The potential of a single half cell cannot be measured on its own. Only a potential difference between two half cells can be measured, so every half cell is compared with the same reference standard, the standard hydrogen half cell (standard hydrogen electrode). Its standard electrode potential is defined as exactly 0.00 V.The standard hydrogen half cell is made of hydrogen gas at 100 kPa bubbling over a platinum electrode dipped in a solution of H+(aq) ions at 1.00 moldm-3 (for example 1.00 moldm-3 HCl(aq)), at 298 K. The half equation is 2H+(aq) + 2e- ⇌ H2(g).The platinum electrode is inert. It does not take part in the reaction but lets electrons pass into or out of the half cell through the wire. It is coated with platinum black, a spongy layer with a large surface area for the transfer of electrons, so equilibrium between H2(g) and H+(aq) is set up quickly.To find the standard electrode potential Eθ of another half cell, it is connected to the standard hydrogen half cell with a wire and a salt bridge, and a high resistance voltmeter measures the e.m.f. when no current flows. All measurements are made under standard conditions: 298 K, solutions at 1 moldm-3 and gas pressure of 100 kPa.
Because the hydrogen half cell is 0.00 V, the voltmeter reading is the standard electrode potential of the other half cell. The sign shows whether that half cell is the positive or negative terminal compared with the hydrogen half cell.
Because the hydrogen half cell is 0.00 V, the voltmeter reading is the standard electrode potential of the other half cell. The sign shows whether that half cell is the positive or negative terminal compared with the hydrogen half cell.