H2O2, HNO3, Br2, Cl2, KMnO4, etc. You can choose at will, and the concentration is also optional.
In acidic environment, no oxidant can oxidize Co2+ into Co3+, and all oxidants with strong oxidation (such as F2, etc.). ) will react with water first.
It is troublesome in alkaline environment, so sodium tetrasulfate, a very weak oxidant, can be considered. But I don't think you mean alkaline environment.