Dissolved CO2 participates in CO2+H2O ⇌ H2CO3 and H2CO3 ⇌ H++ HCO3−. Added CO2 can raise [H+] and lower pH. Acidification means decreasing pH, even while water remains above pH 7. Use pH=−log10[H+]. At 25∘C,Kw=[H+][OH−]=1.0×10−14.
H++ CO32− ⇌ HCO3− removes carbonate ions as acidity rises, reducing carbonate available for calcium-carbonate formation. The overall acidic dissolution rule is CaCO3+2H+→Ca2++ CO2+H2O. One mole carbonate consumes two moles H+. A one-unit pH decrease means ten times greater [H+].
Example 1
Calculate pH when [H+]=2.0×10−8 mol L−1.
pH=−log10(2.0×10−8)=7.70 to two decimal places. The solution remains alkaline even though increasing [H+] would lower its pH.