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 similar to 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.