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pH at a glance ...

Many of natures processes are highly dependent on pH and this is also the case for the chemical reactions that take place in industry or in a laboratory. In 1909, the Danish founder of the modern pH theory, S.P.L. Sørensen, proved that pH is essential for many enzymatic processes. pH is an abbreviation of "pondus hydrogenii' and was proposed in order to express the very small concentrations of hydrogen ions.

In 1909, pH was defined as the negative base 10 logarithm of the hydrogen ion concentration. However, as most chemical and biological reactions are goverened by the hydrogen ion activity, the definition was quickly changed. As a matter of fact, the first potentiometric methods used actually resulted in measurements of ion activity. The definition based on hydrogen ion activity is the definition we use today. pH=-log₁₀a_H⁺

In more simple terms, pH is a measurement for the acidity or alkalinity of a solution and in pure water the hydrogen ion (H+) and hydroxyl ion. (OH-) concentrations are equal at 10-7 M (25°C). The heart of a pH measuring system is a membrane made from special pH-selective glass on which a very thin layer of hydrogen ions is formed when dipped in water. At high pH values, this layer will have a low hydrogen concentration. However, at low pH values a large number of H+ ions diffuse in the layer. By measuring the generated electrical potential (E) in the layer the corresponding pH can be computed.

The potential (E) between both wires will vary with the pH difference between sample and known buffer according the Nernst-equation (-59.2 mV/pH at 25°C). A salt-bridge around each wire prevents direct metal contact with the solutions by using a wet junction for a stable electrical behaviour.

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