How does the pH environment affect bioavailability of Iron?

Author(s): John C. Rivera, Lillian Seu, Juliet Rose Girard, Anthony Shiver

Lesson Overview

Grade level(s):

Middle School (6-8), High School (9-12), Grade 8, Grade 9, Grade 10

Subjects(s):

Biology/Life Science, Chemistry

Topic:

Bioavailability, Acid/Base chemistry, Metabolism, Nutrition

Big ideas(s):

Iron is an important mineral nutrient for health and can be acquired via many natural food sources as well as in foods that have been enriched and in vitamin supplements. We will determine the effect that pH has on the bioavailability of the different forms of iron. Iron is found in nature in three different forms: Fe0, Fe+2 and Fe+3.

Vocabulary words:

bioavailability, reduce, oxidize, absorbed, isolate, metabolism, ingestion, digestion

What you need:

Part One:
Materials:

  • Iron filings
  • Aqueous solution of HCl (pH 1) -  You need a 0.1M solution of Hydrochloric Acid (HCl) with a pH=1. Available at chemicals suppliers ( Science Lab.com, Fisher scientific, etc.  or ask for a donation from a university in your vicinity)
  • Aqueous solution of NaOH (pH 8) - You need a 0.01N sodium hydroxide stock solution to which you do two serial 1:100 dilutions to achieve pH =8. Available at chemicals suppliers ( Science Lab.com, Fisher scientific, etc.  or ask for a donation from a university in your vicinity)
  • goggles
  • glove
  • test tube rack
  • test tubes  (15 ml) (2 test tubes/student pair)
  • Litmus paper
  • Litmus paper color coding key
  • Petri Dishes
  • Pipet and bulbs

Part Two                                                                                                                                                                                            Materials:

  • 2-3 cups fortified cereal (with reduced Fe added such as Raisin Bran or shredded wheat)
  • Junk cereal, such as Fruit Loops (optional)
  • light-colored magnet
  • bowl
  • spoon
  • blender (optional)
  • or mortar and pestle
  • water

 

Grouping:

Student pairs

Author Name(s): 
John C. Rivera, Lillian Seu, Juliet Rose Girard, Anthony Shiver
Summary: 

Dietary minerals are available through ingestion of food and supplements.  In this lesson, students first examine the chemical reaction of two forms of iron, Fe0 and F+2 with various pH conditions of either the stomach or intestine to determine how it gets absorbed and eliminated in the body. Then students isolate iron from the foods we eat (such as cereal) using a magnet to attract elemental iron or Fe0.

Prerequisites for students: 

A basic understanding of chemical equations.

A simple understanding of positive and negative ions to help understand the chemical formula of most acids, bases and salts.

A basic understanding of the three forms of iron.

A basic understanding of acids and bases

Learning goals/objectives for students: 

-Determine how iron (Fe) gets absorbed and eliminated in the body by examining chemical reactions of two forms of iron, with various pH conditions of either the stomach or intestine.

-Isolate iron from foods we eat (such as cereal and vitamin supplements) by isolating free iron (element) with a magnet.

Content background for instructor: 

Nutritional Iron: Iron is mostly found in nature in three forms: First, elemental iron (Fe0), and in two oxidation states, Fe+2 and F+3. These forms are also known as iron II and iron III. Elemental iron or Fe0 (also called reduced iron), as iron filings, are used often as supplement to enrich cereals. The elemental iron is converted in the stomach into Fe+2 the form of iron that is absorbed and useful to the body cells. This conversion is possible because of the acidic conditions in the stomach.

Almost two thirds of iron in the body is found in hemoglobin, the protein in red blood cells that carries oxygen to tissues.

Cast iron (Fe0) pans when used with acidic foods (tomato sauce or lemon juice) can release Fe+2 and if the food remains for a long time in the pan it will be a visible color change in the food.  Would this process be used as a source of dietary iron Fe+2?  Not really. There will be some Fe+ available in the food but long time contact of the food in the pan will give an unpleasant color and taste to the food.

Vocabulary:
bioavailability: the proportion of a nutrient that enters the bloodstream and thus reaches cells,  the tissues and organs of the body.
reduce: to add an electron to an atom or ion
oxidize: to lose an electron from an atom or ion

litmus: Litmus paper indicates the acidity or alkalinity of the solution. Litmus paper contains a water-soluble mixture  of dyes that are a pH indicator - a mixture of 10-15 natural dyes obtained from lichens (mainly Roccella tinctoria) that turns red in response to acidic conditions (pH < 7) and blue under alkaline conditions (pH > 7). When the pH is neutral (pH = 7) then the dye is purple.

Resources: Please see attachment "Minerals & Iron background" & "Minerals_chart"

Getting ready: 

 Prepare before class for Part One

1. Buy and /or prepare your solutions the day before. For pH =1 solution (0.1 M HCl solution) and for the     pH =8 solution (0.01N NaOH solution then two serial 1:100 dilution)

2. Prepare two test tubes (15 ml), one labeled “Stomach” and another “Intestine”, to give to each student pair.

3. Make a solution of Fe(s) in HCl, the day before class, to distribute to sudents.  They will not use the solution that they "make" during class, but will instead use the premade solution, that has had time to react. About 300 ml of 0.1 M HCl pH =1 with about 1/2 tsp of Iron should be enough solution for about 34 students with 5-8 ml per test tube ("Stomach" tube)

4. Make your pH=8 NaOH solution. Make two serial dilutions of 1:100 of a 0.01N NAOH stock solution. You will need about 300 ml of final solution to use for the "Intestine" tubes (5-8 ml per tube)

3. Students should wear gloves and lab goggles because of the use of the solutions of acid (HCl) and base (NaOH) that are corrosive

 

Lesson Implementation / Outline

Introduction: 

 

Introduce the class with the following three questions. These questions are also on the worksheet "Iron Bioavailability Worksheet."

1. Before class starts, assign students to a "DO NOW" task: Think about the kinds of minerals in our diet that are important for our health. What are sources of these minerals?

For discussion with the class:

1. Discuss the "Do Now" task.   List minerals and their sources on the board.

2. Why is it important to ingest iron? What are some sources of dietary iron?

3. Discuss the term bioavailability. Bioavailability means the amount of a nutrient that enters the bloodstream and thus reaches the tissues and organs of the body.

4. Introduce the question, "How does the pH environment affect bioavailability of Iron?"

Give students a quick explanation of the lab experiment, including necessary safety precautions and lab protocol.

Activity: 

Part One: How does the pH environment affect bioavailability of Iron?

1. Distribute the materials. Students should gather all of the necessary equipment and put on personal protective equipment.

2. Using a pipette or dropper, teachers should help the students fill the test tubes with the aqueous solutions.  The HCl solution (pH = 1) should be put in the stomach test tube, and the NaOh solution (pH=8) should be put in the "intestine" test tube. Using the litmus paper, students will record the pH of the “stomach” and “intestine” solutions.

3. Draw a diagram on the board that shows the crude anatomy of the stomach and the intestine. Ask students about the values of pH that they found for each compartment. Why does the pH correspond to these specific compartments?

The pH is optimal for the activity of the digestive enzymes in each of these compartments.

4.Students add iron filling (a small pinch) to a 15ml test tube, with 5-8 ml of HCl solution in the “stomach” tube. Students will add iron fillings to the test tube labeled “stomach” and will observe what happens. Then the students will take an aliquot (a small sample, in this case 3ml) of the reaction solution from “stomach” using a dropper or a pippette and add it to the “intestine” solution. (Note, because the reaction takes a while to complete, a pre-made solution of Fe(s) in HCl should be made and distributed to use instead of the student made solution.)

5. Teachers draw on the board the chemical reaction for the process, and students will also record this as well.

In the stomach:

Stomach acid: ~pH 2 HCl (Hydrochloric Acid)

Add reduced iron:

2H+(aq) +Fe0 => H2 (g)+ Fe2+(aq) (Iron II)

Aqueous Fe2+ , or iron II in solution, then gets transported to the intestine by carrier proteins where it can be absorbed.

6. Distribute the pre-made solution of Fe0 in HC. Students will add an aliquot of this solution to the “intestine” tube an

d observe what happens (color, precipitate formation?)

7. Teachers  draw on the board the chemical reaction for the process, and students will also record this as well.

In the intestine: ~pH 8

The oxidized iron, iron II  or Fe+2 reacts under basic conditions to form a solid precipitate:

Fe2+ (aq)+ 2OH- => Fe(OH)2 (s) (Iron II hydroxide)

Aqueous oxidized Fe2+ can get oxidized further and lose an electron:

Fe2+(aq) => Fe3+(aq) + e-

Fe3+(aq)+ 3OH- => Fe(OH)3 (s) (Iron III hydroxide)

What students see in the tube is a precipitate that is a mixture of iron(II) and iron (III) hydroxide. Explain that this form of iron is insoluble (as evidenced inside their test tube) and passes through the intestines as waste, in a bio-unavailable form.

Data

A) Quantitative data:

Using litmus paper and a pH chart, students will record the pH of the two solutions ("stomach" and "intestine").

~Stomach: pH 1-2

~Intestine: pH 8

B) Qualitative data:

i) Students will add iron fillings to the test tube labeled “stomach” and will observe what happens. Then the students will take an aliquot of the reaction solution from “stomach” and add it to the “intestine” solution. (Note, because the reaction takes a while to complete, a pre-made solution of Fe(s) in HCl will be made and distributed) Students will observe any color changes, precipitate, etc.

ii) Give students a magnet to isolate iron fillings from either cereal or vitamin supplement. Students will record the amount and consistency of the fillings.

Part Two: Isolate iron from cereal (or multi-vitamins).

  1. Pour the cereal into the bowl or blender.
  2. Add sufficient water to completely cover the cereal. (it's not an exact measurement - you can add as much as you like as iron doesn't dissolve in water.)
  3. Mash the cereal with a spoon or mix it with the water using a blender. The more finely ground the cereal is, the easier it will be to get the iron.
  4. Stir a light-colored magnet  through the crushed cereal. Iron is heavy and will sink, so be sure to pay attention to the bottom of the bowl. If you used a blender, make sure you can get to the particles at the bottom of the jar.
  5. Look for the black 'fuzz' or iron on the magnet. It's easiest to see the iron if you wipe the iron on a white napkin or paper towel. Alternatively, you can use a light colored magnet.
Checking for student understanding: 

Cast iron (Fe0) pans when used with acidic foods (tomato sauce or lemon juice) can release Fe+2 and if the food remains in the pan for a long time there will be a visible color change in the food.  Would this process be used as a source of dietary iron Fe+2? Explain your reasoning

Wrap-up / Closure: 

Wrap up and discussion of the importance of minerals in our diet, as well as the significance of bioavailability and how it is dependent on factors such as pH.

Extensions and Reflections

Reflections: 
AttachmentSize
Minerals & iron background.doc49.5 KB
Iron Bioavailability Worksheet.doc22 KB
Minerals chart.doc60.5 KB
NGSS Topics
High School (9-12) Physical Sciences: 
NGSS Disciplinary Core Ideas
High School (9-12): 
NGSS Performance Expectations
NGSS Performance Expectations: 
HS-PS1-2
NGSS Science and Engineering Practices
NGSS Crosscutting Concepts
NGSS Crosscutting Concepts: