Kinetics of Dye Fading
Kinetics of Dye Fading
Introduction
Phenolphthalein is a dye that is used as an acid-base indicator. It is colorless in acidic or neutral
solutions and turns bright red-violet (fuschia) as the solution becomes basic. In strongly basic
solutions the red color slowly fades and the solution again becomes colorless. The kinetics of this
‘fading” reaction can be analyzed by measuring the intensity of the red color and graphing the results.
Concepts
Kinetics
Order of reaction
Reaction rate
Colorimetry
Background
Phenolphthalein is a large organic molecule. In solutions where the pH < 8, it has the structure shown
in Figure 1, which is colorless. As the solution becomes basic and the pH increases, the
phenolphthalein molecule (abbreviated H2P) loses two hydrogen ions to form the red-violet dianion
(abbreviated P2-) shown in Figure 2.
The colorless-to-red transition of H2P to P2- (Equation 1) is very rapid and the red color develops
instantly when the pH reaches the indicated range. Gradually, however, if the concentration of
hydroxide ions remains high, the red P2- dianion will combine with hydroxide ions to form a third
species, POH3- (Equation 2), which is also colorless. The rate of this second reaction is much slower
than the first and depends on the concentration of phenolphthalein and hydroxide ions.
fast
H2P  P2- + 2 H+
colorless red
Equation 1
slow
P2- + OH-  POH3Red
colorless
Equation 2
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Kinetics of Dye Fading
The kinetics of the “fading” reaction can be followed by measuring the concentration of P2- versus time
and graphing the results. Figure 3 illustrates how the concentration of a reactant decreases with time
over the course of a reaction. Notice that the graph of concentration versus time is a curved line, not a
straight line. The curve levels off as it approaches the x-axis. This means that the reaction slows down
as the reactant concentration decreases.
Exactly how much the rate decreases as the reactant concentration decreases depends on the rate law
for the reaction. In the case of the reaction of P2- with OH- ions, the rate law has the general form
Rate = k[P2-]m[OH-]n
Equation 3
The exponents m and n are defined as the order of reaction for each reactant and k is the rate constant
for the reaction at a particular temperature. The values of the exponents m and n must be determined
by experiment. If the reaction is carried out under conditions where the concentration of OH- does not
change – by using a large excess of hydroxide ions – then the rate law will reduce to the form
Rate = k’[P2-]n
Equation 4
Where k’ is a new “pseudo” rate constant incorporating both the “true” rate constant k and the
experimentally constant [OH-]m term.
Mathematical treatment of the equations for the reaction rate and the rate law predicts the following
outcomes:
 If the fading reaction is first order in [P2-] (that is, n = 1), a graph of the natural log (ln) of
[P2-] versus time will give a straight line. The slope of the line is equal to –k’.
 If the fading reaction is second order in [P2-] (that is, n = 2), a graph of 1/[P2-] versus time
will give a straight line. The slope of the line is equal to –k’.
Experiment Overview
The purpose of this experiment is to use colorimetry and graphical analysis to determine how the rate
of the phenolphthalein fading reaction depends on the concentration of the dye. A colorimeter is a
special instrument that measures the absorbance of light. A known amount of phenolphthalein will be
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Kinetics of Dye Fading
added to a large excess of sodium hydroxide, and the absorbance (Abs) of the red solution will be
measured at specific time intervals. Absorbance is directly proportional to concentration, and so a
graph of absorbance versus time has the same characteristics as a graph of concentration versus time
(Figure 3). Graphing the absorbance data (ln Abs versus time and 1/Abs versus time) should reveal
whether the fading reaction is first or second order in phenolphthalein.
Pre-Lab Questions
Crystal violet (CV) is another indicator dye that combines with hydroxide ions to form a colorless
product (Equation 5). Crystal violet was added to 0.10 M NaOH and the solution immediately turned
violet. After 10 minutes, the color faded and the solution was almost colorless. The following
absorbance measurements were recorded.
CV+ + OH-  CVOH
violet
colorless
Reaction Time
1 min.
2 min.
3 min.
4 min.
5 min.
6 min.
7 min.
8 min.
9 min.
10 min.
Equation 5
Absorbance
0.366
0.251
0.176
0.124
0.089
0.065
0.048
0.037
0.029
0.023
ln(Abs)
1/Abs
1. Calculate the values of ln(Abs) and 1/Abs for each absorbance measurement to complete the table.
2. Create two graphs – plot ln(Abs) versus time on your first graph and 1/Abs versus time on your
second graph.
3. Which graph more closely approximates a straight line?
Is the reaction of crystal violet with hydroxide ions (Equation 5) first or second order in crystal
violet?
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Kinetics of Dye Fading
Materials
Dilute phenolphthalein solution, 1 drop
Sodium hydroxide, NaOH, 0.2 M, ~5 mL
LabQuest
Colorimeter
Digital thermometer
Tissues
Wash bottle and distilled water
Cuvette
Computer (for data analysis)
Safety precautions
Sodium hydroxide is a corrosive liquid. Avoid contact with eyes and skin and clean up all spills
immediately. Phenolphthalein is moderately toxic by ingestion. Wear chemical splash goggles and
apron. Wash hands thoroughly with soap and water before leaving the laboratory.
Procedure
1. Obtain and wear goggles.
2. Plug the LabQuest into power. Connect a Colorimeter to Channel 1 of the LabQuest interface.
3. Set up the LabQuest for the Colorimeter.
a. Turn on the LabQuest.
b. Make sure the live readout on the screen is Absorbance.
4. Set up the data collection mode. Adjust time by clicking on “length” and entering 60 (sec) in the
Interval box and 960 (sec) in the length box (total = 16 minutes).
5. Calibrate the Colorimeter.
a. Prepare a blank by filling an empty cuvette ¾ full with 0.2 M NaOH. Wipe the clear sides of the
cuvette with tissue, then place it in the cuvette slot of the Colorimeter with one of the clear sides
next to the arrow inside the compartment. Close the lid.
b. Set the wavelength on the Colorimeter to 565 nm using the arrows on the Colorimeter pad, then
press the CAL button. Calibration takes a few seconds. You will see the red light on the
colorimeter blink, then go dark.
6. Remove the cuvette from the Colorimeter compartment. Measure and (manually) record the initial
temperature of the sodium hydroxide solution in the cuvette using a digital thermometer.
7. Add one drop of dilute phenolphthalein to the cuvette and immediately press the Collect button on
the LabQuest (directly below the screen, or touch on the bottom left of the screen. This ensures
that the absorbance versus time measurement will accurately reflect the time of reaction from the
time of mixing.
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Kinetics of Dye Fading
8. Place the lid on the cuvette and carefully invert the cuvette several times to mix the solution.
9. Place the cuvette in the Colorimeter compartment and close the lid. Measurements will continue
for a total of 16 minutes.
10. Remove the cuvette. Measure the temperature of the solution in the cuvette, using the same digital
thermometer. Record the temperature.
11. Put your thumb drive into the USB port, then press FILE, then SAVE. Click on the box at the top
of the screen and type in your chosen name for the file. A keyboard will automatically appear,
allowing you to type in the name of the file. Be sure to click on the USB icon to save the file. If
the USB icon does not appear, wait a minute or so, then try again.
12. At home, upload your data to your computer in Logger Pro. Label the graph with your name.
Data Analysis and Discussion
1.
Delete the first data point by highlighting the first row, then holding down the alt button while
clicking on “-“, so that it does not interfere with your calculations. You may instead use the
Edit drop-down menu and “strike through data cells” to do the same thing.
2.
Examine your graph of absorbance versus time. Does the “rate of fading” of phenolphthalein
depend on the concentration of the dye? Explain.
3.
Calculate the values of ln(Abs) and 1/Abs for each absorbance measurement: create a new
“calculated column” under the “Data” drop-down menu. From the “Insert” menu, choose “new
graph”. Dr. Hart found creating the new graph for ln(Abs) before creating a second calculated
column for 1/Abs worked best.
4.
Which graph more closely approximates a straight line? Use the “linear fit” function (top
menu) to determine and demonstrate the linearity of the graphs.
5.
Is the reaction of phenolphthalein with hydroxide ions (Equation 2) first or second order in
phenolphthalein?
6.
What effect, if any, would an increase in temperature have on the results of the experiment?
7.
The concentration of sodium hydroxide is assumed to be constant throughout the reaction and
is thus included in the “reduced” rate law expression (See Equation 4 in the Background
section). Is this assumption valid? Prove it.
8.
Be sure to include a printout of your graphs when you turn in your lab report.
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