Carbonic Acid (acid-base example)

• pH of most natural waters controlled by CO₂
• Carbonate system is specific example of one type of acid-base reaction
• Can be generalized to other systems: Phosphoric, Sulfuric, Nitric, Silicic etc.

 
Model:

• When atmospheric CO₂ comes in contact with water, it dissolves

Can write a dissolution reactions:

 CO₂(aq) + H₂O = H₂CO₃*

Here CO₂(aq) is amount of CO₂ in water
H₂CO₃* is the true amount of carbonic acid
 
 
 

K_eq = 2.6 x 10^-3 at 25ºC

 • less than 0.3% of CO₂ present is H₂CO₃

BUT-

 • Generally designate all of CO₂ present as H₂CO₃º
 • We’ll say CO₂(aq) + H₂CO₃* = H₂CO₃
 

 
Henry’s law:

• Concentration of gas in a solvent proportional to pressure in gas phase
 • Proportionality is Henry’s Law Constant:

CO₂(g) + H₂0 = H₂CO₃* + CO₂(aq) = H₂CO₃º

Proportionality constant is an equilibrium constant:
 
 
 

At 25ºC K_CO2 = 10^-1.47

 
H₂CO₃ is an acid, tends to dissociate: