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A pH meter is a scientific instrument that measures the hydrogen-ion activity in water-based solutions, indicating its acidity or alkalinity expressed as pH. The pH meter measures the difference in electrical potential between a pH electrode and a reference electrode, and so the pH meter is sometimes referred to as a "potentiometric pH meter".
Potentiometric pH meters measure the voltage between two electrodes and display the result converted into the corresponding pH value. They comprise a simple electronic amplifier and a pair of electrodes, or alternatively a combination electrode, and some form of display calibrated in pH units. It usually has a glass electrode and a reference electrode, or a combination electrode. The electrodes, or probes, are inserted into the solution to be tested. pH meters may also be based on the antimony electrode (typically used for rough conditions) or the quinhydrone electrode.
In order to accurately measure the potential difference between the two sides of the glass membrane reference electrodes, typically a silver chloride electrode or calomel electrode are required on each side of the membrane. Their purpose is to measure changes in the potential on their respective side. One is built into the glass electrode. The other, which makes contact with the test solution through a porous plug, may be a separate reference electrode or may be built into a combination electrode. The resulting voltage will be the potential difference between the two sides of the glass membrane possibly offset by some difference between the two reference electrodes, that can be compensated for.
In general there are three major categories of pH meters.
- Benchtop pH meters are often used in laboratories and are used to measure samples which are brought to the pH meter for analysis;
- Portable, or field pH meters, are handheld pH meters that are used to take the pH of a sample in a field or production site;
- In-line or in situ pH meters, also called pH analyzers, are used to measure pH continuously in a process, and can stand-alone, or be connected to a higher level information system for process control.
pH meters range from simple and inexpensive pen-like devices to complex and expensive laboratory instruments with computer interfaces and several inputs for indicator and temperature measurements to be entered to adjust for the variation in pH caused by temperature. The output can be digital or analog, and the devices can be battery-powered or rely on line power.
pH meters can be bought from lab suppliers or online. Good quality ones are expensive.
DIY pH meter
To be added
Handling and maintenance
pH meters must periodically be calibrated using standard pH solutions of known concentration (usually pH = 4.00, pH = 7.00, pH = 10.00), solutions which must also be verified periodically. The pH = 10.00 solution tends to degrade the fastest, when exposed to air.
The pH electrode must always be kept in a saline solution when not in use, to prevent the electrode from drying, which will cause irreversible damage to the probe.