Reactions Engineering Lab:
Kinetics of dye-fading reactions
(Updated 6/1/07)
Introduction
In this lab, you will determine the rate constants for the fading reaction of
phenolphthalein with NaOH. Note, this is not the fast color-change
reaction that occurs near pH 8, but a slower fading reaction from pink to
colorless that occurs at higher pH.
Refer to the references cited on this page and in class notes (in lab packet
handed out in class) before planning your experiment and be sure you
understand 1) the chemistry of the reaction 2) how to operate the Ocean
Optics Spectrometer- especially pages 9-13, 22, 33 in the on-line
spectrometer manual and the 1-page on-line dip probe manual.
Be sure to have calculations and procedures completed before coming to
class.
DAY 1: BATCH REACTIONS
OBJECTIVES
In week one you will perform an experiment to confirm the (pseudo) first order nature of
the reaction, determine k’ (pseudo-first order forward rate constant) and kf ( kf = k'[OH] )
as a function of temperature. You will also determine reverse rate constant k and the
equilibrium constant K (K = kf / kr ) at room temperature. Your analysis will also include
determination the Arrhenius constant (Ao) and activation energy (Ea). Procedures for
analysis of the data closely follow that outlined in the journal article by Andres and Hile
( Alkaline Fading of Organic Dyes: An Ideal Reaction for Homogeneous Reactor
Experiments). You will also add to that an alternative non-linear fit of room temperature
data using the program TableCurve 2-D.
Experiment Preparation- Instrument Setup
1. Start the data acquisition program by clicking the OIChem icon on the Windows
Desktop.
2. Turn on the light source using the toggle switch on the light source (blue aluminum
block) and let warm up for at least 5 minutes.
3. Add 248 ml of tap water and 250 ml of 0.2 M NaOH stock solution to the reactor
vessel. Cover with the plastic cover and insert the dip probe. Jiggle the probe to
remove any bubbles that may be in the light path. Start the stirrer slowly to prevent
splashing.
4. Put the spectrophotometer in "Kinetics Mode" and make sure the instrument is set
to plot at 550 nm.
5. Set the output to "Scope Mode" and begin a continuous scan.
6. Adjust the integration time so that the 550 nm transmission peak is discharging at a
charge count of about 2500- 3000. Keep the "averaging" to one point only.
7. Click "Scan Reference" and then OK to accept the reading. Put your finger in front
of the light source to block light from entering the optical fiber-keep finger there
until completely finished with the dark current procedure. Click "Scan Dark" and
then "OK" to accept the reading.
8. Change from "Scope Mode" to "Absorbance Mode". You are now ready to begin
reading absorbance of the reaction mixture at 550 nm. The instrument will
automatically subtract out any electronic noise present in no-light conditions using
the information from the "Dark Scan". The instrument will use the readings from
the "Reference Scan" to determine 100% Transmittance = zero Absorbance. We
will use the knowledge that our phenolphthalein concentration at time zero is 2 x
10-5 to convert absorbance readings to concentration during analysis of the data by
assuming a linear 2-pt calibration according to Beer's law.
Rate Experiments
1. Place the cover on the reaction vessel and allow it to come to reaction
temperature. Keep the probe out of the solution and in clean water until ready to
take data.
2. When the solution is at the desired temperature, place the probe in solution.
"Jiggle" the probe to remove any bubbles that may have entered the solution. Redo the probe reference procedure if much time has elapsed since the last
reference.
3. Simultaneously add quickly 2.0 ml of the concentrated (0.005 M) stock
phenolphthalein solution to the reactor using a 3.0 ml syringe and start the
spectrophotometer data acquisition program (continuous scan). Allow the
reaction and data collection to proceed until the solution is very close to
equilibrium concentration (approx. 55 minutes).
4. Repeat steps 1-5 for three other temperatures, for a total of 4 runs. For these runs
it will be sufficient to collect initial rated data only- about 100 points should be
sufficient.
5. Make plots to check for consistency of first order (in phenolphthalein) behavior of
your reaction data before leaving lab (plot ln Abs. vs. time and check linearity).
What is Ca @ t = 0 in terms of Absorbance at t = 0? (Assume first order behavior
in OH concentration)- you can get this from extrapolation of initial rate data to
time zero and assuming Beer's law (absorbance directly proportional to
concentration in dilute solution). You may have to remove the first few points
before analyzing data (when adding the phenolphthalein it may take a few
seconds go mix). Decide which points fall into the linear "initial rate" region(why will the ln plot be non-linear as the reaction proceeds to later times)? What
concentration do you expect for Ao?
REPORT ITEMS- BATCH*
1) Construct a linear plot to evaluate the forward rate constant at each temperature for a
given NaOH concentration (consider placing all three temperatures on the same plot to
aid in comparison). (Note: do this before the CSTR lab) Make sure that you use the
linear part of the plot only, when using linear regression to find initial rates.
2)Analyze the room temperature equilibrium data set (except for any extraneous points
at, or immediately after injection) in TableCurve 2-D in order to get forward and reverse
rates constants as well as the equilibrium constant, K. Equation number 8142 is the
solution to the kinetics for a first order reversible reaction. When data is plotted as time
(s) vs. concentration (mol/ l), the y-intercept is parameter "a" (initial concentration), the
forward pseudo 1st order reaction constant in units of L/sec is parameter "b", and the
reverse rate constant in units of L/sec is parameter "c".
2) Using your experimental values for forward rate constants, find the activation energy
and frequency factor (or "Arrhenious constant") for the above. Use 95%
confidence intervals to aid in estimating error (note: do this before the CSTR lab).
3) Use eqns. 23 and 24 from the reactions notes to model the CSTR reactor startup and
steady state concentration at a total flow rate of 100 ml/min and a total volume of 500 ml.
4) Use Polymath to construct a model showing concentration of phenolphthalein in the in
the CSTR as a function of time and temperature using forward rate constants. Add the
reverse rate constant to your model at room temperature (you should have kf(T) and kr(T)
from your batch data k vs 1/T plots) when starting with a water hydroxide solution in the
reactors, and beginning the flow of phenolphthalein solution at time zero. This should be
done prior to running the CSTR experiment and presented during "pre-lab" for day 2 of
the reactions lab. After the CSTR lab you may want to modify this model to take into
account any inconsistencies in flow rates, etc.