What is a pH electrode?

An industrial on-line pH measuring system generally consists of four parts, pH electrode (pH sensor), pH meter (pH transmitter), electrode sheath and cable. A pH electrode is a part of the pH meter contacting with measured substance. It's used to measure the electrode potential. Generally, there are two methods to measure the pH value of the aqueous solutions, namely Colorimetry (pH test paper and cuvette) and potentiometric method. The latter is the only way to realize continuous on-line measurement and process monitoring. Moreover, it can obtain accurate and repeatable pH value. The electrode used in the potentiometric method is called as primary battery, which is a system used to converse the chemical reaction energy into electrical energy, voltage of the primary battery is called for electrodynamic force (EMF). EMF consists of two half-cells, one of the half-cell is called for measuring electrode, whose potential is related to specific ionic activity. Another is a reference half-cell, which is also generally called for reference electrode, it generally communicates with measured solutions and connects with the measuring instrument.
The electrode combined from pH glass electrode and reference electrode is called combination pH electrode. In case the shell is made of plastic, it’s called plastic combination pH electrode. If the shell is made of glass, it’s called glass combination pH electrode. The biggest advantage of the composite electrode is combining two electrodes in one and easy to use.
A combination pH electrode mainly consists of electrode bulb, glass support rod, internal reference electrode, internal reference solution, shell, external reference electrode, external reference solution, liquid junction boarder, electrode cap, electrode cable and sockets.

1. Electrode bulb: It is made of molten lithium glass with hydrogen function in a bubble shape, the film thickness is about 0.1~0.2mm and its resistance value is lower than 250 megohm (at 25℃).
2. Glass support rod: It's a glass tube supporting electrode ball. .Made of lead glass with excellent electrical insulation, its expansion coefficient is consistent with the electrode ball glass.
3. Internal reference electrode: It's a Ag/AgCl electrode. Its main function is to apply electrode potential. It should have a stable potential and small temperature coefficient.
4. Internal reference solution: The internal reference solution, with a 7pH at zero potential, is a mixture of neutral phosphate and potassium chloride. Glass electrode and reference electrode constitute the battery and pH value of zero potential. It mainly depends on the pH value of internal reference solution and chloride ion concentration.
5. Shell: It's a case supporting glass electrode and liquid junction boarder, containing the external reference solution. It’s shaped from PPS/PC.
6. External reference electrode: Ag/AgCl electrode, its function is to provide and maintain a constant reference potential. It should have a stable potential, good reproducibility and small temperature coefficient.
7. External reference solution: Potassium chloride gel electrolyte with a concentration of 3.3mol/L, it’s not east to lost and doesn’t need to add.
8. Sand core liquid junction boarder: Liquid junction boarder is a connecting part of the external reference solution and tested solution, it should have a stable infiltration capacity.
9. Electrode cable: Low noise metal shielded wire, its inner core is connected with internal reference electrode while the shielding layer is connected with the external reference electrode.