Dalton’S Law. John Dalton (1766–1844) was an English chemist and physicist who studied the behavior of gases. His law of partial pressures, established in 1801, describes how ideal gases behave in a mixture. It states:
In essence, the total pressure in a closed system containing a mixture of gases is equal to the sum of the partial pressures of each of the individual gases.
The implications for brewers are found in the areas of carbonation, dispense, and oxidation of beer where mixed gas systems are found. A mixture of carbon dioxide and air is present in the headspace of a beer bottle. According to Dalton’s law, each gas in that mixture contributes to the total pressure in the bottle headspace, while reaching its own partial pressure equilibrium with the gas in solution in the beer below. Thus, oxygen in solution in beer will depend only on the partial pressure of the oxygen in the headspace and not the total pressure inside the bottle, most of which is caused by carbon dioxide. This also explains why most beers, if served through draught systems using a nitrogen-rich gas blend, will eventually lose their carbonation. Though nitrogen pressure can push the beer to the tap, only carbon dioxide pressure can keep carbon dioxide in solution. A failure of the operator of an establishment to understand this principle is often responsible for the service of beer that lacks its original level of carbonation.