Hanna Instruments HI 904, Uick Start Manual, Page: 118 / 180

9-6
METHODS OPTIMIZATION
9.1.2.2.2 Absolute Drift Stop
Under this criteria, a titration reaches an endpoint successfully when the drift falls below a
predefined threshold called the absolute drift stop value.
When setting the absolute drift threshold, a balance must be struck between the titration
speed and accuracy. Choosing a threshold slightly higher than the initial drift rate will result
in high reproducibility and relatively slow titrations. Setting the threshold higher (>30 mg/min)
will result in very fast titrations and reduced titration reproducibility.
The current drift rate must be considered before selecting an absolute drift stop value.
Setting to low of a value relative to the starting drift rate may cause the titration to continue
indefinitely without reaching a valid endpoint.
9.1.3 Method Options
9.1.3.1 Pre-analysis Stir Time
When analyzing samples with limited miscibility or release water slowly, the sample must be
stirred prior to the start of a titration, to avoid erroneously low titration results or unreachable
endpoints. The pre-analysis stir time option ensures that after the sample is added, the
titration mixture is stirred for a period of time before any iodine is generated in the cell. The
pre-analysis stir time can be set between 0 and 1000 seconds.
9.1.3.2 Stirring Speed
The HI 904 ’s stirring speed can be set between 200 and 2000 RPM with 100 RPM resolution.
The stirring system is equipped with an optical feedback mechanism to ensure that the
stirring motor is rotating at the speed set by the user.
The optimum stirring speed is obtained when a small vortex is visible. If the stirring speed is
too low, the titrant will not react with the sample before reaching the electrode resulting in
over-titration and poor titration reproducibility. If the stirring speed is too high, bubbles will
form in the solution. Bubbles can destabilize or falsify the measured electrode potential.
The default stirring speed for commercially available standard Karl Fischer reagents used
within the operable volume range of the standard Hanna Instruments cell and with the
supplied magnetic stirring bar is 900 RPM. As the liquid level within the titration vessel
increases, it may be necessery to increase the stirring speed to sufficiently mix the solution.
9.1.3.3 Background Drift Rate Entry
This option provides a choice between the HI 904 ’s automatic drift rate determination and
assigning a fixed value to be used by the titrator as the drift rate.
The primary benefit of bypassing the automatic drift rate feature is saving time. This is
appropriate when titrating samples with high water content where the drift rate is too low to
affect titration results or in diagnostic situations where there is no advantage in waiting for
the
HI 904 to conduct a drift rate analysis.