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

Abstract

AbstractFree 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. CO2− reduces Ag+ in a complex manner. The rate constants of reactions of both Ag0 and Ag2+ 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 (Ag0) and ‐1.2 V (Ag2+). Ag2+ reacts faster in the presence than absence of ammonia.  Ag2+ radical cations react with 2k = 3.0 ·108 M−1 s−1 (or 5.0 · 109 M−1 s−1 in the presence of ammonia) to form dimeric silver. Ag2+ whose standard reduction potential is approximately −0.5 V is reduced by 1‐hydroxyalkyl radicals; the reaction with CH2OH proceeds through the complex Ag2CH2OH+ (life time: 3.3 · 10−4s). 2‐hydroxyalkyl radicals oxidise Ag2+ ‐ Dimeric silver whose standard redox potential for two‐electron transfer is about − 0.9 V is strongly complexed by Ag+ ions to yield Ag42+. Ag0, Ag2+, and Ag42+ absorb in the near UV, the maximum of the band shifting towards shorter wavelengths in this order. Ag42+ disappears by reaction with another Ag42+ (2k = 1.1 · 107 M−1 s−1 or reaction with colloidal silver particles (produced by preirradiation). In the presence of small sulfate concentrations, the reaction between two Ag42+ 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.