What's the difference between a pH electrode and a pH probe? What do you need to consider when interfacing pH measurements to a computer? Just a couple of the questions answered in this article on measuring pH.
Various ways of measuring pH are available, but the most common one used in laboratory and industry is the glass electrode method. Here, the pH of a known reference solution is compared to the pH being measured. Two electrodes are used: a glass measurement electrode and a reference electrode.
The measurement electrode comprises a glass bulb attached to a glass stem. The bulb is a pH sensitive membrane filled with a conducting buffer solution. A silver wire is enclosed in the glass.
The difference in pH between the solutions inside and outside the thin glass membrane creates an electrochemical force (voltage) proportional to the difference in pH. This is passed via the silver wire. The reference electrode has a stable potential and also features a silver wire, enabling a complete circuit to be made and the voltage generated by the glass electrode to be measured.
The measurement and reference electrode may be individual and separate, or may be combined into one probe. (There may also be a temperature compensating electrode.) Individual electrodes are less practical than a combined probe, but may be more precise.
The probe may be connected to a pH meter that displays the current pH reading. See below for how to interface pH electrodes to a PC.
A pH meter is a measuring device which displays the pH of a sample. It comprises a pH electrode connected to a meter that measures and displays the pH reading. pH meters range from simple pen-like devices to laboratory instruments with computer interfaces. See below for how to interface pH meters to a PC.
First lets cover connecting pH electrodes to the PC. To do this you need a data acquisition (DAQ) device. This unit links the electrodes to the computer, via one of the PC's communication ports: USB, RS232 serial port, Ethernet or RS485 for example. You connect the electrodes to the DAQ device. The device then regularly takes pH readings and passes them to the computer.
pH electrodes have a very high output impedance and you cannot just connect them to a normal voltage input on your data acquisition unit. You will need instead to choose a DAQ unit that will amplify the signal to the appropriate level. For example, our small, portable, Microlink 751-pH unit - which plugs into the PC's USB port - has its own pH conditioning unit. The Microlink 751-pH uses an integrating Analogue-to-Digital converter which reduces internal noise, and a differential amplifier which helps remove external noise. (See below for tips on trouble-shooting noisy systems.) The Microlink also lets you monitor temperature, flow, level, gas concentration, resistance, etc.
For distributed systems we recommend the Microlink 851-pH, which has similar facilities to the 751 but is controlled over an Ethernet network.
The larger Microlink 3000 system comprises a frame of modules. Each module is dedicated to a specific task: voltage input, counting, current output and so on. It has a special module dedicated to pH signal conditioning that you slot into the frame (https://www.microlink.co.uk/3800.html). You connect the electrodes to this module. You then connect the pH module to the normal voltage measuring module and take pH readings at the same time as other measurements.
What if you wish to connect a pH meter to the PC? This will often have an RS232 or USB interface. This means that you can connect it to the serial COM or USB port on your computer.
The Windmill software regularly collects the pH readings, every 5 seconds say, and saves them on the computer's hard disk. Windmill can also show live charts of the data, pass data in real-time to other programs like Excel and collect pH data alongside other data such as temperature or flow rate.
pH monitoring systems are very sensitive and prone to interference. Small current or voltage surges can cause large fluctuations in readings.
Here are some simple tests that you can follow to determine the cause of pH (or other) measurement problems. We've used the Microlink 751-pH USB hardware as an example but many of the tips also apply to other manufacturers' systems. Work through the tests one-by-one, eliminating sources of error as you go.
Discovering at which stage the problem occurs will let you identify the source and take remedial action.
For more details of measuring pH: