Hanna Instruments HI 903, Быстрый старт

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9-7
METHODS OPTIMIZATION
drift-based termination, the HI 903 incorporates two drift stop termination criteria which
terminate titrations when the drift rate passes below a specified threshold. The methods
can be distinguished by the way in which the drift rate thresholds are specified.
9.1.2.2.1 Relative Drift Stop
The relative drift stop termination parameter should be the first choice termination criteria.
It is the most universally applicable, easiest to use and results in fast, repeatable titrations.
This parameter has the advantage over other termination criteria in that the relative drift rate
termination value can be set independently from the titrant concentration and the initial drift
rate.
Under this criteria a titration reaches an endpoint successfully when the
HI 903 titrates all of
the water introduced with the sample and maintains a drift rate which is equal to the sum of
the initial drift (drift rate when the titration was initiated) and the set ‘relative drift stop’ value
(i.e. a slightly higher drift than the initial drift rate).
The choice of relative drift stop value influences the titration duration and reproducibility.
Choosing low relative drift stop values (5 to 10
µ
g/min) will result in titrations with high
reproducibly and long durations. Setting high relative drift stop values (20 to 30
µ
g/min)
will result in fast titrations with potentially reduced reproducibility. Reduced reproducibility
at higher drift stop values is of particular concern when using reagents that have slower
reaction rates (one-component or aldehyde and ketone reagents).
It is important to set an appropriate relative drift stop value when working with insoluble or
sparingly soluble samples. During these types of titrations, the final traces of water are
released very slowly. If the sample contains a small amount of water (the final traces are a
large fraction of the total water), the relative drift stop value should be set very low. If the
final traces can be ignored because the sample water content is large, then the titrations can
be terminated at a higher drift rate termination value.
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.
The absolute drift stop value does not take the initial drift rate into account but does have the
advantage of being able to be set without consideration of the titrant concentration. In
addition, for a titration to reach endpoint, the absolute drift stop threshold must be set
higher than the initial drift rate value.
The primary disadvantage associated with the absolute drift rate termination criteria is that
the actual background drift rate must be considered before setting the absolute drift rate
threshold.
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
µ
g/
min) will result in very fast titrations and reduced titration reproducibility.