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pH is measured in many industries, including nearly all handling water. In fabric dyeing the permanence of the colour and the speed of the process depend on pH. pH determines product quality in paper mills. A change in pH in a lake or river is an early indicator of pollution. And the correct pH is essential to ensure proper beer production.
pH indicates the acidity of a solution. The term "pH" was first used by W. M. Clark in 1920. Previously the Danish biochemist Sorenson had used the term PH - H being a subscript of P. The H stands for hydrogen. (An acid forms hydrogen ions in solution.) It's unclear where the p came from. It's often said that the p refers to the German word protenz, which apparently means power. This makes sense because the pH scale is a logarithmic one, so the p would refer to the power or exponent of 10. However, in that case why was the German word for hydrogen not used?
Another view is that the p is for the French word poudre, also meaning power. Or from the Latin, pondus Hydrogenii, which means "weight of Hydrogen". Or perhaps the p stands for potential.
Jens Norby published a paper in 2000 arguing that the little p was simply a constant (Trends in Biochemical Sciences 25 (1) (2000) pp. 36-37.)
Various methods of measuring pH are available, but the most common one used in laboratory and industry is the glass electrode method.
In this, 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.
First let's take the case of a pH meter - a measuring device which displays the pH of the sample. 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 6 COMIML program which reads data from the PC's RS232 COM port (http://www.windmill.co.uk/comiml.html).
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.
What if you are not using a pH meter but instead wish to connect the pH electrodes to the PC? To do this you need a data acquisition (DAQ) device. This device may be a card that you plug into the PC. More commonly, it is a unit sitting between the electrodes and the computer, connected to one of the PC's communication ports: USB, RS232, 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, the 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.
Similarly, our small, portable, Microlink 751 unit - which plugs into the PC's USB port - has its own pH conditioning unit.
For more details of using a Microlink to measure pH contact sales@microlink.co.uk or see Microlink 3800, Microlink 751, or Microlink pH-USB. You may also find our notes on debugging noisy pH systems useful.
Copyright Biodata Ltd 2006
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