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What is the Henry's Law Constant?

William Henry

Henry's Law describes how an amount of a given gas dissolved in a given type and volume of liquid is proportional to the partial pressure of the gas in equilibrium with the liquid. Another way many people understand this would be to say that the solubility of a gas within a liquid will be proportional to the pressure of the gas above the liquid. This is actually a phenomenon that we see almost every day when we open up a can of soda or a bottle of champagne. When the bottle is closed, there is a very high pressure section of carbon dioxide present on top of the champagne. As soon as the bottle is opened, that pressure is lowered and the carbon dioxide dissolved in the champagne begins to come out of solution. Over time, if one were to leave a glass of champagne, the drink would lose all of its carbonation and go flat.

The Henry's law constant appears in the following mathematical formula that relates a partial pressure p to the concentration of a solute in solution.

p = kh.c

where c is the concentration and kh is the Henry's law constant. The "constant" is actually dependent on the solvent, the gas solute and the temperature. Henry's law constant for some common gases are listed:

To complicate matters further, Henry's law is often written in different ways and uses different units. Many of these use mass rather than number of moles, or they arrange them as different ratios. For example:

kh,pc = p/c

In the above equation, the units for kh are the same as the first equation, having a value of l.atm/mol. The next equation switches them:

kh,cp = c/p

In this case, the units on the bottom move to the top and the units on the top move to the bottom, so we get mol/(l.atm). There are others that have units of atmospheres or are dimensionless as well.

Some people prefer to call the Henry's law constant the Henry's law coefficient because of its temperature dependence. For a given temperature T, and a known temperature Tref, the Henry's law constant is given by:

kh = kh,ref Tref.exp(-C.((1/T) - (1/Tref)))

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