Unlike base metals which are readily soluble in mineral acids, noble metals like gold and silver require the presence of oxidizing and complexing agents for dissolution. Silver approaches base metal behavior in acidic thiourea solutions (Ag + 3Thio ↔ Ag(Thio)3+ + e, E° = 0.025 V) requiring only mild oxidizing conditions for dissolution. Accordingly, the following reactions characterize the dissolution of silver in acidic thiourea with various oxidants
Ferric sulfate and chloride have been frequently employed as oxidants. Other oxidants that appear compatible with acidic thiourea systems include Na2O2, O3, KBrO3, and K2Cr2O7.
Like the metallic form, certain silver minerals require oxidizing conditions for dissolution in acidic solutions. Argentite (Ag2S) in the presence of thiourea is leached by ferric ions according to the following overall reaction
It is reasonable to believe that refractory silver minerals like Hessite (Ag2Te) would respond to acidic thiourea leaching similar to the chemistry represented in Eq. (6).
Cerargyrite (AgCl) is a silver mineral that will dissolve in acidified thiourea solutions under nonoxidizing conditions. This mineral proceeds to solution by simple complexation dissolution. The following reaction represents the dissolution of AgCl in thiourea