Standard electrode potentials are measured relative to the Standard Hydrogen Electrode (SHE), defined as E = 0.00 V under standard conditions (1 M H+, 1 atm H2, 25 degrees C). All tabulated values are reduction potentials. To find E_{cell}_{standard}: E_{cathode}_{standard} - E_{anode}_{standard} (both as reduction potentials — do NOT change sign of the anode value in this formula). The electrochemical series arranges metals by their reduction potentials. Key rule: a metal can reduce the cation of any metal below it in the series. Zn (E = -0.76 V) can reduce Cu2+ (E = +0.34 V). Metals above hydrogen can displace H2 from dilute acids; metals below cannot (Cu, Ag, Au don't dissolve in dilute HCl).