Reactions Engineering Lab:
Kinetics of dye-fading reactions
(phenolphthalein)
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) , kf ( kf = k'[OH] ),
the equilibrium constant K (K = kf / kr ) and the reverse rate constant kr as functions of
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 the entire data set on the program TableCurve 2-D.
Experiment Preparation- Instrument Setup
1. Turn on the light source using the toggle switch on the light source (blue
aluminum block). The light should warm up for several minutes before taking
readings.
2. Start the data acquisition program by clicking the “OIChem” icon on the
Windows Desktop.
3. Wearing gloves and eye protection (for this step and all subsequent steps), unplug
the quick-release hose connection to the NaOH feed tank. This valve is valved on
both ends and will not leak more than a drop or two. The feed tank contains a
solution of 0.40 M NaOH and will be used for both the batch and CSTR
experiments.
4. Make sure the NaOH stopcock is in the fully “off” position and is not leaking.
Remove the tank from the secondary containment tray and place securely on the
countertop with the spigot hanging over the edge of the sink.
Note: There is no sink at Station #2- for that setup carefully carry the NaOH tank to
the sink located on the counter against the wall behind station number one. You will
dilute your NaOH at this location- use extreme caution when carrying containers of
your dilute NaOH between the sink and the batch reactor back at station #2.
5. Carefully collect 200 ml of tap water in a 250 ml graduated cylinder. Measure as
accurately as possible by sighting in on the meniscus from a position level with
the 200 ml mark. Add the water to your thoroughly rinsed and dried reactor
vessel.
6.
Carefully collect 200 ml of 0.4 M NaOH solution from the tank spigot and add to
the reactor vessel.
Note: We are shooting for a total solution volume of 400 ml containing 0.2 M
NaOH and 1 x 10-5 M NaOH. Proper volumetric procedure would usually call for
the 200 ml of NaOH to be added to a larger cylinder or volumetric flask, and then
a quantity of water and phenolphthalein stock be added to achieve a total volume
of 400 ml. This procedure eliminates inaccuracy in total volume due to nonadditive effects of the different liquids. However, experimentation with our
system has shown however that by the time you add the 200 ml water, 200 ml
NaOH and 0.40 ml of 0.01 M phenolphthalein (in 50% ethanol solution), the total
volume ends up being very close to 400 ml. Also, we do not want to add our
phenolphthalein until the moment we begin data collection. Therefore we are
using the simpler procedure of step 6.)
7. Slowly start the stirrer. Try to get reasonably good mixing, but do not operate the
stirrer at such a high speed that a vortex forms and bubbles are drawn into
solution from the headspace. Avoiding bubbles that may become lodged in the
spectrophotometer dip probe tip is much more important than vigorous mixing in
the batch reaction. Mixing will be more of an issue in the CSTR experiment.
8. Swish the spectrophotometer dip probe around in the reactor solution to
thoroughly wet the interior of the probe tip. Place the lid on the reactor and jiggle
the spectrophotometer probe to remove any additional bubbles.
9. Make sure the spectrophotometer is in "Kinetics Mode" and check to see that the
instrument is set to plot the absorbance vs time data (bottom screen) at 554 nm.
10. Set the output to "Scope Mode" and begin a continuous scan. You are now
looking at the quantity of electrical charge coming off the capacitors at each
wavelength on the top screen.
11. Adjust the integration time so that the 554 nm transmission peak is discharging at
a charge count of about 3000. Keep the "averaging" to one point only.
12. Hold your finger over the light source and click the “Dark Current” button. Hold
your finger there until the computer prompts you so accept the reading.
13. Remove your finger and again jiggle the probe to make sure it is free of bubbles.
Click “Reference” and accept the reading when the computer prompts you.
14. Put the Spectrophotometer in “Absorbance” mode. You are now ready to begin
taking data.
15. Change from "Scope Mode" to "Absorbance Mode". You are now ready to begin
reading absorbance of the reaction mixture at 554 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
1 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. Set the bath to the desired temperature (or use room temperature for your first
run).
2. When at the desired temperature, simultaneously quickly add 0.40 ml of the
concentrated (0.01 M) stock phenolphthalein solution to the reactor using a 1.0 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. 25-30 minutes at room temperature).
3. Repeat for a total of 4 runs: one at room temperature, one approximately 10
degrees below room temperature, and two 10 to 20 degrees above room
temperature.
4. 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 to 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 entire 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.
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). Also check the
values of the reverse rate constants and see if they follow a similar relationship. These
calculations should allow you to predict conversion in the CSTRs at any temperature.
4) Use Polymath to construct a model showing concentration of phenolphthalein in the in
the 3 CSTRs as a function of time and 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.