Faraday's laws of electrolysis

Faraday's laws of electrolysis are quantitative relationships based on the electrochemical research published by Michael Faraday in 1833.[1][2][3]

First lawEdit

Michael Faraday reported that the mass (m) of elements deposited at an electrode is directly proportional to the charge (Q; SI units are ampere seconds or coulombs).[3]


Here, the constant of proportionality, Z, is called the electro-chemical equivalent (ECE) of the substance. Thus, the ECE can be defined as the mass of the substance deposited/liberated per unit charge.

Second lawEdit

Faraday discovered that when the same amount of electric current is passed through different electrolytes/elements connected in series, the mass of the substance liberated/deposited at the electrodes is directly proportional to their chemical equivalent/equivalent weight (E).[3] This turns out to be the molar mass (M) divided by the valence (v)



A monovalent ion requires 1 electron for discharge, a divalent ion requires 2 electrons for discharge and so on. Thus, if x electrons flow,   atoms are discharged.

So the mass m discharged is


Mathematical formEdit

Faraday's laws can be summarized by


where M is the molar mass of the substance (usually given in SI units of grams per mole) and v is the valency of the ions .

For Faraday's first law, M, F, v are constants; thus, the larger the value of Q, the larger m will be.

For Faraday's second law, Q, F, v are constants; thus, the larger the value of   (equivalent weight), the larger m will be.

In the simple case of constant-current electrolysis, Q = It, leading to


and then to



  • n is the amount of substance ("number of moles") liberated:  
  • t is the total time the constant current was applied.

For the case of an alloy whose constituents have different valencies, we have


where wi represents the mass fraction of the i-th element.

In the more complicated case of a variable electric current, the total charge Q is the electric current I(τ) integrated over time τ:


Here t is the total electrolysis time.[4]

See alsoEdit


  1. ^ Faraday, Michael (1834). "On Electrical Decomposition". Philosophical Transactions of the Royal Society. 124: 77–122. doi:10.1098/rstl.1834.0008. S2CID 116224057.
  2. ^ Ehl, Rosemary Gene; Ihde, Aaron (1954). "Faraday's Electrochemical Laws and the Determination of Equivalent Weights". Journal of Chemical Education. 31 (May): 226–232. Bibcode:1954JChEd..31..226E. doi:10.1021/ed031p226.
  3. ^ a b c "Faraday's laws of electrolysis | chemistry". Encyclopedia Britannica. Retrieved 2020-09-01.
  4. ^ For a similar treatment, see Strong, F. C. (1961). "Faraday's Laws in One Equation". Journal of Chemical Education. 38 (2): 98. Bibcode:1961JChEd..38...98S. doi:10.1021/ed038p98.

Further readingEdit