Mixed oxides of nitrogen are produced by dissolving nitric oxide (NO) gas in liquid dinitrogen tetroxide. Nitric oxide reacts with nitrogen dioxide, present in dinitrogen tetroxide, to from dinitrogen trioxide. Resulting mixture is greenish blue, while dinitrogen tetroxide is colorless or brownish yellow. Liquid phase of MON contains no nitric oxide.[2]
N2O4 ⇌ 2NO2
NO2 + NO ⇌ N2O3
A broad range of compositions is available, and can be denoted as MONi, where i represents the percentage of nitric oxide in the mixture (e.g. MON3 contains 3% nitric oxide, MON25 25% nitric oxide). An upper limit is MON40 (40% by weight). In Europe MON 1.3 is mostly used for rocket propulsion systems, while NASA seems to prefer MON 3. A higher percentage of NO decreases the corrosiveness of the liquid, but decreases oxidation potential and increases costs.
The addition of nitric oxide also reduces the freezing point to a more desirable temperature. The freezing point of pure nitrogen tetroxide is −9 °C (16 °F), while MON3 is −15 °C (5 °F) and MON25 is −55 °C (−67 °F).[3]
