Why can pure gold be dissolved in seawater mixed with aqueous nitric acid?

Abstract

In order to further demonstrate the oxidation ability of diluted aqueous nitric acid containing abundant amounts of salts, the dissolution of precious metals (Au, Pt, and Pd), especially of gold, has been examined in 0.1‒2 mol dm‒3 HNO3 accompanied by alkali metal, alkaline earth metal, and aluminum chlorides. The complete dissolution time of a pure gold plate (20±2 mg, 0.1 mm thickness) in 2.0 mol dm‒3 HNO3 accompanied by 1.0 mol dm‒3 AlCl3 was shortened remarkably with a temperature increase from 15 to 80 °C. The dissolution rate constants, log (k/s‒1), of a piece of gold wire (19.7 ± 0.5 mg) in 20 mL of 2.0 mol dm‒3 HNO3 accompanied by the metal chlorides, in general, increased with increasing salt concentrations at 40 and 60 °C. For instance, the log (k/s‒1) values linearly increase from ‒4.15 via ‒3.77, ‒3.45 to ‒3.14 by the addition of 1.0, 2.0, 3.0, and 4.0 mol dm‒3 LiCl in a 2.0 mol dm‒3 HNO3 solution at 60 °C. The complete dissolution experiments of the gold wire in much lower HNO3 concentrations (down to 0.1 mol dm‒3) with the chloride salts have also been successfully performed. Gold could be dissolved in a solution of 1.0 mol dm‒3 HNO‒3 and 1.0 mol dm‒3 HCl, i.e. a “dilute aqua regia”. We have achieved a total dissolution of five pieces (0.10 g) of the gold wire in 100 mL of a 1: 1 mixture between seawater and 2.0 mol dm‒3 HNO3 at ca. 100 °C, with a complete dissolution time of within 17 hours and log (k/s‒1) = ‒4.52. The distortion of bulk water and the alternation of water properties are discussed based on the Raman spectra with increasing HCl concentrations.