pKa H2CO3 is the cologarithm of the acid dissociation constant of carbonic acid. It is equal to 6.1. [HCO−3] is the molar concentration of bicarbonate in the blood plasma.
Henderson–Hasselbalch equation
The Henderson–Hasselbalch equation, when applied to the carbonic acid-bicarbonate buffer system in the extracellular fluids, states that:
  
    
      
        
          p
          H
        
        =
        
          p
        
        
          K
          
            
              a
            
             
            
              
                H
              
              
                2
              
            
            
              
                C
                O
              
              
                3
              
            
          
        
        +
        
          log
          
            10
          
        
        ⁡
        
          (
          
            
              
                [
                
                  
                    H
                    C
                    O
                  
                  
                    3
                  
                  
                    −
                  
                
                ]
              
              
                [
                
                  
                    H
                  
                  
                    2
                  
                
                
                  
                    C
                    O
                  
                  
                    3
                  
                
                ]
              
            
          
          )
        
        ,
      
    
    {\displaystyle \mathrm {pH} =\mathrm {p} K_{\mathrm {a} ~\mathrm {H} _{2}\mathrm {CO} _{3}}+\log _{10}\left({\frac {[\mathrm {HCO} _{3}^{-}]}{[\mathrm {H} _{2}\mathrm {CO} _{3}]}}\right),}
  
where:

pH is the negative logarithm (or cologarithm) of molar concentration of hydrogen ions in the extracellular fluid.