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Hydrogen Ion Activity

• Hydrogen ion activity (a_H+) closely relates to the free hydrogen ion concentration ([H⁺]).
  
  
• pH is often defined in terms of the free hydrogen ion concentratration ([H⁺]) by -
  
  
  
  pH = - log₁₀[H⁺]
  
  
  
• This definition only applies when ideal solutions are involved, with no other ions present to interfere with or shield the activity of the free H⁺ (hydrogen ions) present.
  
  
• In non-ideal, complex solutions the pH should be defined in terms of hydrogen ion activity (a_H+) by -
  
  
  
  pH = - log₁₀ a_H+