The Design of a Single probe Twin Electrode pH Measuring Unit

The modern commercially available pH sensor is a somewhat complex in design but very simple in form and very easy to operate. As already stated the measurement of pH is vital to many, many industries particularly the food industry, pharmaceutical industry, water supply and sewerage plants, and in pollution monitoring. It follows that many pH probes will need to be used in the field and, thus, the probe needs to be robust, well made and as impervious to the elements and environment as possible. 

A diagram of a commercial pH probe is shown in figure 13. The sensing part of the electrode system is a bulb made of special ion permeable glass the properties of which have been described already in the section on the glass electrode. The bulb often contains some silver chloride to maintain saturation and a silver/silver chloride reference electrode. The inner tube is completely insulated from the outer tube that contains a 0.1M HCl solution for pH measurement. The outer wall of the outer tube contains a small panel of ion permeable material that may be sintered glass, some form of ceramic, or porous TeflonÒ. This ‘ion permeable window’ completes the circuit from the silver/silver chloride reference electrode to the measuring electrode that consists of a platinum electrode immersed in the 0.1M HCl. solution in the outer tube. The complete probe can be very compact, stable to temperature and provide adequately accurate results.

The electrometer circuit usually consists of the customary high impedance FET operational amplifier with a temperature compensation circuit and outputs that can be displayed as volts (millivolts) or pH units on a standard meter or, alternatively, the data can be transferred directly via an RS232 connection to a computer via a USB hub and recorded and processed in any way required.

Figure 13. The Design of a Commercial pH Probe