Question

What is the purpose of sodium hydrosulfite addition? What is the purpose of potassium ferricyanide in the conversion of luminol to an excited state?

Answer 1

Sodium hydrosulfite is used as a reducing agent in dying application. It undergoes reduction reaction with water-insoluble vat dye and sulfur dye to form water-soluble alkali metal salt of the dye (leuco form ) so that they have affinity for the textile fiber. The reductive decomposition of the excessive dye by sodium hydrosulfite improve the colour fastness. Sodium hydrosulfite's reduction reaction removes residual oxide and wrong pigments. Sodium hydrosulfite is a reductive bleaching agent. It reduces carbonyl and alcohol groups, which function as colorants of the substances. It is used in bleaching mechanical paper pulp, cotton, wool and kaolin clay. Additional applications include water treatment, leather processing, food processing, gas purification, cleaning, printing and stripping. In addition to a reducing agent function, sodium hydrosulfite functions as a sulfonating agent and sodium ion source in a variety of chemical reactions.

To exhibit its luminescence, the luminol must be activated with an oxidant. Usually, a solution containing hydrogen peroxide (H₂O₂) and hydroxide ions in water is used as the activator. In the presence of a catalyst such as an iron or periodate compound, the hydrogen peroxide is decomposed to form oxygen and water:

2 H₂O₂ ? O₂ + 2 H₂O

H₂O₂ + KIO₄ ? KIO₃ + O₂ + H₂O

In a laboratory setting, the catalyst used is often potassium ferricyanide or potassium periodate. In the forensic detection of blood, the catalyst is the iron present in hemoglobin.^[6]Enzymes in a variety of biological systems may also catalyze the decomposition of hydrogen peroxide.

When luminol reacts with the hydroxide ion, a dianion is formed. The oxygen produced from the hydrogen peroxide then reacts with the luminol dianion. The product of this reaction, an organic peroxide, is very unstable and is made by the loss of a nitrogen molecule, the change of electrons from excited state to ground state, and the emission of energy as a photon. This emission produces the blue glow.