<Dl> <Dd> 2 H (aq) + 2 e → H (g) </Dd> </Dl> <Dd> 2 H (aq) + 2 e → H (g) </Dd> <P> This redox reaction occurs at a platinized platinum electrode . The electrode is dipped in an acidic solution and pure hydrogen gas is bubbled through it . The concentration of both the reduced form and oxidised form is maintained at unity . That implies that the pressure of hydrogen gas is 1 bar (100 kPa) and the activity of hydrogen ions in the solution is unity . The activity of hydrogen ions is their effective concentration, which is equal to the formal concentration times the activity coefficient . These unit-less activity coefficients are close to 1.00 for very dilute water solutions, but usually lower for more concentrated solutions . The Nernst equation should be written as: </P> <Dl> <Dd> E = R T F ln ⁡ a H + p H 2 / p 0 (\ displaystyle E = (RT \ over F) \ ln (\ frac (a_ (\ mathrm (H ^ (+)))) (\ sqrt (p_ (\ mathrm (H_ (2))) / p ^ (0))))) </Dd> </Dl>

If the pressure of h2 gas is increased from 1 atm to 100 atm