Name: _________________________________________
Block: ________
Date: _____________
Titration Curve of a Weak Acid with Strong Base
Objective: To obtain and analyze a pH versus volume curve for the titration of ethanoic acid with
sodium hydroxide.
Materials:
pH sensor (Pasco)
SPARK (Pasco)
graduated cylinders (100-cm3 and 10-cm3)
Beaker (250-cm3 and 400-cm3)
Small glass funnel
50 cm3 buret
ring stand and buret clamp
Test tube clamp
Plastic pipets
Volummetric pipet (20.00 cm3)
phenolphthalein indicator solution
ethanoic acid solution, 0.1 mol dm-3 CH3COOH
buffer solutions (pH 4 and pH 10)
Standardized sodium hydroxide solution (~0.1 mol dm-3)
magnetic stir plate
Stir bar
Wash bottle with distilled water
Pre-Lab:
1. Watch the following video, which shows how to properly perform a titration:
http://www.dlt.ncssm.edu/core/Chapter16-Acid-Base_Equilibria/Chapter16-Labs/titration-lg.htm
2. Learn how to calibrate the pH sensor and SPARK by watching the video at
http://www.pasco.com/prodCatalog/PS/PS-2102_pasport-ph-sensor/index.cfm#resourcesTab
Procedure:
A. Buret set-up
1. Rinse the buret with ~ 5 cm3 of NaOH solution.
2. Rinse and fill the buret past the 0 cm3 mark with NaOH solution. Use a funnel for this step and
make sure the stopcock is closed before filling.
3. Drain some of the NaOH through the tip into a waste beaker to fill the tip and bring the level of
NaOH to exactly 0.00 cm3. If the 0.00 cm3 mark is passed, use a pipet to add more NaOH until the
meniscus is exactly at 0.00.
4. Place a clean 250-cm3 beaker on the balance and ZERO.
5. Using the volumetric pipet, place exactly 20.00 cm3 of ethanoic acid into the 250-cm3 beaker.
6. Add ~ 80 cm3 of distilled water to the 250-cm3 beaker.
7. Add 3 drops of phenolphthalein indicator to the beaker.
8. Add a stir bar to the beaker.
B. Trial run


No pH measurements need to be made in this run.
The purpose of this run is to determine the approximate amount of titrant needed to reach the endpoint.
This amount will be reffered to as V cm3.
MPHS Chemistry
1. Turn on the stirrer.
2. Open the stopcock valve and add sodium hydroxide until you reach you reach a permanent faint
pink end-point.
3. Record the volume needed (V cm3). This value (V) will be used
later to help you judge the size of volume additions.
C. Calibrate the pH sensor
1. Obtain small samples of pH 4 and pH 10 buffer solutions (enough
so that the pH sensor’s glass electrode will be fully submerged)
2. Following the instructions on p. 13-14 of the SPARK User’s Guide,
perform a two-point calibration using the two buffer solutions.
3. Allow the sensor to rest in the pH 4 solution while you move on to
the next section.
D. Buret and pH sensor set up
1. Repeat all steps in Part A and turn on the stirrer.
2. Clamp the pH electrode so that the bulb is in the solution but above
the stir barn (see diagram).
3. Record the initial pH for 0.00 cm3 of NaOH added.
4. Add sodium hydroxide solution in 5 cm3 portions until about (V - 5)
cm3, recording the pH and buret volume after each addition. V is
the volume determined in Part B Trial Run.
5. Now add sodium hydroxide in 1 cm3 increments and then 0.1 cm3
increments as you see the color change of the indicator start to
persist. Do this through the end point.
6. After the equivalence point, continue adding sodium hydroxide in
appropriate volumes while continuing the record the pH and buret
volume until a pH of 12 is obtained or until the buret reading =
50.00 cm3. Do not allow the NaOH to go below 50.00 on the buret.
Analysis:
1. Enter your data into a spreadsheet program such as Microsoft Excel or
Open Office Calc. (Column A = volume; column B = pH)
2. Construct the following:
a) A plot of the pH as a function of volume of base added.
b) A plot of the slope of the curve (first derivative) as a function of the volume of base added.
3. Determine the pH and volume at the equivalence point.
Helpful videos for performing the analysis.
How to find the equivalence point using slopes: http://youtu.be/DMqY3hSSrw0 (start at about 3:30)
How to generate the two graphs:
http://youtu.be/Cbo_h6r1Ric