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Characteristic Reactions of Magnesium Ions (Mg²⁺)

  • Page ID
    97272
    • Most common oxidation state: +2
    • M.P. 650º
    • B.P. 1120º
    • Density 1.74 g/cm3
    • Characteristics: Magnesium is a silvery metal that is quite active, reacting slowly with boiling (but not cold) water to give hydrogen and the rather insoluble magnesium hydroxide, \(\ce{Mg(OH)2}\). It combines easily with oxygen and at high temperatures reacts with such nonmetals as the halogens, sulfur, and even nitrogen.

    Characteristic Reactions of Mg²⁺

    Magnesium ion rarely forms complex ions. All salts are white; most are soluble in water.

    Aqueous Ammonia

    Aqueous ammonia precipitates white gelatinous \(\ce{Mg(OH)2}\):

    \[\ce{Mg^{2+}(aq) + 2NH3(aq) + 2H2O(l) <=> Mg(OH)2(s) + 2NH4^{+}(aq)} \label{eq1}\] Al3ppt.gif

    Ammonium salts dissolve \(\ce{Mg(OH)2}\) or prevent its precipitation, when added to aqueous ammonia. This is a buffer effect and shifts the pH to a lower value, causing a shift of the precipitation equilibrium in Equation \ref{eq1} to the left.

    Sodium Hydroxide

    Sodium hydroxide gives the same precipitate as aqueous ammonia:

    \[\ce{Mg^{2+}(aq) + 2OH^{-}(aq) <=> Mg(OH)2(s) }\] Al3ppt.gif

    Sodium Monohydrogen Phosphate

    \(\ce{Na2HPO4}\) gives a characteristic crystalline precipitate in an ammonia-ammonium chloride buffer.

    \[\ce{ Mg^{2+}(aq) + NH3(aq) + HPO4^{2-}(aq) <=> MgNH4PO4(s) }\] Hg25b.gif

    Magnesium Reagent

    Solid magnesium hydroxide forms a blue "lake" with a dilute solution of 4-(p-nitrophenylazo)resorcinol (magnesium reagent).

    Mg5b.gif

    No Reaction

    \(\ce{Cl^{-}}\), \(\ce{SO4^{2-}}\)