Re: Why do nitrate salts behave as bases when dissolved in liquid N2O4?

Nitrate ion (NO₃^-) is always a base--just an extremely weak one in an aqueous envrionment. In the Bronsted acid/base definition, nitric acid is a proton donor and nitrate ion is a proton acceptor.

HNO₃ + H₂O <===> H₃O⁺ + NO₃^- K_a = -1.4
(reference: Chemist's Companion)

Thus, nitrate can be considered an extremely weak base in water:

NO₃^- + H₂O <===> H₃O⁺ + OH^- (K_b = 15.6)

Since water is a stronger base:

H₂0 + H₂O <===> H₃O⁺ + OH^- (K_w = 14.0)

in water, nitrate is considered a non-base.

In an aprotic solvent (one with no readily ionizable hydrogens or protons), nitrate can acts as a base because it has electron pairs to donate to an electron pair acceptor. (The Lewis definition of acids and bases.)

Most aprotic solvents are non-polar (e.g. hydrocarbons), but to dissolve an ionic salt such as a nitrate, you need a polar solvent, hence N₂O₄.

N₂O₄ exists in equilibrium with NO₂ and has a boiling point of 21 ^oC. (reference: Merck Index, 11th Ed., #6524) My inorganic chemistry is rusty from years of disuse, so I consulted "Advanced Inorganic Chemistry" (Cotton and Wilkinson, 3rd Ed.), which mentions that N₂O₄ is widely used as a solvent, but doesn't go into a great deal of detail.

Hope this helps!