1.pH meter
A pH (acidity) meter is an instrument that measures the H+ activity in a solution using the potentiometric method. The value measured by a pH meter is a method used to express the pH of a solution. It 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 actual measurement, the electrode is immersed in the solution to be measured, and the H+ ion concentration in the solution is converted into a millivolt-level voltage signal, which is input into the electric meter. The electric meter amplifies the signal and logarithmically converts it into pH, and then the millivolt level display meter displays the pH.

The pH meter uses a glass electrode as the measuring electrode and a calomel electrode as the reference electrode. The change in the potential of the glass electrode caused by the change in hydrogen ion concentration causes a change in the potential between it and the calomel electrode, which should comply with formula (2-6)

E=E0-2.3026RT/F…………(2-6)

E - electromotive force, mV;

E0 - electrode standard electromotive force, mV;

F - Faraday's constant.

It can be seen from equation (2-6) that temperature is also an important factor when measuring pH. When measuring, you should pay attention to the change of temperature and take it into consideration. At present, most of the commonly used electrodes for pH are composite electrodes, which combine glass electrodes and calomel electrodes.

2.pH electrode

(1) The pH measuring electrode, also called the pH indicating electrode, 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 serves as a half-cell with constant potential and forms 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 measurement electrode and a reference electrode. Composite electrodes are not affected by oxidizing and reducing substances, have a fast equilibrium speed, and are widely used. Currently, these electrodes are basically used for pH measurement. Regardless of whether the glass indicator electrode or the glass composite electrode is a thin film electrode, the membrane potential has a Nernst relationship with the hydrogen ion activity in the solution being measured, reflecting the unique properties of the thin film electrode. These properties are affected by the electrode itself and the conditions of use. , and have an impact on the measurement results.