Lecture Notes and Lab Procedure for Reactions Lab Experiments
Kinetics of Dye-Fading Reactions (Phenolphthalein)
CHEN4860 Chemical Engineering Lab II
Revised by: Dr. Y.Y. Lee and Dr. Timothy Placek
February 21, 2006
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.
Experiment 1.1: Batch Reactions
Objectives
During this experiment you will collect data to confirm the (pseudo) first order nature of
the reaction, determine k’ (pseudo-first order forward rate constant) , kf ( kf = k'[OH] ),
equilibrium constant K (K = kf / kr ) and the reverse rate constant kr as functions of
temperature. You will also determine the Arrhenius constant and activation energy.
Procedures
Procedures for analysis of the data closely follows 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.
Rate Experiments
1) Verify that that 550 nm is the maximum absorbance wavelength for the
phenolphthalein using the Ocean Optics spectrophotometer and dip probe in scanning
mode.
2) Rinse the batch reactor thoroughly with water to make sure no residue remains from
the previous lab group.
3) Make up the solution for your first run. The TA will make up a stock solution of 1.25
M sodium hydroxide, and a stock solution of 0.01 M phenolphthalein.. Your total
reaction volume for the batch reactors should be 500 ml. Verify that 80.0 ml of NaOH
stock in 500 ml total volume will give you 0.20 M NaOH. Verify that 0.5 ml of
phenolphthalein stock will give you a 1 x 10-5 M solution if injected into the reactor to
give 500 ml total volume. Operate the spectrometer software in the "kinetics mode" to
efficiently record data. Use a NaOH solution of 0.20 M and a phenolphthalein solution
of 1.0 x 10-5 at 3 different temperatures including room temperature.
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 to mix). Decide which points fall
into the linear "initial rate" region- (why will the log plot be non-linear as the reaction
proceeds)? What concentration do you expect for Ao?
Report Items for Experiment 1.1
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". You will get much better results if you
modify eqn. 8142 using the "user defined" functions in the software and setting
the y-intercept equal to the known starting concentration of 2.0 x 105
. Uncertainty estimates for the rate constants can be found using confidence
intervals in the fit parameters.
3. Using your experimental values for forward rate constants, find the activation
energy and frequency factor (or "Arrhenius 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 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.
Experiment 1.2: CSTR Experiment
1. Use flow rates of 0.25 L/min 2.0 M NaOH and 0.0001 M 0.25 L/min
Phenolphthalein solution. You can set this flow rate on the peristaltic feed
pump. The feed solutions will already be made up. Use a water flow rate of 2.0
L/min. This should correspond to a setting of 68.0 on the rotometer. These
settings should correspond to batch concentrations. Be sure to verify before
coming to lab, that these calculations to make sure they are correct (that they
give you the same initial reaction conditions as the batch reactors).
2. Carefully examine the reactor system prior to coming to lab and write a brief
operating procedure, with safety being the primary consideration. Make sure you
have run prior to coming to lab, a simulation considering both forward and
reverse rate constants (you can run this simulation in Polymath on the lab
computer also, once you measure operating temperature- bring a copy of your
simulation to lab on a diskette.
3. Run the 3 CSTRs in series at the operating conditions from (1). The reactors
should be filled with 0.2 M NaOH solution before feeding the three solutions of
step (1). Record the transient data to SST in reactor 1. Using the transient data in
reactor 1, perform a non-linear regression of Polymath, and determine the first
order rate constant. Use Eq. 4-47, p.217, textbook for regression analysis. This Eq.
is based on irreversible first order reaction. The reaction conversion in reactor 1 is
low enough to assume that the reaction is irreversible. Note that the CA0 in the
equation is the concentration of Phenolphthalein in the feed, the concentration
after mixing of the three liquid streams. The UV absorption value of CA0 may be
measured separately. At SST you can measure the concentration in each reactor
by placing the spectrometer probe in the reactor. Do the conversions correspond
to what was predicted by theory? Why or why not? Consider error analysis and
results of your tracer experiments in (4) below.
(Suggested) Report Items for Experiment 1.2
1. Report the non-linear analysis results including the graphical comparison of data
and the predicted equation. Comment on the degree of fitness (correlation
coefficient) and standard deviation of k value. Compare the k value obtained here
with that of batch reactor experiments. Explain the discrepancy, if any.
2. Set up an unsteady-state model for three CSTRs in series using Polymath. Your
model (Polymath) may include reverse reaction using the kr value (for reverse
reaction) obtained from your batch experiments. Allow enough reaction time in
the model to attain steady state values of CAs in each reactor. Compare the steadystate concentration in each reactor predicted by your model to what you measured
in lab. If significantly different, fully explain why yours was different. Back up
your explanation with evidence, calculations, sound reasoning, etc.
3. Top quality reports will also address other items of interest particular to your
experiment resulting in superior grades.