Iron Corrosion

Equipment

Petri dish, iron wire or nail.

Reagents

Agar, potassium chloride, potassium ferricyanide ( K₃Fe(CN)₆ ), phenolphthalein solution, 6 M HNO₃.

Presentation

Solution preparation: This makes 60 mL of solution, which is sufficient to make up two petri dishes. The solution will be 1% w/w agar, 0.3 M KCl.

1. Add 0.6 g of agar, 1.3 g KCl, 2-3 mL of 0.1 M K₃Fe(CN)₆, 20 drops of 0.01% phenolphthalein solution and 60 mL of DI water to a 150-mL beaker.
2. Heat gently with stirring until everything is dissolved.
3. Pour the solution into the petri dishes until the level is approximately half full.
4. Place a lid on the petri dish and allow it to cool. As long as the dishes remain covered and the solution doesn’t dry out, these may be stored until needed.

Demonstration:

1. Clean an iron nail or piece of wire by briefly dipping it in 6 M HNO₃.
2. After cleaning, cut a length of wire ~ 5 cm in length. Be sure to cut both ends freshly.
3. Push the iron down into the agar solution, approximately in the center of the dish.
4. Observe what the iron and agar solution look like. This may be done using an overhead projector for larger classes.
5. The colors develop over time. Typically they are quite distinct after 9 minutes. Examine the iron and agar solution after the colors have developed.

Hazards

Contacts with solid potassium chloride or with its solutions may lead to an allergic skin reaction, dust may cause throat irritation.
Concentrated nitric acid is both a strong acid and a powerful oxidizing agent. Contact with combustible materials can cause fires. Contact with the skin can result in severe burns. The vapor irritates the respiratory system, eyes, and other mucous membranes, and therefore, concentrated nitric acid should be handled only in a well-ventilated area.
Potassium ferricyanide may cause eye and skin irritation, causes digestive and respiratory tract irritation. It may be harmful if swallowed or absorbed through the skin.

Discussion

The piece of iron appears uniform on the large scale, but at the atomic level it is quite irregular. Regions of the iron which have been subjected to intense stress, like the stamped head and point of the nail, or the sheared ends of the wire, contain atoms that have a higher energy than there unstressed neighbors. These regions lose electrons or undergo oxidation slightly more readily than the unstressed regions. The reaction is shown below.

Fe (s)    Fe2+(aq)  +  2 e-

These electrons are readily taken up during the reduction of water, according to the reaction below.

2 H2O (l)  +  2 e-    2 OH-(aq)  +  H2 (g)

alternatively

2 H2O (l)  +  O2 (g)  +  4 e-    4 OH-(aq)

If we can keep water and oxygen away from our iron surfaces, we can minimize the amount of corrosion that can occur, since oxidation cannot occur without reduction. This is why we paint and wax our cars. The colors are created from two additional reactions as shown below. Phenolphthalein is a weak acid and will be denoted as HPh.

HPh (aq)  +   OH-(aq)    Ph-(aq)  +   H2O (l)
colorless                           pink                         

Wherever there are significant quantities of OH^- the region will turn pink.

H2O (l)  +  K+(aq)  +  Fe2+(aq)  +  Fe(CN)63-(aq)  ®  KFe[Fe(CN)6] · H2O(s)
                               pale yellow                   blue

This last compound is call variously Turnbull’s blue or Prussian blue. It is somewhat unusual in that it is a mixed valence compound containing iron in both the +2 and +3 oxidation states. The agar solution will turn blue in any region with a supply of Fe²⁺.

References

1. Lee R. Summerlin, Christy L. Borgford, and Julie L. Ealy, Chemical Demonstrations, A Sourcebook for Teachers, Washington D.C., American Chemical Society, Second Edition, 1988, Vol. 2, p. 191-192.