Titration results are varying, bad repeatability and accuracy.

Titration results are too varied, the repeatability of a sample series is bad (> 2%), the expected results are sometimes met, sometimes not. What is the reason, how can this be improved?

Possible reasons:

Air bubbles are trapped in burette and tubes. They lead to inaccurate dosing where the system believes the bubble is titrant and gives falsely high results.
Furthermore air bubbles of different sizes lead to result variation. Typically, over a series one will see the results trend down as the air bubbles are removed with titration (e.g. 5.8%, then 5.75%, etc…).

Cross contamination or sample carry over: Previously analyzed samples may stick to the equipment (sensor surface, dosing tube, magnetic stirrer bar) and therefore lead to wrong titrant consumption and therefore to wrong results.

Recommendations:

Air bubble check and removal: Check whether your burette and all tubes are air bubble free. Perform a rinse procedure by activating a manual dispense function on the titrator. Ideally, two rinse functions at the beginning of every shift (8 hour period) is recommended.

Make sure that the titration components (stirrer, sensor, tubes, etc…) are thoroughly rinsed with deionized water and therefore released from any adhering sample.