CHEM 203 Lab 4: Titration for Unknown Acid Identification Winter 2025
Lab 4: Identification of unknown acids by titration
In this experiment, you will use titration to determine the identity and concentration of one unknown
weak monoprotic acid.
You will work and discuss in pairs for this lab, Before your lab...
BUT you will individually submit a completed
report. 1. Read briefly through…
✓ Lab 4 (this document)
Complete your lab Worksheet at home within
2. Complete…
one week after your lab session. Upload the
Worksheet file (as a PDF) to Gradescope by ✓ Pre-lab 4 (D2L quiz), with a score of
the starting time of your lab section. 50% or higher
✓ Questions P1 and P2 in this
document – Page 12. You will need
What laboratory techniques and skills will you these ideas and values in the lab, so
practice? come prepared! The pre-lab quiz is
• Preparing solutions in volumetric closely related and will help.
glassware
• Performing acid-base titrations You will not be allowed to
enter the lab have not passed
• Determining the identity of an unknown the pre-lab quiz, which closes
acid 1-h before lab. Contact your
• Calibrating and using a pH meter TA or lab coordinator if you
• Managing your lab time have issues
What chemical concepts will you apply? If concepts are unclear or new, refer to the
course textbook to review them.
• Solution concentration and dilutions
• Section 3.3 (Open Stax) On your lab day...
• Calculating pH of solutions of weak • Wear clothes & footwear that covers
acids and bases any exposed skin below the hem of the
• Section 14.3 (Open Stax) lab coat, including your ankles.
• Interpreting pH curves for acid-base • Bring your lab coat & safety glasses
titrations • Bring printed copies of...
• Section 14.7 (Open Stax) ✓ Lab 4
✓ Lab Manual Appendix H
What communication and reporting skills will (Mettler-Toledo pH meter)
you use? • Bring a pen (not pencil) and scientific
calculator
• Recording data and observations
• Arrive at least 5-minutes early
• Presenting data and results in tables
• Presenting and interpreting data from
graphs
Lab 4: Background & Procedures Do Not Scan Me to Gradescope Page 1
CHEM 203 Lab 4: Titration for Unknown Acid Identification Winter 2025
,CHEM 203 Lab 4: Identification of unknown acids by titration Winter 2025
Key Safety Information
All solutions in this lab must be disposed of in the aqueous (inorganic) waste container.
Unknown Acid, since the acid you are working with is unknown you will
want to work with it very carefully as you cannot be sure of the hazards.
Corrosive Aquatic Sodium hydroxide, NaOH (aq), is corrosive and can cause skin burns and
toxicity eye damage. It also is harmful to aquatic life.
Background
In this experiment, you will use both qualitative and quantitative properties to determine the identity and
concentration of an unknown acid. To do this analysis, you will perform a titration of your unknown acid
sample, specifically a potentiometric titration where you use a pH meter and record pH values during titration,
combined with a visual titration using a colour indicator to show the progress of the reaction.
Using your findings, you will be able to comment on the equilibrium acid ionization constant (Ka) and compare
the effectiveness of the two titration methods.
What is a titration?
Titration is a method used to determine the concentration of one
solution by reacting it with a second solution of very precisely known Figure 1: Titration setup
concentration.
A typical set-up for a titration is shown in Figure 1. Usually, the
burette is filled with titrant (the solution of known concentration
shown as yellow in the diagram), while some of the analyte (the
solution that is being analyzed) is placed in a flask. You will add the
titrant slowly, until exactly enough titrant has been added to react
with all of the analyte in the flask - this is the equivalence point.
Usually, the equivalence point cannot be seen directly. To help make
the endpoint visible, a colour indicator may be used. This kind of
compound is one colour before the equivalence point and a different
color after. One common colour indicator is phenolphthalein, which is
colourless in an acidic solution but changes to bright pink in a basic
solution. Since the colour change of the indicator may not happen
exactly at the equivalence point, the point where the indicator changes color (and where the titration is halted)
is called the endpoint. The volume of titrant needed to reach this point is called the titre.
Lab 4: Background & Procedures Do Not Scan Me to Gradescope Page 2
, CHEM 203 Lab 4: Identification of unknown acids by titration Winter 2025
For any titration, knowing the relationship between the moles of analyte in your titration sample and the moles
of titrant used to reach the equivalence point will allow you to determine the concentration of your analyte.
With care, manual titrations can be extremely accurate; although many modern laboratories carry out analyses
instrumentally, manual titrations are still commonly performed.
One real-life example of using a manual titration would be if some small farms outside of Calgary were having
unusual livestock illnesses and small-scale crop failures. The owners of the farms could send in some samples of
liquid that they found on-site to a lab for analysis. Because they are dealing with a time-sensitive issue, titration
may provide useful information more rapidly and allow for preliminary determination of the liquid. More
complex analyses, which take a few days, cannot be used as effectively.
What are acid-base titrations?
In this experiment, you will perform an acid-base titration, where the reaction involved is a neutralization
reaction.
Neutralization of a weak acid ‘HA’:
HA(aq) + OH⁻(aq) → A⁻(aq) + H2O (l) [1]
Neutralization is an ideal type of reaction to use in a titration due to the fast reaction speed and the large
equilibrium constant (K). K is so large that the reaction will proceed nearly to “completion”. At each step, you
can assume that all of the available reactants are consumed. Since these titrations involve a change in pH as the
reaction proceeds, you will use a pH probe to monitor the reaction in a potentiometric titration.
What can you learn from titration curves?
In a potentiometric titration, pH of the analyte is plotted against the amount of titrant added. By using a pH
probe to monitor the acidity of the solution, the endpoint can be determined without relying on a colour
indicator. A typical plot of pH vs. mL of titrant added for the titration of a weak acid is shown in Figure 2 so that
we can highlights some useful regions on the titration curve. All the points described below would also exist for
the titration of a weak base (with a strong acid). However, you will only perform a titration of a weak acid during
Lab 4.
A. Initial pH: Before any titrant is added, the major species in solution is the weak acid; therefore, the pH is
determined only by the equilibrium ionization of the weak acid as it reacts with water to produce the conjugate
base:
Equilibrium ionization of HA: HA(aq) + H2O(l) ⇋ A⁻(aq) + H3O+(aq) Ka
B. Buffer region: Once some titrant is added, there will be significant amounts of both the weak acid and its
conjugate base present in the flask. This solution is a buffer. Notice that the pH changes slowly as titrant is
added in this region. The point on the titration curve marked with a star (⋆) is the half-equivalence point. At this
point, enough base has been added to react with exactly half of the weak acid available.
Lab 4: Background & Procedures Do Not Scan Me to Gradescope Page 3
Lab 4: Identification of unknown acids by titration
In this experiment, you will use titration to determine the identity and concentration of one unknown
weak monoprotic acid.
You will work and discuss in pairs for this lab, Before your lab...
BUT you will individually submit a completed
report. 1. Read briefly through…
✓ Lab 4 (this document)
Complete your lab Worksheet at home within
2. Complete…
one week after your lab session. Upload the
Worksheet file (as a PDF) to Gradescope by ✓ Pre-lab 4 (D2L quiz), with a score of
the starting time of your lab section. 50% or higher
✓ Questions P1 and P2 in this
document – Page 12. You will need
What laboratory techniques and skills will you these ideas and values in the lab, so
practice? come prepared! The pre-lab quiz is
• Preparing solutions in volumetric closely related and will help.
glassware
• Performing acid-base titrations You will not be allowed to
enter the lab have not passed
• Determining the identity of an unknown the pre-lab quiz, which closes
acid 1-h before lab. Contact your
• Calibrating and using a pH meter TA or lab coordinator if you
• Managing your lab time have issues
What chemical concepts will you apply? If concepts are unclear or new, refer to the
course textbook to review them.
• Solution concentration and dilutions
• Section 3.3 (Open Stax) On your lab day...
• Calculating pH of solutions of weak • Wear clothes & footwear that covers
acids and bases any exposed skin below the hem of the
• Section 14.3 (Open Stax) lab coat, including your ankles.
• Interpreting pH curves for acid-base • Bring your lab coat & safety glasses
titrations • Bring printed copies of...
• Section 14.7 (Open Stax) ✓ Lab 4
✓ Lab Manual Appendix H
What communication and reporting skills will (Mettler-Toledo pH meter)
you use? • Bring a pen (not pencil) and scientific
calculator
• Recording data and observations
• Arrive at least 5-minutes early
• Presenting data and results in tables
• Presenting and interpreting data from
graphs
Lab 4: Background & Procedures Do Not Scan Me to Gradescope Page 1
CHEM 203 Lab 4: Titration for Unknown Acid Identification Winter 2025
,CHEM 203 Lab 4: Identification of unknown acids by titration Winter 2025
Key Safety Information
All solutions in this lab must be disposed of in the aqueous (inorganic) waste container.
Unknown Acid, since the acid you are working with is unknown you will
want to work with it very carefully as you cannot be sure of the hazards.
Corrosive Aquatic Sodium hydroxide, NaOH (aq), is corrosive and can cause skin burns and
toxicity eye damage. It also is harmful to aquatic life.
Background
In this experiment, you will use both qualitative and quantitative properties to determine the identity and
concentration of an unknown acid. To do this analysis, you will perform a titration of your unknown acid
sample, specifically a potentiometric titration where you use a pH meter and record pH values during titration,
combined with a visual titration using a colour indicator to show the progress of the reaction.
Using your findings, you will be able to comment on the equilibrium acid ionization constant (Ka) and compare
the effectiveness of the two titration methods.
What is a titration?
Titration is a method used to determine the concentration of one
solution by reacting it with a second solution of very precisely known Figure 1: Titration setup
concentration.
A typical set-up for a titration is shown in Figure 1. Usually, the
burette is filled with titrant (the solution of known concentration
shown as yellow in the diagram), while some of the analyte (the
solution that is being analyzed) is placed in a flask. You will add the
titrant slowly, until exactly enough titrant has been added to react
with all of the analyte in the flask - this is the equivalence point.
Usually, the equivalence point cannot be seen directly. To help make
the endpoint visible, a colour indicator may be used. This kind of
compound is one colour before the equivalence point and a different
color after. One common colour indicator is phenolphthalein, which is
colourless in an acidic solution but changes to bright pink in a basic
solution. Since the colour change of the indicator may not happen
exactly at the equivalence point, the point where the indicator changes color (and where the titration is halted)
is called the endpoint. The volume of titrant needed to reach this point is called the titre.
Lab 4: Background & Procedures Do Not Scan Me to Gradescope Page 2
, CHEM 203 Lab 4: Identification of unknown acids by titration Winter 2025
For any titration, knowing the relationship between the moles of analyte in your titration sample and the moles
of titrant used to reach the equivalence point will allow you to determine the concentration of your analyte.
With care, manual titrations can be extremely accurate; although many modern laboratories carry out analyses
instrumentally, manual titrations are still commonly performed.
One real-life example of using a manual titration would be if some small farms outside of Calgary were having
unusual livestock illnesses and small-scale crop failures. The owners of the farms could send in some samples of
liquid that they found on-site to a lab for analysis. Because they are dealing with a time-sensitive issue, titration
may provide useful information more rapidly and allow for preliminary determination of the liquid. More
complex analyses, which take a few days, cannot be used as effectively.
What are acid-base titrations?
In this experiment, you will perform an acid-base titration, where the reaction involved is a neutralization
reaction.
Neutralization of a weak acid ‘HA’:
HA(aq) + OH⁻(aq) → A⁻(aq) + H2O (l) [1]
Neutralization is an ideal type of reaction to use in a titration due to the fast reaction speed and the large
equilibrium constant (K). K is so large that the reaction will proceed nearly to “completion”. At each step, you
can assume that all of the available reactants are consumed. Since these titrations involve a change in pH as the
reaction proceeds, you will use a pH probe to monitor the reaction in a potentiometric titration.
What can you learn from titration curves?
In a potentiometric titration, pH of the analyte is plotted against the amount of titrant added. By using a pH
probe to monitor the acidity of the solution, the endpoint can be determined without relying on a colour
indicator. A typical plot of pH vs. mL of titrant added for the titration of a weak acid is shown in Figure 2 so that
we can highlights some useful regions on the titration curve. All the points described below would also exist for
the titration of a weak base (with a strong acid). However, you will only perform a titration of a weak acid during
Lab 4.
A. Initial pH: Before any titrant is added, the major species in solution is the weak acid; therefore, the pH is
determined only by the equilibrium ionization of the weak acid as it reacts with water to produce the conjugate
base:
Equilibrium ionization of HA: HA(aq) + H2O(l) ⇋ A⁻(aq) + H3O+(aq) Ka
B. Buffer region: Once some titrant is added, there will be significant amounts of both the weak acid and its
conjugate base present in the flask. This solution is a buffer. Notice that the pH changes slowly as titrant is
added in this region. The point on the titration curve marked with a star (⋆) is the half-equivalence point. At this
point, enough base has been added to react with exactly half of the weak acid available.
Lab 4: Background & Procedures Do Not Scan Me to Gradescope Page 3
