This discrepancy in results is primarily noticeable when performing acid/base titrations using one of the pH indicators. The first reason for this is that these pH indicators change color over a pH range rather than at a fixed value. The actual point at which the color change occurs is very much sample dependant and may not coincide with the chemical equivalence point. This can result in a small discrepancy in result which is easily nullified by standardizing the titrant using a similar method as is used for samples. The second reason for this difference is primarily one of the sensitivity of the human eye to color change. While a color change may have already started to occur, the human eye has still not detected any change. This can be demonstrated by using a photometric sensor such as the
METTLER TOLEDO DP5 Phototrode. Using one of these sensors there is a clear change in light transmittance long before the human eye detects any color change. In the typical acid/base titration using potentiometric indication with a pH sensor, the sharp change in signal occurs at the first trace of excess acid (or base) and is therefore a more true indication of the end point.