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Winter 2013
Experiment 4
Preparation of Ammonium Iron(II) Sulfate Hexahydrate,
(NH4)2Fe(SO4)2. 6 H2O, from Iron Metal
Introduction
Iron is a d-block element and a transition metal. Like most d-block elements, it exhibits
various oxidation states. Compounds are known that contain iron in all of the oxidation states
from 0 to +7. Iron reacts with acids that have non-oxidizing anions, such as hydrochloric acid or
sulfuric acid, evolving hydrogen gas and forming iron(II) salts. If iron is dissolved in an acid that
has an oxidizing anion, the iron(III) salt is obtained. Thus when iron is dissolved in nitric acid, it
is oxidized to iron(III) and the product is iron(III) nitrate. At the same time, the nitric acid is reduced to various oxides of nitrogen including NO2 (brown fumes!).
The iron(II) ion is not very stable under neutral conditions and is slowly oxidized to iron(III)
ion (and precipitates as the hydroxide). Aqueous solutions of iron(II) ions are more stable in
acidic solution, so the aqueous iron(II) sulfate that is provided in the laboratory is acidified with
sulfuric acid.
Iron(II) sulfate as the solid or in solution slowly becomes oxidized to iron(III). The double
salt, ammonium iron(II) sulfate, is much more stable towards oxidation both as the solid and in
solution. This makes ammonium iron(II) sulfate suitable for use as a primary standard in volumetric analysis. The ammonium ion reacts with water to give hydrogen ions, thus making the
solution acidic and stabilizing the iron(II) ions.
Chemicals Required
Ammonium sulfate
6.0 g
Iron
6.0 g
2.0 mol / L sulfuric acid
50.0 mL
Procedure
In an 250 mL Erlenmeyer flask, dissolve the ammonium sulfate, (NH4)2SO4, in 12 mL of
deionized water. Add the iron to a second 250 mL Erlenmeyer flask with the sulfuric acid. Fit
the flask containing the solid iron with a cotton wool or glass wool plug. Using a hot plate, heat
the flask gently until about 2 cm of foam forms on the surface of the liquid. Maintain this rate of
reaction for about 20 minutes by adjusting the heat as required. This may mean periodically
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Winter 2013
Procedure (continued)
removing the flask from the hot plate. Do NOT leave your bench until the reaction is complete.
A little iron may remain. The flask may be swirled a few times (care!!) during the reaction but
too much agitation will lead to excess evaporation of water and precipitation of the intermediate
product (FeSO4).
Filter the solution directly into the flask containing the ammonium sulfate solution. Discard
the unreacted iron. Mix the solution by gentle swirling and then allow it to cool first to room temperature and then in an ice bath. After a minimum of 30 minutes of cooling in the ice bath, swirl
the mixture vigorously and then return it to the ice bath for a further 10 minutes.
Collect the crystalline precipitate by suction filtration using a Buchner funnel. Do NOT
wash the crystals with water! Wash the crystals once with 10 mL of 50% methanol (prepare this
yourself: 5 mL of methanol mixed with 5 mL of deionized water) and then once with 10 mL of
methanol. Air dry on a watch glass in your locker for a week. Determine the mass of the dry
product and submit it in a labeled sample vial (see page 18 of the “W2013 Introduction” document ) with your report.
Questions
1. What is a primary standard? When could your product be used as a primary standard.
Include a balanced reaction equation for the appropriate reaction.
2. Define double salt and complex ion. Give an example of each and explain how they
differ.
References
1.
G. Svehla. Vogel's Qualitative Inorganic Chemistry, sixth edition.
2.
Petrucci, Harwood et al., General Chemistry, 9th or 10th edition.
3.
D.C. Harris, Quantitative Chemical Analysis, any edition.