As an Applied Chemist who spent my career in Industrial Chemistry with a few years as a Chemistry and Physics Teacher, I often conjectured that planned or unplanned chemical reactivity is often the direct result of matter attaining stability. When considering matter and its stability, we must reference our understanding of atomic structures.
Fundamentally, the negatively charged electrons and their ‘shells’ or ‘energy levels’ orbit the protons and neutrons held within the nucleus. Stability is sometimes attained when two or more atoms combine to form one or more molecules, termed covalency; while other times, it comes from a transfer of electrons from the outer shells or energy levels, termed an ionic reaction. This brings us to Oxidation, Reduction, and Redox Reactions.
For instance, rusting is a chemical reaction that requires water and oxygen to convert elemental Iron into molecular Iron (III) Oxide: 4Fe+3O2+6H2O→4Fe(OH)34Fe + 3O_2 + 6H_2O → 4Fe(OH)_34Fe+3O2+6H2O→4Fe(OH)3
Oxidation describes a loss of electrons or a gain of oxygen, while Reduction describes a gain of electrons or a loss of oxygen. An oxidizing agent causes another species to be oxidized and is itself reduced as it gains electrons. Conversely, a reducing agent causes another species to be reduced and is itself oxidized as it loses electrons.
When iron reacts with oxygen, each iron atom loses 3 electrons, forming an iron ion (Fe3+). The iron has been oxidized in this process and has acted as the reducing agent. The oxygen atom gains electrons, forming an oxide ion (O2-), meaning the oxygen has been reduced and has acted as the oxidizing agent.
Oxygen, a highly reactive non-metal and oxidizing agent, readily forms oxides with most elements and other compounds. By mass, it is the third-most abundant element in the universe. Di-atomic oxygen gas (O2) constitutes 20.8% of the Earth’s atmosphere.
As a compound, oxygen makes up almost half of the Earth’s crust. The name oxygen was coined by Antoine Lavoisier, who first recognized it as a chemical element and correctly characterized the role it plays in combustion.
Common uses of oxygen include steel production, welding, rocket propellant, oxygen therapy, and life support systems. Oxygen must be absent for certain processes, such as manufacturing, food packaging, or preservation, to prevent oxidation reactions.
Oxygen is produced commercially by air separation, which involves purifying and cooling air cryogenically into liquid form, followed by fractional distillation to separate and collect liquid Nitrogen, Oxygen, and Argon. The produced oxygen, known as LOX, can be supplied in bulk or compressed into high-pressure gas cylinders as GOX.
Oxygen needs to be present to allow combustion to occur. A fire starts by a spark or heat generated by a physical or chemical reaction, requiring fuel at or above its flash point. Oxygen, whether as a liquid in cryogenic tanks or compressed into gas cylinders, must be stored in ventilated spaces to reduce the risk of oxygen enrichment. Rising above the nominal 21% oxygen level in the atmosphere increases the fire hazard, as oxygen aids combustion. An atmosphere enriched with oxygen will ignite faster as the oxygen percentage rises.
Additional concerns include:
The risk of chemical reactions with the presence of oxygen gas is evident in processes like oxy-acetylene welding. Runaway or uncontrolled reactions can occur with compounds containing oxygen, classified as Class 5.1 Oxidizing Agents or more reactive 5.2 Organic Peroxides. These substances react readily with organic matter, such as hydrocarbons, skin tissue, and greases. An exothermic reaction generates heat, which, if not dissipated, can lead to a thermal runaway or explosion.
Common oxygen-containing compounds used in industry include:
The growth in supply-chain volumes of oxygen-containing cleaning/disinfecting products is partly due to the COVID-19 pandemic.
Oxygen must be stored in well-ventilated areas to avoid oxygen enrichment. Conversely, oxygen depletion can lead to hypoxia, anoxia, and anoxemia. Oxygen elimination occurs in processes like nitrogen blanketing (used to prevent oxidation) and modified atmosphere packaging (MAP) in food manufacturing. MAP increases shelf life by reducing or removing oxygen, slowing oxidation.
Equipment used with oxygen must be “Oxygen Cleaned,” meaning all dirt, hydrocarbon-based lubricants, greases, and waxes must be removed from surfaces that may come into contact with oxygen. Oxygen reacts aggressively with these materials, especially under pressure or elevated temperatures. Incidents have occurred in hospitals when moisturizers accidentally smeared on valves caused reactions with oxygen.
There are security concerns regarding oxygen as LOX and GOX, as well as Oxidizing Agents (Class 5.1) and Organic Peroxides (Class 5.2). These products, when consigned in bulk, are termed High Consequence Dangerous Goods (HCDG) under ADR/IMDG regulations and can pose significant threats if used nefariously.
Examples of oxidizing agents include:
Compounds containing functional groups like azide, acetylide, diazo, nitroso, and peroxide are shock and heat-sensitive and can explode violently. Some organic compounds such as ethers and tetrahydrofuran (THF) can form unstable peroxides with oxygen, leading to explosion and fire.
Some compounds react with water or moisture, releasing heat or flammable, toxic gases. Toxic gases like arsine, hydrogen cyanide, chlorine, and others may be evolved following the inadvertent mixing of chemicals.
Bulk and packaged chemicals, commonly referred to as Dangerous Goods, are categorized into eight main classes based on their generic types and associated risks. These classes are further identified by four-digit UN numbers used in industry and logistics.
Class 1 includes explosive substances or articles, while Class 2 encompasses various gases. Flammable liquids fall under Class 3, and Class 4 is subdivided, with Class 4.1 covering flammable solids, self-reactive substances, and de-sensitized explosives. Class 4.2 addresses substances liable to spontaneously combust, and Class 4.3 includes those that emit flammable or toxic gas upon contact with water.
Oxidizing substances are classified under Class 5.1, and organic peroxides are categorized in Class 5.2. Toxic substances belong to Class 6.1, while Class 6.2 is reserved for biologically infectious substances. Radioactive materials are classified under Class 7. Class 8 includes corrosive substances, and finally, Class 9 covers miscellaneous dangerous substances and environmentally hazardous materials.
This information is provided in good faith, but you must always consult with your Dangerous Goods Safety Advisor or Competent Chemist/Hazmat Officer regarding the specifics of your operations.