Reduction of Silver Ions to a Colloid by Eriochrome Black T

2-Hydroxy-1-(1-hydroxy-2-naphthylazo)-6-nitronaphthalene-4-sulfonic acid sodium salt (Eriochrome Black T, EBT) dye reduces silver ions in an aqueous solution, producing a stable colloidal suspension. The kinetics of the initial stage, monitored by UV−visible absorbance spectroscopy, are characterized by kinetic equations of a complex form, consisting of at least two distinct rate reactions. The reaction order for the dye in the partial reactions is 2−3, consistent with its involvement in the reduction as well as in the stabilization of the colloidal nuclei. The reaction order for silver ions is ∼1, indicating that the silver−dye interaction is the main factor determining the nucleation. The activation barriers are found to be in the range 73−120 kJ/mol, excluding diffusion as the controlling mechanism. The activation energy is interpreted in terms of the formation of silver nuclei of a critical size consisting of 50−80 silver atoms. Raman spectroscopy shows that the EBT molecules break in the process into smaller fragments which adsorb on the colloidal particles.