The Youden pH Meter

Sørensen used a slide-wire potentiometer to make his pH measurements. Here a known potential is applied to counterbalance the potential of the hydrogen cell (i.e. the hydrogen electrode plus the counter electrode) and to reduce its current output to zero, at which point the value of the applied counter-potential is equal to that of the cell potential. Since the point of balance is indicated by a galvanometer reading of zero, this technique became known as the “null-point method” and would form the basis of most commercial pH meter design for almost 40 years. By 1920 pH was becoming sufficiently important to biochemists that various laboratory supply houses began offering to sell, as a single package, all of the electrical and electrode components required to make the necessary electrometric measurements (figure 2). Not unexpectedly, it was not long before it was realized that all of these electrical components could be housed in a single convenient box, thus giving rise to what most chemists now think of as the modern pH meter (5-8). From this point on, the evolution of the meter it self was largely an issue of improved electronics rather than chemistry, though the latter would continue to play an important role in the choice of the electrodes used. As it turned out, the hydrogen electrode is not very convenient to work with. It requires a tank of ultra pure hydrogen gas and the necessary equipment to regulate both the pressure and flow rate. Equilibration times are slow and the Pt, which not only conducts the current but is also an essential catalyst for the electrode reaction itself, is susceptible to catalytic poisoning by various solutes and can also act as a surface for the competing electrochemical reduction of others. These problems led to a search for alternative