Coulometry:

In the bulk electrolysis method, when the amount of the constituent to be determined is not weighed but calculated by measuring the quantity of electricity and making use of Faraday's law the technique is called coulometry, the term derived from coulomb, the quantity of current. In these methods, an analyte is quantitatively converted to a various oxidation state through passing the current by the electrolytic cell.

In basic coulometric analysis the substance being determined might directly undergo reaction at one of the electrodes. In secondary coulometric analysis the substance being determined may react in solution with another substance which is generated by an electrode reaction. Additional, in coulometric analysis which considers the reactions proceeding with 100% current efficiency, so in which the quantity of the species reacted can be calculated via Faraday's law from the quantity of electricity passed.

According to Faraday's law, a given amount of chemical change caused by electrolysis is direct proportional to the amount of electricity passed through cell. For a general reaction,

O + ne → R

where n is the number of electrons in the involved reaction, O is the reactant to be reduced and R the reduction product. Faraday's law associate the number of moles of the analyte n_A to the charge

n_A  = Q/nF                                                                                        ... (1.46)

where,   n = the number of moles of electrons in the analyte half-reaction, F = Faraday, this is the quantity of charge that corresponds to one mole or 6.022×10²³ electrons. Since each electron has charge of 1.6022×10^-19 C, the faraday also equivalent 96,485C, and

Q = Quantity of electricity consumed in coulomb.

The value of Q can be determined as:

i)  for a constant current, I amperes, operate for t seconds , Q = It, and

ii) for a variable current, i, Q = ∫ ^t₀idt

In coulometry we can directl y calculate the amount of the reactant/product by the relation (1.46) and the calibration or standardisation is not ordinarily required.

Coulometry can be classified as (a) potentiostatic coulometry, and (b) amperostatic coulometry.