Abstract
The equilibrium solubility of crystalline cuprous oxide, cuprite, was measured in
liquid water and steam using two flow-through reactors and a conventional batch autoclave.
These measurements were carried out from 20 to 400 °C. Different batches of pretreated
cuprite were thoroughly characterized prior to and following each set of experiments. Metallic
copper beads were added to the inlet end of the reactors and to the solid charge in the
autoclave to preserve the Cu(I) oxidation state, although one series of experiments produced
some results which were only compatible with CuO(cr) as the solubility limiting phase.
Comparison of the solubility data for Cu2O(cr) in aqueous solution with those from the
only available high-temperature dataset (Var’yash, Geochem. Int. 26:80–90, 1989) showed
that in near-neutral solutions the new data are lower by about four orders of magnitude at
350 °C. Moreover, the dominant species in solution at temperatures ≥100 °C were found
to be only Cu+ and Cu(OH)
−
2 with Cu(OH)0 occurring over a narrow pH range at ≤75 °C
rather than the reverse trend reported previously. Solubility equations were developed as a
function of temperature and pH, based on these new results, which showed increased solubility
with temperature in acidic and basic solutions. The solubility of Cu2O(cr) in steam
decreased slightly with temperature and as expected increased with increasing pressure to
supercritical conditions where limited, compatible data were available in the literature. The
solubility at subcritical conditions was on the order of one to several parts per billion, ppb.
A simple empirical fit was derived for the solubility in steam as a function of temperature
and pressure.