Cornell Science Inquiry Partnerships
Cornell University
http://csip.cornell.edu
Detection of Ions in Solutions Using Acid/Base Chemistry:
A Quality Control Test
Teacher’s Guide
by Tammy Hanna, CSIP Graduate Student Fellow, Cornell University
Objective
To use titrations to determine the amount of Mg(OH)2 in Milk of Magnesia and
Ca2+ in Tang®.
Subject: Chemistry
Audience: High School
Time required: Five 40-minute class periods if students standardize solutions
or two 40-minute class periods if solutions are standardized previously.
Background
This laboratory-based curriculum introduces students to the concept of titrations
and describes the techniques necessary to determine an unknown concentration of ions in
solution. Students make use of pH meters or pH paper to determine the pH of their
solutions, giving them an introduction to acids and bases and teaching them the proper
way to calibrate a pH meter. Topics covered include preparation and standardization of
solutions, determination of pH, introduction to indicators and their uses, as well as
calculation of solution concentrations and weight percentages of ions. The lab is
formatted so that students first become familiar with titrations while standardizing their
solutions. They learn how to reach the endpoint without over-titrating as well as how to
read volumes on the burets. After standardizing the solutions, the students are given the
opportunity to be quality control testers and analyze the percent Mg(OH)2 (the active
ingredient) in Milk of Magnesia, and the amount of calcium in Tang®. They must
determine if drug companies and food manufacturers put the right values on their labels,
and if their results don’t match, think about where the experiment or company went
wrong. This curriculum exposes students to analysis of “real-world” samples and gives
them an idea of how chemists evaluate components in drugs.
Learning and Behavioral Objectives
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Students will learn the basics of titrations and how to calibrate a pH meter or use
pH paper.
Students will gain practice in preparing and standardizing solutions.
Students will use algebraic representations to describe data.
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Students will learn how to use indicators to determine when a titration is
complete.
Students will develop an appreciation for drug analysis.
National Science Education Standards
This activity addresses several National Science Education Standards.
Physical Science
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Understanding of structure and properties of matter.
Understanding of chemical reactions.
Science as Inquiry
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Skills necessary to do scientific inquiry.
Assessment Strategy
In the pilot classes, each pair of students was required to standardize each solution and an
overall class average was taken. Each student was then given an unknown amount of
Milk of Magnesia and directed to determine the percent of Mg(OH)2. Assessment was
done by means of a class discussion, where each student displayed their value and the
class conversed about the accuracy of their results and what the results meant. This
facilitated a dialogue about drug companies and their goals, and allowed the students to
discuss what may have gone wrong during their experiments and what could be
improved, or why their results differed from what was expected. Evaluation was based
on understanding of the methods as well as ability to think about the experiment and
rationalize the results; less emphasis was placed on the final result. Questions at the end
of each experiment probed additional understanding and allowed students to think
analytically. Calculations required math skills and manipulation of data.
Teaching Tips and Additional Background Information
Motivation
I designed/modified this lab in response to the need for students to learn about acids and
bases in a Regents level course, and the desire to give them something interesting and
tangible to study. This curriculum gives students the chance to gain an understanding of
material they will be responsible to know, while allowing them to be quality control
testers. This method could be extended to determine the amount of many ions in
solution, as long as a suitable indicator and standard solution were determined. Tang®
was chosen for study due to high school students’ love of the beverage and its claim to be
“a good source of calcium.” Milk of magnesia was chosen due to its milky consistency,
which allowed the students to learn about back titrations.
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Supplies and Preparation of Solutions
Milk of magnesia and Tang® can both be purchased at a grocery store for approximately
$3.99 each. One container of each will last a very long time. Hydroxynaphthol blue
indicator is essential for the titration of Tang®; a 5.0 gram container of the disodium salt
can be purchased from Acros Organics (catalog number 20488 0050) and costs $11.20.
This provides enough material for a lifetime of students to do this experiment, as 10 mL
of indicator requires only 100 mg of hydroxynaphthol blue. Potassium hydrogen
phthalate can also be purchased from Acros Organics (catalog number 42406 1000),
where a 100 gram container costs $10.80. EDTA (ethylenediaminetetraacetic acid) can
be purchased from Acros Organics (catalog number 11843 2500) and costs $12.00 for
250 grams.
The master standard of NaOH was prepared to be approximately 0.10 M, so for a class of
25 students, about 3 L should be prepared (there will probably be extra). The solution
was prepared by weighing 12 grams of NaOH into a large screw cap bottle or a carboy
and dissolving the pellets in 3 liters of distilled water, measured with a large graduated
cylinder. Since the students will be standardizing the solution, it is not crucial that the
solutions be prepared perfectly, although you should make enough that the students use
the same solutions for the entire length of the set of experiments. The HCl solution was
prepared to be approximately 0.10 M, so in a similarly large container, 4 L of solution
were made by mixing 33.3 mL of concentrated HCl (12 M from the bottle) with 3.9667 L
of distilled water. (Due to the difficulty of standardizing HCl, your students may
determine the concentration of HCl to be as high as 0.18 M, so you should discuss why
this occurs!)
Students should prepare their CaCO3 and Tang® solutions by following the directions set
out in the experiment, using calibrated flasks and pipettes. If there is not enough
calibrated glassware in the classroom, have groups of students share the solutions.
In order to prevent students from comparing the volume of solutions needed to titrate the
milk of magnesia, samples were pre-weighed and distributed to students marked with
numbers, i.e. 1-25. The students were asked to weigh the vial of solution at the beginning
of the experiment, rinse out the milk of magnesia with distilled water, and then heat the
vial in a microwave to remove any excess water and reweigh. Samples ranging between
1.0-1.5 grams were distributed in 20 mL scintillation vials.
Safety Considerations and Waste Disposal
All of the solutions used in this lab can be dumped down the drain of a sink with water.
The standardized solutions have very low concentrations of acid or base so any contact
with skin can be rinsed briefly with running water. Concentrated HCl is needed to
dissolve the CaCO3 (about one pipette full per dilution), so oversee any measuring. 1 M
NaOH needs to be added to the CaCO3 solution to raise the pH to 12, so make a small
bottle of solution at this concentration. The EDTA solution should be prepared in a hood
and any skin that comes in contact with the solution should be washed with lots of soap
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and water. As always, use of gloves in addition to regular lab protection such as goggles
will help to ensure safety.
This material was developed through the Cornell Science Inquiry Partnership program (http://csip.cornell.edu), with
support from the National Science Foundation’s Graduate Teaching Fellows in K-12 Education (GK-12) program
(DGE # 0231913 and # 9979516) and Cornell University. Any opinions, findings, and conclusions or recommendations
expressed in this material are those of the author(s) and do not necessarily reflect the views of the NSF.
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