The versatile gas
Carbon dioxide is a colourless, odourless gas which is toxic and can have a damaging effect on health long before it would have displaced enough oxygen to asphyxiate someone. It has a long-term workplace exposure limit of 5000 parts per million (0.5%).
Most people know that people breathe it out, trees breathe it in and it’s the fizz in beer and soft drinks, but carbon dioxide can crop up in some very unexpected places, so here are the ten most unusual places you might find it;
1. You probably wouldn’t expect to find a gas like CO2 underwater but scientists diving off the coast of the Philippines have just found a huge soda stream below the waves, 60 metres (200 feet) down, the ocean floor is bubbling like champagne with vast amounts of carbon dioxide. Measurements revealed concentrations between 60,000 and 95,000 parts per million, the higher end of which is up to 200 times higher than atmospheric concentrations of carbon dioxide. That level quickly falls away as the gas is diluted in the surrounding waters, but even so, the concentrations remain higher than average and if they were present in the atmosphere around us would be completely poisonous! Fortunately for us the carbon dioxide dissolves in water, so we don’t feel the impact of the gas here on dry land.
2. Carbon dioxide does have quite an impact on people. If you’ve ever felt lethargic or dull in a long and busy meeting, it isn’t just because someone is rattling on about finding an underwater stream of CO2 and it’s terminally boring, it is probably because the carbon dioxide in the air has increased from the people present breathing out. The air outside typically has a carbon dioxide level around 380 ppm, but because we exhale the gas when we breathe, levels indoors are generally much higher—in crowded classrooms or meeting rooms, levels frequently reach 1,000 ppm and sometimes exceed 3,000 ppm. The long-term exposure limit may be 5000ppm, but people will start to feel drowsy at levels lower than that and the ability to think clearly and perform various tasks can be impaired.
3. You might decide to step out and wake yourself up with a hit of coffee, however choose carefully. Carbon dioxide can also be used to decaffeinate coffee. In this process green coffee beans are steamed and then added to a high-pressure vessel. A mixture of water and liquid CO2 is circulated through the vessel at 300 atmospheres pressure and 65 ºC the caffeine dissolves into the CO2. The compounds which create the flavour of the brewed coffee are largely insoluble in CO2 and remain in the bean. Giving you all the flavour without the caffeine hit! In a separate vessel, caffeine is scrubbed from the CO2 with additional water and the CO2 is then recyclded to the pressure vessel to start all over again.
4. Fancy something a little sweet to go with your coffee? Maybe something exotic like popping candy (or space dust as it used to called back in the day) would excite your pallet. To make Popping candy a hot sugar mixture is allowed to mix with carbon dioxide gas at about 600 pounds per square inch (psi). The carbon dioxide forms tiny, 600-psi bubbles in the candy. Once it cools the pressure is released and the candy shatters, but the tiny pieces still contain the high-pressure bubbles and when you put the candy in your mouth, it melts and releases the bubbles. What you are hearing and feeling is the 600-psi carbon dioxide gas being released from each bubble. But if all that pop and crackle is enough to make you snap let’s think about a more serious use for the gas.
5. Carbon dioxide is used in certain types of nuclear power stations. In the UK these were known as Magnox Power stations and there were 11 of them around the country producing electricity and also weapons grade plutonium for the nuclear weapons programme. The nuclear reactor heated the gas, which was then transferred to the heat exchanger where the hot gas heated water to produce steam which was used to generate electricity. The cooled gas was then returned to the reactor to start all over again.
6. Carbon dioxide is used on a large scale as a shield gas in MIG/MAG welding, where the gas protects the weld puddle against oxidation by the surrounding air. A mixture of argon and CO2 is commonly used today to achieve a high weld rate and to reduce the need or post weld treatments.
7. Carbon dioxide is also used in certain types of fire extinguisher. These are commonly used for class B flammable liquid fires which includes petrol and other flammable liquids, but also electric equipment. This is because CO2 extinguishers do not contain any water and do not leave any residue as would be expected with a foam extinguisher. The carbon dioxide quashes the fire in the same way as foam, but does not leave any mess. This makes it the ideal choice for offices and other locations with lots of electrical equipment. A carbon dioxide fire extinguisher has a black label to distinguish it from other fire extinguishers.
8. The food industry uses carbon dioxide in many ways, Modified Atmosphere Packaging (MAP) is well established and continues to gain in importance. MAP means, simply put, that the natural ambient air in the package is replaced by a gas or gas mixture, often nitrogen and carbon dioxide. This packaging under a protective atmosphere preserves the quality of fresh produce over a longer period of time, prolongs shelf-life, and gives food producers access to a geographically larger market for perishable products. This is suitable for meat and sausage products, dairy products, bread, fruits and vegetables, fish or convenience products. So, each time you pop open a pack of pork chops, you are also releasing a small amount of carbon dioxide, which was added when the package was sealed.
9. By coincidence, carbon dioxide is also used in slaughterhouses. Prior to animals such as chickens and pigs being taken to slaughter they are immersed in concentrations of up to 80% by volume carbon dioxide as this “stuns” them. The animals are rendered unconscious and so can be slaughtered in a way which causes less anxiety, fear, pain, suffering and stress. Not everyone is convinced by this and animal welfare groups have concerns about the use of CO2 because it takes time and the animal’s response to the CO2 may vary. Compared to the use of electrical stunning which is more invasive and can only be done on a one to one basis the use of carbon dioxide is often seen as the least bad option as it enables the animals to be stunned in a group together with minimal handling and restraining.
10. Dry ice (solid carbon dioxide) has been the mystical material of choice in the discotheques of the land for many many years, rapidly evaporating to create a fog which deflects the lights and lasers and makes everyone look a little bit lovelier than they perhaps really are. But dry ice pellets are also used to replace sandblasting when removing paint from surfaces. This is much gentler on the surface being stripped and the problems associated with residual deposits of paint are greatly reduced. It naturally also adds an air of mystery to the art of sandblasting!
So, carbon dioxide crops up in lots of interesting places, and if your idea of fun is a bit of snap, crackle and pop, shaking your bits to the hits or simply slaughtering a pig, carbon dioxide is there to help you make the most of it.