Synthesis of Submicron Silver Powder by the Hydrometallurgical Reduction of Silver Nitrate with Hydrazine Hydrate and a Thermodynamic Analysis of the System

Abstract

A silver powder of submicron size was produced from the aqueous solutions of its compounds. The silver compounds tried out were silver nitrate and silver oxide, and the reducing agents employed were dimethyl formamide (DMF), hydrazine hydrate, and sodium azide. The solvent mediums were distilled water for the reductions with DMF and sodium azide, and a 2:1 (by volume) mixture of distilled water and ethanol for the reductions with hydrazine hydrate. Of the three reductants, hydrazine hydrate (N2H4·H2O) alone was successful in reducing both the silver compounds to a submicron (<500 nm) metallic silver powder, as revealed by X-ray diffraction (XRD) studies and scanning electron microscopy (SEM) analyses. Additionally, the thermodynamic equilibrium of the system AgNO3-N2H4·H2O in the water–ethanol mixture (2:1) was studied at 298 K; the equilibrium constant data so generated was found to compare very well with those derived from the established data of enthalpies and free energies of formation, and half-cell potentials. The following activity coefficient (Raoultian)–composition relationship for hydrazine hydrate in its dilute solution in water (plus ethanol) at 298 K is proposed: $$ \ln (\gamma_{{{\text{N}_2\text{H}_4}}{\text{.H}_2\text{O}}} ) = 1862( \pm 371) - 2055( \pm 424)(1 - X_{{{\text{N}_2\text{H}_4 \cdot \text{H}_2\text{O}}}} )^{2} $$