Reactivity of Silver Atoms in Aqueous Solution II. A Pulse Radiolysis Study

Abstract

Abstract Free silver atoms were produced by reducing Ag + with hydrated electrons or hydrogen atoms. AgH + with a lifetime shorter than 10 −8 s is postulated as an intermediate of the H atom reaction. Organic free radicals cannot reduce Ag + in a single electron transfer process. CO 2 − reduces Ag + in a complex manner. The rate constants of reactions of both Ag 0 and Ag 2 + with a number of inorganic and organic compounds were measured. Both particles were found to be electron donating agents with approximate standard redox potentials of ‐1.8 V (Ag 0 ) and ‐1.2 V (Ag 2 + ). Ag 2 + reacts faster in the presence than absence of ammonia.  Ag 2 + radical cations react with 2 k = 3.0 ·10 8 M −1 s −1 (or 5.0 · 10 9 M −1 s −1 in the presence of ammonia) to form dimeric silver. Ag 2 + whose standard reduction potential is approximately −0.5 V is reduced by 1‐hydroxyalkyl radicals; the reaction with CH 2 OH proceeds through the complex Ag 2 CH 2 OH + (life time: 3.3 · 10 −4 s). 2‐hydroxyalkyl radicals oxidise Ag 2 + ‐ Dimeric silver whose standard redox potential for two‐electron transfer is about − 0.9 V is strongly complexed by Ag + ions to yield Ag 4 2+ . Ag 0 , Ag 2 + , and Ag 4 2+ absorb in the near UV, the maximum of the band shifting towards shorter wavelengths in this order. Ag 4 2+ disappears by reaction with another Ag 4 2+ (2 k = 1.1 · 10 7 M −1 s −1 or reaction with colloidal silver particles (produced by preirradiation). In the presence of small sulfate concentrations, the reaction between two Ag 4 2+ particles is accelerated. Sulfate also catalyses subsequent agglomeration processes. Colloidal particles with an agglomeration number of about 200 are present after a few milliseconds. They strongly absorb at 390 nm. Larger particles which still absorb close to 390 nm but cause much stronger scattering of red light are present after a few seconds. Free radical reactions can be influenced by the presence of colloidal silver particles; several examples are discussed. – The hydrogen yield from γ‐irradiated silver solutions (25 to 130°C) is equal to the yield of 0.6 molecules/100 eV known as direct molecular yield. It is concluded that the smaller silver aggregates formed in the reduction of Ag + do not react with water in spite of their negative redox potentials.