Oxygen Balance
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Oxygen balance is an expression that is used to indicate the degree to which an explosive can be oxidized. If an explosive molecule contains just enough oxygen to convert all of its carbon to carbon dioxide, all of its hydrogen to water, and all of its metal to metal oxide with no excess, the molecule is said to have a zero oxygen balance. The molecule is said to have a positive oxygen balance if it contains more oxygen than is needed and a negative oxygen balance if it contains less oxygen than is needed. The sensitivity, strength, and brisance of an explosive are all somewhat dependent upon oxygen balance and tend to approach their maximums as oxygen balance approaches zero.
The oxygen balance (OB) is calculated from the empirical formula of a compound in percentage of oxygen required for complete conversion of carbon to carbon dioxide, hydrog-en to water, and metal to metal oxide.
The procedure for calculating oxygen balance in terms of 100 grams of the explosive material is to determine the number of gram atoms of oxygen that are excess or deficient for 100 grams of a compound.
- 1600 Y
OB (%) = Mol. Wt. of Compound 2X + 2 + M - Z
where
X = number of atoms of carbon
Y = number of atoms of hydrogen
Z = number of atoms of oxygen
M = number of atoms of metal (metallic oxide produced).
In the case of TNT (C6H2(NO2)3CH3),
Molecular weight = 227.1
X = 7 (number of carbon atoms)
Y = 5 (number of hydrogen atoms)
Z = 6 (number of oxygen atoms)
Therefore
OB (%) = -1600 [14 + 2.5 - 6]
227.1
= - 74% for TNT
Because sensitivity, brisance, and strength are properties resulting from a complex explosive chemical reaction, a simple relationship such as oxygen balance cannot be depended upon to yield universally consistent results. When using oxygen balance to predict properties of one explosive relative to another, it is to be expected that one with an oxygen balance closer to zero will be the more brisant, pow-erful, and sensitive; however, many exceptions to this rule do exist. More complicated predictive calculations, such as those discussed in the next section (go here), result in more accurate predictions.
One area in which oxygen balance can be applied is in the processing of mixtures of explosives. The family of explosives called amatols are mixtures of ammonium nitrate and TNT. Ammonium nitrate has an oxygen balance of +20% and TNT has an oxygen balance of -74%, so it would appear that the mixture yielding an oxygen balance of zero would also result in the 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.
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