chemistry student handout

8/13/2019 Chemistry Student Handout

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The packet label also indicates that Total cereal also contains 100% of the daily-recommended

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 Spec 20 instrument.

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 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



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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. When as much iron as possible has

been collected the stirrer bar is removed using another magnet on the outside of the beaker, and

the excess cereal from the stirrer bar removed by spraying it with distilled water. (Warning: Do

this over a beaker and not 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 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.

You are going to use a method similar to the previous lab utilizing the Spec 20 instruments to

analyze the solution quantitatively for iron. The chemical reaction between Fe2+

and 2,2 -

dipyridyl produces an intensely red product. You will react the Fe2+

in your solution with 2,2 -

dipyridyl and compare the color with a calibration line made from known standard solutions of

Fe2+

and 2,2 -dipyridyl. The standard solutions are made as follows. Obtain 6 test tubes and mark

on the outside a line where 10 mL would be. Add 1.0 mL 10 ppm Fe2+

solution to tube #1, 2.0

mL 10 ppm Fe2+

solution to tube #2, 4.0 mL 10 ppm Fe2+

solution to tube #3, 5.0 mL 10 ppm



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Fe2+

solution to tube #4, and 8.0 mL 10 ppm Fe2+

solution to tube #5, as accurately as possible.

Add 2 drops 1.0 M HCl(aq), then 0.50 mL 10% hydroxylamine (to reduce any Fe3+ to Fe2+ ), 6

drops 2.0 M sodium acetate(aq), and 1.0 mL 2,2 -dipyridyl solution to each tube. Dilute to the 10

mL line and mix well. 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. You will need to calculate the concentration of your

standard solutions, in ppm. The equation c1v1 = c2v2 will be useful.

Determine the absorbance of your solutions using the Spec 20 instrument at the analytical

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.

Record the absorbance of your solution which should fall somewhere on 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.

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.

chemistry student handout

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