THE hydrogen electrode is the primary standard to which all electrode potentials are referred and is used in cells without liquid junction for measurements of high accuracy. The potential of the hydrogen electrode is corrected to the standard pressure of one atmosphere on the basis that the electrode is in equilibrium with the measured partial pressure of hydrogen above the solution. It is, however, not the gas phase which directly determines the potential, but the hydrogen in solution ; and a tacit assumption that the concentration of dissolved hydrogen is unequivocally defined by the gas pressure is false. This error can arise in two ways.