EDB workshop
The Hong Kong University of Science and Technology
Department of Chemistry
MICROSCALE ANALYTICAL EXPERIMENT
Determine the Order of Reaction of Phenolphthalein in Alkaline Solution
Dr TSUI, W. M.
Basic Safety & Waste Disposal Procedures
Personal Safety
1. Safety glasses must be worn at all times in the labs. Contact lenses should not be worn
in the lab because chemicals and particulates can get caught behind them, causing severe
eye damage.
2. Lab gown must be worn in the labs where an unexpected chemical spill may expose you
to the risk of injury. The following clothing is not permitted in the labs unless covered
by protective clothing:
 Open-toed shoes, sandals or other uncovered footwear; clothes that expose above the
ankles;
 Eating, chewing gum, and drinking in the lab.
 Untied long hair, dangling jewelry, loose clothing, and anything else that may get
caught in equipment, or dipped in chemicals.
3. Never work alone or unsupervised in the labs. Work only during the scheduled
laboratory periods and perform only authorized experiments.
 Wash your hands, arms, and then face, with soap and water as soon as possible after
leaving the lab.
 If you are uncertain about any safety aspect of an experiment, please ask your TA.
Lab Safety
 Make sure you know the exact locations of the safety features of the lab; e.g.,
eyewash fountains, safety showers, chemical spill kits, fire extinguishers, fire alarms,
fire blankets.
 Whenever possible, do not deal with incidents on your own. Your TA, the lab
instructor and the technician are all trained to respond to the sort of incidents that
may occur in this lab; e.g. chemical spills, cuts, burns, fires, medical emergencies, etc.
 Keep your work area clean and organized to reduce the possibility of accidents.
Know what you are doing and don't be careless.
 Avoid unnecessary exposure to chemicals. Never pipette by mouth. Never taste or
inhale a chemical on purpose. Wear gloves when directly working with hazardous
chemicals. Use hoods when appropriate.
1
EDB workshop



Take appropriate precautions. Keep flammables away from hot plates and open
flames. Wear gloves when using toxic, carcinogenic, or other hazardous chemicals.
Take care with corrosive acids and bases. Always pour concentrated acid slowly into
water (never water into acid). Read the Safety Issues section at the beginning of each
experiment.
Be informed. Material Safety Data Sheets (MSDS) summarize known hazards
associated with every chemical are available from the lab.
Chemicals & equipment may not be removed from the lab without permission from
the Lab Instructor.
Disposal of Chemical Waste
It is very important to properly dispose the chemical waste you generate. Follow these
guidelines and dispose of your waste properly, to avoid adding to the contamination of our
environment.
• Generate as little waste as possible. It is expensive to have hazardous waste removed and
disposed. Don't prepare more of a chemical than you expect to use.
• Never return unused portions of chemicals to the reagent bottle. At the end of your
experiment, unused reagent must be disposed of as waste, so don't pour out more than
you need.
• Don't discard chemicals down the sink or in the wastebasket, unless you are explicitly
told that it's okay to do so. Most of your chemicals will pose a threat to the environment
if disposed improperly.
• Place chemical waste only in the appropriate container. Often, more than one waste
container is provided to separate certain chemicals for safety or easier disposal. Pay
attention to the Waste Disposal information for each experiment in this lab manual, and
use the waste containers indicated. If you cannot find a waste bottle labeled with your
particular chemical, ask your TA where to dispose it.
• Fill in the appropriate waste inventory sheet. There is separate inventory sheet for each
waste container. Use it to record the chemical(s), concentrations and volume you dispose.
• Do not over-fill a waste container. Tell the technician the bottle is getting full and they
will replace it.
• Use the clearly marked GLASS containers to dispose of broken glass and Pasteur
pipettes. Do not place broken glass in the sink or wastebasket, to avoid serious injury to
an unsuspecting person.
• Use the clearly marked WASTE SOLIDS wide-mouth bottles to dispose of waste solids.
Waste solids include solid chemicals, filter paper, and weighing paper.
• If you realize that you disposed of a chemical in the wrong container, use the waste
inventory list provided by the waste container to let us know.
• If you have waste whose identity you can’t recall, you can often test your waste, e.g.,
with litmus paper, to deduce its identity. Do not add unidentified waste to the waste
bottles. You will force us to categorize the entire waste container as “unknown” which
becomes extremely expensive to identify and dispose of.
2
EDB workshop
DEPARTMENT OF CHEMISTRY
Hazard Identification Sheet
Name: ________________________
Experiment Title:
MICROSCALE ANALYTICAL EXPERIMENT – Determine the Order of Reaction of
Phenolphthalein in Alkaline Solution
Date of Experiment: ________________________________
 I have read and understand the instructions stated in ‘Basic Safety & Waste Disposal
Procedures’ concerning the proper use of the chemicals.
 I shall work in compliance with the aforementioned instructions made by the ‘Basic
Safety & Waste Disposal Procedures’.
SECTION I:
General Hazards Identified for the Experiment

I shall handle hazardous inorganic chemicals during the experiment. I shall follow
the instructions specified in ‘Basic Safety & Waste Disposal Procedures’.

I shall handle organic solvents and chemicals during the experiment. I shall follow
the instructions specified in ‘Basic Safety & Waste Disposal Procedures’.

I shall dispose hazardous waste chemicals during the experiment. I shall follow the
instructions specified in ‘Basic Safety & Waste Disposal Procedures’.
[please tick  where it is appropriate]
3
EDB workshop
SECTION II: Specific Chemical Hazards
For classification of hazards, please check the Material Safety Data Sheet (MSDS)
established
by
The
Sigma-Aldrich
Company
at:
http://www.sigmaaldrich.com/chemistry.html
Hazard Flammable Corrosive
Toxic
Irritant
Harmful
Oxidizing Explosive
Others
Chemical
Phenolphthalein
Sodium chloride
Sodium hydroxide
[please tick  where it is appropriate]
* Others: Carcinogenic, Allergic, Lachrymatory, Narcotic, Teratogenic, Reproductive
hazard, Central nervous system depression, Dangerous to the environment, Asphyxiant,
Poison
SECTION III: Declaration
This form to be completed by participant:
I am well aware of the hazard(s) of the experiment(s) to be carried out in the Chemistry
Laboratory.
Name:
Signature:
Date:
(in block letters)
4
EDB workshop
The Hong Kong University of Science and Technology
Department of Chemistry
MICROSCALE ANALYTICAL EXPERIMENT
Determine the Order of Reaction of Phenolphthalein in Alkaline Solution
Dr TSUI, W. M.
Introduction
Phenolphthalein is a common indicator to determine the end point of titrating an
acid with a base by observing the colour change from colourless to pink. If excess base is
present in the mixture, the pink colouration fades gradually. This is due to the reaction
between the dianion of phenolphthalein (Ph2-) and hydroxide ion to form a colourless
complex.
Phenolphthalein is colourless at pH 8 and lower, it becomes pink as the pH rinses,
and the rate of such conversion is extremely rapid. Decolourization results when the pH
is higher than 12, the rate of such reaction is sufficiently slow to be measured.
Reaction: Ph2- + OH-  POH3Rate law: rate = k [OH-]m [Ph2-]n
To determine the reaction rate, strongly basic solutions containing a trace of
phenolphthalein are used in this experiment. The concentration of OH- is in largely
excess (by a factor of at least 104 in any mixture) in any mixture, the OH- concentration,
therefore, can be assumed to remain essentially constant during each run.
So the rate law becomes:
Rate = k1 [Ph2-]n
, where k1 = k [OH-]m
Rearrange the equation:
ln [Ph2-] = -k1t + ln [Ph2-]o
A plot of ln [Ph2-] versus time should give a straight line if the reaction is pseudo
first order with respect to phenolphthalein, and the slope of the graph is equal to –k1.
According to Beer Lambert Law, A = bc, the absorbance of the solution is directly
5
EDB workshop
proportional to the concentration of dianion of phenolphthalein, so the ln [Ph2-] in the
above plot can be replaced by ln A, and the graph should give a straight line with the
slope of –k1. One can also determine the half life (t1/2) of the reaction, it is the time
needed for a reactant concentration to decrease by half compared to its initial
concentration. The length of half life of first order kinetics will be constant for a
particular concentration of reactant.
To determine the reaction order with respect to NaOH, recall
k1 = k [OH-]m
a straight line is given in the plot of k1 versus [NaOH] if the reaction is pseudo first order
in NaOH, and the slope of this line gives the overall rate constant k of the reaction. An
alternative approach is to determine the slope in the plot of ln k1 against ln [NaOH],
based on the equation ln k1 = ln k + m ln [OH-], the slope m should be the order with
respect to OH-.
Procedure
A. Calibration (set zero):
1.
Open the compartment of the colorimeter, insert into the compartment a cuvette
filled with 0.3M NaCl solution (This serves as the blank of the measurment).
2.
Close the compartment and press the green button on the colorimeter. The light on
the green button will be turned on. Wait until the light is turned off.
B. Record a UV spectrum of phenolphthalein
3.
Turn on the UV spectrometer instrument.
4.
After initialization, press “F4” (PC Ctrl) on the instrument.
5.
Then, turn on the computer, click the “UV Probe” icon
and the following
screen can be seen:
6
EDB workshop
6.
Press Connect button.
7.
Put the cuvette containing around 4 mL of blank solution, i.e. 0.4M NaOH solution
in this experiment) into the compartment.
8.
Then press Baseline to set zero.
9.
Select the wavelength by typing the wavelength in the box as shown in following
picture:
10. Press the method button
to set the scan range.
7
EDB workshop
11. Take out the cuvette from the compartment, add 2 drops of 0.1% phenolphthalein in
ethanol to the blank solution in the cuvette, invert the cell 2-3 times to mix the
solution properly. Put the cuvette into the compartment again.
12. Press Start button, and choose the specific location to save:
13. Then, press save button
spectral data and press
to save the spectrum. Press
button to see the
to locate the peak maximum.
14. Press file and then save as to save the “data print table” and “peck pick table”.
C. Setup of the colorimeter
15. Plug in the colorimeter into the PASPort interface.
16. Connect to the computer by the way of the USB slot.
17. Double click the icon of DataStudio. Then, click Create experiment.
18. In the Windows of DataStudio, close the graph windows and click Table under the
displays menu.
19. Then, press the Setup button. Under Measurement tab, based on your UV spectrum
obtained in Part B, tick only the appropriate filter (the absorbance), and leave the
others un-ticked.
20. Under the same windows, select the sampling rate according to the procedure.
21. After setting the sampling rate, close the windows.
8
EDB workshop
22. Back to the DataStudio windows, drag the absorbance of the filter you have chosen
in step 16 to the table. After doing this, you can see “Colour (xxx nm) Absorbance”
on the top of the table.
D. Preparation of sample solutions
23. Place 6 mL of the 0.4M standardize sodium hydroxide solution to a 10-mL
volumetric flask, add 0.3M NaCl solution up to the mark of the volumetric flask to
make a total volume of 10 mL solution. Shake well.
24. Repeat the above procedure according to the following table to prepare sodium
hydroxide solution with various concentrations.
Solution
NaOH / mL
NaCl / mL
A
6
4
B
7
3
C
8
2
D
9
1
E
10
0
E. Recording the data
25. Transfer about 4 mL of Solution A to a colorimeter cuvette, add 3 drops of 0.1%
phenolphthalein solution and invert the cell 2-3 times to mix the reactants properly.
26. Immediately place the cuvette into the sample compartment of the photometer and
close the lid.
27. Press the Start button to collect data, take the absorbance reading for every 30
seconds for a total of 10 minutes. End the data logging by pressing the Stop button.
28. From the File menu, select Export Data to save the data to your personal data disc,
using the file name Soln-A. Discard the solution and rinse the cuvette with D. I.
water.
29. Repeat step 25 – 28 to collect the data for Solution B – E. (NOTE: take the
absorbance reading for every 30 seconds for a total of 15 minutes for solution E).
9
EDB workshop
Data
Complete the following table
Solution
NaOH / mL
NaCl / mL
A
6
4
B
7
3
C
8
2
D
9
1
E
10
0
Molarity / M
Plot
i) A graph of absorbance (A) against time (t) and for Solution E.
ii) A graph of ln absorbance (ln A) against time (t) for Solution A – E.
Questions
1. From the plot of A vs t, determine the half life (t1/2) of the reaction in Solution E.
2. From the plot of ln A vs t, determine
a)
the reaction order with respect to phenolphthalein.
b)
the rate constant, k1, for each NaOH concentration.
3. Using the result in (2b), plot a graph of ln k1 against ln [NaOH].
a)
From the graph, determine the reaction order with respect to NaOH.
b)
What is the total reaction order?
10