The pH meter's structure and operating mechanism
The pH (acidity) meter is an instrument for measuring the H+ activity in the solution by the potentiometric method. The value measured by the pH meter is a method used to express the pH of the solution, which is expressed by the negative logarithm of the H+ ion concentration in the solution, that is: pH= ー lg[H+].
The pH meter we use is composed of two parts: an electric meter and an electrode. In the actual measurement, the electrode is immersed in the solution to be measured, and the concentration of H+ ions in the solution is converted into a millivolt level voltage signal, which is input to the electric meter. The electric meter amplifies the signal and converts it into pH through logarithm, and then the pH is displayed by the millivolt level display instrument.
The pH meter uses a glass electrode as the measuring electrode and a calomel electrode as the reference electrode. The change of the potential of the glass electrode caused by the change of the concentration of hydrogen ions causes the change of the potential between it and the calomel electrode, which should conform to the formula (2-6)
E=E0-2.3026RT/F………(2-6)
In the formula:
E-electromotive force, mV;
E0-electrode standard electromotive force, mV;
R a molar gas constant;
T-absolute temperature in Kelvin;
F - Faraday's constant.
It can be seen from the formula (2-6) that the temperature is also an important influencing factor when measuring pH. We should pay attention to the change of temperature and take it into account when measuring. At present, most commonly used pH electrodes are composite electrodes, which have combined glass electrodes and calomel electrodes.
2. pH electrode
Electrodes are an integral part of a pH meter. pH measurement is accomplished by a chemical battery composed of a measuring electrode and a reference electrode.
(1) The pH measuring electrode is also called the pH indicating electrode, which responds to the activity of hydrogen ions in the solution, and the electrode potential changes accordingly. Almost all modern pH meters use glass electrodes as measuring electrodes.
(2) The reference electrode (inert electrode) has no response to the activity of hydrogen ions in the solution. In pH measurement, it is used as a half-cell with constant potential to form a chemical battery with the pH measurement electrode, providing and maintaining a fixed reference potential.
(3) A composite electrode is an electrode that combines a pH measuring electrode and a reference electrode. Composite electrodes are not affected by oxidizing and reducing substances, have a faster equilibrium speed, and are widely used. Currently, these electrodes are basically used in pH measurement. Regardless of the glass indicator electrode or the glass composite electrode, they are all thin-film electrodes. The membrane potential has a Nernst relationship with the hydrogen ion activity in the measured solution, which reflects the unique properties of the thin-film electrode. These properties are affected by the electrode itself and the conditions of use. , and have an effect on the measurement results.
