Test for Iron II and III

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Test for Iron II and Iron III Ions

Process Objectives

To design an experiment to determine if an unknown solution contains iron II or iron III ions.
To organize the data collected into a chart.

Learning Objectives

To list compounds useful in identifying the iron II and the iron III ions.
To learn how to confirm the presence of the iron III ion.

Introduction

In the identification tests for the Fe²⁺ and Fe³⁺ ions shall use the complex ferrocyanide, Fe(CN)₆^4-, and ferricyanide, Fe(CN)₆^3-_, ion. The complex ion charges clearly indicate the difference in the oxidation number of the ion present in the two complexes. Look at eh table below. The (CN) group in each complex has a charge of 1^-. Thus, iron II is present in the ferrocyanide, group, [Fe²⁺(CN-)₆]^4-. Iron III is present in the ferricyanide group, [Fe³⁺(CN-)₆]^3-.

^{Fe⁺²

CN^-

CN^-

CN^-

CN^-

CN^-

CN^-

Oxidation # = -4

Fe⁺³

CN^-

CN^-

CN^-

CN^-

CN^-

CN^-

Oxidation # = -3}

A deep-blue precipitate results when either complex ion combines with iron in a different oxidation state from that present in the complex. So if a Fe(CN)₆^4- which has an Fe²⁺in it where to combine with another iron with an Fe³⁺ a deep-blue precipitate forms. An iron of a different oxidation state would have to combine in order for the precipitate to occur. The deep-blue color of the precipitate is caused by the presence of iron in both oxidation states.

This provides us with the means of identifying either iron. If the deep-blue precipitate is formed on addition of [Fe²⁺(CN-)₆]^4- the complex indicates the presence of the iron II ion. Similarly, a deep-blue precipitate formed with the [Fe³⁺(CN-)₆]^3-complex indicates the presence of the iron (II) ion.

When the iron of a different oxidation number is added to either the ferricyanide or the ferrocyanide, both deep-blue precipitates are now recognized as having the same composition. The potassium salts of the complex ions are commonly used, in which case the deep-blue precipitate may be considered to have the composition, KFeFe(CN)₆^. H₂0. The thiocyanate ion, SCN-, provides an excellent confirming test for the Fe³⁺ ion. The soluble FeSCN²⁺ complex is formed, imparting a rich blood-red color to the solution.

The deep-blue precipitate produced in this experiment is found in a number of products. This insoluble KFeFe(CN)₆ . H₂O complex is the “blue” of blueprint paper. It is the pigment in Prussian blue oil paint, and typewriter ribbon. As the main ingredient in certain brands of laundry bluing, it is used to counteract the yellowing of white clothes.

Safety

Take the necessary safety precaution before beginning this experiment. Wear Safety goggles and apron. As you conduct this experiment, you are required to handle various chemicals. Do not touch the chemicals with your hands. Carefully check the labels on the reagent bottles before removing any of their contents. Observe all safety precautions while conducting experiments. It is important to use good safety techniques while conducting experiments.

Apparatus

Graduated cylinder, 10ml 6 test tubes

Stirring rod 150 ml beaker

Materials

0.1 M iron (III) chloride 0.2 M potassium thiocyanate

0.1 M potassium ferricyanide 0.1 M postassium ferrocyanide

iron (II) ammonium sulfate

Recording Your Observations

After completing each of the procedures, record you observations in the Data Table and in the spaces below.

Procedures

1. Dissolve 0.9 grams of iron (II) ammonium sulfate in approximately 25 mL of distilled water in a 150 mL beaker.

Observation ___________________________________________________

______________________________________________________________

2. To a 5-ml portion of the iron (II) ammonium sulfate solution, add 3 or 4 drops of a 0.2 M solution of potassium thiocyantate, KSCN. Record your observation.