CHEMISTRY OF COLLOIDAL SILVER - REACTIONS OF LEAD ATOMS AND SMALL LEAD AGGREGATES WITH AG-N

The reactions of lead atoms with 70 Angstrom A colloidal silver partic les in aqueous solution are investigated. The lead atoms are generated by a short pulse of high energy radiation. The formation of a lead ma ntle around the silver particles is recognized by the changes in the s hape of the surface plasmon absorption band of the silver particles. A t low intensities (low lead atom concentration of 5.10(-8) M) almost a ll the lead atoms generated reach the silver particles before they for m larger aggregates by coalescence reactions in the bulk of the soluti on. The specific rate of the reaction of lead atoms with silver partic les is about 5 times smaller than expected from a diffusion controlled rate. The plasmon absorption band is blue-shifted when the lead atoms react with the silver particles. It is concluded from the details of this shift that the first Pb atoms reaching the silver particles trans fer an electron to them, and that the atoms, which arrive later, are j ust deposited. Time resolved measurements show that small aggregates o f lead, which are formed at higher intensities, can still contribute t o mantle formation around the silver particles. Lead particles with ag glomeration numbers larger than 10 to 20 do no longer contribute to le ad deposition; they form colloidal lead.