Modeling the Detection of Dissolved Substances and the Water Treatment with
Iodine
Andrew Camp
University of North Carolina at Chapel Hill
[email protected]
Abstract
We all require water to drink; controlling its quality and supply is a pressing issue facing
many areas of the world, including the United States. This utilizes the 5E’s to have students
examine what is in the water supply and how do we control its purity. Students will specifically
explore the detection of invisible chemicals dissolved in water in this activity. Furthermore,
students will design their own filter apparatus and attempt to purify water. Using data from
these experiments, students will explain what factors affect water quality and elaborate ways
to improve it.
Target Grade Level: 8th grade, options to target 3rd-9th
Standards
NC Science Essential Standards:
8.E.1.4 Conclude that the good health of humans requires:
Water quality standards
Methods of water treatment
Next Generation Science Standards:
MS-ETS1-1
MS-ETS1-2
MS-ETS1-3
MS-ETS1-4
MS-ESS3-4
Safety
Iodine solutions are generally safe at low concentrations; in fact, it is commonly used as an
antiseptic in medicine and sanitizing agent in drinking water. Excess consumption (such as
drinking significant amounts of the concentrated bottle) can lead to ulcers and other negative
symptoms (see Lugol’s Solution MSDS).
Materials
2% Lugol’s Solution – Available for about $12 from Amazon
Starch indicator (bought or homemade) – I mixed ¼ tsp of cornstarch in 1 cup of hot water,
then cooled. If exact concentration of starch is desired, you can use a kitchen scale (try 1g
dissolved in a cup).
Water
Clear Containers – I would recommend 5 small containers for the calibration curve; 6 larger
cups to filter into and make stock solutions.
Filter Materials – See “Extend Section” I used coffee filters as my base, rubber banded the filter
to a cup with coffee filter slightly depressed, and spread a layer of the filter material evenly.
Rubber bands
Masking Tape – for labels
Measuring Cups
Graduated syringe or dropper (I used a baby medicine syringe)
Scientific Background
Many halogen-containing substances already exist in our water supply. Chlorine is used
as a common sanitizing agent, and fluoride is added to most municipal water supplies to
strengthen enamel. Metal salts can also be present depending on where the water is drawn
from, how it is treated, and the pipes through which the water flows. This lab will use dissolved
iodine, a fairly nontoxic and commonly used sanitizing agent, as a model of these invisible
dissolved substances.
Lugol’s solution will be employed in this lab. This solution contains both dissolved iodine
(I2) and potassium iodide (KI) some of which reacts to form trace amounts of triiodide (I 3-).
I2(aq) + KI(aq)
KI3(aq)
Triiodide is what gives Lugol’s solution and tinctures of
iodine their distinctive yellow-brown color. With
starch, triiodide behaves as a colorimetric substance,
changing to dark-black blue color upon interacting
with the starch. The linear I3- makes its way into the
coils of amylose (a helical component of starch,Figure
1). There, the I3- forms a charge transfer complex with
water inside the coil, giving rise to the vibrant blue
color observed.
Figure 1. Structures in Starch
Engage (15 min)
Students will begin the lesson by drawing from their own experience with water quality.
Several examples are given below:
“Why does water taste like it does?”
“What constitutes potable water? How do we make it drinkable?”
“Where does the water that we drink come from?”
“What health hazards does contaminated water create?”
Place students in small groups (3-5) and assign each group a question. Have them discuss their
answers to these questions within the group and have each group explain their answer to the
class.
Question
Answer
Revision
Figure 2. Class Discussion Activity. Have students
from each group come up to a large easel pad and
write the question and answer given to each group.
After the lab is completed, have a different student
from each group revise the answer based on
knowledge garnered from the lesson.
Alternatively, students could be given an assignment to bring in water from a certain
water source, and the discussion questions above could center around the various types of
water brought in (sink, river, lake, etc.).
Next, take a colored liquid, the “contaminant”, and serially dilute it until you have a
spectrum of colors from dark to completely clear (this can be done prior to class). Compare the
glasses of colored liquid with that of pure water and have the students guess which one
contains the “contaminant”. As the “contaminant” becomes more dilute this task will become
increasingly difficult until it is a matter of guessing. This activity demonstrates that the eyes can
only detect so much of the colored liquid in solution. This would introduce the concept of a
limit of detection, the lowest concentration of a substance that can be detected with an
instrument – the human eye in this case.
A
B
Figure 3. Example of a Serial Dilution. B was made by
taking 2 mL of A and adding 1/8 cup of water. This
was repeated using 2 mL of B to generate C. Notice
the solution becomes increasingly hard to
differentiate from pure water as the concentration of
the “contaminant” drops. C is near the limit of
detection.
C
Explore (15-20 min)
Students will rejoin their groups for the next activity. Demonstrate the colorimetric
properties of iodine and starch solution for the class, then have the students guess how many
drops the method will be able to detect when dissolved in one cup of water. Have the students
write down their hypotheses in their notebooks, then generate a calibration curve using
different amounts of Lugol’s solution in water (Figure 4). Sketch the setup, including the shade,
number of drops, and amount of water used to dilute for each container.
A
B
III.
II.
I.
≈ 1.5 𝑚𝑔 𝐼3− 𝑖𝑛 𝑜𝑛𝑒 𝑑𝑟𝑜𝑝
𝑚𝑔
≈ 0.006
𝑜𝑟 6 𝑝𝑝𝑚
≈ 237 𝑚𝐿 𝑖𝑛 𝑎 𝑐𝑢𝑝
𝑚𝐿
Figure 4. Generating an Iodine-Starch Calibration Curve. A
contains 3 solutions with only Lugol’s solution. I. contains 1
drop per cup of water, II. two drops per cup, and III. three
drops per cup. 1/8 cup each solution was put into the
containers seen in the figure, which gave, to my eyes at least,
3 identical looking solutions. To each of these 4 mL of starch
solution (homemade, ¼ tsp of cornstarch in 1 cup hot water)
was added to each container to generate picture B. As you can
see, II. and III. take on a purple color, indicating that iodine is
in solution. I. was too faint for me to reliably see a color
change, so I consider this point to below the limit of detection
for this method. I recommend having at least 5 samples
containing 0, 1, 2, and 3 drops of Lugol’s solution in the
calibration curve. Leave the 5th container as optional for
student determined.
Optional math incorporation: In the lab notebooks,
have students calculate Iodine concentration for each solution
in the calibration curve; I’ve include an example calculation for
solution I. This calculation approximates the I3- as being the
only species in the 2% solution. We can see a color change at
about 12 ppm.
Next, students will explore various methods to purify their iodine solutions. Have several
materials available (suggestions in Figure 5) to allow construction of filters. Ask students to
predict which materials will be the best filters for iodine and have them record their answers in
the lab notebook. Pick an iodine solution that will give decent color - I recommend 4 or 5 drops
of Lugol’s solution per cup of water - and use that as stock to purify. Allow a testing period of
various materials.
Explain (10- 12 min)
Allow students time to discuss their findings. What was the best material for purifying iodine?
What was the worst? Extend the discussion beyond iodine: “What other impurities in water
need to be removed?” “Is one kind of filter going to work for every impurity?” “How do we
target specific agents in the water?” After discussion, a video walkthrough of a water treatment
plant will help connect the activity to the methods used in a municipal water plants.
(Resources). Do any of the methodologies resemble materials used in this lab?
Extend (10 -15 min)
Ask each group to compete to build the best composite filter for iodine (Figure 5). Ask them to
record results for the material test and the composite filter, including predictions about how
many drops of iodine per cup are in solution based on each group’s calibration curve.
Figure 5. Comparison of different filters.
A stock solution containing 3 drops of
Lugol’s solution in 1 cup of water was used
in this demo Left. 1/8 cup of this solution
was filtered through 2 coffee filters, then
4mL of starch added. The solution is
slightly paler than the stock example,
indicating some of the iodine was
removed. Center. 1/8 cup Iodine solution
was filtered through a coffee filtered that
was stuffed with flour. Since flour contains
starch, it adsorbed the I3- and left a clear
solution after starch was injected. Right. A
wire mesh was used. No Iodine was
removed based on color. comparison with
stock solution.
Example Materials for Filters:
-Sand -Flour -Mesh -Coffee Filters -Cloth
-Activated Carbon (Found in Pet Stores)
-Commercial Filters
-Gravel
One final filter product can be discussed, or an iterative approached used if time permits to
perfect the iodine filter. Student’s will reinforce concepts learned through both experiments
with a video on waste water treatment in municipal systems (see Resources).
Evaluate
Students will already have generated a lab notebook product, containing hypotheses,
apparatus, and data from the previous activity. This could be taken up and graded, or can be
supplemented with discussion questions (revised individual answers to the class questions in
the engage section, unique questions, assignment to research water treatment strategies, or
discussions about the video).
Differentiation
For students who need accommodations, a typed out walkthrough of the lab would help
guide thinking and keep them focused on the task.
Resources
“Why does water taste like it does?”
http://www.livescience.com/54521-tap-water-tastes.html
http://www.popsci.com/article/science/ask-anything-what-does-water-taste
https://www.quora.com/Does-ordinary-water-really-have-no-taste-or-is-it-just-that-humanslack-the-ability-or-the-necessary-taste-buds-to-perceive-it
Check out the dissolved salts and minerals on water bottle labels
“What constitutes potable water? How do we make it drinkable?”
http://www.owasa.org/drinking-water
“Where does the water that we drink come from?”
http://www.cdc.gov/healthywater/drinking/public/water_sources.html
http://water.usgs.gov/edu/drinkseawater.html
“What health hazards does contaminated water create?”
http://www.cdc.gov/healthywater/drinking/public/water_diseases.html
http://www.cdc.gov/healthywater/drinking/private/wells/diseases.html
Waste Water Treatment Video
https://www.youtube.com/watch?v=20VvpASC2sU
Increasing the Difficulty for High School Chemistry
http://www.outreach.canterbury.ac.nz/chemistry/documents/vitaminc_iodine.pdf
http://mccscience.yolasite.com/resources/EXP%204.7.pdf
Decreased Difficulty for Younger Students
See “Exploration of Plant Structure, Function, and Nutrition using a Simple Starch Test“ on the
SciRen Portal
References
Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 03 Nov.
2015. Web. 06 Sept. 2016.
"Sugars and Starches | Background." Colorado State Univerity. Colorado State University, n.d.
Web. 06 Sept. 2016.
Haneef, Deena T Kochunni Jazir. "Major Differences." Difference between Amylose and
Amylopectin. N.p., n.d. Web. 06 Sept. 2016.
Real Science Connection
The sensitive detection of ions in solution has become a more pressing issue as the
desired purity for liquid solvents has increased. Development and manufacturing increases the
presence of transition metals (cadmium, chromium, lead, mercury, etc.) in the environment,
many of which have detrimental effects to human health1.
Metal contamination is also important in organic solvents (acetone, methanol,
methylene chloride, acetonitrile, pentanes, etc.). Metals can infiltrate solvents used to wash
silicon wafers used in microchips and semi-conductors, negatively affecting device lifetime and
performance2,3. Other potential applications could include quality control of chemical feedstock
and testing of plant, medicine, and food extracts in laboratory settings4.
Most current chemical detection methods involve one signal response for every
association between the detector and dissolved metals in solution. My project in the Miller Lab
at UNC5 focuses on designing new detector molecules that can produce thousands of signals in
response to a single interaction with a metal. A system of this type would require less detector
material, leading to cheaper and more sensitive detection methodologies.
(1)
(2)
(3)
(4)
(5)
Nriagu, J. O. Envron. Pollut. 1988, 50, 139.
Wendt, H.; Bergholz, W.; Zoth, G.; Cerva, H.; Kolbesen, B.O. 2nd International
Symposium on “Orbital Welding in High Purity Industries. 1-7.
Takiyama, M. Nipon Steel Tech. Rep. 2001, 83, 95-99.
Ernst, E. Trends Pharmacol. Sci. 2002, 23 (3), 136.
Kita, M. R.; Miller, A. J. M. J. Am. Chem. Soc. 2014, 136 (41), 14519.