METTLER TOLEDO Produces Guide to pH Measurement

pH measurement and control is vital in many industrial processes, but what actually is pH, why is it so important, and how is it measured? In a new guide, METTLER TOLEDO answers all these questions, and explains the theory behind pH measurement.

Of all the analytical parameters measured in the process industries, pH is by far the most common and one of the most important. Among other reasons, control of pH is often required to ensure that an end product meets the producer's specification, that a product is safe to use, fulfills regulatory requirements, or to protect equipment from corrosion.

All aqueous solutions contain hydrogen ions and hydroxide ions. pH is the measurement of the hydrogen ion activity relative to hydroxide ion activity. Aqueous liquids that have a hydrogen ion concentration greater than the hydroxide ion concentration are called acidic. If the hydroxide ion activity is greater than the hydrogen ion activity the liquid is called alkaline.

The well-known pH scale of 0 to 14 is logarithmic, and it takes 10 times the quantity of chemicals to adjust a solution from pH 8 to 9, compared to an adjustment of pH 7 to 8. Therefore, pH measurement is also important for ensuring expensive reagents are not being dosed in excess.

In industry, the most common method of measuring pH is by a glass electrode that is sensitive to hydrogen ions. The signal output from this electrode is compared with that from a reference electrode that is in contact with the solution but is not sensitive to hydrogen ions. The difference in the output signal from the two electrodes represents the pH level and is displayed on a meter or transmitter. For convenience, modern pH sensors have the pH electrode and reference electrode combined into a single unit. Most are also equipped with a temperature sensor, as temperature has a significant effect on the output of the pH sensor and must be compensated for to obtain an accurate pH measurement.

pH sensors are sensitive instruments and should be treated with care, and regularly cleaned and calibrated if they are to provide reliable measurements. Despite this, pH sensors are available that can tolerate the extreme conditions found in many chemical and petrochemical processes.

METTLER TOLEDO has released "A Guide to pH Measurement – the Theory and Practice of pH Applications", which explains in detail what pH is, the theory behind its measurement, the practicalities of its measurement, and how to select the correct pH sensor for an application.

The guide can be downloaded from