Reference electrodes must be carefully maintained, so that the buffered system inside it, which ensures the constancy of the potential, remain stable, and the concentrations of the chemicals inside do not change. Electrodes come with refill solutions precisely for that purpose. The worst and most common mistake operators make is let an electrode go dry.

Dear Aiman,

look at:

http://www.consultrsr.net/resources/ref/agcl.htm

the reversible potential Er, is given:

Er = 0.199-(2.3RT/F)log a(Cl-)

The potential of the reference electrode depends mainly on the concentration of the ions Cl-, so basically either a saturated p-or KCl is taken, or a 3 M KCl solution is prepared. The addition to concentration, in purity of the solution, the noise of the reference electrode relative to the working electrode and the temperature have a strong influence. To adjust the purity of solutions, it is better to prepare solutions in bidistilled and de-watered water, to eliminate measurement noise in the Faraday cage, to eliminate the influence of distance, bring the reference electrode as close as possible to the working electrode (through the Lungin capillary), and to eliminate the temperature factor, conduct under normal conditions (101 325 Pa and 293.15 K)

Reference electrodes maintain constant potential by the vertue of its composition. For the Silver-silver cloride reference electrode or the calomel reference electrode the potential is constant because it contains a fixed chloride concentration.