Hanna Instruments HI 903 - Manual (Page 134)

9-6
METHODS OPTIMIZATION
9.1.1.4 Start Mode
The HI 903 can be set to either normal or cautious start mode. The cautious start feature is
designed to prevent the accidental over-titration of a sample with very low water content. In
cautious start mode, the HI 903 starts a titration using the minimum dose size specified by
the user rather than starting with half of the maximum dose size as with normal start mode.
9.1.1.5 Signal Averaging
The chosen value for the signal averaging setting determines how many readings the electronics
will average to produce a single data point on the titration curve. While higher values of 3 or
4 readings reduce the response time of the electrode, they also result in a ‘smoother’ titration
curve which may result in a faster titration (single unstable readings may cause the dose size
to be reduced).
9.1.1.6 Flow Rate
The flow rate setting specifies the volume of titrant delivered per minute. The default flow
rate should be used for the majority of titrations. In cases where the titrant is more viscous,
the flow rate can be reduced.
9.1.2 Termination Parameters
HI 903 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.
In order for this criteria to successfully terminate a titration the stability time and the minimum
dose size must be set such that, at the end of a titration, the minimum dose size is large
enough to react all of the water leaking into the cell due to drift during the set stability time.
If the minimum dose volume is too small to compensate for the water introduced by the
drift, the titration will never be terminated.
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