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colorimetryUse this method if you have access to a colorimeter
at least 6 boiling tubes (or large test tubes)
Iron is one of the many minerals required by the
human body. It is used in the manufacture of the oxygen-carrying proteins haemoglobin and myoglobin.
A deficiency of iron in the body can leave a person feeling tired and listless, and can lead to a disorder
called anemia. Many of the foods we eat contain small
NB: It may take several days to dissolve the Fe3+ salt
used here, so carry out this preparation well in advance
In this analysis the iron present in an iron tablet (dietary
of the rest of the experiment. Weigh out about 3.0 g
supplement) or a sample of food is extracted to form
of ferric ammonium sulfate (FeNH (SO ) •12H O). Use
a solution containing Fe3+ (ferric) ions. To make the
a mortar and pestle to grind the salt to a fine powder.
presence of these ions in solution visible, thiocyanate
Accurately weigh 2.41 g of the powder into a 100 mL
ions (SCN−) are added. These react with the Fe3+ ions to
beaker and add 20 mL of concentrated sulfuric acid
(Caution: concentrated acids are highly corrosive − wear safety glasses and rubber gloves and take care when
handling). Leave powder to soak in acid overnight. The
By comparing the intensity of the colour of this solution
next day, carefully pour the acid/powder slurry into a
with the colours of a series of standard solutions, with
500 mL volumetric flask, rinsing the beaker into the
known Fe3+ concentrations, the concentration of iron
flask a few times with water, then make up to the mark
in the tablet or food sample may be determined. This
with distilled water. Let this solution stand for several
days until the ferric ammonium sulfate powder has fully dissolved. If possible, insert a magnetic stirrer bar and
stir the solution to speed up this dissolving process.
ferric ammonium sulfate FeNH (SO ) •12H O
Use a pipette to transfer 20 mL of ferric ion solution to a
200 mL volumetric flask and make up to the mark with distilled water. This gives a solution with [Fe3+] = 0.001
2, 4, 6, 8 and 10 × 10−5 mol L−1 (see below for preparation)
mol L−1. To prepare a 2 × 10−5 mol L−1 standard solution
iron tablet (dietary supplement) or sample of
pipette 10 mL of the 0.001 mol L−1 solution into a 500
iron-containing food (e.g., silverbeet, spinach, raisins,
mL volumetric flask, add 10 mL of 1 mol L−1 sulphuric
acid, and then make up to the mark with distilled water. Repeat this procedure in separate 500 mL volumetric
1 mol L−1 ammonium thiocyanate solution (see
flasks, pipetting in 20, 30, 40 and 50 mL of 0.001 mol L−1
Fe3+ solution in turn, to obtain 4, 6, 8 and 10 × 10−5 mol L−1
(NB: if you do not have five 500 mL volumetric flasks
0.15 mol L−1 potassium permanganate solution (see
you can use one flask to prepare each standard in turn.
After preparing each standard, pour the solution into a labelled glass vessel which has a lid (eg: a glass bottle).
Then rinse your 500 mL volumetric flask thoroughly
100, 200, 250 and 500 mL volumetric flasks
with distilled water before using it to prepare your next standard solution.)
Preparation of 1 mol L−1 ammonium thiocyanate solution
Weigh 38 g of solid ammonium thiocyanate into a 500
combusting your sample. Also beware that the crucible
mL volumetric flask and make up to the mark with
will become very hot during this process, so handle it
only with crucible tongs − or preferably not at all − until it has cooled.
When the sample and crucible have cooled, use a
stirring rod to crush the ash to a fine powder (see Figure
Only required for analysis of iron tablet.
Weigh 2.4 g of
3). Use a measuring cylinder to add 10 mL of 1 mol L−1
solid potassium permanganate into a 100 mL volumetric hydrochloric acid and stir for 5 minutes, making sure
flask and make up to the mark with distilled water.
4. Add 5 mL of distilled water and then filter the
Place iron tablet in a 100 mL beaker and use a
solution into a 100 mL conical flask to remove the
measuring cylinder to add 20 mL of 1 mol L−1 sulfuric
ash. This filtered solution will be used for colorimetric
acid. Allow the tablet’s coating to break down and its
contents to dissolve. You may help this process by
using a stirring rod to carefully crush the tablet and stir the solution. (NB: iron tablets sometimes contain filler
Accurately measure 10 mL of your sample solution
materials that may not fully dissolve in acid)
into a clean, dry boiling tube. (NB: a boiling tube, or large test tube, is ideal; however any small vessel −
Once the iron tablet is dissolved, add 0.15 mol L−1
eg: a beaker, flask or glass vial may also be used). This
potassium permanganate solution dropwise, swirling
measurement is most accurately made using a 10 mL
the beaker after each addition. Iron tablets usually
pipette; however, it is possible to do this accurately
contain ferrous sulfate, with iron present as Fe2+ ions.
enough (and with less hassle) using a clean 10 mL
Since Fe2+ does not form a coloured complex with
measuring cylinder if you measure carefully.
thiocyanate, permanganate ions are added to oxidise all the Fe2+ to form Fe3+ ions. For the first few drops
Next, measure 10 mL of each Fe3+ standard solution
of permanganate, the purple colour will disappear
into separate boiling tubes (one standard per tube) in
immediately upon addition to the iron solution;
order of increasing concentration, beginning with the
however, as further drops are added the colour will
2 × 10−5 mol L−1 standard. It is a good idea to first rinse
begin to linger for a little longer. Stop adding potassium
your pipette or measuring cylinder with a few mLs of the
permanganate drops when the purple colour persists for 2 × 10−5 mol L−1 standard. NB: Make sure you label each several seconds after addition − usually no more than
boiling tube appropriately with the name of the sample
about 2 mL of 0.15 mol L−1 permanganate solution will
or standard it contains. A test tube rack is very useful
for holding and transporting your tubes (see Figure 4). Alternatively you can use a large beaker to hold them.
Transfer the iron solution to a 250 mL volumetric
flask, rinsing the beaker with distilled water a few times
Using a 10 mL measuring cylinder, add 10 mL of 1
and transferring the washings to the volumetric flask.
mol L−1 ammonium thiocyanate solution to each iron
Make up to the mark with distilled water, stopper the
solution in sequence, with 2 minutes between each
addition. These additions must be carefully timed so that all samples react for the same period of time.
4. Use a pipette to transfer 5 mL of iron solution to a 100 mL volumetric flask and make up to the mark with
4. Mix the solutions by swirling. A stable red colour
distilled water. This diluted solution will be used for
will appear over the next few minutes.
As near as possible to 15 minutes after adding
thiocyanate, measure the absorbance at wavelength
of 490 nm for each coloured solution using your
Accurately weigh a few grams (typically 2 − 5 g is
colorimeter. These measurements will be made in
required, depending on iron content of sample) of your
sequence − one sample every two minutes − reflecting
the timing of the thiocyanate additions above. The
Heat the crucible over a bunsen burner (see Figure
measured absorbance of light is a direct measure of
1) until the sample is reduced completely to ash, or
the intensity of the solution’s colour. Figures 4 and 5
(preferably) combust the sample directly in the bunsen
illustrate the range of colour intensities that you can
flame (as shown in Figure 2), reducing it to ash. NB:
expect from your set of Fe3+ standards.
be very careful with the bunsen flame while heating/
The same boiling tubes as in Figure 4 (prepared using Fe3+ standard solutions with concentrations of 2 − 12 × 10−5 mol L−1) viewed from above, looking directly down the length of the tube. This view provides the most accurate “eyeball” comparison of the solutions’ colour intensities.
Using only the absorbance results obtained for
your Fe3+ standard solutions (not your unknown iron sample), prepare a graph with [Fe3+] (in mol L−1) as the horizontal axis and absorbance (at 490 nm) as the
Set up used for heating a silverbeet sample in a crucible.
Draw a line of best fit for your data points that
goes through the origin (because absorbance must be zero when Fe3+ concentration is zero.
Now identify the point on your line of best fit
which corresponds to the absorbance measured for your unknown iron sample. By drawing a vertical line to the horizontal axis you will be able to determine the concentration of Fe3+ in your unknown solution.
4. Use this concentration to calculate the mass of iron (in mg) in your original tablet or food sample (NB: the molecular weight of iron is 55.8 g mol−1). Remember to take into account any dilutions that you performed while preparing your sample solution.
If the absorbance value you measured for your
unknown iron sample is greater than the absorbance
Set up used for combusting a silverbeet sample directly in the
value for your highest concentration Fe3+ standard you
will need to modify the above procedure. In the case of an iron tablet, you should repeat the analysis with a more dilute solution of the dissolved iron tablet. In the case of a food sample, you should repeat the analysis using a smaller mass of your food.
Contact UsIf you have any questions or comments relating to this
Left photo: ash remaining after combustion of a silverbeet sample.
Right photo: silverbeet ash which has been ground to a fine powder using a stirring rod.
Outreach College of Science University of Canterbury Private Bag 4800 Christchurch New Zealand
Phone: +64 3 364 2178 Fax: +64 3 364 2490
Blood-red coloured solutions produced by the ferric thiocyanate
complex ion (Fe[SCN]2+). These solutions were prepared using the following range of Fe3+ standards: 2, 4, 6, 8, 10, 12 × 10−5 mol L−1.
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