Parathion with time

Practically all toxicokinetic properties reported are based on the results from acute exposure studies. Generally, no information was available regarding intermediate or chronic exposure to methyl parathion. Because methyl parathion is an enzyme inhibitor, the kinetics of metabolism during chronic exposure could differ from those seen during acute exposure. Similarly, excretion kinetics may differ with time. Thus, additional studies on the distribution, metabolism, and excretion of methyl parathion and its toxic metabolite, methyl paraoxon, during intermediate and chronic exposure are needed to assess the potential for toxicity following longer-duration exposures. 

Figure 12. Parathion degradation with time in the presence of five other formulated pesticides. , amount in soil and water o, amount in water.

Note that the reaction at the phosphorus atom is postulated to occur by an SN2 (no intermediate formed) rather than by an addition mechanism such as we encountered with carboxylic acid derivatives (Kirby and Warren, 1967). As we learned in Section 13.2, for attack at a saturated carbon atom, OH- is a better nucleophile than H20 by about a factor of 104 (Table 13.2). Toward phosphorus, which is a harder electrophilic center (see Box 13.1), however, the relative nucleophilicity increases dramatically. For triphenyl phosphate, for example, OH- is about 108 times stronger than H20 as a nucleophile (Barnard et al., 1961). Note that in the case of triphenyl phosphate, no substitution may occur at the carbon bound to the oxygen of the alcohol moiety, and therefore, neutral hydrolysis is much less important as compared to the other cases (see /NB values in Table 13.12). Consequently, the base-catalyzed reaction generally occurs at the phosphorus atom leading to the dissociation of the alcohol moiety that is the best leaving group (P-0 cleavage), as is illustrated by the reaction of parathion with OH ... 

Figures 1A, 2A and 3A give representative dissipation curves for parathion, azinphosmethyl and methidathion on orange trees in California (6). Parathion dissipates with the formation of considerable amounts of paraoxon. Low volume application (100 gal/acre) of these insecticides results in high levels of OP residues and thus longer dissipation times to safe levels. Azinphosmethyl does not dissipate as rapidly as parathion under field conditions. Azinphosmethyl oxon is formed during the process and dissipates slowly with time. Azinphosmethyl oxon levels were determined only for azinphosmethyl at 6.0 lb AI per 100 gal/acre. Methidathion dissipates on citrus also with the formation of its oxon.

Formulations of parathion with mineral oils have proved to be very effective as winter sprays against various hibernating forms of orchard pests. It is particulary potent against plant lice, flies and caterpillars, killing these pests in concentrations as low as 0.0001-0.0008%. It is hazardous to bees. Spraying with parathion at blossom time is therefore not permitted. 

Using gas-liquid and thin layer chromatography techniques, the only intermediate product detected and identified during the oxidation of parathion with CI2 or CIO2 was paraoxon. Paraoxon accumulates in the system from the parathion-Cl2 or parathion-C102 reactions, but not in a stoichiometric amount. The amounts of CI2 and CIO2 required to oxidize completely 1.012 mg/liter paraoxon were determined graphically to be equal to 38.95 mg/liter and 44.50 mg/liter, respectively, within a contact time of 60 minutes. 

The rat LD qS are 13, 3.6 (oral) and 21, 6.8 (dermal) mg/kg. Parathion is resistant to aqueous hydrolysis, but is hydroly2ed by alkah to form the noninsecticidal diethjlphosphorothioic acid and -nitrophenol. The time required for 50% hydrolysis is 120 d ia a saturated aqueous solution, or 8 h ia a solution of lime water. At temperatures above 130°C, parathion slowly isomerizes to 0,%diethyl 0-(4-nitrophenyl) phosphorothioate [597-88-6] which is much less stable and less effective as an insecticide. Parathion is readily reduced, eg, by bacillus subtilis ia polluted water and ia the mammalian mmen to nontoxic 0,0-diethyl 0-(4-aminophenyl) phosphorothioate, and is oxidized with difficulty to the highly toxic paraoxon [511-45-5] diethyl 4-nitrophenyl phosphate d 1.268, soluble ia water to 2.4 mg/L), rat oral LD q 1.2 mg/kg. 

Erythrocyte cholinesterase levels were monitored in two men exposed dermally to methyl parathion after entering a cotton field that had been sprayed with this pesticide (Nemec et al. 1968). The field was entered on two separate occasions twice within 2 hours after an ultra-low-volume spraying and a third time within 24 hours after spraying. Dermal methyl parathion residues 2 hours after spraying were 2-10 mg on the arms dermal residues 24 hours after spraying were 0.16-0.35 mg on the arms. The exposed individuals did not have signs of cholinergic toxicity, but erythrocyte cholinesterase levels after the third exposure were 60-65% of preexposure levels. 

In multi-residue analysis, an analyte is identified by its relative retention time, e.g., relative to aldrin when using ECD or relative to parathion or chlorpyrifos when using a flame photometric detection (FPD) and NPD. Such relative retention times are taken from corresponding lists for the columns used. Further evidence for the identity of an analyte is provided by the selectivity of the different detectors (Modules D1 to D3), by its elution behavior during column chromatography (Modules Cl and C2) and in some cases even by the peak form in a gas chromatogram. In a specific analysis for only some individual analytes, their retention times are compared directly with the corresponding retention times of the analytes from standard solutions. 

The substrate was Valencia orange leaves, with 2500 leaves per sample selected in a carefully prescribed manner (4). The trees involved were field sprayed in a conventional manner with 4 pounds of a 25% wettable powder of parathion, then sampled after 7 days and again after 11 days. Each sample was mixed thoroughly and subsampled into 125-leaf units in 2-quart Mason jars. To all units were added 250 ml. of benzene each, and they were sealed, stripped for various lengths of time, then restripped with fresh benzene, again for various lengths of time. The strip solutions were analyzed in the usual manner. 

Table IV gives the data from seven plots of Winesap and Rome Beauty apples in the Mississippi Valley. The spray schedules are similar to those used for the plots included in Table III, except that an additional parathion spray was applied on plots 9, 11, 14, and 4 on August 19, and the final harvest sample was taken on October 5. Only on the plot that was sprayed seven times with the 8-ounce strength of parathion (plot 14) did the spray residue at harvest approximate 0.1 p.p.m.

The parathion residue at harvest time resulting from a program of 0.5 to 1 pound of 25% wettable powder applied 45 to 60 days before harvest could be expected to be 0.10 p.p.m. or less. The application of sprays with a concentration in excess of 0.5 to 1 pound (25%) would result in no measurable increase in insect control, waste of materials, and higher parathion spray residues. 

Pure parathion is a pale yellow, practically odorless oil, which crystallizes in long white needles melting at 6.0° C. (17). It is soluble in organic solvents, except kerosenes of low aromatic content, and is only slightly soluble in water (15 to 20 p.p.m. at 20° to 25° C.). Peck (35) measured its rate of hydrolysis to diethyl thiophosphate and nitro-phenate ions in alkaline solutions. He found that the reaction kinetics are first order with respect to the ester and to hydroxyl ion. In normal sulfuric acid the rate of hydrolysis was the same as in distilled water. Peck concluded that hydrolysis takes place by two mechanisms—a reaction catalyzed by hydroxyl ions and an independent uncatalyzed reaction with water. He calculated that at a pH below 10 the time for 50% hydrolysis at 25° C. is 120 days in the presence of saturated lime water the time is 8 hours. The over-all velocity constant at 25° C. is k = 0.047 [OH-] + 4 X 10-6 min.-1... 

There is a significant amount of data from other countries on the effects on human health of large-scale pesticide production and use, in particular of OPPs and OCPs. Even one-time, accidental contact with some OCPs and OPPs such as dieldrin, malathion, and parathion, can lead to changes in the encephalogram (which remain for a year after exposure), disruptions of sleep patterns and memory, loss of libido, and difficulties in concentration [3]. Global practice shows that all pesticides are toxic to humans. 

In the adsorption with Tenax alone satisfactory results were obtained, while in the presence of mineral oil a considerable proportion of the organophos-phorus pesticides (particularly Malathion and Parathion-methyl) was not adsorbed and was recovered in the filtered water. This drawback can be overcome by adding a layer of Celite 545 which, in order to prevent blocking of the column, is mixed with silanised glass wool plugs. A number of analyses of surface and estuarine sea waters were carried out by this modified Tenax column and simultaneously by the liquid-liquid extraction technique. To some of the samples taken, standard mixtures of pesticides were also added, each at the level of 1 xg/l (i.e., in concentration from 13 to 500 times higher than that usually found in the waters analysed). One recovery trial also specifically concerned polychlorobiphenyls. The results obtained in these tests show that the two extraction methods, when applied to surface waters that were not filtered before extraction, yielded very similar results for many insecticides, with the exception of compounds of the DDT series, for which discordant results were frequently obtained. 

With respect to C-parathion and Cl-toxaphene, protease-liberated flavoprotein was significantly more active than phosphate buffer in photodegrading these chemicals to ater-soluble products (Tables II and III). The amount of C-water-soluble products formed from parathion was 5-7 times greater in the presence than in the absence of flavoprotein. It should be noted that the presence of FMN in the mixture caused a slight grange in amount of water-soluble products formed (Table II). 

Walter Reed-Wistar and Charles River male adult rats were exposed to oral doses of turpentine or to turpentine vapors, which consisted of a- and p-pinene. These exposures were followed by oral administration of heptachlor epoxide or of one of three pesticides, paraoxon, heptachlor, or parathion, or by an intraperitoneal injection of hexobarbital. The studies revealed that pretreatment with turpentine reduced hexobarbital sleeping time, reduced the parathion LDso, and increased the heptachlor LDso. The paraoxon and heptachlor epoxide LOo values were unchanged. a-Pinene and P-pinene vaporized from turpentine had no effect on either hexobarbital sleeping time or parathion, paraoxon, or heptachlor epoxide mortality but did increase the heptachlor LDso (Sperling et al. 1972). The authors speculated that increases in hepatic microsomal enzyme activity are responsible for these differences. 

The hydration status of the clay or earth material may affect the adsorption capacity of nonpolar (or slightly polar) toxic chemicals. Continuing with parathion as a case study, Fig. 8.33 shows the increase adsorbed parathion on attapulgite from a hexane solution, as the adsorbed water on the clay surface decreases. This behavior may be explained by the competition for adsorption sites between the polar water and the slightly polar parathion. Possibly, however, the reduction in adsorption due to the presence of water is caused by the increased time required for parathion molecules to diffuse through the water film to the adsorption sites. 

The relative importance of the two processes in a model evaporation pond, along with the time lor 97% loss of the applied pesticide (system purification time), were calculated (Table V). This calculation confirmed that mevinphos and malathion dissipated primarily by hydrolysis, with malathion the more rapid of these two chemicals. For methyl and ethyl parathion, both processes were significant, although volatilization was the dominant dissipation route. However, since both processes were relatively slow for these pesticides, the purification time was fairly long. Diazinon was predicted to be lost primarily via volatilization, and the purification time was relatively short. 

The 2003 ACGIH threshold limit value-time-weighted average (TLV-TWA) for methyl parathion is 0.2 mg/m with a notation for skin absorption. 

Figure 6. Elution profile of protein, radioactivity, and thiocyanate from a Sepha-dex G-25 column of reconstituted monooxygenase system from rat liver that had been incubated with [ 5] parathion. The 5-mL incubation mixture contained 20 nmol Cytochrome P-450 (specific activity 16.4 nmol/mg protein), 5 units NADPH-Cytochrome c reductase, 600 fig dilauroyl L-3-phosphatidylchoUne, 600 fig sodium deoxycholate, and 1 X IO M p 5] parathion. The remainder of the incubation mixture is described in Figure 4. The incubation time was 5 min. One-milliliter fractions were collected. The radioactivity (x) represents cpm/0.1 mL. The OOggo (o) was measured on each 1-mL fraction (20).

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