# Signs of electrode potentials

Why is the oxidation potential given a positive sign and the reduction potential a negative one?

Also, in my book a bunch of formulas are given without any explanation:

$E^{o}_{cell}=$ Oxidation potential of anode + Reduction potential of cathode

$E^o_{cell}=$ Standard reduction potential of cathode - Standard reduction potential of anode

$E^o_{cell} =$ Oxidation potential of anode - Oxidation potential of cathode

I am unable to get any intuition for this and the physics behind these signs. Could someone please provide an intuitive explanation of what's going on?

• Why, that's simple: negative sign means that the reaction is running in reverse direction. – Ivan Neretin Aug 29 '18 at 7:10

We use a convention for the sign of potentials. The standard reduction potentials are referenced wrt hydrogen reaction $\ce{2H+ + 2e^- <=>H2_{(gas)}}$ which is given the value $0.0$ V. Oxidising agents have positive values, for example $\ce{Ag^+ + e^- <=> Ag_{solid}}$ $E^\mathrm{o}=+0.799$ V, or $\ce{Cl2_{(gas)} + 2e^- <=> 2Cl^-}$, $E^\mathrm{o}=+ 1.36$ V, and reducing agents negative values, e.g. $\ce{Cr^{2+} + 2e^- <=> Cr_{solid}}$, $E^\mathrm{o}=-0.91$ V.
( Note that in some papers and books you may come across the redox potential values quoted wrt the saturated calomel electrode, not hydrogen, and this has a value $+0.246$ V so its always worth a quick check. )