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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