Reaction:

On electrolysis the following reactions occur at anode and cathode

Anode                        :           4OH ^- →        2H₂O + O₂ + 4e

Cathode                     :           4H ⁺ + 4e        →        2H²

Overall reaction          :           2H₂O  → 2H² + O²

There are three moles of gases (2 moles of hydrogen and one mole of oxygen) are generates through consumption of 4 Faradays that is equal to 0.l74l cm³ at N.T.P. of mixed gas per coulomb. Lehfildt and Lingane found the actual volume evolve to be 0.l739 cm³.

Like coulometers show an accuracy of about -0.l % at a current density of 0.l A cm^-2, falling to - 4 % at 0.0l A cm^-2 and the loss of effectiveness increases as the current density reduces. On low current strengths below 50 milliamperes, that cause negative errors.  The relationship among gas production and current density has been examined.

It has been reported in which the loss of efficiency is because of the formation of hydrogen peroxide at the anode that aggravates the situation through being reduced at the cathode, so in which the cathodic current competence also suffers. The other general gasometric coulometer is the hydrogen-nitrogen coulometer, where as 0.l M hydrazine sulphate is used as the electrolyte. The electrode reactions are same except nitrogen gas is evolved at the anode.

Anode                        :           N₂H₅⁺ → N₂ + 5H⁺ + 4e

Cathode                     :           4H⁺ + 4e → 2H₂

 Overall reaction         :           N₂H₅⁺ → N₂ + 2H₂ + H⁺

Three moles of gas are produced by consumption of 4 Faradays and the yield is theoretically 0.l74l cm³ at NTP of mix gas per coulomb. The Lingane reported an average yield of 0.l738 ml over the current density range is 0.l - 0.007 A cm^-2.