Principle and structural components of pH meter
1. pH meter
A pH (acidity) meter is an instrument for determining the activity of H+ in a solution by potentiometric method. pH meter measures the value used to indicate the acidity or alkalinity of a solution in terms of the negative logarithm of the concentration of H+ ions in the solution, i.e., pH = ーlg[H+].
The pH meter we use is composed of two parts: the meter and the electrode. In the actual measurement, the electrode is immersed in the solution to be measured, the concentration of H + ions in the solution is converted into millivolt voltage signal, input meter. The meter will amplify the signal, and after logarithmic conversion to pH, and then by the millivolt level display meter shows the pH.
The pH meter uses a glass electrode as the measuring electrode and a calomel electrode as the reference electrode. Changes in hydrogen ion concentration caused by changes in the glass electrode potential, resulting in changes in the potential between its electrode and the calomel electrode, should be consistent with the formula (2-6)
E=E0-2.3026RT/F............(2-6)
In the formula:
E a electric potential, mV;
E0 a electrode standard electric potential, mV;
R a molar gas constant;
T a Kelvin absolute temperature;
F a Faraday constant.
From the formula (2-6) can be seen in the measurement of pH temperature is also an important factor, in the measurement should pay attention to the temperature change, and take it into account. Currently used pH electrodes for composite electrodes, glass electrodes have been compounded together with the electrode of calomel.
2.pH electrode
Electrode is an indispensable part of the composition of the pH meter. pH measurement is through the measurement of electrodes, reference electrode composed of a chemical battery to complete.
(1) pH measurement electrode, also known as pH indicator electrode, the hydrogen ion activity in the solution has a response, the electrode potential changes. Almost all modern pH meters use glass electrodes as the measuring electrode.
(2) Reference electrode (inert electrode), no response to the hydrogen ion activity in the solution. In the pH measurement, it as a constant potential half-cell and pH measurement electrode form a chemical cell, to provide and maintain a fixed reference potential.
(3) Composite electrode is a combination of pH measurement electrode and reference electrode. Composite electrode is not affected by oxidising and reducing substances, balance faster, more widely used, the current pH measurement are basically such electrodes. Whether the glass indicator electrode or glass composite electrode, is a film electrode, the film potential and the measured hydrogen ion activity in the solution is energy-starter relationship, reflecting the unique performance of the film electrode, these properties by the electrode itself and the use of the conditions of the impact, and on the results of the role of the measurement.
