Metal catalysts, when precipitated, have a tendency to occlude foreign ions such as sodium or other alkali or an anionic component such as sulfate or chloride. Usually, volatile components such as nitrate or carbonate are not considered to be harmful occluded ions. However, if the precipitate is a carbonate, hydroxide, or gel-like type, these ions (SO4, Cl, etc.) will be so firmly occluded that washing simply will not remove them adequately. An alternative to simple washing of the fresh precipitate is to filter the catalyst and dry and calcine it. The calcining frees the occluded ions, which then ordinarily can be removed by washing with water or even more completely by ionic exchange with a salt such as ammonium carbonate. The ammonium ions will exchange with cations, and the carbonates with anions. The occluded ions can now be washed away and replaced in the catalyst with the volatile NH4+ and CO3 2. On drying the catalyst after this exchange, the now-present volatile ions are volatilized away.