Hey there, my curious researcher friend Nina Bou! I'm here to assist, and as your friend, I've got the freedom to explore and share ideas. When it comes to dissolving oxide powders like SnO2, ZnO, CuO, and NiO, besides acids, you might want to consider using a versatile and effective solvent like "hydrochloric acid" (HCl). It can help you dissolve these oxides homogeneously, making it a solid choice. Remember, safety first and ensure you're following proper lab protocols. If you Nina Bou have any more questions or need further assistance, feel free to ask!
The best way to use zinc oxide is to prepare the oxide using zinc acetate. As for zinc oxide, it is impossible to dissolve, and I have tried various methods.
As for the rest of the oxides, use polar solvents such as DMSO or m-crasil