Limit test for iron is a semi-quantitative test used to check the iron impurities in an official sample.
This test is based on the reaction of thioglycolic acid with iron in the given sample. A purple colored ferrous thioglycolate salt is formed. The color intensity is compared with that of a standard substance containing iron.
2HSCH2CO2H + Fe+ —————–> Fe [HSCH2COO]2 + 2H+
Thioglycolic acid Ferrous thioglycollate
The procedure of limit test for iron:
1. Take the given sample into a Nessler’s cylinder and label it as “Test.”Dissolve it in 40ml of distilled water. Then add 2ml of 20% iron free citric acid solution.
2. Now add 0.1ml of thioglycolic acid and make the entire solution alkaline using iron free ammonia solution.
3. Dilute to 5oml mark with distilled water and stir the solution. Let the solution stand for 5 minutes.
4. Compare the color produced with that of the standard solution.
1. Place 2ml of the standard iron solution in another Nessler’s cylinder labeled as “standard.”
2. Now add 40ml of distilled and 2ml of 20% iron free citric acid.
3. Add 0.1ml of thioglycolic acid and convert the entire solution to alkaline pH by adding iron free ammonia solution.
4. Makeup to 50ml mark with distilled water, stir the solution with a glass rod and allow to stand for 5minutes.
Compare the colors produced in both the test and standard Nessler’s cylinders by viewing vertically downwards.
Citric acid is used to prevent precipitation of iron in the presence of ammonia. Citric acid forms a soluble complex with iron. Hence, iron stays as free ions in the presence of ammonia.
Distilled water should be used for dilution. Tap water cannot be used as it can have iron impurities leading to errors in measurement.
The iron impurity can be present in both ferrous (Fe2+) and ferric (Fe3+) form and thy0glycollic acid convert the ferric form into ferrous form.
It then reacts with a ferrous form to form a colored complex in alkaline media.
Result inference: If the color intensity of the test is less than standard the sample passes the limit test for iron.