Salts are sometimes added to pharmaceutical buffer formulations to maintain the pH necessary for product stability. Sometimes these buffers can give potential instability due to insoluble salts formed with metal ions. This problem can occur especially with phosphate buffers, as most heavy metal phosphates are insoluble.
If this insoluble phosphate salt buffer comes from a colloidal dispersion, it can co-precipitate the particles next to it. To avoid this, one could use chelating operators which in particular respect the metal particles and therefore prevent them from reacting with the phosphate. Non-phosphate buffers could then be replaced with phosphate buffers, if possible, to avoid instability.