Air Solubility in Water

Air Solubility in Water Amount of air that can be dissolved in water - decrease with temperature - increase with pressure The amount of air that can be dissolved in water increase with the system pressure and decrease with the temperature.

Deaeration When fresh water is heated up, air bubbles start to form. The water can obviously not hold the dissolved air with increasing temperature. At 100 °C (212 °F) Water starts to boiI - the bubbles are formed by evaporated water or steam. lf the water is cooled down and then again reheated, bubbles will not appear until the water starts to boil. The water is deaerated.

Solubility Ratio The solubility of air in water can be expressed as a solubility ratio:

Sa = ma/mw Where:

Sa = Solubility ratio; ma = Mass of air (g); mw = Mass of water (g).

Henry's Law Solution of air in water follows Hanry's Law - "The amount of air dissolved in a fluid is proportional with the pressure of the system"- and can be expressed as:

C = Pg/kH Where:

C = Solubility of dissolved gas; kH = Proportionality constant depending on the nature of the gas and the solvent; Pg = Partial pressure of the gas. The solubility of oxygen in water is higher than the solubility of nitrogen. Air dissolved in water contains approximately 35,6% oxygen compared to 21,0% in air.

Solubility of Air in Water Example - Calculating Air Dissolved in Water Air dissolved in water can be calculated with Henry's law. Henry Law's Constants at a system temperature of 25°C (77°F) • Oxygen: 769,23 litre·atm / mol • Nitrogen: 1639,34 litre·atm / mol Molar Weights • Oxygen: 31,9988 g/mol • Nitrogen: 28,0134 g/mol Partial fraction in air • Oxygen: ~ 0,21 mol • Nitrogen: ~ 0,79 mol Oxygen dissolved in the water at atmospheric pressure can be calculated as: Co

= 1 atm · 0,21 mol · 31,9988 g/mol / 769,23 litre·atm/mol = 0,00874 g/litre

Nitrogen dissolved in the water at atmospheric pressure can be calculated as: Cn

= 1 atm · 0,79 mol · 28,0134 g/mol / 1639,34 litre·atm/mol = 0,01350 g/litre

Since air is the sum of nitrogen and oxygen: Ca

= (0,00874 g/litre) + (0,0135 g/litre) = 0,02224 g/litre

Some forms of Henry's law and constants (gases in water at 298,15 K)

Equation

C

Units

K

O2

769,23

1700

H2

1282,05

500

CO2

29,41

2400

N2

1639,34

1300

He

2702,7

230

Ne

2222,22

490

Ar

714,28

1300

CO

1052,63

1300

Where: Caq = Concentration of gas in solution (mol/litre) p = Partial pressure of gas above the solution (atm)

Where:

kH,pc = For a given temperature is Henry's constant; T

= Temperature, in K; = Refers to the standard temperature (298,15 K).