PROPERTIES & UNITS

 

Hydrolysis constant (K_H)
          -d(RX)/dt=K_H[RX], where K_H = 0.693/half-life of substrate (days)

Photolysis constant (K_p)
          -d(RX)/dt = K _p[RX], where K _p= 0.693/half-life of substrate (days)

Vapor Pressure

mPa = milliPascals [1 mm Hg=133,000 mPa]

Solubility

ppm = part per million = ug g -1 (or ug ml -1 in water)

Henry's Law constant - air-water partioning coefficient (H_c)

H_c = p/c, where p = pesticide vapor pressure (Pa)
c = solubility in water (mol m^-3 )

        H_c=Pa/moles M^-3=Pa M³/moles

H_c=P¹ x MW x 10^-3/c¹

                  ^H_c=mPa x MW x 10^-3/PPM where

p^' = pesticide vapor pressure (mPa)

mw = Molecular weight of pesticide

c^' = solubility in water (ppm)

^{Acid Dissociation constant} (pK_a, shown as pK_a in database)

RH =R- + H+

K = [R-] [H+]/[RH]

pK_a=-log₁₀ K_a

 

^{Octanol-water partition coefficient} (Kow and pKow, shown as log Kow in database)

K_ow ⁼ C octanol/Cwater, where C = molar concentration

pK_ow ⁼-log₁₀ K_ow

 

^{Sorption coefficient (K}_D, shown as Kd in database)

K_D=C ^' _soil /C ^' _water, where C ^' = concentration (ppm)

 

Organic carbon sorption coefficient ( shown as Koc in database)

Koc = KD x 100 / % oc,

where % oc = percent organic carbon in the soil = % organic matter/1.72

 

 

 

 

Field dissipation half-life (days)

Measure of overall rate of disappearance of pesticide from soil-
includes leaching, runoff, hydrolysis, photolysis, microbial degradation,

vaporization function of pesticide, site, climate, soil. Related to
"Persistence" is equivalent to about 4 half-lives, and is the time taken for
about 90% of the pesticide to dissipate.

 

Soil half-life (days)

Microbiological (aerobic and anaerobic) degradation in the soil. It is a
function of pesticide, soil, soil temperature, soil water content and
micro-organisms present.