RX + HOH ' ROH + HX.
Equation 1
-d[RX]/d= = kh[RX] = kA[H=] [RX]
+ kB[OH-] [RX] + k'N [H2O] [RX],
where KA, kB and k'N are the second-order rate constants for acid and base catalyzed and neutral water processes, respectively. In dilute solutions, such as are encountered in following this Test Guideline, water is present in great excess and its concentration is, thus, essentially constant during the course of the hydrolysis reaction. At fixed pH, the reaction, therefore, becomes pseudo first-order, and the rate constant (kh) can be written as:
Equation 2
kh = kA [H=] + kB [OH-] + kN,
where kN is the first-order neutral water rate constant. Since this is a pseudo first-order process, the half-life is independent of the concentration and can be written as:
Equation 3
t1/2 = 0.693/kh.
At constant pH, Equation 1 can be integrated to yield the first order rate expression
Equation 4
log10C = - (kh t/2.303) + log10Co,
where C is the concentration of the test chemical at time t and Co is the initial chemical concentration (t = 0).
Equation 5
kA = 103 [kh (3)-kh (7) + 10-4 kh (11)]
kB = 103 [kh (11)-kh (7) + 10-4 kh (3)]
kN = kh (7)-10-4 [kh (3) + kh (11)]
The calculated rate constants from equation 5 under this paragraph can be employed in equation 2 under paragraph (a)(2)(v)(B) of this section to calculate the hydrolysis rate of a chemical at any pH of environmental concern.
Equation 6
-d[RX]/dt = [RX] = k2 [RX] + . . . . + kn
[RX] = (k1 + k2 + . . . . . kn) [RX] = kh [RX].
Equation 6 applies to the hydrolysis rate of a molecule having n hydrolyzable groups, each of which follows first-order reaction kinetics. The measured kh is now the sum of the individual reaction rates and is the only rate constant required in this section.
40 C.F.R. §796.3500