Henrys

First formulated by William Henry (1775-1836) in 1803 this law states:

'The mass of a gas (C) that dissolves in a defined volume of liquid is directly proportional to the pressure of the gas (P) (provided the gas does not react with the solvent)'.

where: p = partial pressure of the gas above the liquid C = concentration of the gas in the liquid a = Bunsen's solubility coefficient (specific for gases and liquids)

Figure 1.1-2. Principle of Henry's gas law (Welslau, 2004)

Figure 1.1-2. Principle of Henry's gas law (Welslau, 2004)

The solubility coefficient for gases in liquids a [millilitres of gas/atm/litre of fluid] described by Robert Wilhelm Bunsen (1811-1899) may also be expressed as Henry's law constant (k). As a basic principle, the solubility of gases is greater in cold liquids.

Table 1.1-4. Bunsen's solubility coefficient a for different gases in water

Temp. [°C]

Air

Oxygen

Nitrogen

Helium

Carbon dioxide

0

29.2

48.9

23.5

9.5

35.4

5

25.7

42.9

20.9

9.2

31.5

10

22.8

38.0

18.6

9.0

28.2

15

20.6

34.2

16.9

8.8

25.4

20

18.7

31.0

15.5

8.7

23.2

25

17.1

28.3

14.3

8.5

21.4

30

15.6

26.1

13.4

8.4

20.0

35

14.8

24.4

12.6

8.3

18.8

40

14.1

23.1

11.8

8.3

17.6

Practical relevance: The pressure dependent solubility of inert gases (e.g. nitrogen) in body liquids and tissues is crucial for the development of decompression sickness (DCS) due to supersaturation of tissues in relation to reduced ambient pressure after exposure.

Fick's Laws of Diffusion were derived by Adolf Fick in 1858. Fick's First Law is used in steady state diffusion. This law gives rise to the formula below, which states the rate of diffusion of a gas across a membrane.

where:

K = constant (determined by experiment, gas and temperature specific) A = surface area over which diffusion is taking place AP = difference of gas partial pressure on both sides of the membrane D = distance over which diffusion takes place, ie membrane thickness

Practical relevance: At various places in the human body partial pressures (or concentrations) of dissolved gases, such as oxygen or nitrogen, depend on diffusion. According to Fick's First Law of Diffusion we can identify the variables for diffusion of gases as size of diffusion area, thickness of diffusion barrier (or distance), and differential gas partial pressure. According to Fick's Second Law of Diffusion, the time needed for diffusion is dependent on size of molecules, allowing smaller gas molecules like helium to diffuse faster than larger ones.

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