Part 1 - Get a Lab Appointment and Install Software:
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/students-scheduling-labs)
Part 2 – Before Lab Day:
Read your lab experiment background and procedure below, pages 1-8.
Submit your completed Pre-Lab Questions (page 4-5) per your faculty’s instructions.
Watch 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: Citric Acid in Popular Drinks – Titration
Lab format: This lab is a remote lab activity.
Relationship to theory (if appropriate): In this lab you will apply your knowledge of acid/base
titrations to determine the amount of citric acid in various drinks. This activity is meant to be
done after a student has a basic understanding of titrations.
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........................................................................................................ 2
Background Information ............................................................................................... 2-3
Estimating Error ............................................................................................................ 3-4
Pre-lab Questions .......................................................................................................... 4-5
Equipment ..................................................................................................................... 5
Preparing for this NANSLO Lab Activity ........................................................................ 6
Experimental Procedure ............................................................................................... 6
Exercise 1: Titration of two drinks ................................................................................ 7
Post Lab Analysis ........................................................................................................... 7
Creative Commons Licensing ........................................................................................ 8
U.S. Department of Labor Information ......................................................................... 8
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LEARNING OBJECTIVES:
After completing this laboratory experiment, you should be able to do the following things:
1. Understand the role of citric acid in food products.
2. Determine the concentration of an unknown concentration of citric acid using titration.
o Report the uncertainty (experimental error) in this result.
3. Describe the purpose and mechanism of a titration.
4. Describe the interactions of multiple acidic protons in a polyprotic weak acid.
5. Explain the conditions at the equivalence point for the titration of citric acid with
sodium hydroxide.
BACKGROUND INFORMATION:
Citric acid is a naturally-occurring weak acid that is not only critical to the energy product cycle
of most cells (known as the Krebs cycle) but is also used in thousands of food, drinks, and
household products. Like any acid, citric acid has a sour taste and is the reason that citrus fruits
(for which the acid is named) taste sour. Citric acid is commonly confused with so-called
“Vitamin C”, which is a different weak acid,
ascorbic acid (Error! Reference source not
found.). Although citric and ascorbic acids
are both found in most citrus fruits, they are
very different from one another. In most
citrus fruits, citric acid is present in
concentrations of 20 to 100 times that of
ascorbic acid (Nour). Although it looks like
citric acid might be tetra-protic, only three
of the protons are acidic, enough to be
removed during a normal titration with a
strong base. Notice that each of the three
acidic protons in citric acid look very similar to one another in terms of the chemical bonds
around them. This means that they will have similar acidic characteristics. Since the “middle”
acidic proton (at the top of Error! Reference
source not found.) is closer to the non-acidic
hydroxide group (at the middle bottom of
Error! Reference source not found.), it is the
easiest to remove from the molecule, because
the oxygen in the hydroxide group is pulling
electron density away from it more than from
the other two acidic protons on either side of
the molecule. The acid equilibrium constants
for the three acidic protons in citric acid are
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(Weast):
Ka1
Ka2
Ka3
8.4 x 10-4
1.8 x 10-5
4.0 x 10-6
Table 1: Acid Equilibrium Constants for Citric Acid
Citric acid is used in a very large variety of food products for various reasons. Citric acid can be
used to provide sourness in many drinks and foods, as a way to help curdle cheese, as an
anticoagulant, to help preserve dye colors, and also can be used in various industrial processes
such as electroplating and waste-stream conditioning (NCBI). Citric acid content in juices and
drinks is important to know, because it can be an aid for people who suffer from kidney stones
and other diseases of the renal system (Penniston). There are many methods of determining
the citric acid content of foods and drinks, but we are going to focus on one of the oldest:
titration with a strong chemical base.
Like any weak acid, citric acid can be titrated with a strong base of known concentration.
Unlike monoprotic weak acids, however, there are three protons that must be removed during
the titration process. Citric acid is kind of unique among polyprotic acids in that its three
protons are so similar to each other. This gives some unique features to its titration curve,
which you will discover as you do this activity.
ESTIMATING ERROR:
You can always estimate the error in any measurement by taking several measurements of it
and calculating the standard deviation of the set of results. In other words, if you measure
something several times, you are likely to end up with a set of results that are not all exactly the
same number. The standard error of that set of numbers is an estimate of how much error
there is in your results, and it is equal to the standard deviation divided by the square root of
the number of samples in the data set. Here’s an example. Let’s say we measured something
five times, and the result was:
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Trial #1
1
Measurement
0.324
2
0.332
3
0.325
4
0.328
5
0.322
We would report that the value was: average +/- standard error, which in this case is: 0.326
+/- 0.002. This means that the uncertainty in your result (0.326) starts in the third decimal
place, because that’s the first digit in your standard error. Since the uncertainty starts in the
thousandths place, we have to limit the average value to the thousandths place as well. When
reporting uncertainty in this way, you always report only one digit in the standard error.
The standard deviation can be calculated manually (you’ve probably done this in a math
course), or you can use a spreadsheet program like Excel. If you have fewer than 100 samples
to work with, use the “stdev.s” function, which is more accurate for small samples.
Sources:
Nour, V., Trandafir, I., Ionica, M. E. HPLC Organic Acid Analysis in Different Citrus Juices under
Reversed Phase Conditions. Not. Bot. Hort. Agrobot. Cluj 2010, 38, 1, pp. 44-48
Weast, R. C., Ed. Handbook of Chemistry and Physics, The Chemical Rubber Company, Ohio, U.S.
1968
NCIB - National Center for Biotechnology Information. Compound Summary for Citric Acid,
http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=311#x94 (accessed Oct 2, 2014)
Penniston, K. L., Nakada, S. Y., Holmes, R. P., Assimos, D. G. Quantitative Assessment of Citric
Acid in Lemon Juice, Lime Juice, and Commercially-Available Fruit Juice Products. J Endourol.
Mar 2008; 22, 3, pp. 567–570.
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PRE-LAB QUESTIONS:
1. When you titrate citric acid with a strong base, do you expect the titration curve to look
more like a or b below?
2. Explain the reasoning for your choice in question 1.
3. Why do the second and third protons on the citric acid molecule have different values of
Ka, even though they are basically equivalent?
EQUIPMENT:
1.
2.
3.
4.
Paper
Pencil/pen
Computer with Internet access (for the remote laboratory and for data analysis)
A spreadsheet program such as Excel
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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.
Once you have logged onto the remote lab, you will perform the following laboratory
procedures:
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EXERCISE 1: Titration of Two Drinks
Before you begin, ask the Lab Technicians to tell you the name and standardized concentration
of the strong base that you will be using to titrate with.
You will have two samples of orange juice and two samples of a sports drink that you can titrate
with the strong base.
Hint: For each addition of titrant, add small quantities of the strong base titrant (less than 0.5
mL) so you don’t over-shoot the equivalence point.
1. Titrate the orange juice samples and download the graph image and data and paste
them into a document for later analysis.
2. Titrate the sports drink samples and download the graph image and data and paste
them into a document for later analysis.
3. Ensure that each person in the group gets a chance to do at least one full titration.
4. Ensure that each person in the group downloads and saves all the graph images and
data sets.
POST-LAB ANALYSIS
(Can be done offline if you run out of time)
1. Each drink sample you titrated was composed of 5.0 mL of the original drink (orange
juice or sports drink) and some water to provide enough volume to do the titration. Use
the data you collected to determine the moles of citric acid in each sample you titrated.
2. Use the data you calculated in the first step to determine the average concentration of
citric acid in the original drinks.
3. Report the experimental error in each average value.
4. Does the titration curve match the expectation you had in the pre-lab assignment? Do
some research (cite your sources) and try to explain why the titration curve of citric acid
has the shape that it does.
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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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