The reference. The left cell starts as the standard hydrogen electrode: platinum in 1 M H⁺ with H₂ bubbling over it at 1 atm. Its potential is defined as 0.000 V, so every reading against it is the reduction potential of the right cell.

The meter. It reads E = E(right) − E(left). A positive reading means the right cell takes electrons (it is the cathode). A negative reading means the right cell gives them up (it is the anode). The meter draws almost no current, so nothing is used up while you measure; at each electrode the couple is turning over in both directions at the same rate.

E°′. Biochemistry tables list the potential with every solute at 1 M, gases at 1 atm and pH 7. Set the right cell to those conditions and the meter shows the table value. Because the reference stays at pH 0, the hydrogen couple itself reads −0.414 V when it is moved to pH 7.

Away from standard conditions. The Nernst equation corrects E°′ for the actual concentrations and pH. A tenfold change in the ratio of oxidized to reduced form moves E by 59 mV divided by the number of electrons.

Two couples. Electrons go from the couple with the lower E to the couple with the higher E. ΔE = E(acceptor) − E(donor) and ΔG = −nFΔE, so a positive ΔE is a negative ΔG.

Under each beaker. The half-reaction is written the way it is running, and E is given for that direction: the reduction potential in the cathode cell, and the same number with its sign reversed in the anode cell, where the couple is being oxidized. Those two values add up to ΔE. The meter and the tiles always use reduction potentials.

Running the cell. Run to equilibrium closes the circuit through a resistor. Reactant is used up, Q climbs, and E falls until Q equals K′_eq and E is zero. The Speed switch moves in tenfold steps. It sets how fast the run plays and how fast the dots move; the time on the plot is the same at every setting.

• 25 °C. Activities are taken as concentrations (and as pressures in atm for gases). Solids and water have activity 1.
• The number of H⁺ in each half-reaction is fixed as written at pH 7. Real couples change slope where a species gains or loses a proton, so readings far from pH 7 are a guide only.
• Every cell except the hydrogen reference is buffered, so its pH stays where the slider puts it. The reference is 1 M strong acid with no buffer.
• Standard state is a reference point, not a recipe. Nobody makes 1 M cytochrome c.
• Most of these couples do not exchange electrons with bare platinum at a useful rate. In the laboratory an enzyme or a mediator dye carries the electrons. The potentials are the same.
• When the cell runs: 100 mL in each half-cell, a 10 Ω load, no internal resistance. Gases are supplied at constant pressure and metal electrodes do not run out.
• The dots are a picture, not a count. Their number follows the logarithm of concentration. A tenfold step on the Speed switch plays the run exactly ten times faster, but moves the dots only about three times faster so they stay visible.