Abstract

Objective

Use a color-based chemical test to measure the rate at which solid iron dissolves.

Introduction

Iron is essential to most forms of life and to healthy human physiology. It is a key part of many proteins and enzymes that help you maintain good health. In humans, iron is an essential part of proteins that are involved in oxygen transport. Almost 66 percent of the iron in your body is found in hemoglobin, which is the protein in red blood cells that carries oxygen to tissues. Smaller amounts of iron are found in myoglobin, a protein that helps supply oxygen to muscle. A deficiency of iron limits oxygen delivery to cells, resulting in fatigue, poor work performance, and decreased immunity. On the other hand, excess amounts of iron can result in toxicity, and even death.

Even though iron is the second most abundant metal in Earth's crust, iron deficiency is the world's most common cause of anemia. As far as your body is concerned, iron is more precious than gold. The body stores iron so effectively that there is no biological process for the excretion of iron. The amount of iron in your body is regulated by how much iron is absorbed from food by your intestines. The recommended daily allowance (RDA) for iron depends on your age and gender. According to the Office of Dietary Supplements-National Institutes of Health, for people between the ages of 14 and 18, the minimum daily requirement is 11 milligrams (mg)/day for males and 15 mg/day for females.

Scientists have developed various chemical tests for determining the amount of iron in a substance. For this chemistry science fair project, you will use a colorimetric test for iron. The chemical test is based on an indicator molecule called 1,10-phenanthroline. 1,10-phenanthroline produces a clear solution when no iron is present, but produces an orange-red color in the presence of iron. The greater the concentration of iron in a solution, the more the solution will turn orange-red (in other words, the color change is proportional to the iron concentration). The test is designed to detect all soluble forms of iron (iron that is dissolved in the solution) and most insoluble forms of iron (iron that is still in solid form). In the presence of iron, phenanthroline molecules combine with iron atoms to form an orange-red complex. The structure of the phenanthroline-iron complex is shown in Figure 1, below.

Figure 1. Structure of phenanthroline molecule complexed with iron. Note that there are three phenanthroline molecules for every iron atom in a phenanthroline-iron complex. (Wikipedia, 2009.)

In this chemistry science fair project, you will use the colorimetric test to measure the rate at which solid iron is dissolved in an indicator solution. As the iron dissolves, it reacts with phenanthroline in the solution to form the orange-red complex. The rate will be measured as the increase in the concentration of iron (as determined by the color) divided by the time. For example, say the iron in solution goes from 2 mg/liter (L) to 5 mg/L in 30 minutes (min.). The change in concentration is 3 mg/L (from 2 to 5 mg/L). The time it took for this increase in concentration is 30 min., so the rate is given by the equation below:

Equation 1:

Rate = (3 mg/L)/30 min. = 0.1 mg/L per minute

• The rate of the reaction equals the change in concentration, divided by the time for the change to occur.

The test device is called an iron test cube. The amount of iron in the test solution can be determined by comparing the color of the test solution with the scale on the test cube. (See Figure 2.)

Figure 2. The Hach iron test cube. The test liquid is poured into the clear square tube on the left and its color is compared to the color scale on the right. Higher concentrations of iron create darker shades of orange. The test cube has five levels of orange, which correspond to 1, 2, 3, 4, and 5 mg/L (milligrams per liter) of iron.

The chemicals that are used for the reaction come in premixed packets, called reagent pillows. To make the indicator solution, you simply open the packet and pour the contents into water. Along with the color cube, this is a particularly user-friendly way to measure iron concentration. Let's get started!

Terms, Concepts, and Questions to Start Background Research

• Iron
• Physiology
• Enzyme
• Oxygen transport
• Hemoglobin
• Myoglobin
• Toxic
• Recommended daily allowance (RDA)
• Colorimetric test
• Indicator molecule
• 1,10-phenanthroline
• Solution
• Proportional
• Soluble
• Insoluble
• Complex
• Reaction rate
• Control

Questions

• What is an indicator solution?
• Based on your research, what percent of the total mass of Earth is made up of iron?
• What illness is caused by too little iron in your blood?
• What foods are good sources of dietary iron?
• Based on your research, what are the most common mineral forms of iron?
• What is the definition of a reagent?

Bibliography

• Office of Dietary Supplements-National Institutes of Health. (n.d.). Dietary Supplements Fact Sheet: Iron. Retrieved October 10, 2009, from http://dietary-supplements.info.nih.gov/factsheets/iron.asp#h4
• Harvard Medical School. (2001). Iron Absorption. Retrieved October 10, 2009, from http://sickle.bwh.harvard.edu/iron_absorption.html
• National Library of Medicine. (2009). Iron in Your Diet. Retrieved October 10, 2009, from http://www.nlm.nih.gov/medlineplus/ency/article/002422.htm
• Wikipedia Contributors. (2009, September 26). Phenanthroline. Wikipedia: The Free Encyclopedia. Retrieved October 10, 2009, from http://en.wikipedia.org/w/index.php?title=Phenanthroline&oldid=316316879
• Wikipedia Contributors. (2009, October 10). Solubility. Wikipedia: The Free Encyclopedia. Retrieved October 12, 2009, from http://en.wikipedia.org/w/index.php?title=Solubility&oldid=318986625

Materials and Equipment

• Newspaper
• Styrofoam^TM cups (6)
• Permanent marker
• Graduated cylinder, 50-mL; available from Home Science Tools at www.hometrainingtools.com, catalog # CE-CYGL050
• Distilled water; available at most grocery stores
• Safety goggles
• Disposable gloves
• Iron Test Kit ("Test Cube"); available online from Hach at www.hach.com, catalog # 14008-00
• Pipet, 5-mL (10-pack); available from Home Science Tools at www.hometrainingtools.com, catalog #CE-PIPET
• Paper towels (1 roll)
• Steel wool, superfine; available at hardware stores
• Stopwatch or other timer
• Scissors
• Lab notebook

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Experimental Procedure

Important Notes Before You Begin:

• In the procedure below, iron in the form of steel wool is added directly to the indicator solution. The indicator solution contains chemicals that enhance the rate at which the iron dissolves. This method is designed to produce quick and dramatic results. See the Variations, below, for alternative procedures.
• Be sure to read the Science Buddies Chemistry Safety Guide before you begin.
• Cover your work space with newspaper before you begin.

Making the Indicator Solution

1. Label two Styrofoam cups 1 and 2.
2. Fill the graduated cylinder to 30 mL with distilled water.
3. Pour the 30 mL of water into Styrofoam cup #1.
4. Put on your safety goggles and disposable gloves.
5. Open the contents of six reagent "pillows" and pour them into the 30 mL of water (each pillow has enough chemicals for 5 mL).
6. Swirl the water to dissolve the chemicals.
7. Use a 5-mL pipet to transfer 15 mL of indicator solution from cup #1 to cup #2.

Adding the Steel Wool

1. Pull off three small pieces of steel wool, all approximately the same size.
1. You do not need much. Each piece should be about the size and thickness of a quarter ($0.25 coin).
2. Steel wool is used because it has a large surface area relative to its mass.
2. Place one piece of steel wool into the solution in cup #2. It should be submerged.
3. Start the timer.
4. Swirl the cup with the steel wool for 5 seconds every 5 minutes. This is to mix the dissolved iron in the solution. Continue swirling, as stated, but move on with the following steps.
1. Feel free to vary the time that the cup is swirled. Record the details of the procedure and your observations in your lab notebook.

Measuring the Iron Concentration

1. Rinse the reservoir in the test cube with clean water.
2. Carefully pipette 5 mL of the solution from cup #1 into the square test cube reservoir.
3. Hold the test cube up to a light source, such as a lamp shade.
4. Record the color of the indicator solution.
1. This is the negative control, so it should be clear.
2. The reason this step is included is to show that the color that develops in cup #2 is due to the steel wool.
5. Pour the negative control contents of the test cube back into cup #1.
6. Gently tap the test cube upside-down on several sheets of paper towel to remove as much of the liquid as possible.
7. Watch the solution in cup #2. When it develops a light orange color, pour 5 mL into the square test tube reservoir.
8. Read the iron concentration in milligrams/liter based on the color scale. Hold the vial up to a white light source, if needed. The lightest orange equals 1 mg/L and the darkest orange equals 5 mg/L.
9. Record the concentration and time in your lab notebook.
10. Pour the solution from the test cube reservoir back into cup #2.
11. Gently tap the test cube on several sheets of paper towel to remove as much of the liquid as possible.
12. Repeat steps 7–11 as the solution color develops. The time it takes for the color change to develop will vary. Watch the color change carefully. It will probably be complete (over 5 mg/mL iron) in less than 30 minutes.
13. Stop taking readings when the color of the indicator solution shows that the iron in the water has reached a concentration equal to or greater than 5 mg/L.
14. Clean the iron test cube with distilled water.
15. Pour the solutions in the Styrofoam cups down the sink.
16. Repeat the entire procedure at least two more times with fresh and clean materials. This will ensure that your results are repeatable.

Analyzing Your Results

1. Graph the concentration of iron on the y-axis in mg/liter and the time in minutes on the x-axis. Make separate graphs for each trial.
2. Calculate the rate at which the iron dissolved for each trial, using Equation 1 from the Introduction to calculate the reaction rate.
3. Average the reaction rates for your final result.

Variations

• Vary the temperature of the solution. How does temperature affect the reaction rate?
• Try dissolving the iron in water and then testing the water for the presence of soluble iron. What effect do salt (sodium chloride), pH, and temperature have on the rate at which the iron is dissolved?
• Investigate other kits that will allow more precise measurement of the concentration of iron in solution, such as the Hach (www.hach.com) color comparator kit, catalog # 1465-00.
• Develop a procedure to determine how the surface area of the iron affects the rate of dissolution.
• Develop a procedure to determine how stirring the solution affects the rate of dissolution.
• Use the kit to measure iron in water from around your house. Compare tap water, bottled water, well water, and stream, lake, or ocean water.
• Develop a procedure to measure iron in cereal using the iron test cube.

• For more science project ideas in this area of science, see Chemistry Project Ideas.

Credits

David B. Whyte, PhD, Science Buddies

• Styrofoam^TM is a registered trademark of The Dow Chemical Company.

Last edit date: 2009-12-21 12:00:00