Reference Electrodes, Calomel and Ag/AgCl

 Both leach out a small amount of filling solution during operation which provides electrical contact with the sensing electrode and helps provide a stable unchanging potential.

Both do the same job in principle but careful consideration is needed to select the right reference for the right application. Silver Chloride references do not contain mercury and are often considered more suitable for environmental applications. They are also  less effected by higher temperatures than the Calomel electrode.

The Calomel electrode is refillable with Potassium Chloride  and as such is less prone to reference junction blockage. With the Ag/AgCl references the filling solution has to be saturated with Silver Chloride which is slightly soluble in KCl. In some circumstances Silver salts can be deposited in the Ceramic reference junction leading to blockage and electrode failure. Calomel reference electrodes may also be used in TRIS applications unlike the Ag/AgCl equivalent. Calomel electrodes tend to have a longer lifetime.

The reference junction type is another option  requiring consideration. Ceramic junctions are rugged and ideal for  general applications. Annular junctions give a slightly higher leak rate and contact area and are therefore good for  low ionic strength applications and offer greater stability.  Teflon junctions are resistant to blockages and are great for harsh applications.

Finally Double junction references have the advantage of having an outer chamber which can contain an electrolyte of the users choice. This means that the electrolyte leaching into the sample during measurement can be selected to be inert and therefore not affect the result.  The most common application is for use with Ion Selective Electrodes or in cases where chloride interferes with the result.