pH/ORP

pH

Why measuring pH?

pH is the most important most frequently measured value in aqueous solutions. Roughly speaking pH defines if a solution is acid, neutral or alkaline. It is bases on the dissociation of the water molecule H₂O ⇔ H⁺+ OH^- and describes the activity of the hydrogen ion. The scale it includes according to the definition 14 orders of magnitude from 0 to 14. Solutions between 0 and 7 are acid, exactly at 7 they are neutral, between 7 and 14 alkaline. The smaller the pH the higher the activity of the hydrogen ions and vice versa. In practice values between -1 and 15 can be measured.

The sensor for pH is the so-called pH electrode, consisting of a measuring electrode and a reference. The measuring electrode consists of a special glass sensitive for hydrogen ions. It is filled with a buffer solution with pH 7. Immersing the electrode into a sample leads to change of the potential of the measuring electrode in comparison to the reference. This change will be detected by appropriate instruments and converted tot eh pH reading.
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ORP

Why measuring ORP?

The measurement of the oxidation/reduction potential (ORP) is based on reactions taking place in aqueous solutions, where the reactants exchange electrons. The uptake of electrons by one reactant is called reduction, the donation of electrons by the other oxidation.

An ORP electrode makes this reaction visible. It consists of an inert precious metal electrode (gold, platinum) and a reference electrode. The reaction of the reactants on the metal surface causes an electrical potential in comparison to the reference electrode which can be detected by appropriate meters. A typical application in practice is the monitoring of the efficiency of a disinfectant dissolved in water like chlorine. The potential of an ORP electrode will always be noted considering the reference system used (typically Ag/AgCl). If the conversion into the potential versus a hydrogen electrode is required a temperature dependent potential has to be added (e.g. 207 mV at 25 °C).