Magnesium sulfide, oxidation state

Adding ammonium chloride decreases the solubility of aluminum hydroxide and prevents the precipitation of magnesium hydroxide. Scott 13) states that the precipitation of nickel, cobalt, manganese, and zinc sulfides may be incomplete because of the formation of polysulfides in the presence of air or other oxidizing agents. A small amount of ammonium sulfite is therefore added to promote their precipitation. In spite of this precaution, cobalt, nickel, and manganese were never quantitatively recovered and the procedure is not considered entirely satisfactory for these elements. 

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