At low pH values (< pH 2) the potential difference between measurement and reference electrode will not conform exactly to the Nernst equation. Through experiments it has been proven that the gel layer of the pH sensitive glass membrane will absorb acid molecules at very low pH values.
This absorption decreases the activity of the H+ ions and results in a lower potential at the outer membrane phase boundary. The pH measurement shows a higher pH value than the actual pH value of the measured liquid solution. This effect is known as the acid error. As with the alkaline error, manufacturers supply specialized measuring electrodes with glass membranes having specially low acid errors.
Hamilton membrane glass shows no acid error above pH 1. Please refer to the listing of pH glass membrane types for specific application information.