Chemical Indicators
Chemical indicators reveal the presence of residual oxidizing agents on bleached cotton fabrics before the dyeing cycle begins. Running a potassium iodide spot test allows technicians to detect leftover hydrogen peroxide that would otherwise oxidize and destroy sensitive dyestuffs during coloration. A prompt color change to blue or purple on the fabric surface warns the operator that a neutralizing wash is necessary.
The Reaction
The reaction relies on the oxidation of iodide ions to iodine by residual peroxides, which then forms a colored complex with starch on the fabric or in the test reagent. This rapid potassium iodide spot test prevents batch failures and shade variation across the production run. It is carried out at the wet-processing checkpoint immediately after the rinsing stage of the peroxide bleach cycle.
Standard Procedure
Standard procedure requires the operator to apply a few drops of the prepared starch-iodide solution directly to the damp fabric. A negative result shows no color change, confirming that the bleach has been fully neutralized or washed out.
This Step
This step protects the subsequent reactive or direct dyes from chemical degradation. When the reaction shows high levels of residual oxidant, a reducing agent such as catalase or sodium bisulfite must be added to the bath.
