Aim: Study of laboratory equipments – pH meter
PRINCIPLES
The pH of a solution may be accurately and easily determined by electrochemical measurements with a device known as a pH meter with a pH (proton)-sensitive electrode (usually glass) and a reference electrode (usually silver chloride or calomel). Ideally, the electrode potential, E, for the proton can be written as
where E is a measured potential, E0 is the standard electrode potential at aH+= 1 mol/L, R is the gas constant, T is the temperature in kelvin, F is the Faraday constant.
The pH electrode uses a specially formulated, pH-sensitive glass in contact with the solution, which develops the potential (E) proportional to the pH of the solution.
The reference electrode is designed to maintain a constant and known potential at any given temperature.
The difference in the potentials of the pH and reference electrodes provides a millivolt (mV) signal proportional to pH.
In practice, a combined glass electrode has a built-in reference electrode. It is calibrated against buffer solutions of known hydrogen ion activity. Most pH sensors are designed to produce a 0-mV signal at 7.0 pH. Two or more buffer solutions are used to calibrate the pH and mV readings. Commercial standard buffer solutions usually come with information on the pH value at 25 °C.
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