How does a soil pH meter work?
How does a soil pH meter work? |
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A pH meter relies on a voltage test to determine hydrogen ion levels and thus pH. | |||
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The more hydrogen ions in a solution, the more conductive it will be. So the more acidic a solution, the more electricity it will conduct. This is what the pH meter relies on. | |||
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In order for electricity to flow around a circuit, it must be complete. A pH tester circuit is completed by the test sample. The sample soil comes into contact with two electrodes which are found on the probe. | |||
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An electrode is a part of a circuit which comes into contact with non-metallic elements. The electrons travelling around the circuit are attracted to the electrodes’ membrane. | |||
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pH meters have two electrodes, a glass electrode and a reference electrode.
The glass electrode has a permeable membrane made from specialised glass, which houses a chemical solution and a silver-based wire. The reference electrode is also made up of a wire in a chemical solution. |
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The reference electrode is stable and works as a buffer and a reference against which the other electrode can be compared, and the pH determined.
The glass electrode attracts the hydrogen ions. This then creates a small voltage which can be compared to the reference electrode. |
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The voltage difference between the two electrodes is then sent as a signal to the indicator, which is translated into a pH reading. | |||
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For analogue pH meters this signal acts upon an electromagnetic coil which causes the needle to rotate about its hinge and give a pH reading on the scale.
Digital pH meters require an analogue-to-digital converter to produce the number displayed on the LCD screen. |
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The reference electrode is calibrated before the devices are sold, which means it is set at a certain pH (generally 6.86 – close to neutral). Calibration is usually set using a buffer solution with a known pH, which the device can then be altered to.
Most basic soil pH meters cannot be calibrated after purchase. However, they can be placed in a buffer solution to check the device is indicating correctly. Buffer solutions are not widely available and can cost more than the meter itself, so this test is not usually deemed necessary. |
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pH meter results may be affected by temperature, but pH itself is temperature-dependent and so the results are a true reflection of these changes. However, the probes may also have slight discrepancies determined by temperature. These will be minimal but should be considered if an unexplained change occurs. | |||
