Calculating the first-order kinetics of three coupled, reversible processes.

Calculation of the concentration-time profile in four consecutive, well-mixed compartments that are connected by diffusional transport is a frequently occurring problem for chemists and engineers. Mathematically this is equivalent to many other problems such as the concentration profiles of a parent compound and its three consecutive reaction products resulting from reversible first-order kinetics. Here we present an analytical solution to this problem implemented in a Microsoft Excel spreadsheet (available for download) and we discuss various examples of how this simple-to-use tool can be applied to very different scenarios from various fields of science.

Photodegradation of malachite green and malachite green carbinol under irradiation with different wavelength ranges.

The dye malachite green (MG) is used worldwide as a fungicide in aquaculture. It is a toxic substance which in aqueous solutions is partly converted into its non-ionic colorless form (leucocarbinol). The equilibrium between these two forms is pH-dependent (pK=6.9). To assess the photodegradation of MG under sunlight conditions, both species were irradiated separately in aqueous solutions with different pH values (4.0 and 12.0) using various ultraviolet and visible wavelength ranges (UV/VIS). A 700 W high-pressure mercury lamp with special filters was used. No artificial photooxidizers such as H2O2 or TiO2 were added. MG leucocarbinol proved to be much more sensitive to irradiation than the dye form. Quantum yields Φ were calculated for some wavelength ranges as follows: MG carbinol: Φ((280-312 nm)) is 4.3 × 10⁻³, Φ((313 - 410 nm)) is 5.8 × 10⁻³, and MG dye: Φ((280 - 312 nm)) is 4.8 × 10(-5), Φ((313-365nm)) is 1.1×10⁻5, and Φ((> 365nm)) is 0, respectively. Therefore, the solar photolysis of MG is an important sink and primarily depends on the photodegradation of the colorless leucocarbinol. During the irradiation of MG leucocarbinol with wavelengths > 365 nm, an intermediate was formed which has photocatalytical properties.

Linear free energy relationships used to evaluate equilibrium partitioning of organic compounds.

In environmental chemistry, one often wants to interpret or predict equilibrium partitioning of organic compounds between any two phases. Hence, one needs to understand the partition variability that stems from using different types of compounds and the variability that arises from looking at different natural phases, e.g. different soil organic matter. It is current practice in environmental chemistry to evaluate equilibrium partitioning with the help of double logarithmic correlations between the unknown partition constant and a well-known partition constant of the compounds, e.g., partitioning between natural organic matter and water or air is correlated with the octanol/water or octanol/air partition constant, respectively. However, these relationships (in the following called one-parameter LFERs) can only predict the compound variability within a single substance class. They supply no means to understand the variability between substance classes or the variability between different natural organic phases. The reasons for these limitations are that (a) the complete compound variability cannot be described by a single parameter because partitioning results from different kinds of interactions that vary independently from each other and (b) the specific properties of the studied phase are represented in the slope and intercept of the double logarithmic correlation and not in a variable parameter. In contrastto one-parameter LFERs, polyparameter LFERs are based on a concept that considers all interactions involved in partitioning by separate parameters. They allow for predicting the complete compound variability by a single equation, and they also provide the possibility to evaluate and predict the variability in the sorption characteristics of different natural phases. Thus future research in the field of environmental partition processes should focus on adapting and improving the more comprehensive polyparameter LFERs rather than trying to refine existing one-parameter LFERs.

Adsorption of organic vapors on polar surfaces- Recent advances.

This paper summarizes recent research on the adsorption of organic vapors on surfaces. Since the low gas phase concentration range is typical for environmental situations, this review is restricted to these adsorption coefficients.Two environmental parameters have a strong influence on the adsorption of organic vapors on polar surfaces:temperature andrelative humidity (which is the most suitable parameter for describing the influence of ambient moisture). An exponential relationship was found for the adsorption coefficientversus relative humidity and the reciprocal temperature, respectively. Comparing the heats of adsorption, two different groups of substances emerged: polar chemicals exhibited heats of sorption which were higher than their corresponding heats of condensation due to their ability to form hydrogen bonds, while for the nonpolar compounds the opposite was true. Sorption takes place on the surface of an adsorbed water film when the relative humidity exceeds the value which is necessary to form a monomolecular layer of water on the surface of the adsorbent (≥ 30 % relative humidity). Therefore, at temperature below 0 °C, a change in the adsorption behavior might be expected due to a change of properties of the adsorbed water film. However, no alterations were observed at temperatures from -12 °C to + 4 °C (adsorption on quartz sand). The results were comparable to those at much higher temperatures (50 - 80 °C).A statistical approach for the prediction of the adsorption coefficients from physico-chemical parameters of the substances (vapor pressure, polarizability, and electron-donating capability) was developed and good agreement was found with experimental results and independent data from the literature. Finally, two special cases, the adsorption on bulk water and ice, are discussed.