Silver(III) chemistry concerns the trivalent oxidation state of silver, Ag³⁺, which appears as a brown ion in acidic solution and is the key intermediate in the preparation of Tetrasilver Tetroxide (Ag₄O₄) (Ag₄O₄).
The brown Ag³⁺ ion
The dossier's informal preparation write-up describes the behavior directly. When solid sodium persulfate is added to a solution of silver nitrate or silver oxide in dilute nitric acid (1–2 mol/L), the liquid turns brown and remains clear. The brown color is due to silver(III) ions. The color forms quickly, though not instantaneously — it takes a few seconds.
The write-up notes a caveat: whether the brown color is due to plain Ag³⁺ or to some mixed-valency complex of silver(I) and silver(III) is not clear. It suggests the brown color might be due to a mixed-valency complex, and points to analogous copper(I)/copper(II) and titanium(III)/titanium(IV) examples.
Instability and hydrolysis
Silver(III) ions are not very stable. Even in fairly strongly acidic liquids, the compound slowly decomposes: a black precipitate forms and oxygen is released slowly. The black precipitate results from combined hydrolysis and reduction of the silver(III) ions, forming a mixed silver(I) silver(III) oxide.
Ag+ + S2O8^2- → Ag3+ + 2 SO4^2-
4 Ag3+ + 6 H2O → 2 Ag(I)Ag(III)O2 + 12 H+ + O2
4 Ag3+ + 12 OH- → 2 Ag(I)Ag(III)O2 + 6 H2O + O2
2 Ag(I)Ag(III)O2 → 2 Ag(I)2O + O2
When the brown liquid is added to sodium hydroxide, the formation of the black mixed oxide is almost immediate. A dark brown, very finely divided precipitate forms; the particles stick together quickly into larger black particles. The black solid slowly evolves oxygen and periodically rises to the surface on trapped bubbles before sinking again — a visible "dance" repeated several times.
Remarkably, when persulfate is added to a neutral solution of silver nitrate, no brown color forms at all. The liquid first remains colorless, then slowly turns turbid and a dark brown/black precipitate forms. Apparently at higher pH the brown silver(III) ion is not formed and the mixed oxide precipitates directly.
Catalytic role
Silver nitrate is a catalyst for many persulfate reactions in acidic media. Persulfate is a strong but somewhat sluggish oxidizer; the reaction between Ag(I) and persulfate in acidic media is quite fast. Ag(III) in turn reacts quickly with Mn(II) or Cr(III) to form permanganate or dichromate, itself being converted back to Ag(I). The catalytic action of silver is based on providing an alternative pathway for the final redox reaction, with Ag³⁺ as intermediate species.
Both silver(III) compounds and NiO₂ are strong enough oxidizers to take Mn(IV) and Mn(II) to the +7 state as permanganate, and Cr(III) to the +6 state as dichromate or chromate.
Chloride interference
Chloride ions must be avoided in the silver experiment. They make the liquid cloudy through formation of silver(I) chloride, and they interfere through oxidation to chlorine.
Nomenclature
The dossier cites David Tudela's 2008 J. Chem. Educ. paper, "Silver(II) Oxide or Silver(I,III) Oxide?" (85(6), 863, doi:10.1021/ed085p863), which argues via a thermochemical cycle with volume-based lattice energies that the mixed-valence silver(I,III) oxide is more stable than a hypothetical silver(II) oxide. The coordination geometries of silver and copper in their known oxides correlate with their electron configurations in coordination compounds; the second ionization energy is higher for Ag than for Cu, related to the small size of 3d orbitals and the resulting high electron repulsion for first transition series elements.
The dossier also points to Greenwood & Earnshaw, Chemistry of the Elements, 2nd ed., pages 1181 and 1188, for further reading on silver(III) chemistry.
Related pages
Source notes & attribution
- https://rexresearch.com/AntelmanTSTO/AntelmanTSTO.html