General Chemistry Laboratory
Fall 2009
THE QUANTITATIVE ANALYSIS OF IRON IN TOTAL CEREAL
Adapted from material published by Paul Hooker to accompany his article
“Mineral analysis of total whole grain cereal,” JCE 82:1223-1225.
Objectives
1. To determine how much iron metal is in a sample of Total cereal and whether this is
consistent with the manufacturer's claims.
2. To understand the difference between elements in their elemental states and ionic
forms.
Introduction
Consumers are now asking themselves the all-important question, “Just what is in that
stuff, anyway!” Many people want to know the vitamin and mineral content of foods, as
well as which preservatives and colors are present. Most foods are now labeled
accordingly to give the relevant information, but is the information accurate?
Reading the label on a packet of Total cereal tells us that Total cereal contains 40
mg of potassium per serving. Potassium is an extremely reactive metal, made of
potassium atoms that will produce potentially explosive hydrogen gas, H2(g) when it
comes into contact with water. Imagine your surprise when as a result of pouring milk
onto a bowl of Total, it promptly blows up in your face. The information on the packet is
misleading. Total cereal does indeed contain potassium, but in the form of potassium
ions, K+ cations, as opposed to potassium metal. K+ ions have very different chemical
properties from K metal and will not endanger your life. In fact, they are essential for life.
(Note: K+ ions must be combined with anions when in the solid state, for example,
1
chloride ions, Cl- ions, to give the compound KCl, potassium chloride.) The only
difference between a K atom and a K+ cation is that K+ cations have one less electron.
The packet label also indicates that Total cereal also contains 100% of the dailyrecommended intake of iron. Elemental iron metal is not as reactive as potassium metal
and is added to Total cereal as the pure element and not in the form of ions. In fact, any
food label that indicates iron present in a reduced form, i.e., reduced iron, contains
metallic iron in the form of iron filings. The term "reduced" is used in chemistry to
indicate the gain of electrons and the term "oxidized" the loss of electrons. Fe2+ and Fe3+
ions are the oxidized forms of Fe because they have lost two and three electrons
respectively.
Elemental iron is ferromagnetic (attracted to a magnet) and there is enough iron
in one flake of Total cereal that a magnet can move the flakes themselves. Your task is to
take a sample of Total cereal and extract and determine the amount of iron metal present.
The amount is small and great care must be taken experimentally so that the sample is not
lost. Using your skill, judgment, and laboratory experience etc. it is up to you to develop
a satisfactory method for the analysis using the suggestions in the Experimental
Guidelines. Having extracted the Fe metal, you will dissolve it in HCl(aq), and perform a
quantitative analysis using a spectrophotometer.
Experimental Guidelines
To release the iron from the flakes it is necessary to grind the cereal into as fine a powder
as possible using a mortar and pestle. Approximately 10 g is a recommended sample size
but it is important to measure the mass of cereal accurately. Once ground, the powder is
2
placed in a beaker and distilled water added to make a slurry. The slurry can be stirred
using a magnetic stirrer bar and plate. The stirrer bar is a white Teflon coated piece of
magnetic material 1-2 cm in length, which is simply placed in the mush. The beaker plus
contents is positioned on the stirrer plate so that the rotating magnet inside the plate will
turn the stirrer bar inside the beaker. When adjusted to the appropriate setting and the
slurry to the correct consistency (by adding the appropriate amount of water), the stirrer
bar in the beaker will rotate at a constant, rapid speed collecting small pieces of iron as it
moves through the slurry.
While the stirrer bar is mixing the slurry of cereal, prepare the standard solutions
that you will use to construct a ‘standard curve.’ Use the same set of standards that you
prepared for the analysis of iron in surface waters. The solutions in the tubes should be
various shades of red. The units, ppm, refer to parts per million. These units are often
used for dilute solutions and can be also be written as 1 mg solute/L of solution (mg/L).
You will need to calculate the concentration of each of your standard solutions, in ppm.
The equation c1v1 = c2v2 will be useful.
After about 15-20 minutes remove the stirrer bar using another magnet on the
outside of the beaker, and remove the excess cereal from the stirrer bar by gently
spraying it with distilled water. (Warning: Do this over a beaker and not over the sink. Do
not risk sending your sample to the sewer.)
Having extracted the iron metal from the cereal the task is now to find out how
much iron is stuck to the stirrer bar. The stirrer bar plus iron metal should be placed in a
test tube so that the iron can be dissolved in concentrated HCl(aq). Do this in a fume hood,
adding just enough HCl(aq) to cover the stirrer bar. A little heat may be necessary to speed
3
up the dissolution. Record your observations as the iron dissolves. Once dissolved,
transfer the solution to a 100 mL volumetric flask, ensuring that every last drop is
transferred, and dilute to the mark on the flask.
Determine the absorbance of your standard solutions and your cereal samples
using the spectrophotometer at the wavelength of 522 nm. To prepare your sample,
transfer 2.0 mL from the 100 mL volumetric flask into a test tube, and add all of the
reagents listed above, omitting the 1.0 M HCl(aq), which is already present. Add
NaOH(aq), if necessary, to bring the pH of the solution to above 2.0.
To record the absorbance of your solutions (standards and sample) add 200 µL of
each standard and your sample to the well of a microplate, in duplicate. Write notes on
which standards and samples were placed in which wells of the microplate. Place the
microplate in the spectrophotometer and read the plate at 522 nm. Once you have your
raw data, use Excel to subtract the absorbance value of the blank from each of the other
absorbance values for your standards and samples. Again, with Excel, use this adjusted
absorbance data to make a graph of absorbance vs. concentration. Add a ‘trendline’ to
your graph to obtain an equation for the line that best fits the data points on your graph.
The absorbance of your sample should fall somewhere near the middle of your
calibration line. If the absorbance does not fall on your calibration line, quantitatively
dilute the sample so the absorbance will fall somewhere on your calibration line. When
you have an absorbance for your sample that does fall somewhere on your calibration line
you can use the equation for your calibration line, and the absorbance for your sample, to
calculate the concentration of iron in your sample.
4
Caution!
This laboratory exercise requires the handling of concentrated acids. Due care and
attention must be exercised at all times. Safety goggles are mandatory at al times and
gloves are recommended. All solutions must be disposed of according to your instructor’s
directions.
Pre-lab assignment
1.
What is the dietary reference intake for iron? List your reference source and be as
specific as possible.
2.
Assuming your sample of cereal had a mass of 10.00 g, and a recommended
serving of Total cereal is 30.0 g, how many mg of iron would you expect from your
approximately 10 g sample?
5
3.
The label on a packet of Total cereal states that it contains 200 mg of sodium per
serving. Is the sodium present in the form of sodium atoms or ions?
Is the sodium present as reduced or oxidized sodium?
4.
Develop an Experimental Procedure and Data Sheet, which will be checked
before the lab. Write the Experimental Procedure as a series of steps that you will carry
out in the lab. Refer to your Lab Manual for examples. For the Data Sheet, determine the
measurements needed for you to be able to determine the amount of iron in the 10 g
sample. This will enable you to have a clear idea of where to start and what to do in the
lab and you will be able to work efficiently. However, it may be necessary to modify the
6
procedure as you perform the experiment to take into account any unforeseen problems
that can arise. Note that this is common practice in all "real-life" science experiments
ranging from simple chemical analyses or launching the space shuttle. This is called
"problem solving" and requires initiative. This is one of the most important skills that you
can learn in a chemistry laboratory.
7