Variation in result while switching from manual titration to an automatic titrator.

What causes result differences between visually detected, color-indicator based manual titrations and end point titrations performed on an automatic titrator?

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This discrepancy in results is primarily noticeable when performing acid/base titrations using a pH indicator. 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 and indicator amount dependent and may not coincide with the automatically detected end point at a defined pH value.

The second reason for this difference is due to 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. 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.

Another point is that an indicator consumes titrant to make a complete change. When using an indicator, blanks should be performed to compensate for how much titrant is consumed past neutralizing the sample to make the indicator change color. If an indicator consumption is not compensated for, the results may be too high.