section_path: Chemical and physical properties/In aqueous solution
section_name: In aqueous solution
paragraph_text: In aqueous solution
  
  Carbon dioxide is soluble in water, in which it reversibly forms H CO (carbonic acid), which is a weak acid, because its ionization in water is incomplete.
  
  The hydration equilibrium constant of carbonic acid is, at 25 °C:
  
  Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as CO molecules, not affecting the pH.
  
  The relative concentrations of CO, H CO, and the deprotonated forms HCO − 3 (bicarbonate) and CO 2− 3 (carbonate) depend on the pH. As shown in a Bjerrum plot, in neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater. In very alkaline water (pH > 10.4), the predominant (>50%) form is carbonate. The oceans, being mildly alkaline with typical pH = 8.2–8.5, contain about 120 mg of bicarbonate per liter.
  
  Being diprotic, carbonic acid has two acid dissociation constants, the first one for the dissociation into the bicarbonate (also called hydrogen carbonate) ion (HCO − 3):
  
  This is the true first acid dissociation constant, defined as
  
  where the denominator includes only covalently bound H CO and does not include hydrated CO (aq). The much smaller and often-quoted value near 4.16 × 10 (or pK = 6.38) is an apparent value calculated on the (incorrect) assumption that all dissolved CO is present as carbonic acid, so that
  
  Since most of the dissolved CO remains as CO molecules, K (apparent) has a much larger denominator and a much smaller value than the true K.
  
  The bicarbonate ion is an amphoteric species that can act as an acid or as a base, depending on pH of the solution. At high pH, it dissociates significantly into the carbonate ion (CO 2− 3):
  
  In organisms, carbonic acid production is catalysed by the enzyme known as carbonic anhydrase.
  
  In addition to altering its acidity, the presence of carbon dioxide in water also affects its electrical properties.
  
  When carbon dioxide dissolves in desalinated water, the electrical conductivity increases significantly from below 1 μS/cm to nearly 30 μS/cm. When heated, the water begins to gradually lose the conductivity induced by the presence of C O 2 {\displaystyle \mathrm {CO_{2}} }, especially noticeable as temperatures exceed 30 °C.
  
  The temperature dependence of the electrical conductivity of fully deionized water without C O 2 {\displaystyle \mathrm {CO_{2}} } saturation is comparably low in relation to these data.