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United States Department of Agriculture

Agricultural Research Service

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

Photolysis constant (Kp)
          -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]


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

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

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

        Hc=Pa/moles M-3=Pa M3/moles

Hc=P1 x MW x 10-3/c1

                  Hc=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 (pKa, shown as pKa in database)

RH =R- + H+

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

pKa=-log10 Ka


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

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

pKow =-log10 Kow


Sorption coefficient (KD, shown as Kd in database)

KD=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.


Last Modified: 9/27/2006