Product of Rusting

Aim: The purpose of this experiment is to detect the metal oxidation that will lead to rust by using a ferroxyl indicator solution in the presence of Fe2+ ions.

Hypothesis: The reaction would most probably occur in an acidic solution and it would be more pronounced on the nails where the surface area is greater and the metal is weaker.

General Background:

The rusting of iron is primarily due to attack by hydrogen ions.

“In order that rust should be formed iron must go into solution and hydrogen must be given off in the presence of oxygen or certain oxidizing agents. This presumes electrolytic action, as every iron ion that appears at a certain spot demands the disappearance of a hydrogen ion at another, with a consequent formation of gaseous hydrogen. The gaseous hydrogen is rarely visible in the process of rusting, owing to the rather high solubility and great diffusive power of this element. Substances which increase the concentration of hydrogen ions, such as acids and acid salts, stimulate corrosion, while substances which increase the concentration of hydroxyl ions inhibit it.”1

Hence, in order for the experiment to be conducted, an acidic solution would be required. This would be done by using the oxidizing agent – potassium ferricyanide (K3Fe(CN)6).

Control variables:

* Concentration and volume of NaCl used

* Concentration and volume of K3Fe(CN)6 used

* Volume of phenolphthalein used

Independent variables:

* Iron (II) Sulphate FeSO4 crystal

* NaOH solution

* Iron nails – one bent and one straight

Dependent variables:

* Colour changes in the various reaction

* Amount of rusting on the different nails

Materials:

Apparatus

1. 1 � measuring cylinder (50cm3)

2. 2 � dropper

3. 1 � beaker (50cm3)

4. 2 � test tubes

5. Paper napkin

Reagents

1. 15cm3 of 3% NaCl solution

2. 10% of K3Fe(CN)6 solution

3. Phenolphthalein indicator

4. FeSO4 crystal

5. NaOH solution

6. 2 � iron nails

Method:

1. First, 15cm3 of 3% NaCl solution, 4 drops of 10% K3Fe(CN)6 solution and 2 drops of phenolphthalein indicator were combined in a small beaker and labeled as solution A.

2. Then, a sample of solution A was taken in a test tube and reacted with a crystal of FeSO4. The resulting colour change was then noted.

3. After that, another sample of solution A was taken in a test tube and a drop of NaOH solution was added. The resulting colour change was noted.

4. Next, a small plain paper napkin was folded in half and soaked in solution A. Then, two clean nails – one straight and one bent were wrapped in the paper napkin and kept in a petri dish.

5. The nails were kept apart and left there for 10 minutes. After that, the napkin was opened and the colour change formed on the tissue paper was observed. Furthermore, the area in which the colour was more pronounced was noted.

Controlling the variables:

The original solution A was made once only, and little amounts of it were used for each trial hence the amount of NaCl, K3Fe(CN)6 and phenolphthalein remained the same.

Data collection

The following are the colour changes obtained in the experiment:

Step 1: formation of solution A

Observation: colour of the solution was orange-yellow

Step 2: addition of FeSO4 to solution A

Observation: the solution turned deep blue and an insoluble precipitate was noticed

Step 3: addition of NaOH of solution A

Observation: solution turned pinkish – red

Step 5: reaction of iron nails with solution A

Observation: a deep blue outline of the nail of similar colour to the one in step 2 was obtained. Furthermore, it was noticed that the colour was darker on the outside part of the bent nail. Moreover, there were tiny pink spots in the shape (outline) but were not easily distinguishable.

Data Processing

Analysis of the various steps:

Step 1:

15cm3 of 3% NaCl solution, 4 drops of 10% K3Fe(CN)6 solution and 2 drops of phenolphthalein indicator were combined to form a ferroxyl indicator solution.

Rusting is a slow process when it undergoes naturally, hence in order to perform a quick study, it was necessary to stimulate the rusting to take place faster.

Hence, K3Fe(CN)6 was used as an acidic oxidizing agent. The acidity of the solution would allow the dissociation of ions in the NaCl solution to give Na+, Cl-, H+ & OH-. This is important because in order for rusting to take place, hydrogen ions are needed so that Fe can go into solution and H2 gas is liberated.

Furthermore, “the more ionized the solution is, the higher its electrical conductivity, and the more rapid damage to the underlying iron.”2 Thus, a more acidic solution would lead to faster ionization of the NaCl solution.

Hence, the acid salt was needed to stimulate the corrosion process i.e. make it faster as explained above.

The phenolphthalein indicator was used to make sure that the solution was acidic. Phenolphthalein stays colourless when the solution is acidic and turns pink when the solution is basic. In this case, when solution A was formed, initially the NaCl solution was colourless, the potassium ferrocyanide solution was orange – yellow and the phenolphthalein was colourless and hence the colour of solution A was that of potassium ferrocyanide – orange – yellow.

Although, this does not confirm that the solution was acidic, it indicates the absence of OH- ions. (This would be discussed further in Step 3 analysis.)

Step 2:

A crystal of FeSO4 was added to solution A. After a few moments, the solution turned a deep blue colour and an insoluble pigment was noticed at the base of the test tube. This pigment is known as Prussian blue – Fe4[Fe(CN)6]3.3

The short time in which the colour change was obtained indicated that iron went into solution immediately to give Fe2+ ions. Furthermore, it signified that the solution was acidic as it allowed rapid ionization of FeSO4.

Step 3:

A drop of NaOH solution was added to solution A. after a few moments, the colour turned pinkish – red indicating the presence of OH- ions. Hence, this confirms that solution A must be slightly acidic because a single drop of NaOH solution resulted in the colour change of the indicator. This step was done to ensure the validity of the earlier trial and assumptions.

Step 5:

In this step, the colour formed on the part of the paper napkin in direct contact with the nails was the same as that obtained in step 2 – deep blue. Moreover, it was noticed that the outside part of the bent nail showed localized corrosive effects by depicting a darker colour. This was probably due to two reasons – the first being that the nail was exposed to a greater surface area at that point hence more corrosion took place there. The second reason is that due to the bending, that part of the nail became weaker due to stress and hence it was easily corroded. This effect is known as pitting.

On the other hand, the straight nail had no such darker spots in its outline.

In addition to that, the slight pink spots observed on the paper napkin indicate that some parts of the nails contained more iron and hence were slightly more electropositive, thus attracting the OH- ions and giving the pinkish colour.

The deep blue colour also indicates that the ions of iron were Fe2+.

Conclusion and Evaluation

From my experiment, I conclude that the product of rusting in this case was Fe2+ ions. Furthermore, I noticed that rusting occurs faster when the iron is in contact with an acidic salt solution. In addition to that, when parts of the object are weaker, they are corroded more easily and rapidly. Moreover, the amount of surface area exposed also affects the rate of rusting.

Overall, the experiment performed was quite adequate and all the factors were taken into account. Although, the two nails could be kept for a longer time in the petri -dish to obtain more distinct results.

In order to further study the action of rusting, the iron nails could be compared with steel nails and/or galvanized nails to see how fast or slow rusting occurs in each case.

Bibliography

Gardner, Cushman &. Corrosion doctors. 24 November 2008

<http://corrosion-doctors.org/Corrosion-History/Rusting.htm>.

Potassium ferricyanide – Wikipedia, the free encyclopedia. 25 November 2008 <http://en.wikipedia.org/wiki/Potassium_ferricyanide>.