▶ Reference Electrode 

Reference electrode is constitute the standard of potential to obtain the occur potential of a direction electrode. and have constant potential.

If make a battery out of a standard electrode and another electrode and measure its electromotive force,

Since the electric potential of the reference electrode is already known, the electric potential of the other electrodes being measured is known.

※ electromotive force : A driving force that maintains a constant potential difference between conductors and allows current to flow.

① Ag/AgCl Reference Electrode

Electrode of KCl and AgCl saturated solution that contains AgCl solids

For electrodes with saturated KCl, at 25℃ 

E(Ag/AgCl) = +0.196 [V]

E° = +0.222 [V]

● This electrode depends on the Cle-ion concentration, so if the KCl concentration is high, As [ AgCl + Cl- → AgCl2- ] tends to melt AgCI, must be saturated AgCI in the solution.

● AgCl(s) also causes prevents reference junction and causes electrode failure.

 Therefore, to melt AgCl, the electrodes must be immersed in a concentrated ammonia solution..

● Less hysteresis of potential about temperature cycle. and stable potential to high temperature. ( Available use at high temperature )

※ hysteresis : The physical quantity is not determined by the conditions and the current state depends on the past state of the object.

② (Calomel Electrode)

Electrode that mercury, Hg and Cl mixture is contained in a saturated solution of KCl.

For electrodes with saturated KCl, at 25℃ 

E(Hg/Hg₂Cl₂) = +0.241 [V]

E° = +0.268 [V]

● Use a KCl solution on the Metallic insoluble salt electrodes.

Therefore potential will depend on the concentration of the anionic Cl-.

● Usually, the saturated calomel electrode(SCE, Assuming a constant ion concentration of Cl- using a saturated solution of KCl. ) is used the most. 

● The electrodes are slow to respond to changes in temperature, and in particular, The decomposition reaction near 70℃ makes it impossible to use at higher temperatures.

● Stable in acidic solutions.

※ Insoluble salt : Properties that are not easily soluble in water or solvents.

③ Murcury Sulfate Electrode

Electrode is composed of Hg/Hg2SO4/H2SO4 or K2SO4 solution 

For electrodes with saturated KCl, at 25℃ 

E(Hg/Hg₂Cl₂) = +0.615 [V]

Used to avoid leakage of the measured solution Cl- ions or to involve SO42- ions. (from the reference electrode)

④ Murcury - Oxide Mercury Reference Electrode)

Electrode is composed of Hg / HgO / 1M NaOH solution.

For electrodes with saturated KCl, at 25℃ 

E(Hg/HgO) = +0.114 [V]

Stable in strong alkaline solutions.

Therefore, it can be used as a reference electrode it in alkaline solutions.

※ How to Keep Reference Electrode