Hanna Instruments HI 903 - Manual (Page 136)

9-8
METHODS OPTIMIZATION
9.1.3 Method Options
9.1.3.1 Pre-dispensing Amount
It is possible to shorten titration times by adding a large fraction of the titrant at the start of
the analysis if the approximate water content of the sample is known.
When activated, the pre-dispensing amount can be set to deliver between 1% and 90% of
the titrant required to reach the titration endpoint.
A high pre-dispensing amount (around 90%) increases the chances of erroneous results.
Pre-dispensing amounts above 50% should only be used if the reaction is very rapid.
9.1.3.2 Pre-analysis Stir Time
When analyzing solid samples with limited solubility or release bound water slowly, the
sample must be stirred in the chosen solvent 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 titrant
is added to the cell. The pre-analysis stir time can be set between 0 and 1000 seconds.
9.1.3.3 Stirring Speed
The HI 903’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. Samples which result in a titration solution with
higher or lower viscosity may require stir speed adjustment.
9.1.3.4 Background Drift Rate Entry
This option provides a choice between the HI 903’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 903 to conduct a drift rate analysis.