VAPOR PRESSURE
Many factors interact to determine the fate of chemicals in the environment, but the inherent physical properties of the chemical are among the most important. Knowledge of these physical properties is especially relevant to the prediction of the environmental fate of a chemical. Among the physical properties vapor pressure is of primary importance. The vapor pressure of a compound determines its rate of loss from soil and surfaces by volatilization, the rate of diffusion within the soil, i.e., its potential for use as a fumigant, and its tendency to volatilize from a body of water. It is not surprising, then, that vapor pressure determination is one of the "benchmark" measures1 for predicting rates of pesticide dissemination under field conditions.
Most pesticides have fairly low vapor pressures, in the range 10-2 to 10-7 Pascals. A convenient method for measurements in this range involves passing an inert gas over a non-adsorptive surface containing the chemical, and analyzing the chemical's concentration in the flowing air volume.2 This method presumes equilibrium conditions, that is, that the gas flow rate is slow enough and the gas-surface contact sufficient so that the flowing gas becomes completely saturated with chemical. Providing this condition occurs, the vapor pressure may be calculated from the perfect gas law, PV = nRT, manipulated such that the vapor density (m/V) is equal to PM/RT.
Several techniques have been used to scrub or trap organic vapors from the gaseous phase as required in vapor pressure determination by the gas saturation technique. The present experiment utilizes a lipophilic polystyrene resin, XAD-4, as the vapor trap (resins of this nature have an extremely high affinity for pesticide vapors3), such that essentially quantitative collection occurs. Alternative methods, including solvent-filled scrubbers or impingers and activated charcoal, are less efficient or less convenient to use. A simple rinse of the XAD-4 with solvent serves to extract the chemical in a form amenable to quantitative analysis by gas- liquid chromatography. From the concentration in the extract, the amount of vapor trapped by the XAD-4 may be calculated; then, knowing the air sampling rate (mL/min) and the duration of sampling (minutes), the vapor density (µg/m3) and vapor pressure (Pascals) may be calculated.
Factors of environmental importance which influence vapor pressure include temperature, relative humidity, and type of surface. By holding most variables constant and changing only a single variable, much valuable information can be gained.
Increasing temperature, for example, greatly increases vapor pressure, as might be expected intuitively.4 Vaporization tendency from dry soil surfaces is much less than that from moist soil due to the greater adsorptivity of the former5. Also, water evaporation tends to facilitate movement of chemicals from soil depths to the top surface, from which evaporation may occur, by what is termed the "wick effect." As a matter of fact, Hartley6 takes the interesting approach that evaporation of the pesticide probably is proportional to the evaporation of the water from a field, providing one takes into account the difference in vapor pressure and in molecular weight:
See references below for more details on this point. VP is in
mm Hg. 1 mm Hg = 133.32 Pa.
References for Vapor Pressure
1. C.A.O. Goring. Agricultural chemicals in the environment. Chapter 13 in Organic Chemicals in the Soil Environment, Goring and Hamaker (eds.), Dekker, New York, 1972.
2. J.W. Hamaker. Vapor Pressure (Category Recommended for EPA Evaluation).
3. T.C. Thomas and J.N. Seiber. Chromosorb 102, an efficient medium for trapping pesticides from air. Bull. Environ. Contamin. Toxicol. 12, 17 (1974).
4. W.F. Spencer and M.M. Cliath. Vapor density of dieldrin. Environ. Sci. and Technol. 3, 670 (1969).
5. W.F. Spencer and M.M. Cliath. Pesticide volatilization as related to water loss from soil. J. Environ. Quality 2, 284 (1973).
6. G.S. Hartley, Evaporation of pesticides. In Pesticidal Formulations Research, R.F. Gould (ed.), ACS Advan. Chem. Ser. 86, pp. 115-134.
7. J.W. Hamaker. Diffusion and volatilization. In Organic Chemicals in the Soil Environment, Goring and Hamaker (eds.), Dekker, New York, 1972, pp. 384-385.
8. L.R. Suntio, W.Y. Shiu, D. Mackay, J.N. Seiber, and D. Glotfelty. Critical review of Henry's Law constants for pesticides. Rev. Env. Contam. Toxicol. 103.
9. W.F. Spencer and M.M. Cliath, Measurement of Pesticide Vapor Pressures. Residue Reviews, 85, 57-71 (1983).
10. Y.H. Kim, J.E. Woodrow, and J.N. Seiber. Evaluation of a gas
chromatographic method for calculating vapor pressures with
organophosphorous pesticides. Journal of Chromatography,
314, 37-53 (1984).
Procedure
The object of the present experiment is to obtain the vapor pressure of two pesticides of moderate volatility (lindane, and trifluralin) under ambient conditions.
Coat 50 g of sand with 5.0 mg each of lindane and trifluralin by adding 10 mL of hexane containing the 5 mg of each pesticide to the sand in a 100 mL round bottom flask. Mix well! Using a rotary evaporator SLOWLY evaporate the solvent until the sand is dry and free flowing.
Using the apparatus shown in Fig. 1 place ca. 40 g of sand in the low joint of the two joint vapor tube assembly.
Place a 2-inch plug of XAD-4 resin in the upper joint as in Fig. 1. Use a minimum of glass wool. Take care not to contaminate the XAD-4. When the assembly is completed, pass dry nitrogen through at the rate of approximately 125 mL/min. Check the flow rate several times during the experiment with the soap bubbler and timer provided and record the flow rates. Allow vaporization to proceed for a known period of time (usually 1 hour). Be sure to record starting and stopping times. While waiting, construct a standard curve (see below).
After an hour, disassemble the vapor tube apparatus and transfer the XAD-4 and glass wool from the upper joint to a 125 mL Erlenmeyer flask. The XAD-4 is not free flowing and tends to jump around, so the transfer is most easily accomplished by pushing the glass wool and XAD-4 with a stirring rod directly into the Erlenmeyer flask. Rinse the upper tube with 50.0 mL of ethyl acetate directly into the Erlenmeyer flask. Cover the flask with aluminum foil and swirl by hand for at least 10 min, and allow contents to settle.
Standard curve. Because the amount of pesticide volatilizing is quite small, a highly sensitive detector is required for its measurement. The electron capture detector responds with extremely high sensitivity to compounds such as lindane and trifluralin.
To construct the standard curve, inject 25, 50, 75 and 100 picograms (from the available mixed standards of lindane and trifluralin) on the GC equipped with a nonpolar column such as DB-1. Exact conditions vary with the instrument and will be set up beforehand. Make certain that the injecting syringe is clean before use (check by injecting solvent only). Make duplicate injections of each amount of the mixed standard; the standard curve of response (peak) versus amount (pg) should be linear.
Record all GC operating conditions (column length, type, carrier gas type and flow rate, column, injector and detector temperatures, and attenuation and chart speed).
Samples. Once a satisfactory standard curve has been obtained, inject (in duplicate) 1 or 2 µL of the 50 mL ethyl acetate extract of the lindane and trifluralin vapor trap. Compare peak height of sample with standard curve, and from the latter read off the pg injected. Calculate µg in XAD-4 extracts; then the vapor pressure from PV = nRT in the form of vapor density, m/V = PM/RT. Use the ambient temperature (in K) in the equation. Note that R = 8.314 Païm3/mol K. Record the room temperature.
Compare your experimental values with those reported in the literature.
Lindane Trifluralin
Figure 1. Diagram Of Apparatus For Vapour Pressure Determination.
PARTITION (KOW)
A physical property of extreme importance in determining environmental behavior of a chemical is that of solubility in water, and the relative water-organic solubility. The latter is particularly pertinent to predicting ability for biomagnification or bioconcentration in aquatic food chains, since it is simple partitioning between a polar phase (water) and nonpolar phase (the fate or lipid matter of a living organism) which governs uptake. Of course, a chemical may be very hydrophobic and still not accumulate because it is chemically not very stable; however, a significant number of our synthetic organic chemicals (DDT, PCB's, etc.) are both hydrophobic and stable enough to present environmental problems.
Partition coefficients have been tabulated for many years in pharmaceutical work as an index of drug absorption and translocation ability. An excellent review of partition coefficients and their uses has been published.1
A simple working definition is the following:
When the volume of organic phase is equal to the volume of the polar phase then:
Kp = p/q
where
K = partition coefficient
p = fraction in the nonpolar phase
q = fraction in the polar phase (usually water)
The partition coefficient (Kp) for a compound distributing between 1-octanol and water has been used by Neely et al. and others to estimate the bioconcentration potential (BF) for that compound in fish in the environment. It is often unnecessary to experimentally determine Kp as it may be calculated from tabulated data based on known Kp's for related compounds. However, since these calculations are only approximations, the experimental determination of Kp is still necessary in many cases.
In the present experiment the partition coefficient between octanol
and water of two pesticides, lindane and trifluralin will be
determined. This determination is a companion to the vapor pressure
experiment since it utilizes the same quantitative techniques,
namely, nitrogen-phosphorus GLC. The experiment will include
calculation of Kp values from tabulated data for comparison with
your experimental values.
References for Partition
1. A. Leo, C. Hansch and D. Elkins. Partition coefficients and their uses. Chem. Revs. 71, 525 (1971).
2. W.B. Neely, D.R. Branson and G.E. Blau. Partition coefficient to measure bioconcentration potential of organic chemicals in fish. Env. Sci. and Tech. 8, 1113-1115 (1974).
3. L.R. Suntio, W.Y. Shiu, D. Mackay, J.N. Seiber, and D. Glotfelty. Critical review of Henry's Law constants for pesticides. Rev. Env. Contam. Toxicol. 103.
4. Lyman, Reehl, and Rosenblatt. Handbook of Chemical Property
Estimation Methods. 1990.
Procedure
CAUTION! Test all your centrifuge tubes for leakage before you begin the partitioning step.
Pipette 1 mL of a 1 mg/mL standard of lindane and trifluralin in hexane into a 15 mL screw-cap calibrated centrifuge tube. Add 2 drops of octanol and evaporate the hexane under nitrogen stream just until all of the hexane has evaporated (leaving ca. 2 drops).
Pipette 5 mL of octanol (i.e., the top layer from a stock of 1:1 equilibrated octanol-water) into the centrifuge tube, cap and shake. Then pipette in 5 mL of water (i.e., the bottom layer from the same stock), cap, and gently tumble end-over-end for at least two minutes.
Centrifuge briefly. Be sure to place another tube of equal weight in the rotor to balance the centrifuge. Remove and discard most of the octanol (upper layer) with a disposable pipette. Centrifuge again. Next, with a clean pipette transfer 2 to 4 mL of clear water from the bottom layer into a second centrifuge tube, and centrifuge this one. Then with a clean pipette remove 1 mL of clear water from the bottom of the second tube and transfer it to a third clean, dry tube. Make sure no octanol gets transferred.
To the third tube add 1.0 mL of hexane, and tumble the capped tube end-over-end for at least 2 min. Then inject (in duplicate) 1 or 2 µL of the hexane phase onto the electron capture gas chromatograph and compare with standard curve as described in vapor pressure experiment.
From the concentration in hexane (µg/mL) calculate the amount present (µg) in the aqueous phase of the partition. Calculate your experimental Kp from:
*Assume almost all is left in octanol, which is the case for very hydrophobic substances.
*1000 µg is not a constant, therefore will vary.
In your lab write-up consider the structural components of each
of the pesticides that leads to their relative hydrophobicities.