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Concentrated Potassium Chloride (KCl)

• The reference electrode of a pH meter usually contains an silver/silverchloride (Ag/AgCl) wire electrode and a concentrated potassium chloride electrolyte.
  
  
• This arrangement provides a stable electrical reference potential against which the electric potential of the measuring electrode can be measured against.
  
  
• The electrical potential of Ag/AgCl and KCl reference electrode half cell depends on the KCl concentration
  
  @ 25^o C Ag/AgCl - 3.5 M KCl ……..0.205 V (Volts)
  
  @ 25^o C Ag/AgCl - supersaturated KCl ……..0.197 V (Volts)
  
  
• A supersaturated KCl solution is guaranteed if a persistent presence of KCl crystals is evident.
  
  
• The KCl concentration is not critical as long as it remains above a 3 molar concentration (3 M).
  
  
• A supersaturated solution guarantees that the solution will not fall below the 3 molar concentration (3 M) and hence is the recommended concentration for the reference electrode of a pH meter.