Pure gold dissolution in dilute chloric, bromic or iodic acid solution containing abundant halide ions

Abstract

The pure gold dissolution has been explored in (dilute) chloric, bromic, or iodic acid (HXO3, X = Cl, Br, I) aqueous solution containing abundant halide ions at 60 °C. It has been found that mixtures between sodium chlorate and HCl (NaClO3 + HCl ⇆ HClO3 + NaCl) in 20 mL are effective media for the dissolution of pure gold-wire (99.95 %, ca. 20 mg, 0.25 mm diameter). The gold-wire dissolution is much more enhanced by HBr than HCl in NaClO3 solution. At 0.50 mol dm-3 NaClO3, for instance, log (k/s-1) values are -5.11 and -3.40 (ca. 50-fold increase) for 1.0 mol dm-3 HCl and HBr, respectively, where “pseudo” first-order dissolution rate constants (k/s-1) have been evaluated by the changes in the mass of gold-wire. In HClO3 solution containing abundant halide ions (X- = Cl-, Br-), the Au metal should be oxidized mainly by X2 to be AuX4-, where X2 is produced by the oxidizing power from HClO3, just as dilute HNO3 (< 2 mol dm-3) containing abundant NaCl. In NaBrO3 solution, however, HBr does not acquire any superiority to HCl in the gold dissolution. In NaIO3 solution, the effect of HX at the lower concentrations (< 0.03 mol dm-3) of NaIO3 increases in the order of HCl < HBr < HI. The gold dissolution in hypochlorous acid (HClO) containing halide ions has been also examined as the reference. A new diagram has been proposed in order to verify that even the conjugate anion (ClO3- or NO3-) from a strong acid (HClO3 or HNO3) is subject to be protonated in the “aqueous solution” modified by electrolytes of very high concentrations (> ~ 5 mol dm-3).