Question

Redox Reaction: Detecting Blood Traces Experiment.

Give an example of a free radical discussed in this experiment

Answer 1

The Kastle–Meyer test is a presumptive blood test, in which the chemical indicator phenolphthalein is used to detect the possible presence of hemoglobin. The Kastle-Meyer test relies on the peroxidase-like activity of hemoglobin , which is the iron-containing portion of a red blood cell, to promote the oxidation of phenolphthalin to phenolphthalein. Phenolphthalin is colorless, but in the presence of blood and hydrogen peroxide, it changes to phenolphthalein, which makes the solution pink.

Equation given below describes how phenolphthalin (colorless) is oxidized by hydrogen peroxide and heme (which comes from the blood cells) to form phenolphthalein (pink) and water.

Equation :

heme iron Fe⁴⁺ + phenolphthalin (colorless) + H₂O₂ → phenolphthalein (pink) + H₂O + heme iron Fe³⁺

• Heme iron Fe⁴⁺ = heme complexed with iron that has four positive charges
• Phenolphthalin = colorless indicator molecule
• H₂O₂ = hydrogen peroxide
• Phenolphthalein = pink indicator molecule
• H₂O = water
• Heme iron Fe³⁺ = heme complexed with iron that has three positive charges

The Kastle-Meyer test “is all about transfer of electrons through reduction and oxidation (called redox for short). If something gains electrons, it is reduced, if something loses electrons, it is oxidized.” Oxygen free radicals which are produced in this experiment are cleaved from peroxide group to oxidize phenolphthalin (colorless) to phenolphthalein (pink).

Step 1:

Heme has peroxidase activity as a result of which oxygen free radicals are cleaved from peroxide group.

Step 2:

Oxygen free radicals generated above react with phenolphthalin (colorless) and oxidise it to form phenolphthalein (pink).

This test is very sensitive. One drop of blood diluted in 10,000 drops of water can still be detected by the Kastle-Meyer test.