Oxygen balance
Encyclopedia
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 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, 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 combustion will then be incomplete, and large amount of toxic gases like carbon monoxide
will be present. 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 is calculated from the empirical formula of a compound in percentage of oxygen required for complete conversion of carbon to carbon dioxide, hydrogen 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 mole
s of oxygen that are excess or deficient for 100 grams of a compound.
where
X = number of atoms of carbon, Y = number of atoms of hydrogen, Z = number of atoms of oxygen, and 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
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, powerful, and sensitive; however, many exceptions to this rule do exist. More complicated predictive calculations, such as those discussed in the next section of the explosive materials article, 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 amatol
s 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.
Examples of materials with negative oxygen balance are eg. trinitrotoluene (-74%), aluminium
powder (-89%), sulfur
(-100%), or carbon
(-266.7%).
Examples of materials with positive oxygen balance are eg. ammonium nitrate (+20%), ammonium perchlorate
(+34%), potassium chlorate
(+39.2%), sodium chlorate
(+45%), sodium nitrate
(+47%), tetranitromethane
(+49%), lithium perchlorate
(+60%), or nitroglycerine (+3.5%).
Commercial explosive materials should have oxygen balance close to zero, in order to minimize the production of nitrogen oxide
s and carbon monoxide
; the gaseous products of incomplete combustion are especially dangerous in confined spaces, eg. coal mines.
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
from carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, water from hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
molecules, all of its sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
from sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
, and all metal oxides from metals 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 combustion will then be incomplete, and large amount of toxic gases like carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
will be present. The sensitivity, strength
Strength (explosive)
In explosive materials, strength is the parameter determining the ability of the explosive to move the surrounding material. It is related to the total gas yield of the reaction, and the amount of heat produced...
, and brisance
Brisance
Brisance is the shattering capability of an explosive. It is a measure of the rapidity with which an explosive develops its maximum pressure. The term originates from the French verb "briser", which means to break or shatter...
of an explosive are all somewhat dependent upon oxygen balance and tend to approach their maximums as oxygen balance approaches zero.
The oxygen balance is calculated from the empirical formula of a compound in percentage of oxygen required for complete conversion of carbon to carbon dioxide, hydrogen 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 mole
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
s of oxygen that are excess or deficient for 100 grams of a compound.
where
X = number of atoms of carbon, Y = number of atoms of hydrogen, Z = number of atoms of oxygen, and 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% = -74% for TNT
Because sensitivity, brisance
Brisance
Brisance is the shattering capability of an explosive. It is a measure of the rapidity with which an explosive develops its maximum pressure. The term originates from the French verb "briser", which means to break or shatter...
, 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, powerful, and sensitive; however, many exceptions to this rule do exist. More complicated predictive calculations, such as those discussed in the next section of the explosive materials article, 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 amatol
Amatol
Amatol is a highly explosive material made from a mixture of TNT and ammonium nitrate. Its name originates from the words ammonium and toluene...
s 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.
Examples of materials with negative oxygen balance are eg. trinitrotoluene (-74%), aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
powder (-89%), sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
(-100%), or carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
(-266.7%).
Examples of materials with positive oxygen balance are eg. ammonium nitrate (+20%), ammonium perchlorate
Ammonium perchlorate
Ammonium perchlorate is an inorganic compound with the formula NH4ClO4. It is the salt of perchloric acid and ammonia. It is a powerful oxidizer, which is why its main use is in solid propellants...
(+34%), potassium chlorate
Potassium chlorate
Potassium chlorate is a compound containing potassium, chlorine and oxygen atoms, with the molecular formula KClO3. In its pure form, it is a white crystalline substance. It is the most common chlorate in industrial use...
(+39.2%), sodium chlorate
Sodium chlorate
Sodium chlorate is a chemical compound with the chemical formula . When pure, it is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 250 °C to release oxygen and leave sodium chloride...
(+45%), sodium nitrate
Sodium nitrate
Sodium nitrate is the chemical compound with the formula NaNO3. This salt, also known as Chile saltpeter or Peru saltpeter to distinguish it from ordinary saltpeter, potassium nitrate, is a white solid which is very soluble in water...
(+47%), tetranitromethane
Tetranitromethane
Tetranitromethane or TNM is an organic oxidizer with chemical formula C4. Its chemical structure consists of four nitro groups attached to one carbon atom...
(+49%), lithium perchlorate
Lithium perchlorate
Lithium perchlorate is the inorganic compound with the formula LiClO4. This white or colourless crystalline salt is noteworthy for its high solubility in many solvents. It exists both in anhydrous form and as a trihydrate.-Uses:...
(+60%), or nitroglycerine (+3.5%).
Commercial explosive materials should have oxygen balance close to zero, in order to minimize the production of nitrogen oxide
Nitrogen oxide
Nitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:* Nitric oxide, also known as nitrogen monoxide, , nitrogen oxide* Nitrogen dioxide , nitrogen oxide...
s and carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
; the gaseous products of incomplete combustion are especially dangerous in confined spaces, eg. coal mines.