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Microlink Measurement and Control Systems

Technical Notes: How to Measure pH


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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.

pH Electrode and pH Probe | pH Meter | How to Measure pH with a Computer | Trouble-Shooting pH Measurement Systems | More Information on Measuring pH


pH Electrode and pH Probe

pH electrode with BNC connector

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.


pH Meter

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.


How to Measure pH with a Computer

Interfacing pH Electrodes and Probes

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 (http://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.

Interfacing pH Meters

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.

You will also need some software to collect the data from the pH meter: software like the Windmill COMIML program which reads data from the PC's RS232 COM port (http://www.windmillsoft.com/).

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.


Trouble-Shooting pH Measurement Systems

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.

  1. Check that the Microlink is working properly. First remove other factors in the system that might be causing the problem, then check the Microlink itself.
    1. Remove any power signal conditioner. This provides surge protection and noise filtering. It should remove interference from other equipment but a faulty one could cause its own problems.
    2. Move the Microlink to another location. If you are using it in a laboratory for example, take it home.
    3. Connect the Microlink to a different laptop running on battery power, not on mains power.
    4. Now you can test whether the Microlink is the source of the noisy, fluctuating, data. Connect together the first positive input pin, the first negative input pin and the 0 V input pin. With the Microlink 751 these inputs are on pins 20, 1 and 19. You could use paperclips to connect the three pins. Leave all other inputs unconnected.
      s As you have now shorted out the Microlink, if it is working correctly it will produce a reading of zero. If not the fault lies with the Microlink and you should contact Technical Support and send the unit back. If the reading is zero then the problem is very unlikely to be caused by the Microlink and you can go on to step 2.
  2. Once you have eliminated the DAQ unit as the source of the fault, test the power supply to the computer. Plug the laptop into the mains - the reading should still be zero.
  3. Reconnect any power signal conditioner. Again, check that the reading is zero.
  4. Test the probes one-by-one. Place a pH probe in a known solution and see if the reading is as expected. Keeping the signal wires short and far away from electrical machinery helps reduce noise. You may also need to clean, or even change, your probes.
  5. Move everything back to the original location. If the readings become erratic go through the tests again.
  6. Disconnect from the laptop and connect to the original computer. Again, if you now get noisy readings repeat the tests.
  7. Finally make your measurements in a real situation.

Discovering at which stage the problem occurs will let you identify the source and take remedial action.


More Information on Measuring pH

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Last revised: 10 August 2012