Predicting Chemical Hydrolysis

Chemical (abiotic) hydrolysis has been well studied by organic chemists for many years, frequently under environmentally relevant conditions (in water at ambient temperature and pH 5-9) [25]. Testing is sometimes carried out at elevated 

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Relation to Alkalinity. Some explanations for this behavior can be offered. Persistent levels appear to be associated with low pH (4.5-6.5) and alkalinity (<10 mg/L) in the groundwater. Residues disappeared faster from Fields 4 and 5, where pH and alkalinities were high, especially at deeper levels (Figure 5) than in Fields 1 and 2, where pH and alkalinity were low (Figure 6). Rates of chemical hydrolysis of aldicarb and its oxides have been extensively studied (26-29). Alkalinity and pH tend to increase with depth in the aquifer under all fields. However, prediction of rates of chemical hydrolysis based on known rate constants are complicated by two phenomena 1. fluctuations in groundwater... 

The use of internal standards is somewhat controversial.115 There is agreement that an internal standard may be used as a correction for injection volume or to correct for pipetting errors. If an internal standard is included before sample hydrolysis or derivatization, it must be verified that the recovery of the internal standard peak is highly predictable. Ideally, the internal standard is unaffected by sample handling. Using an internal standard to correct for adsorptive or chemical losses is not generally approved, since the concentration of the standard may be altered by the conditions of sample preparation. An example of internal vs. external standards is given in Chapter 4. 

Hydrolysis. NMR results show that TBT carboxylates undergo fast chemical exchange. Even the interfacial reaction between TBT carboxylates and chloride is shown to be extremely fast. The hydrolysis is thus not likely to be a rate determining step. Since the diffusivity of water in the matrix is expected to be much greater than that of TBTO, a hydrolytic equilibrium between the tributyltin carboxylate polymer and TBTO will always exist. As the mobile species produced diffuses out, the hydrolysis proceeds at a concentration-dependent rate. Godbee and Joy have developed a model to describe a similar situation in predicting the leacha-bility of radionuclides from cementitious grouts (15). Based on their equation, the rate of release of tin from the surface is ... 

Such a restriction does not exist for a readily accessible experimental parameter such as the chemical shift of the carbonyl C-atom (A<5). This parameter, as measured by 13C-NMR, expresses a complex mixture of electronic and steric effects, some of which may not be relevant to the mechanism of hydrolysis. Its correlation with log k is not as good as that of Taft s polarity parameter, yet it is of sufficient quality and includes enough variation between substituents to have fair predictive value (Eqn. 8.2) ... 

Chemical/Physical. Hydrolysis in distilled water at 25 °C produced l-chloro-2-propanol and HCl. The reported half-life for this reaction is 23.6 yr (Milano et al., 1988). The hydrolysis rate constant for 1,2-dichloropropane at pH 7 and 25 °C was determined to be 5 x 10 Vh, resulting in a half-life of 15.8 yr. The half-life is reduced to 24 d at 85 °C and pH 7.15 (Ellington et al., 1987). A volatilization half-life of 50 min was predicted from water stirred in an open container of depth 6.5 cm at 200 rpm (Dilling et al., 1975). Ozonolysis yielded carbon dioxide at low ozone concentrations (Medley and Stover, 1983). 

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. 

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