Waterguide - Lime-carbonic acid equilibrium
What is the lime-carbonic acid equilibrium?
The lime-carbonic acid equilibrium (also known as calcite saturation) refers to the chemical equilibrium between dissolved lime/calcium carbonate (CaCO3, CaO oder Ca(OH)2), carbonic acid (H2CO3) and carbon dioxide (CO2).
What is the chemical equilibrium of calcite saturation?
When CO2 comes into contact with rain, 99 % of it physically dissolves. Around 1 % reacts with the water molecules and turns into carbonic acid H2CO3.
CO2 + H2O → H2CO3
Depending on the pH value, the carbonic acid can dissociate, thus forming hydrogen carbonate (HCO3-) and an oxonium (H3O+). At a pH value of 6.5, CO2 and HCO3- occur in equal quantities.
H2CO3 + H2O → HCO3- + H3O+
If the pH value is even higher, hydrogen carbonate dissociates, thus forming the carbonate ion CO32+. At a pH value of 10.5, HCO3- and CO32- occur in equal quantities.
HCO3 + H2O → CO32- + H3O+
When water that contains CO2 seeps into rock such as limestone or gypsum, it causes calcium to dissolve and react according to the following equilibrium reaction:
Ca2+ + 2 HCO3- → CaCO3 + H2CO3
How do shifts in the equilibrium occur?
Increasing the pH value of water causes more HCO3 and CO32+ to form.
Example: Adding sodium hydroxide to water that contains CO2 creates several reactions, as carbonic acid dissociates on a multi-level basis – as mentioned above. The consequence of this is that there is ultimately less CO2 in the water.
Lowering the pH value of water causes more CO2 to form.
Example: By adding hydrochloric acid, the reaction with the sodium hydroxide occurs in the reverse order. This results in a higher amount of CO2 in the water.
Some types of water may have higher quantities of calcium, which imbalances the equilibrium, causing it to shift towards the educt side. If there is more carbon dioxide in the water, the equilibrium shifts towards the product side.
Evaporation can also cause the concentration levels of elements in the water to change.
The figure below illustrates the effect of different pH levels of water.
What is the difference between free and bound carbonic acid?
Free carbonic acid: Is present almost exclusively as CO2 (approx. 1 % as carbonic acid).
Bound carbonic acid: Is contained within carbonates (CaCO3).
What effect does this have on how the equilibrium shifts?
Carbonic acid excess: pH value is low (in acidic area) and causes surrounding trees to die (caustic effect). An excess can also occur if acid rain falls on ground that is deficient in lime and the acid cannot be neutralised.
Acid rain is rainwater that has a low pH value (between 4.2 and 4.8), and is produced when certain environmental conditions cause the natural formation of acid-forming gases. This effect is intensified through human activities, generally through emissions.
Carbon dioxide excess: Has a corrosive effect on metal pipes, and dissolves metal such as copper, lead and nickel.
As the pH level of drinking water has to be within a specific range and must be unable to dissolve calcite, it is essential to ensure the equilibrium in the water treatment process is stable.