pH Theory and Measurement

The most common pH measurement system utilizes glass pH electrodes. The system consists of a pH sensor (whose voltage varies proportionately to the hydrogen ion activity of the solution), a reference electrode (which provides a stable and constant reference voltage), a conductive measurement solution, and a special meter to measure and display the pH.

The pH sensor incorporates a thin membrane of hydrogensensitive glass blown on the end of an inert glass tube. This tube is filled with a buffered electrolyte and an Ag/AgCl wire. This system is called a pH half-cell.

A complementary system produces a constant voltage; it also contains a Ag/AgCl wire and an electrolyte (often a KCl solution saturated with AgCl). A small “filter", often a porous ceramic component, connects this tube to the external sample. This system is called a reference half-cell.

The meter measures the voltage difference between the pH halfcell and the reference half cell in DC millivolts. The measurement is read by the meter and displayed in either mV or pH units. The mV response by a pH electrode follows the Nernst Equation:

Eobs = Ec + ln(10)(RT / nF)(log[aH+])

Eobs = Observed potential (sum of reference and liquid junction potentials)

Ec = Reference potential including other stable and fixed potentials

aH+ = The hydrogen ion activity

T = Temperature in Kelvin (C° + 273.15)

n = Valence of the ion measured (1)

F = Faraday’s constant (9.6485 x 104)

R = Gas constant (8.31432J / KMol)