorbed onto a unit mass of soil, relative to the mass remaining in water solution. It is heavily influenced by the organic carbon content (OC) of soil and the value is also dependent on the type of soil and the soil pH; it must therefore be used carefully and a range of given values is commonplace.

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Vapour pressure (VP)

The vapour pressure of a substance is a measure of how readily it will volatalise and for pesticides can be considered advantageous or in a negative light:

  • a pesticide with fumigant action can have useful penetrative powers, but ...
  • a high VP can cause vapour drift and environmental pollution; first noted with some of the early synthetic auxin herbicides.

The usually used SI unit for vapour pressure is the milliPascal (mPa = gm-1s-2 or 0.001 Nm-2).

Pesticide breakdown

After application, pesticides are degraded by chemical and physical processes in the environment such as sunlight, soil and water (called abiotic degradation) or metabolised within living organisms. Breakdown of a pesticide (and many other substances) in the environment can be thought of as following a decay curve. This is a function of the chemical's half-life, which is the time (most usually expressed in days) required for half of the applied pesticide to become converted into degradation products (which may in turn be active biologically and have substantial half-lives).

The rate of breakdown depends on many factors, not least the chemical stability of the pesticide in question, but factors such as temperature and pH are extremely important, so the half life may be expressed as a range (e.g. 3-10 days). Probably the most important mode of pesticide degradation is oxidation: especially by activated oxygen (e.g. ozone and hydroxyl radicals generated by sunlight, hydrogen peroxide generated in plants, etc.) rather than O2 in the atmosphere.

Allowing sufficient time to elapse between application and harvest enables any residue to degrade to acceptable levels and the pre-harvest interval (PHI) has a built-in safety factor. Reducing the dosage reduces the time to which acceptable levels are reached, but pest control may be impaired. Excessive residues occur with short harvest intervals, overdosing, or worst of all both of these.

QSAR

Quantitative structure-activity relationship (QSAR) is the process by which chemical structure and properties are quantitatively correlated with biological activity - usually as measured in bioassays.


[1] Esser HO (1986) Pesticide Science, 17, 265-276

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