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Experiment 4: Beer-Lambert Law and the Crystal Violet-Sodium Hydroxide Reaction
Background
Crystal Violet, CV, (structure shown on the left) is an intensely coloured dye. Reaction with NaOH results in a colourless product. The absorbance of a reaction mixture containing CV and NaOH will be proportional to the concentration of unreacted dye still present in solution. The reaction can therefore be monitored and the kinetics studied by measuring the absorbance of the mixture as a function of time.
Kinetic Rate Law
The rate of the reaction between CV and NaOH can be
written in a general form which is known as the rate law:
Rate = k [OH-] x [CV] y Equation 1
where k is the rate constant (or rate coefficient) for the
reaction and x is the order with respect to OH- and y is
the order with respect to CV. In this experiment,
the value of x and y will be determined experimentally.
In order to do this the reaction (and the kinetics) must be simplified by using a vast excess of NaOH: it can, therefore,
be assumed that the concentration of hydroxide ion does not change through the course of the reaction even though all of the CV may have been used up.
Consequently, the [OH-] term in the general rate law is
constant and can be grouped with k as in equation 2:
Rate = {k [OH-] x} [CV] y Equation 2
where k’ = k [OH-] x (k’ is termed a pseudo rate constant or the observed rate constant.)
The rate equation can therefore be written:
Rate = k’ [CV] y Equation 3
This approximation simplifies the reaction and allows k’ and y to be determined experimentally by carrying out two reactions: in the first kinetic run the concentration of NaOH is half that used in the second experiment. For both reactions the [OH-] is vastly in excess of that of CV.
Once k’ has been derived it is possible to determine k and x since the concentration of NaOH is known in both experiments.
Experiment 4, the CV/NaOH Reaction
The Beer-Lambert Law is used in the first part of the experiment which relates the absorbance to the concentration of the sample. Students will make up a series of solutions of CV in water using the virtual flask.
The absorbance spectra of the virtual solutions will be recorded using the spectrometer simulator:
A Beer-Lambert plot will be constructed in order to derive the molar extinction coefficient for CV. This information can then be used to determine the concentration of CV remaining in the reaction as a function of time in the second part of the experiment.
The CV/NaOH reaction will be performed in the virtual laboratory at two different NaOH concentrations in part two of the experiment.
Apparatus used in this experiment: Graduated flasks and various pipettes; timer; 1cm path length sample cell; pipette to release
9 cm3 of CV; pipette to release 1 cm 3 of NaOH to start the reaction; absorption spectrometer simulator.
The screen captures below show the experiment being performed:
CV is pipetted into the beaker, followed by NaOH to initiate the reaction. A sample of the CV/NaOH reaction mixture is then extracted from the reaction
and transferred to a cuvette.
The cuvette is then loaded into the spectrometer and absorbance readings taken as a function of time:
This allows the concentration of CV to be monitored as a function of time and for kinetic information about the reaction to be derived.
Any questions? Please contact our experts who will be happy to help.
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