Page 186 - Modeling of Chemical Kinetics and Reactor Design

156 Modeling of Chemical Kinetics and Reactor Design

Chemical methods involve removing a portion of the reacting
system, quenching of the reaction, inhibition of the reaction that occurs
within the sample, and direct determination of concentration using
standard analytical techniques—a spectroscopic method. These methods
provide absolute values of the concentration of the various species that
are present in the reaction mixture. However, it is difficult to automate
chemical methods, as the sampling procedure does not provide a
continuous record of the reaction progress. They are also not applicable
to very fast reaction techniques.
Physical methods involve measuring a physical property of the
system as the reaction progresses. It is often possible to obtain a
continuous record of the values of the property being measured, which
can be transformed into a continuous record of reactant and product
concentrations. Examples of physical methods that vary linearly with
concentrations include conductance (ionic reagents), absorption of
visible or ultraviolet light, optical density, the total pressure of gaseous
systems under nearly ideal conditions, and the rotations of polarized
light. An essential feature of physical methods involves continuous,
rapid response measurement without the need for sampling.
Physical techniques can be used to investigate first order reactions
because the absolute concentrations of the reactants or products are
not required. Dixon et. al [3] studied the base hydrolysis of cobalt
3+
complex, [Co(NH ) L] , where L = (CH ) SO, (NH ) C = O, (CH )O P
3
3 5
3
3 2
2 2
= O in glycine buffers.
( [
Co NH ) ] 3+  → Co NH ) OH ] 2+ + L (3-195)
( [
−
OH
L
3 5
3 5
The rate equation is:
( [ ) ] 3 +
( [
dCo NH L 3 +
− 3 5 = kCo NH ) ] (3-196)
L
dt 3 5
The release of dimethyl sulfoxide is accompanied by an increase
in absorbance (D) at 325 nm. The absorbance D is defined as log(I /I),
o
where I and I are the intensities of the incident and transmitted light,
o
respectively. Figure 3-15 illustrates the relationship between con-
centration and absorbance changes for the hydrolysis of the cobalt