Are Sulfate Salts Acidic or Basic?

  1. Are Li2SO4, Na2SO4, K2SO4, Ag2SO4, (NH4)2SO4, MgSO4, BaSO4, COSO4, CuSO4, ZnSO4, SrSO4, Al2(SO4)3, and Fe2(SO4)3 acidic or basic?

    On the one hand, Wikipedia says that (NH4)2SO4 and Al2(SO4)3 are acidic, and says nothing about the others. On the other hand logic seems to dictate that in a soluble salt, the SO42- would want to grab H+, leaving OH- floating around to basify the waters.

  2. jcsd
  3. Borek

    Staff: Mentor

    When you dissolve salt, anion is not the only ion present in the solution.

    SO42- is a verrrry weak base (something like Kb=10-12). Theoretical pH of 0.1M potassium (or sodium) sulfate is few tenths of the pH unit above 7.

    Last edited: Apr 19, 2010
  4. NH4+ is more acidic than SO42- is basic, so (NH4)2SO4 is acidic.

    As for Al2(SO4)3, Al3+ is an acidic cation, in that it forms complexes with the water molecules such that the H-atoms is pointing outwards. Because of the polarity of the O-H bond, the H-atoms are somewhat like H+-ions. SO42- is all in all a very weak base, so Al3+ is easily a stronger acid and wins this battle, so Al2(SO4)3 is acidic.

    Whether a salt is acidic or basic depends on the acid/base constants for both ions.
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