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Study Notes: pH Meter

Measuring pH is about measuring the acidity (pH 0-7) or alkalinity (pH 7-14) of an aqueous sample. The instrument used to perform this function in the laboratory is the pH meter. The main components of a typical meter are:
  • Electrode (two electrodes can be used but one is more usual)
  • Temperature probe
  • Electronic control unit
pH Meter that is labelled to identify the following: Electrode; Temperature probe; Electornic control unit.

The electrode responds to the concentration of hydrogen ions in the sample solution to produce a chemical signal. This signal is then converted into pH values which are displayed on the control unit.

Variations in temperature can cause pH to change (how much depends on the temperature as well as the nature of the samples). To measure accurately, most pH meters automatically compensate for temperature. For this to occur the control unit needs to know the sample's temperature - this is achieved through the meter's temperature probe, which is immersed in the sample along with the electrode.

When pH meters malfunction, it can often be due to a problem with the electrode. Electrodes are fragile and should be handled and maintained with care. Dirty electrodes or those with scratches or cracks can produce unreliable results. Never leave electrodes to dry out - they should be left immersed in buffer solution between the measuring of samples and during long term storage (never store electrodes in deionised or distilled water). Some electrode types need to be kept filled with an electrolyte solution through a hole near the top of the electrode.

When measuring the pH of a sample/buffer carefully swirl the sample cup around the electrode for a few moments, to ensure the meter responds rapidly and accurately. Always rinse the electrode with distilled water after use and between samples/buffers.

Like any laboratory instrument, pH meters need to be calibrated so that they provide accurate readings. Buffer solutions, which can be made up or purchased for use 'as is', are used for this purpose. These solutions have stable, accurately known pH values, the most common buffers being 4, 7 and 10. Because pH varies slightly with temperature, the pH of the buffer solution may not be exactly 4, 7 or 10 (eg the pH of buffer solution pH 4 has a pH of 4.01 at 25°C, 4.02 at 30°C). They have a certain shelf life after which they must be replaced. This information is supplied with the buffers.

One point calibrations can be performed (where only one buffer is used) but two point calibrations (using two buffers) give better accuracy overall. Depending on the samples, calibrate the meter initially using the pH 4 or 10 buffer then which calibrate with the pH 7 buffer. You would use the pH 4 buffer when the pH is expected to be less than 7 and the pH 10 buffer when the pH is expected to be greater than 7. In this way, you know that the instrument is adjusted best to cover the pH range of the samples you are testing. If the pH is around 7, either pair of buffers would be satisfactory as would pH 7 buffer if used by itself.

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