In the Laboratory
Assessing the Fate of Organic Contaminants in Aquatic Environments: Mechanism and Kinetics of Hydrolysis of a Carboxylic Ester Jörg Klausen,* Markus A. Meier, and René P. Schwarzenbach Swiss Federal Institute for Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETHZ), CH-8600 Dübendorf, Switzerland
The processes that govern the distribution and fate of organic compounds in aquatic environments include transport phenomena as well as biological and abiotic transformations. To assess the behavior of a chemical under certain environmental conditions, it is necessary to collect both compound-specific (e.g., pKa) and system-specific (e.g., pH) properties. The data required today by many authorities for the approval of new compounds include information about the persistence of these chemicals in aquatic environments. One of the important transformation processes to be considered is hydrolysis—that is, the reaction of the compound with water. Hydrolysis and the kinetics of transformations of organic compounds have been dealt with in only a small number of recent papers in this Journal (1– 4), and their focus has been very different. In this paper, we propose an integrated experiment that was developed for an upper-level undergraduate environmental chemistry laboratory. It has been carried out successfully during several one-week laboratory courses in aquatic chemistry given for students of environmental sciences and engineering in their second or third year at university. Requiring relatively simple equipment and using an environmentally significant reaction as an example, the experiment serves mainly 3 purposes: (i) to introduce and apply the basic concepts of chemical kinetics and mechanisms of hydrolysis, (ii) to illustrate what it means to generate reliable kinetic reaction parameters, and (iii) to acquaint students with the practical aspects of some important analytical methods (liquid chromatography, UV-vis spectroscopy, pH measurement). If HPLC equipment is not available, the lab course could still be offered (following the original method [5] using a UV-vis photometer only).
NO2
kh
HO
OH
+ O
O O2N
DNPA
DNP
(1)
HOAc
pH Dependence of Ester Hydrolysis In aquatic environments such as lakes, rivers and aquifers, hydrolysis is usually the most important chemical transformation of organic molecules that are susceptible to nucleophilic attack. In freshwater environments, the hydroxide ion and the water molecule are the dominant nucleophiles available, with OH { being about 10,000 times as strong a nucleophile as H2 O in nucleophilic substitutions at a carbon center (7). The kinetics of hydrolysis at a constant pH (natural systems are usually pH-buffered, and a pH buffer is added to our experimental system) can be described by a pseudo-first-order rate law (eq 2).
rate = {
d[DNPA] = k h ⋅ [DNPA] dt
(2)
where [ ] denotes concentrations in mol L{1 and kh is the pseudo-first-order rate constant for the reaction. (By using concentrations, [ ], instead of activities, { }, we assume activity coefficients to be close to unity.) The hydrolysis of a carboxylic ester can proceed via three different pathways. Depending on pH, one or several of these mechanisms contribute to the observed rate of hydrolysis. Hence, the pseudo-first-order hydrolysis rate constant is given by kh = kA ? {H+} + k N + kB ? {OH{}
(3)
where kA and kB are the second-order rate constants for the acid- and base-catalyzed reaction, respectively, and kN is the pseudo-first-order rate constant for the neutral hydrolysis (i.e., for the attack of the organic compound by the H2 O molecule) (see ref 7 for a detailed discussion of the various mechanisms). The hydrolysis by H2 O is, in fact, also a second-order reaction. However, the concentration of H2 O remains virtually constant during the process, and the reaction can thus be described with a pseudo-first-order rate constant: kN = kH2O ? [H2O]. If kA, kN, and kB are known for a given compound at a specified temperature and pH, the overall rate of hydrolysis can be calculated easily according to eqs 2 and 3. Note that the hydroxide activity at a given pH is calculated using the temperature-dependent protolysis equilibrium constant of H2O:
*Corresponding author.
1440
+ H 2O
O 2N
Choice of Model Pollutant 2,4-Dinitrophenylacetate (DNPA) was chosen as model compound because, first, it hydrolyzes with half-lives on the order of minutes to hours depending on pH and temperature. Thus, the experiments can be carried out easily within 3-4 hr. Furthermore, the hydrolysis product 2,4-dinitrophenol (DNP) exhibits an intense yellow color, thereby allowing the student to actually see the progress of the reaction. Finally, the toxicity (rainbow trout LC50 = 1.2 mg L{1 [6]) and the biodegradability (≥ 99.3 % in biological waste-water treatment plant [6]) of the hydrolysis product DNP are such that the small quantities of micromolar solutions generated in the course of the experiment can be disposed of without causing environmental concern (consult the local authorities for details on waste disposal regulations—e.g., for drinking water limits). DNPA hydrolyzes according to eq 1.
NO2 O
Journal of Chemical Education • Vol. 74 No. 12 December 1997
K W (T) = {H +} ? {OH {}
(4)
In the Laboratory with the following empirical relationship for KW (T ) (8) (T in Kelvin):
log K W (T) = – 4470.99 + 6.0875 – 0.01706T T
(5)
Thus, for example, log KW (298 K) = {14. In the particular case of DNPA, the data reported in the literature indicate that kA ? {H +}
