Corrosion of iron


Aim To determine which factors tend to increase or decrease the rate of the corrosion and to see if the rusting reaction occurs in different places along the iron metal.


· 200 mL de-ionised water
· 2g agar
· 2 mL of 0.1 M potassium hexacyanoferrate(lll)

4 x clean iron nails
zinc strip approx. 6 cm x 0.3 cm bare copper wire or thin copper strip approx. 8 cm long
· 2 mL of 0.1% phenolphthalein indicator
· emery paper
· 2 x Petri dishes and covers
· 10 mL measuring cylinder
· 100 mL measuring cylinder
· 500 mLbeaker
· pliers
· Bunsen burner
· bench mat
· tripod
gauze mat and safety glasses

1 Place 200 ml of de-ionised water in a 500 mL beaker. Heat until the water just begins to boil. Remove the heat and stir in 2 g of powdered agar. Heat gently while stirring until the agar is evenly dispersed throughout the water.
2 Add about I ml of 0.1 M potassium hexacyanoferrate(IIl) (potassium ferricyanide) and I ml of 0.1% phenolphthalein indicator to the agar mixture and stir thoroughly.
3 Allow the agar mixture to cool until it is lukewarm but still liquid.
4 While the agar is cooling, clean the four nails with emery paper and place one of them in a Petri dish.
5 Use a pair of pliers to bend a second nail into a right angle, and then place it flat in the Petri dish (not touching the other nail).
6 Wrap a piece of bare copper wire or strip around the third nail in a tight helix so that the wire and nail are in good electrical contact. Place the nail and wire flat in the second Petri dish.
7 Take a piece of zinc strip and clean its surface thoroughly with emery paper. Wrap the strip around the fourth nail in a tight helix so that they are in good electrical contact. Lay the nail and strip flat in the second Petri dish (not touching the other nail).
8 When the agar mixture is cool, pour it carefully over the nails in the Petri dishes until they are covered to a depth of about 0.5 cm. Allow the agar to cool to room temperature and to solidify. Carefully sketch the arrangement of nails in each dish. Place a labelled cover over each Petri dish.
9 Leave the Petri dishes overnight and observe the nails the next day. The areas of blue colouration indicate the area where iron has had its electrons taken from it, while the areas of pink colouration indicate the area where oxygen has taken the electrons..

In this experiment, the hexacyanoferrate forms a blue colour in the region where oxidation of the iron is taking place (where iron is losing elecrons).
Phenolphthalein turns pink in the region where oxygen is taking electrons.
This experiment indicates that the corrosion of iron involves oxidation and reduction reactions. It also indicates where these take place on iron nails.
When iron is in contact with a more reactive metal, this metal will be oxidised(rusted) in preference to the iron thus protecting the iron from corrosion. In other words the more reactive metal will act as a sacrificial anode.When iron is in contact with a less reactive metal, corrosion of the iron by oxidation will be promoted.

* The reactivity of a metal is a measure of how easily it will give up electrons to other atoms. The more reactive the easier it is to give up electrons.

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