Hanna Instruments HI 904 - Manual (Page 117)

9-5
METHODS OPTIMIZATION
9.1.2 Termination Parameters
HI 904 provides a choice of three criteria by which a titration can be considered to have
reached an endpoint successfully.
9.1.2.1 Stability Time
When this termination criteria is selected, a titration is considered to have reached an endpoint
when the electrode potential stays below the specified endpoint potential for a period of time
called the stability time. Typical endpoint stability times range between 5 and 15 seconds.
9.1.2.2 Drift Stop Termination Criteria
Drift-based termination criteria, or Drift stop, terminates titrations based on the idea that at
the end of a titration, when all of the water due to the sample has been reacted, the titrator
should only be titrating the water seeping into the cell due to the background drift rate (see
section 6.3 for a detailed explanation of background drift).
Ideally, drift stop termination criteria would end a titration when a drift rate identical to that
which preceded the start of a titration is observed at the end of a titration. However, from a
practical standpoint the achievement of an identical drift rate results in very long titration
times.
In order to shorten titration times while still taking advantage of the positive aspects of
drift-based termination, the HI 904 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 initial drift rate.
Under this criteria, a titration reaches an endpoint successfully when the HI 904 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 (3 to 5
µ
g/min) will result in titrations with high
reproducibly and long durations. Setting high relative drift stop values (8 to 15 µ g/min) will
result in fast titrations with potentially reduced reproducibility.
Lower relative drift stop values are required for low-concentration samples. The last few
micrograms of water from a sample are slow to react with iodine. Therefore, it is critical to
allow the last few micrograms time to react since it may be a significant portion of the total
titrated water. For titrations of less than 200
µ
g water, it is recommended to set the relative
drift stop to 3-4
µ
g/min. Titrations of greater than 200
µ
g water can have a relative drift stop
of 8 to 15
µ
g/min.