Oxygen balance (OB or OB%) refers to the amount of oxygen contained in a compound relative to the amount needed to convert all the carbon to carbon dioxide, the hydrogen to water and if present, sulfur, phosphorus or halogen to its respective oxides or non-oxidizable products. To put it shortly, the degree to which an explosive can be oxidized.
Oxygen balance comes in three forms:
- Positive oxygen balance: The explosive molecule contains more oxygen than is needed to fully oxidize its components.
- Example: Erythritol tetranitrate (C4H6N4O12 → 4 CO2 + 3 H2O + N2 + 3/2 O2)
- Zero oxygen balance: The explosive molecule contains just enough oxygen to form carbon dioxide from carbon, water from hydrogen molecules, all of its sulfur dioxide from sulfur, and all metal oxides from metals with no excess.
- Example: Ethylene glycol dinitrate (C2H4N2O6 → 2 CO2 + 2 H2O + N2)
- Negative oxygen balance: The explosive molecule contains less oxygen than is needed, the combustion will then be incomplete, and large amount of toxic gases like carbon monoxide will be present.
- Example: Trinitrotoluene (C7H5N3O6 → CO2 + 2 H2O + 2 CO + 4 C + 1/2 H2 + 3/2 N2)
The brisance, strength and sensitivity of an explosive material are all somewhat dependent upon oxygen balance and tend to approach their maxima as oxygen balance approaches zero. However, this isn't always the case, as seen with explosive peroxides, such as acetone peroxide, which has a negative oxygen balance of -151.19%, and yet it is much more unstable than explosives with a less negative oxygen balance, such as TNT (−74%) or even explosives with a positive oxygen balance such as ammonium nitrate (+20%).
The oxygen balance is very important when creating mixtures of explosives. One example is the family of explosives called amatols, which are mixtures of ammonium nitrate and TNT. Since ammonium nitrate has an oxygen balance of +20% and TNT has an oxygen balance of −74%, it is desirable that the mixture with an oxygen balance of zero would be have best explosive properties. In actual practice a mixture of 80% ammonium nitrate and 20% TNT by weight yields an oxygen balance of +1%, the best properties of all mixtures, and an increase in strength of 30% over TNT.