53
Titration Curves – pH vs Volume of Base Experiment 8
1.0 Introduction
The titration of an acid with a strong base produces a distinctive “S” shaped curve when
the pH is graphed as a function of the volume of titrant base added. The reaction between
the acid and the base is a neutralization with water formation as its driving force. The
general equation for the reaction is:
HX(aq) + MOH(aq) HOH + MX(aq)
As the titration begins the solution of acid has a low pH. As base is added the pH value
slowly begins to rise until near the equivalence point there is a sharp rise in the pH as the
final H+ ions react. Once the acid has completely reacted the pH, climbs into the basic pH
range as more of the strong base is added. The purpose of this experiment is to explore the
variations in these curves for several different categories of acids – strong, weak and
polyprotic. More concentrated solutions of acids and the sodium hydroxide base will be
diluted until they are ready to use in the titration in more dilute form.
2.0 Procedure
There will be 3 acids and a bottle of sodium hydroxide solution that will all be 6 M
concentration in the lab. For the purposes of this experiment we will not be concerned with
exact concentrations so we will use graduated cylinders and beakers (or flasks) to prepare
the solutions we desire. For more exact work we would use pipets and volumetric flasks to
get much more accurate solution concentrations.
First calculate the number of mL of the 6 M sodium hydroxide that must be used to
prepare 100 mL of 0.1 M NaOH solution. Measure this amount of base into a beaker and
add water until a volume of 100 mL is reached. Careful stir the base with a stirring rod to
mix the final solution.
Next calculate the number of mL of each 6 M acid that must be used to prepare 50 mL of
0.15 M acid solution. For each acid add approximately 30 mL of DI water to a 100 mL
beaker then add the calculated amount of 6 M acid to this water (always dilute acids into
water not water into acids) and finally add enough DI water to bring the total solution
volume to 50 mL.
Clean and prepare a buret making sure it is rinsed with DI
water (remember the tip) and then with 3 small (~5 mL)
portions of the diluted NaOH solution. Set the buret up with a
buret clamp and ring stand so that the tip is above the notch
in the drop counter as seen on the left. The pH electrode will
go in the larger holder slot and a 100 mL beaker will be
positioned below and on a stirring plate with a magnetic stir
bar in it. The 100 mL beaker should be clean but not
necessarily dry. Once the stand, buret, drop counter, pH
electrode, beaker, stir plate and magnetic stir bar are set up,
we are ready to titrate. If you wish you can test the buret and
the drop counter with water before you rise and add the
NaOH solution. That way you can practice the control that is
needed to get the drops counted properly without squirting!
College of San Mateo Chemistry Dept
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Titration Curves – pH vs Volume of Base Experiment 8
MeasureNet Task: Setup for pH Titration
Your Actions: MeasureNet Response:
Press: MAIN MENU LCD: Shows Function Choices
Press: F3 – pH/mV LCD: Shows pH & mV options
Press: F2 – pH v VOLUME LCD: Shows SELECT OPTION screen
Press: CALIBRATE LCD: Asks for TEMP in Celsius
Use keypad to set temperature & Press LCD: Asks Enter pH of Buffer
ENTER (Use digital thermometer)
Use keypad to set pH 7.0 & Press ENTER LCD: Asks Put probe in buffer When Value
& Put the pH probe in pH 7.0 buffer soln Stops Changing Press Enter
Swirl the probe in the buffer & When LCD: Shows For Single Point Cal. Press F1
value is steady Press: ENTER etc.
Press: F1 (This is a ONE buffer cal.) LCD: Shows Select Option screen
Press: DISPLAY LCD: Shows pH Measurement screen
Once the station is setup you might want to test the drop counter by allowing a few drops
to fall through it into a waste beaker. Careful – you must keep the drop rate fairly slow and
steady- NO squirts. Practice closing and opening the tip of the buret until you can get the
drop counter to flash at a steady and consistent rate. You want drops at a 1 to 2 per second
rate – careful not too fast and no squirts! When you are ready you can replace the stir plate
with a clean (but not necessarily dry) 100 mL beaker and a stir bar. Now your setup should
look like the picture shown before. Press DISPLAY to go to the measurement screen if you
haven’t already.
With a 10.0 mL pipet and pipet bulb, transfer a 10.0 mL aliquot (a measured liquid
portion) of one of the acid solutions into the 100 beaker under the drop counter. Add
about 20 mL of deionized water so the pH electrode sensor is submerged, begin stirring the
solution with a magnetic stirrer.
MeasureNet Task: pH Titration (Start on the DISPLAY screen)
Your Actions: MeasureNet Response:
Press: START/STOP LCD: Asks INITIAL BURET READING
Respond with Reading (2 decimals) Using LCD: Asks PRESS START to COLLECT
keypad & Press: ENTER DATA
Press: START/STOP LCD: Shows pH & Vol readings
Carefully add NaOH keeping drops steady LCD: Shows the pH titration graph
When the graph clearly tops out STOP the LCD: Asks FINAL BURET READING
buret FIRST then Press: START/STOP
Respond with Reading (2 decimals) Using LCD: Asks SELECT A FUNCTION
keypad & Press: ENTER
College of San Mateo Chemistry Dept
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Titration Curves – pH vs Volume of Base Experiment 8
MeasureNet Task: pH Titration (CONTINUED)
Your Actions: MeasureNet Response:
Press: FILE OPTIONS LCD: Shows Function Choices
Press: F3 – SAVE LCD: Asks Enter File Number
Respond “081” on Keypad & Press ENTER LCD: Shows saving data & Asks SELECT a
(Lab 08 File 1) Function (Choose what to do next)
Now you can discard the titrated acid solution in a waste beaker. As you end a titration
remove the stir plate and lower the titration beaker allowing you to rinse the pH electrode
with DI water from a wash bottle and into the titration beaker. Add this titration waste to a
waste container. Repeat the titration with the other two acid samples. To start a new
titration on MeasureNet press DISPLAY then repeat the procedure above. Save the results
of these titrations as files “082” and “083”.
3.0 Before You Leave
When you have completed the three titrations you may clean your work area. Make sure
to pour your remaining solutions in an appropriate waste container and return your
glassware to its appropriate storage. Rinse everything with DI water FIRST!!!
4.0 Calculations and spreadsheet
There are no calculations for this lab instead you are to print out the final pH titration
graphs for each of your acids. Your instructor can have you print these out directly from the
MeasureNet stations instead of saving the files for later processing (or maybe both). For this
report you should print a graph for each of the acids and label each. The MeasureNet
stations can print by request from the same function page that is used to save files.
The final report for this lab should include the following materials:
1) graphs of each of the 3 pH titrations – label each as strong, weak or polyprotic
2) equivalence points should be marked (What is different about the polyprotic acid?)
3) mark the buffer region(s) of the weak acid (Will the strong acid have a buffer region?)
4) indicate how to find the pKa of the weak acid (Will the strong acid have a pKa?)
If you use the saved files to do plots in Excel the file with all the station data will be
named “Lab08TitrationCurves.csv”. Use the XY Scatter plot type to print titration curves
for each of the acids.
College of San Mateo Chemistry Dept
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Titration Curves – pH vs Volume of Base Experiment 8
Report Sheet NAME__________________________
mL of 6 M NaOH used to prepare 100 mL of 0.1 M NaOH ________ mL
mL of 6 M Acids used to prepare 50 mL of 0.15 M acids _________mL
What are the major differences between the titration curves of the 3 acids?
Which acid(s) have buffer regions?
Does the strong acid have a pKa value? Explain.
Explain how to find a pKa value from a titration graph.
College of San Mateo Chemistry Dept