Hydrolysis half-life time

The compound-specific data required for exposure assessments comprise the 1-octanol/water partition coefficient (log water solubility (S ), vapour pressure (p ), Henry s law constant (H, H ), soil sorption coefficient hydrolysis half-life time, photolysis half-life time and information on biodegradability (OECD, 1993c). These parameters generally relate to steady-state conditions - conditions that are rarely met in the real environment. The experimental data underlying the QSAR models are preferably determined by standardized protocols, but, even then, the absolute values are of variable reliability and precision, which clearly affects the accuracy of the predictions based on the acquired QSARs. The endpoints discussed in the following sections were selected because of their consideration in regulatory evaluation schemes in, for example, the EU (EEC, 1990). 

Table 1—1. Rate constants k [105 sec-1] and half-life times t1/2 [min] for neutral hydrolysis of iV-acylazoles [ conductivity water, pH 7.0, 25 °C] together with IR frequencies v(C=o) in CC14 and enthalpies.

Table 1—3. Rate constants and half-life times t1/2 [min] for hydrolysis of NJJ -carbonylbisazoles and their benzo and thio derivatives in THF/water (40 1 27°C) and IR frequencies V(C=o> in CHC13.

The rate of hydrolysis of various organic chemicals, under environmental conditions can range over 14 orders of magnitude, with associated half-lifes (time for one-half of the material to disappear) as low as a few seconds to as high as 10 years and is pH dependent. It should be emphasized that if laboratory rate constant data are used in soil models and not corrected for environmental conditions — as is often the only choice — then model results should be evaluated with skepticism. 

The degradation half-life time (Tb 1/2) of chlorpyrifos is 7.1 days in seawater (Schimmel et al. 1983), and 53 days in distilled water (Freed et al. 1979). Degradation is usually through hydrolysis to produce 3,5,6-trichloro-2-pyridinol and phosphorthioic acid (Brust 1966 Smith 1966, 1968 Marshall and Roberts 1978). Temperature, pH, radiation, and metal cations all significantly affect chlorpyrifos Tb 1/2 in water half-life is decreased with increasing water pH, temperature, sunlight, and metal cation concentrations (Brust 1966 Mortland and Raman 1967 Smith 1968 Schaefer and Dupras 1969, 1970 Meikle and Youngson 1970 Menconi and Paul 1994). 

Two types of investigations of the alkaline hydrolysis of chlorpyrifos in sediment/water systems were made, all at pH s between 10.6 and 10.8. First, studies were conducted in which the pH was adjusted (using a carbonate buffer) immediately upon mixing the sediments (EPA-23 and EPA-26) with the chlorpyrifos solution. Second, a study using EPA-26 was made in which the alkaline buffer was not added until three days after mixing the sediment with the chlorpyrifos solution. Three days represents a time which is long with respect to the achievement of sediment-water equilibrium for this system, yet short compared to the neutral hydrolysis half life (-50 days). 

At this point in our discussion about chemical bonds and structural formulas, we should stress that structural isomers may exhibit very different properties and reactivities. For example, the rates of hydrolysis (reaction with water, see Chapter 13) of the four butyl chlorides shown in Fig. 2.1 are quite different. While the hydrolytic half-life (time required for the concentration to drop by a factor of 2) of the first and third compound is about 1 year at 25°C, it is approximately 1 month for the second compound, and only 30 seconds for the fourth compound. When we compare the two possible structural isomers with the molecular formula C2H60, we can again find distinct differences in that the well-known ethanol (CH3CH2OH) is a liquid at ambient conditions while dimethylether (CH3OCH3) is a gas. These examples should remind us that differences in the arrangement of a single collection of atoms may mean very different environmental behavior thus we must learn what it is about compound structure that dictates such differences. 

The half-life time of baygon was 16 days at pH 8, 1.6 days at pH 9, and 0.17 day at pH 10. Sevin was also stable to hydrolysis at the acidic pH range of 3.0-6.0. However, at pH 7.0 quick rise in the hydrolysis rate was observed and increased with the increase of the pH. The first order rate constants of hydrolyzing Sevin were therefore determined over the pH range of 7.0-10.0 the data are plotted in Figure 4. The half-life time was 10.5 days at pH 7, 1.8 days at pH 8, 2.5 hours at pH 9, and only 15 minutes at pH 10. Table II shows the ki values and lifetimes of Sevin... 

Both, MCs and NODs, are chemically very stable, considering their peptidic nature. Spontaneous hydrolysis apparently occurs only at negligible rates. Boiling of MC at neutral pH does not lead to considerable decay for weeks and even at pH 1 and 40°C the half-life time is some three weeks. Further, MC was found to be resistant to enzymatic cleavage by common proteases like trypsins. 

Chloroplasts have been prepared from spinach. The ATP-synthase was brought into the active, reduced state by illumination in the presence of thioredoxin and dithiothreitol. Washing of the chloroplasts drastically reduced the content of endogeneous ADP and ATP. Then, the ATP-synthases are activated by a ApH/Areduced state and contain no enzyme-bound ADP and only one enzyme-bound ATP per CFqFq. The maximum rate of ATP hydrolysis (at 1 mM ATP, pH 8.2) is about 100 before and after the washing. The enzyme remains in the active state for about 450 s (half-life time). All following experiments are carried out with such chloroplasts preparations. 

Hydrolysis Half-Life (H-tm) The hydrolysis half-life of a chemical is the time that it takes to reach one-half or 50% of its original concerrtration. The rate of chemical hydrolysis is highly dependent upon the compound s solubility, tempera-ture and pH. Since other envirorunental factors such as photolysis, volatility (i.e., Henry s law constants) and adsorption can affect the rate of hydrolysis, these factors are virtually eliminated by performing hydrolysis experiments under carefuUy controlled laboratory conditions. The hydrolysis half-lives reported in the Uterature were calculated using experimentally determined hydrolysis rate constarrts. 

The environmental fate of plasticizers results from their chemical constitution. Most of them contain the ester group and thus the hydrolysis of ester bond is the main reaction in aqueous medium. According to Wolfe et al. [9] it appears that this reaction at pH 7 may be too slow and is negligible (the half-life time exceeds 100 days). 

Alitame (trade name Adame) is a water-soluble, crystalline powder of high sweetness potency (2000X, 10% sucrose solution sweetness equivalence). The sweet taste is clean, and the time—intensity profile is similar to that of aspartame. Because it is a stericaHy hindered amide rather than an ester, ahtame is expected to be more stable than aspartame. At pH 2 to 4, the half-life of aUtame in solution is reported to be twice that of aspartame. The main decomposition pathways (Fig. 6) include conversion to the unsweet P-aspartic isomer (17) and hydrolysis to aspartic acid and alanine amide (96). No cyclization to diketopiperazine or hydrolysis of the alanine amide bond has been reported. AUtame-sweetened beverages, particularly colas, that have a pH below 4.0 can develop an off-flavor which can be avoided or minimized by the addition of edetic acid (EDTA) [60-00-4] (97). 

CASRN 3337-71-1 molecular formula C8H10N2O4S FW 230.24 Soil Asulum is not persistent in soils because its half-life is approximately 6-14 d (Hartley and Kidd, 1987). The short persistence time is affected by soil temperature and moisture content. The half-life of asulam in a heavy clay soil having a moisture content of 34% and maintained at 20 °C was 7 d (Smith and Walker, 1977). In soil, sulfanilamide was reported as a product of hydrolysis. In nonsterile soils, this compound degraded to unidentifiable products (Smith, 1988) which may include substituted anilines (Bartha, 1971). 

The primary design parameter to be considered in hydrolysis is the half-life of the original molecule, which is the time required to react 50% of the original compound. The half-life is generally a function of the type of molecule hydrolyzed and the temperature and pH of the reaction. Figure 13 shows the elfect of pH and temperature for the degradation of malathion by hydrolysis [11]. 

Valacyclovir is the L-valyl ester of acyclovir (Figure 49-2). It is rapidly converted to acyclovir after oral administration via first-pass enzymatic hydrolysis in the liver and intestine, resulting in serum levels that are three to five times greater than those achieved with oral acyclovir and approximate those achieved with intravenous acyclovir. Oral bioavailability is 54-70%, and cerebrospinal fluid levels are about 50% of those in serum. Elimination half-life is 2.5-3.3 hours. 

For evaluating the major removal mechanism(s), calculate the hydrolysis rate of BzC at 5°C using the half-life at 25°C (i.e., 15 h) and an activation energy, E3 of 80 kJ mol-1. Insertion of a into Eq. 12-30 with 7j = 298 K and T2 = 278 K shows that the hydrolysis rate constant of BzC at 5°C is about ten times smaller (or the half-life is ten times larger) as compared to 25°C (see also Table D1 of Appendix D). Thus, the elimination of BzC by hydrolysis occurs with a rate constant, kh, of... 

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