Part 1 - Get a Lab Appointment and Install Software:
Set up an Account on the Scheduler (FIRST TIME USING NANSLO):
Find the email from your instructor with the URL (link) to sign up at the scheduler.
Set up your scheduling system account and schedule your lab appointment.
NOTE: You cannot make an appointment until two weeks prior to the start date of this lab assignment.
You can get your username and password from your email to schedule within this time frame.
Install the Citrix software: – go to http://receiver.citrix.com and click
download > accept > run > install (FIRST TIME USING NANSLO).
You only have to do this ONCE. Do NOT open it after installing. It will work automatically when you go
to your lab. (more info at
http://www.wiche.edu/info/nanslo/creative_science/Installing_Citrix_Receiver_Program.pdf)
Scheduling Additional Lab Appointments:
Get your scheduler account username and password from your email.
Go to the URL (link) given to you by your instructor and set up your appointment.
(more info at http://www.wiche.edu/nanslo/creative-science-solutions/students-scheduling-labs)
Changing Your Scheduled Lab Appointment:
Get your scheduler account username and password from your email. Go to http://scheduler.nanslo.org
and select the “I am a student” button. Log in to go to the student dashboard and modify your
appointment time. (more info at http://www.wiche.edu/nanslo/creative-science-solutions/studentsscheduling-labs)
Part 2 – Before Lab Day:
Read your lab experiment background and procedure below, pages 1-9.
Submit your completed Pre-Lab Questions (page 6) per your faculty’s instructions.
Watch the Titration Apparatus Control Panel Video Tutorial
http://www.wiche.edu/nanslo/lab-tutorials#titration
Part 3 – Lab Day
Log in to your lab session – 2 options:
1)Retrieve your email from the scheduler with your appointment info or
2) Log in to the student dashboard and join your session by going to http://scheduler.nanslo.org
NOTE: You cannot log in to your session before the date and start time of your appointment. Use
Internet Explorer or Firefox.
Click on the yellow button on the bottom of the screen and follow the instructions to talk to your lab
partners and the lab tech..
Remote Lab Activity
SUBJECT SEMESTER: ____________
TITLE OF LAB: Acid Base Titration
Lab format: This lab is a remote lab activity.
Relationship to theory (if appropriate): In this lab you will learn about how acids and bases
interact in solution and how to interpret graphs of pH when a titration is done.
Instructions for Instructors: This protocol is written under an open source CC BY license. You
may use the procedure as is or modify as necessary for your class. Be sure to let your students
know if they should complete optional exercises in this lab procedure as lab technicians will not
know if you want your students to complete optional exercises.
Instructions for Students: Read the complete laboratory procedure before coming to lab.
Under the experimental sections, complete all pre-lab materials before logging on to the
remote lab. Complete data collection sections during your online period, and answer questions
in analysis sections after your online period. Your instructor will let you know if you are
required to complete any optional exercises in this lab.
Remote Resources: Primary – Titration Apparatus; Secondary – Acetic Acid Titration.
CONTENTS FOR THIS NANSLO LAB ACTIVITY:
Learning Objectives........................................................................................................
Background Information ...............................................................................................
Pre-lab Questions ..........................................................................................................
Equipment .....................................................................................................................
Preparing for this NANSLO Lab Activity .........................................................................
Experimental Procedure ...............................................................................................
Exercise 1: Exploratory Observations ...........................................................................
Exercise 2: Quantitative Measurements .......................................................................
Post-Lab Analysis ...........................................................................................................
(Optional) Advanced Post-lab Analysis .........................................................................
Creative Commons Licensing ........................................................................................
U.S. Department of Labor Information .........................................................................
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LEARNING OBJECTIVES:
After completing this laboratory experiment, you should be able to do the following things:
1.
2.
3.
4.
5.
Perform an acid base titration.
Determine the concentration of an unknown analyte using titration.
Describe the purpose and mechanism of a titration.
Understand what species are present during titration.
Predict the pH regime (acidic, neutral, and basic) at the equivalent point of a titrated
acid.
BACKGROUND INFORMATION:
What is a titration?
A titration is a procedure used to analyze a substance for the concentration of a particular
compound. In this experiment, a titration is used to determine the concentration of an acid in a
sample by adding a known quantity of a base.
What is an acid and what is a base?
In order to fully understand the definition of acids and bases, it is important to review some
other terms. A hydrogen atom contains one electron, one proton and one neutron. When
hydrogen is written H+, that means that one electron is missing and there is only a proton to
produce a positive charge. Therefore, a hydrogen atom without an electron is commonly
referred to as a proton.
There are three ways to describe acids and bases:
1. Arrhenius: An acid is a substance that ionizes in solution to donate an H+, and a base is
a substance that ionizes to produce an OH-.
2. Bronsted-Lowry: An acid is a substance that donates an H+, and a base is a substance
that accepts an H+.
3. Lewis: An acid is a substance that accepts a lone pair of electrons, and a base is a
substance that donates a lone pair of electrons.
Some acids and bases fit one definition over the others better while others may fit more than
one definition at the same time. For example, NaOH can be classified as an Arrhenius base,
because it produces an OH- in water, and it can also be a Bronsted-Lowry base because that OHcan accept a proton. Additionally, NaOH can be a Lewis base because that OH- has a lone pair
of electrons that can be donated. Likewise, acetic acid, HC2H3O2, is a Bronsted acid and an
Arrhenius acid since it donates a proton.
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There are other acids and bases that fit only one description best. For example, AlCl3 is a Lewis
acid, because it can accept a lone pair of electrons. However, AlCl3 does not fit the other two
definitions of acids, because it does not have the ability to donate a proton (notice there is no
hydrogen in the formula).
Once an acid has donated a proton, it is then referred to as the conjugate base. Likewise, when
a base accepts a proton, it is then the conjugate acid.
HA + H2O ∏ A- + H3O+
In the above example, HA is the weak acid, and A- is the conjugate base.
What makes an acid or a base weak or strong?
The strength of an acid is determined by how much of the acid will ionize once it is in water. If the acid
ionizes completely and all of the HA becomes A- in water, then that is a strong acid. There are seven
acids that are strong and will ionize 100% in solution: HCl, H2SO4, HNO3, HBr, HI, HClO4, and HClO3.
If the acid does not ionize completely, then it is considered a weak acid. Some examples of weak acids
are HC2H3O2, HCN, HF, and HClO. The strength of weak acids is determined by the ratio of the product
of the conjugate base and hydronium ion to the acid. This is called the acid ionization constant, Ka.
In the example, acid ionization reaction given above with HA as the weak acid, the Ka would be
determined by the following equation:
[𝐴− ][𝐻3 𝑂+ ]
𝐾𝑎 =
[𝐻𝐴]
The Ka equation follows the same pattern as all equilibrium constant equations with the
concentrations of the products divided by the concentration of the reactants. Notice that
water is left out of the equation even though it is a reactant. Only aqueous (aq) or gaseous (g)
substances appear in equilibrium expressions.
Based on the value of the Ka expression, the strength of the acid is known. If the acid is a very
weak acid, then that would mean very little of the acid will have ionized to form the conjugate
base and the hydronium ion. That would mean that the numerator in the equation is small, and
therefore, the value of Ka will be small. On the other hand, if the acid is not weak, then more of
the conjugate base and hydronium ion will form which means the value of numerator increases,
increasing the value of Ka.
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Kb is the base ionization constant and would be written the same way. For example, in the
ionization of ammonia given below, ammonium and hydroxide will be in the numerator and
ammonia will be in the denominator:
HA + H2O ∏ A- + H3O+
[𝑁𝐻4 + ][𝑂𝐻− ]
𝐾𝑏 =
[𝑁𝐻3 ]
To summarize, the smaller the value of Ka (or Kb) is the weaker the acid (or base).
What about “conjugate” acids and bases?
You’ve probably heard the term “conjugate acid” or “conjugate base”, but what does that
mean? Notice in the above example of ammonia that one of the products that is produced is
ammonium ion. Ammonium ion, NH4+, has an extra proton, so it can act like an acid:
NH4+ + H2O ∏ NH3 + H3O+
Ammonium ion is the conjugate acid of ammonia. And ammonia is the conjugate base of
ammonium ion (“conjugate” just means “related”). Like any acid, it has a value of Ka that
expresses how acidic it is. It turns out that there is a relationship between Ka and Kb for
conjugate acid/base pairs like ammonium ion/ammonia. It is this:
(Ka)(Kb) = Kw = 1.0 x 10-14 (at 25 C
̊ )
So if you know the value of Kb for ammonia, you can calculate the value of Ka for ammonium
ion. The Kb for ammonia is 1.8 x 10-5. So what is the Ka for ammonium ion?
(Ka)( 1.8 x 10-5) = 1.0 x 10-14
Ka = (1.0 x 10-14)/(1.8 x 10-5) = 5.6 x 10-10
Notice that this relationship means that the larger the Ka (stronger the weak acid), the smaller
the Kb has to be. So, when the conjugate acid is very, very strong (like HCl, for example), its
conjugate base
(Cl-) is very, very weak. In fact, Cl- is such a weak base that it has no discernible impact on pH at
all!
How does the titration work?
The base is added from the burette to the acid in the beaker, which is being stirred
continuously with a stir bar. The reaction between the acid and the base is complete when the
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endpoint is reached. The endpoint is the point at which the moles of acid in the beaker will
equal the moles of base that was added from the burette, according to the stoichiometry of
their reaction together.
Below is an example of a strong acid and strong base titration. The first plot shows the pH on
the y-axis and the volume of the strong base added on the x-axis. As more base is added, you
can see the pH increasing steadily. At first, the pH changes are minor, but then as the endpoint
is reached, the pH change is drastic and the solution goes from an acidic pH to a basic pH. See
Figure 1.
Initially, as the base is added, the pH does not drastically change, because the added base
reacts with the weak acid to produce more conjugate base. The conjugate base can then react
with water to produce the weak acid base. Then as the moles of base added begins to equal
the moles of acid already present, the equivalence point is reached (the arrow in the plot below
indicates the equivalence point). At this point, there is no longer any HA left in the solution,
because all of it has reacted with the added base and has been turned into the conjugate base,
A-. The result in this case is a neutral solution, because A- is the conjugate base of a strong acid,
making it a very, very weak base. However, if the acid being titrated was a weak acid, then its
conjugate base would also be a weak base, which means it would react with water and affect
the pH at the equivalence point. After the equivalence point, the solution continues to become
basic as more base is added.
Figure 1: Graph showing increase of pH as more base is added
The endpoint can be determined also by plotting the first derivative of the titration and looking
for the highest peak. The first derivative plot shown below for the same titration shows the
biggest peak at a volume of 4.3 mL which means that the endpoint was reached after 4.3 mL
was added. It may sound complicated, but it is actually quite easy to graph the first derivative
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of the pH curve. If you have the data in a spreadsheet, you can just calculate the slope at each
point (beginning with point #2). This is done by calculating “rise over run” for each point. In
this case, that means:
(pH#2-pH#1)/(vol#2-vol#1)
where “#1” and “#2” refer to the first and second data point in each column. You’ll notice that
right at the equivalence point, the slope numbers get very large. Just graph the slope column
vs. the volume column and you will get a graph like the one shown below. See Figure 2.
Figure 2: Graph of the first derivation of the pH curve
What is standardization?
Before you begin the quantitative analysis, you will standardize the base solution that you will
be using. Standardization is using titration to determine the exact concentration of a solution.
Since you will use a base solution to determine the concentration of an unknown acid solution,
you will first standardize the base so that you know the exact concentration.
Why standardize the base solution? Why is the exact concentration not already known?
The sodium hydroxide solution used in this lab is prepared by dissolving some amount of solid
sodium hydroxide in water and diluting to a certain volume. The solid sodium hydroxide is
hygroscopic, meaning it will absorb water molecules from the air easily. Even if the exact mass
of the sodium hydroxide dissolved in water is known, some of that mass probably contains
water, and there is no quick way to know exactly how much. Therefore, in order to truly know
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the concentration of sodium hydroxide in the base solution that is prepared, standardization
must be performed.
How is the standardization performed?
The sodium hydroxide solution will be titrated with a known concentration of a monoprotic
acid, potassium hydrogen phthalate, also known as KHP. KHP is not as hygroscopic as NaOH,
and it can easily be dried in an oven. So measuring the mass of the solid acid before dissolving
it in water is a good way to know the exact concentration of the acid solution. The sodium
hydroxide solution will be added to the KHP solution of known concentration until the endpoint
is reached.
PRE-LAB QUESTIONS:
1. Refer back to the definitions of acids and bases and indicate the role of water in the
example below. What is the role of H3O+?
HA + H2O ∏ A- + H3O+
2. What is the difference between a strong base and a weak base? Give an example of a
strong base.
3. What do you think the pH will be at the equivalence point with the titration of a weak
acid and a strong base? What about a strong acid and a weak base? Explain your
reasoning.
4. Do you expect the actual concentration of the sodium hydroxide in solution to be higher
or lower than what you would calculate from the mass of dissolved sodium hydroxide in
solution? Explain your reasoning.
5. Which is the stronger acid, one with Ka equal to 1 x 10-5 or one with Ka equal to 1 x 109?
6. Why is the volume of the strong base added on the x-axis in the titration plot?
7. In a standardization of the NaOH solution, you found that a beaker with 25.0 mL of
water with 5.55 g of KHP dissolved required 17.25 mL of NaOH to reach the equivalence
point. How many moles of NaOH are in the solution? What is the actual concentration
of the original NaOH solution? If an extra 25.0 mL of water were added to the NaOH
solution before the titration with KHP, would more, less or the same amount of KHP
have to be added to reach the equivalence point? (The molar mass of KHP is 204.22
g/mol.)
EQUIPMENT:


Paper
Pencil/pen
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
Computer with Internet access (for the remote laboratory and for data analysis)
PREPARING FOR THIS NANSLO LAB ACTIVITY:
Read and understand the information below before you proceed with the lab!
Scheduling an Appointment Using the NANSLO Scheduling System
Your instructor has reserved a block of time through the NANSLO Scheduling System for you to
complete this activity. For more information on how to set up a time to access this NANSLO lab
activity, see www.wiche.edu/nanslo/scheduling-software.
Students Accessing a NANSLO Lab Activity for the First Time
For those accessing a NANSLO laboratory for the first time, you may need to install software on
your computer to access the NANSLO lab activity. Use this link for detailed instructions on
steps to complete prior to accessing your assigned NANSLO lab activity –
www.wiche.edu/nanslo/lab-tutorials.
Video Tutorial for RWSL: A short video demonstrating how to use the Remote Web-based
Science Lab (RWSL) control panel for the titration apparatus can be viewed at
http://www.wiche.edu/nanslo/lab-tutorials#titration.
NOTE: Disregard the conference number in this video tutorial.
AS SOON AS YOU CONNECT TO THE RWSL CONTROL PANEL: Click on the yellow button at the
bottom of the screen (you may need to scroll down to see it). Follow the directions on the pop
up window to join the voice conference and talk to your group and the Lab Technician.
EXPERIMENTAL PROCEDURE:
Read and understand these instructions BEFORE starting the actual lab procedure and collecting
data. Feel free to “play around” a little bit and explore the capabilities of the equipment before
you start the actual procedure.
You will do 4 titrations of a mono-protic weak acid with a strong base. For each titration, the
amount of acid in the beaker will be identical. The first titration will be exploratory, and the
second through the fourth will be quantitative. You will average the quantitative titrations to
get an average value of the acid concentration.
Before you begin, ask the Lab Technicians to tell you these two pieces of information:
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1. The name and standardized concentration of the strong base.
2. The name of the weak acid.
EXERCISE 1: Exploratory Observation
First you will do a qualitative titration where you will see how the acid in the beaker reacts
with the base that you add from the burette. Notice the changes to the pH and estimate any
fluctuations in pH. Add the strong base in differing increments of anything from a few tenths of
a milliliter to a few milliliters and observe what happens to the pH with each addition. See if
you can get a rough idea of where the equivalence point should be.
EXERCISE 2: Quantitative Measurements
Now that you know approximately where the equivalence point is do three more titrations but
this time try to capture as much data near the equivalence point as you can so you can get an
accurate measurement of it. Make sure each student in your group gets the chance to do at
least one full titration.
Download the titration graph data for each titration and save it in a spreadsheet for later
analysis.
POST-LAB ANALYSIS
1. Plot each set of quantitative data. Make a pH graph so you can identify the equivalence
point. If you want to, make a first derivative graph for each set of data too.
2. Using your graphs, determine the volume of base added at each equivalence point:
o Titration #2:
o Titration #3:
o Titration#4:
3. Using the data from question 2, determine the number of moles of weak acid in each
titration:
o Titration #2:
o Titration #3:
o Titration#4:
4. Exactly 1.0 mL of the weak acid was placed in each of these three beakers and water
was then added to the beaker. Using this information, calculate the concentration of
the weak acid (before the water was added):
o Titration #2:
o Titration #3:
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o Titration#4:
5. Report the average concentration of the weak acid:________________________
6. Was the volume of water added to each beaker significant in your calculations? Explain.
(OPTIONAL) ADVANCED POST-LAB ANALYSIS
1. If the acid you titrated was a di-protic acid, would you have used more or less base to
titrate it than you did in the above exercises? Explain.
2. If you re-did the mon-protic titrations with a base that was twice as strong as the one
you used, would you have required more or less of it to reach the equivalence point?
Explain.
3. Calculate the standard deviation of the three quantitative titrations you did above.
Report your average value of concentration with an error of +/- 3 standard deviations.
For more information about NANSLO, visit www.wiche.edu/nanslo.
All material produced subject to:
Creative Commons Attribution 3.0 United States License 3
This product was funded by a grant awarded by the U.S.
Department of Labor’s Employment and Training Administration.
The product was created by the grantee and does not necessarily
reflect the official position of the U.S. Department of Labor. The
Department of Labor makes no guarantees, warranties, or
assurances of any kind, express or implied, with respect to such
information, including any information on linked sites and
including, but not limited to, accuracy of the information or its
completeness, timeliness, usefulness, adequacy, continued
availability, or ownership.
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