Standard reduction potentials are based on which element?

Hydrogen

Explanation:

Standard reduction potential is a measure of the tendency of an atom to attract electrons and get reduced. Higher the reduction potential i.e. more positive the value greater will be the tendency to get reduced. These values are measured at 298 K and 1 atm and expressed in terms of volts.

The standard reduction potential of Hydrogen is taken as zero and the values of all other elements are deduced based on hydrogen. These are tabulated in the thermochemical reduction potential table. The reduction equation for hydrogen is:

H2(g) + 2e-  → 2H+           where E⁰ = 0.00 Volts

your answer would be “dry”