Highly acid and alkaline pH values

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Acid and alkaline errors

When measuring more extreme pH values you have to be aware of the acid and alkaline errors that will influence your measurement.

Alkaline error (Na+ error)

The pH glass electrode responds very selectively to hydrogen ions (H+). However, there is a small interference caused by alkaline ions, particularly sodium ions (Na+) but also to some extent lithium ions (Li+). This effect, called the alkaline error, increases with increasing pH values (pH > 9), higher alkaline concentrations and increasing temperatures.

At high pH value the hydrogen ion activity is low and the sodium ions replace the hydrogen ions in the outer gel layer of the glass membrane. As a result, a pH value that is lower than the actual value of the sample solution will be measured. Under extreme conditions the glass membrane responds only to sodium ions.

In order to minimize the contribution of alkaline errors, pH electrode manufacturers use special glass membranes for electrodes that are used to measure high alkaline values (high pH). The composition of the glass membrane will, to a large extent, determine the electrode's response time and its sensitivity to ions other than H+. However, there is no types of glass membrane currently available that has zero alkaline error. Some error will always exist.

Acid error

At very low pH values acid molecules are absorbed by the gel layer leading to a decrease in the hydrogen ion (H+) activity in the gel layer. The pH measurement, therefore, shows a higher pH value than the actual value of the measured solution. This is according to the definition of pH, if the hydrogen activity decrease the pH value increase.

The acid error changes very little with temperature and is only relevant for very low pH values. Usually below 1.00 pH. Fairly uncommon applications. However, for these situations, you can get measuring electrodes with membrane glasses having specifically low acid errors.

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