Sodium manganate is a chemical compound. Its chemical formula is Na2MnO4. It contains sodium and manganate ions. It is a bright green solid. It cannot be made by reacting manganese dioxide with sodium hydroxide, like potassium manganate is made. It is made by reacting hot sodium hydroxide with sodium permanganate. This releases oxygen and makes sodium manganate. It is more expensive than potassium manganate because it is harder to make.