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Oxyhydrogen

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Nineteenth-century electrolytic cell for producing oxyhydrogen

Oxyhydrogen is a mixture of hydrogen (H2) and oxygen (O2) gases. This gaseous mixture is used for torches to process refractory materials and was the first gaseous mixture used for welding. Theoretically, a ratio of 2:1 hydrogen:oxygen is enough to achieve maximum efficiency; in practice a ratio 4:1 or 5:1 is needed to avoid an oxidizing flame.

This mixture may also be referred to as Knallgas (Scandinavian and German Knallgas; lit. 'bang-gas'), although some authors define knallgas to be a generic term for the mixture of fuel with the precise amount of oxygen required for complete combustion, thus 2:1 oxyhydrogen would be called "hydrogen-knallgas".

"Brown's gas" and HHO are terms for oxyhydrogen originating in pseudoscience, although x H2 + y O2 is preferred due to HHO meaning H2O.

Properties

Oxyhydrogen will combust when brought to its autoignition temperature. For the stoichiometric mixture in air, at normal atmospheric pressure, autoignition occurs at about 570 °C (1065 °F). The minimum energy required to ignite such a mixture, at lower temperatures, with a spark is about 20 microjoules. At standard temperature and pressure, oxyhydrogen can burn when it is between about 4% and 95% hydrogen by volume.

When ignited, the gas mixture converts to water vapor and releases energy, which sustains the reaction: 241.8 kJ of energy (LHV) for every mole of H2 burned. The amount of heat energy released is independent of the mode of combustion, but the temperature of the flame varies. The maximum temperature of about 2,800 °C (5,100 °F) is achieved with an exact stoichiometric mixture, about 700 °C (1,300 °F) hotter than a hydrogen flame in air. When either of the gases are mixed in excess of this ratio, or when mixed with an inert gas like nitrogen, the heat must spread throughout a greater quantity of matter and the flame temperature will be lower.

Oxyhydrogen is explosive and can detonate when ignited, releasing a large amount of energy. This is often demonstrated in classroom environments in which teachers fill a balloon with the gas, due to the easy access of hydrogen and oxygen.

Production by electrolysis

A pure stoichiometric mixture may be obtained by water electrolysis, which uses an electric current to dissociate the water molecules:

Electrolysis: 2 H2O → 2 H2 + O2
Combustion: 2 H2 + O2 → 2 H2O

William Nicholson was the first to decompose water in this manner in 1800. In theory, the input energy of a closed system always equals the output energy, as the first law of thermodynamics states. However, in practice no systems are perfectly closed, and the energy required to generate the oxyhydrogen always exceeds the energy released by combusting it, even at maximum practical efficiency, as the second law of thermodynamics implies (see Electrolysis of water#Efficiency).

Applications

Limelights used an oxyhydrogen flame as a high-temperature heat source

Lighting

Many forms of oxyhydrogen lamps have been described, such as the limelight, which used an oxyhydrogen flame to heat a piece of quicklime to white hot incandescence. Because of the explosiveness of the oxyhydrogen, limelights have been replaced by electric lighting.

Oxyhydrogen blowpipe

Nineteenth-century bellows-operated oxy-hydrogen blowpipe, including two different types of flashback arrestor

The foundations of the oxy-hydrogen blowpipe were laid down by Carl Wilhelm Scheele and Joseph Priestley around the last quarter of the eighteenth century. The oxy-hydrogen blowpipe itself was developed by the Frenchman Bochard-de-Saron, the English mineralogist Edward Daniel Clarke and the American chemist Robert Hare in the late 18th and early 19th centuries. It produced a flame hot enough to melt such refractory materials as platinum, porcelain, fire brick, and corundum, and was a valuable tool in several fields of science. It is used in the Verneuil process to produce synthetic corundum.

Oxyhydrogen torch

See also: Thermal lance and Gas burner

An oxyhydrogen torch (also known as hydrogen torch) is an oxy-gas torch that burns hydrogen (the fuel) with oxygen (the oxidizer). It is used for cutting and welding metals, glasses, and thermoplastics.

Due to competition from arc welding and other oxy-fuel torches such as the acetylene-fueled cutting torch, the oxyhydrogen torch is seldom used today, but it remains the preferred cutting tool in some niche applications.

Oxyhydrogen was once used in working platinum, because at the time, only it could burn hot enough to melt the metal 1,768.3 °C (3,214.9 °F). These techniques have been superseded by the electric arc furnace.

References

  1. Howard Monroe Raymond (1916), "Oxy-Hydrogen Welding", Modern Shop Practice volume 1, American Technical Society, archived from the original on March 6, 2011
  2. Viall, Ethan (1921). Gas Torch and Thermite Welding. McGraw-Hill. p. 10. Archived from the original on August 3, 2016.
  3. W. Dittmar, "Exercises in quantitative chemical analysis", 1887, p. 189 Archived June 27, 2014, at the Wayback Machine
  4. ^ O'Connor, Ken. "Hydrogen" (PDF). NASA Glenn Research Center Glenn Safety Manual. Archived from the original (PDF) on February 2, 2013.
  5. Moyle, Morton; Morrison, Richard; Churchill, Stuart (March 1960). "Detonation Characteristics of Hydrogen Oxygen Mixtures" (PDF). AIChE Journal. 6 (1): 92–96. Bibcode:1960AIChE...6...92M. doi:10.1002/aic.690060118. hdl:2027.42/37308.
  6. ^ Chisholm, Hugh, ed. (1911). "Oxyhydrogen Flame" . Encyclopædia Britannica. Vol. 20 (11th ed.). Cambridge University Press. p. 424.
  7. Calvert, James B. (April 21, 2008). "Hydrogen". University of Denver. Archived from the original on April 18, 2009. Retrieved April 23, 2009. An air-hydrogen torch flame reaches 2045 °C, while an oxyhydrogen flame reaches 2660 °C.
  8. "Adiabatic Flame Temperature". The Engineering Toolbox. Archived from the original on January 28, 2008. Retrieved April 23, 2009. "Oxygen as Oxidizer: 3473 K, Air as Oxidizer: 2483 K"
  9. "Temperature of a Blue Flame". Archived from the original on March 16, 2008. Retrieved April 5, 2008. "Hydrogen in air: 2,400 K, Hydrogen in Oxygen: 3,080 K"
  10. Vernon, Julia (August 2011). Acoustical Characterization of Exploding Hydrogen-Oxygen Balloons (Thesis). Retrieved August 15, 2024.
  11. ^ Tilden, William Augustus (1926). Chemical Discovery and Invention in the Twentieth Century. Adamant Media Corporation. p. 80. ISBN 978-0-543-91646-4.
  12. Hofmann, A. W. (1875). "Report on the Development of the Chemical Arts During the Last Ten Years". Chemical News. Manufacturing chemists.
  13. Griffin, John Joseph (1827). A Practical Treatise on the Use of the Blowpipe in Chemical and Mineral Analysis. Glasgow: R. Griffin & co.
  14. "Verneuil process". Encyclopaedia Britannica. October 22, 2013. Retrieved July 11, 2018.
  15. P. N. Rao (2001), "24.4 Oxyhydrogen welding", Manufacturing technology: foundry, forming and welding (2 ed.), Tata McGraw-Hill Education, pp. 373–374, ISBN 978-0-07-463180-5, archived from the original on June 27, 2014
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