pH meter is a device used for potentiometric pH measurements.
pH is one of the most important parameters of the water solutions. Kinetics and equilibrium of virtually every reaction occurring in the water solution depends on the solution pH, and these reactions are not only responsible for a typical chemical applications (like compound synthesis), but also for the way plants absorb nutrients from the soil, water animals grow their shells, our bodies regulate breathing and so on. In the case of biological systems pH change in the range of few hundredths can have a significant effect. That in turn means knowing pH of the solutions is usually very important if we want to control the situation. In fact pH is probably the most often measured property of water solutions, in laboratories, factories and in the field, with millions of pH meters sold yearly throughout the world.
pH can be measured using either pH indicators (like phenolphthalein) - in form of a solution or pH strips - or using potentiometric method. Using indicator solutions is pretty inconvenient, strips are easy to use but limited in their accuracy to about 0.2-0.5 pH unit, plus they can't be used for a continuous control of the pH. When you need higher precision, or fast results, pH meter is the only way to go. And as they can take any form – starting from high precision, expensive bench units, ending with a handy pocket ones, barely larger then a pen – they became the solution of choice for most applications.
pH meter works by measuring potential difference between known reference electrode and the measuring pH electrode. Potential of the pH electrode depends on the logarithm of the concentration (or more precisely activity) of hydronium ions. This dependence is described by the Nernst equation, thus once the potential has been measured you can directly calculate solution pH.
From the technical point of view, pH meter is nothing else but a precise voltmeter, connected to the pH and reference electrodes, and scaled in such a way that it displays not the measured potential, but ready pH value. In typical today's pH meter reference electrode is built into the pH electrode, which makes the device compact and its use very simple.
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