Mercury Electrodes:

The other significant second order electrode used for the measurement of EDTA and its anion Y4-, is based on the response of mercury electrode within the presence of a small concentration of stable Hg (II) EDTA complex.

An half-cell reactions for this electrode process are similar to the one given above and might be written as

HgY^-2   + 2e ↔ Hg (1) + Y^{- 4} (E^o   = 0.21 V)                                        ... (2.35)

for which

E = 0.21 -(0.0591/2) log [Y^4-]/[HgY^-2]                                               ... (2.36)

For this electrode system, a small concentration of HgY2¯ is introduced into the analyte solution. The complex is so stable, that the dissociation to form Hg2+ is minimal (Kf = 6.3 x 1021 for Hg Y2¯ ) and the concentration of the complex remains constant over a wide range of Y4¯ . Hence, the above equation may be simplified as

 E = K - (0.0591/2) log Y ^-4]                                                                        ... (2.37)

where the constant K is equal to

K = 0.21 - (0.0591/2) log 1/[HgY^2-]                                                           ... (2.38)

That electrode is hugely used in complexometric titrations using EDTA.