Silver(I,III) oxide

Silver(I,III) oxide is the inorganic compound with the formula Ag₄O₄. It is a component of silver oxide-zinc alkaline batteries. It can be prepared by the slow addition of a silver(I) salt to a persulfate solution e.g. AgNO₃ to a Na₂S₂O₈ solution. It adopts an unusual structure, being a mixed-valence compound. It is a dark brown solid that decomposes with evolution of O₂ in water. It dissolves in concentrated nitric acid to give brown solutions containing the Ag²⁺ ion.

Structure

Although its empirical formula, AgO, suggests that silver is in the +2 oxidation state in this compound, AgO is in fact diamagnetic. X-ray diffraction studies show that the silver atoms adopt two different coordination environments, one having two collinear oxide neighbours and the other four coplanar oxide neighbours. AgO is therefore formulated as Ag^IAg^IIIO₂ or Ag₂O·Ag₂O₃. It is being a 1:1 molar mixture of silver(I) oxide, Ag₂O, and silver(III) oxide, Ag₂O₃. It has previously been called silver peroxide, which is incorrect since does not contain the peroxide ion, O₂^2âˆ'.

Preparation

US patent 4003757 (Lux and Chobanov) describes one method for preparing this oxide (then called Ag(II)-oxide) in a form suitable for batteries and gives the following example:

In 1.5 liters of aqueous solution containing 150 grams of sodium hydroxide, 65 grams of silver powder are suspended with continuous stirring. The silver powder has a density of approximately 1.6 grams per cubic centimeter. Its grain size distribution is: 52% under 10 microns; 33% 10 microns to 30 microns, 15% above 30 microns.

The liquid is then heated to about 85° C. Upon reaching this temperature, a total of 200 grams of potassium peroxidisulfate (K2 S2 O8) in portions of about 40 grams each is added at intervals of, for example, 1 hour. After addition of the final portion of oxidant, stirring is continued for 3 hours. The product is then filtered, washed to free it of alkali substances, dried at a temperature of approximately 80° C and reduced to particle form.

The foregoing yields approximately 73 grams of silver-(I,III)-oxide with more than 95% content of pure silver-(I,III)-oxide. The silver oxide produced is characterized by high thermodynamic stability, low internal discharge and consequent long shelf life. The rate of gas evolution of there products in 18% NaOH is below 1 microliter per gram-hour at room temperature. This stability is attributable to the fact that the process embodying the invention produces single cyrstals of exceptionally regular shape and monoclinic form.

References

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