Pure gold dissolution with hydrogen peroxide as the oxidizer in HBr or HI solution

Abstract The pure gold dissolution has been explored for H 2 O 2 as the oxidizer in the presence of HBr or HI at 35 °C and, as the reference, for the dilute HNO 3 system containing NaCl. It has been found that 0.02–0.2 mol dm − 3 H 2 O 2 solutions with 0.2–2.0 mol dm − 3 HBr have provided effective media for dissolution of pure gold-wire (99.95%, 0.25 mm diameter, ca. 20 mg) in a 20 mL reaction vessel: the dissolution rate constant, log ( k /s − 1 ), has increased from − 6.17 to − 3.33 with increasing concentrations of H 2 O 2 and HBr. The influences of “indifferent” salts or supporting electrolytes, LiCO 4 , NaClO 4 , or Mg(ClO 4 ) 2 on the log ( k /s − 1 ) value have been very small. Contrastingly, the log ( k /s − 1 ) value of − 4.22 in 2.0 mol dm − 3 HNO 3 (as the oxidizer) solution containing 2.0 mol dm − 3 NaCl at 50 °C has been accelerated remarkably with increasing electrolyte concentrations, e.g. , Δ log ( k /s − 1 ) = 0.66 ( ca. 4.5 times) at 1.0 mol dm − 3  Mg(ClO 4 ) 2 . The effect of the added electrolytes increases in the order, NaClO 4  ~ LiClO 4 4 ) 2 . In acidic media, H 2 O 2 has naturally strong oxidizing power while dilute HNO 3 ( − 3 ) has no strong oxidizing power by itself. We have discussed that the change of water property along with the destruction of the water structure is essential for dilute HNO 3 to acquire the oxidizing power. The H 2 O 2 -HBr solution has induced the Br 2 ‐Br ‐ system and gold is oxidized and complexed to form AuBr 4 − . Similarly, the I 2 -I - system has been provided by H 2 O 2 with HI (combination between HClO 4 and NaI) and the gold dissolution has been also successfully done at 35 °C. In this case, however, the relation between log ( k /s − 1 ) and the HClO 4 and NaI concentrations has not been so straightforward. The adsorption or covering on gold-wire of solid iodine (I 2 ) has caused severe deceleration (or almost no reaction) when c (NaI)/ c (H 2 O 2 )  ca. 4. An excess amount of H 2 O 2 to I − may cause the shortage of I − to form the soluble species I 3 − in solution.