Methods optimization, 2 titration speed, 3 signal averaging – Hanna Instruments HI 904 User Manual
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9-4
METHODS OPTIMIZATION
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The table that follows correlates endpoint potential ranges for each of the possible polarization
current settings of the HI 904. The suggested endpoints below are applicable for reagents
formulated with methanol. Endpoint potentials should be increased by 20 to 25% when
titrating with reagent systems formulated for use with aldehydes or ketones or where methanol
has been replaced with higher alcohols or substituted ethers like diethylene glycol monoethyl
ether or 2-methoxyethanol.
Note: Cleaning the detector electrode with cleaning agents will remove platinum-iodine
complexes that have formed on the electrode surface. This will lower the resistance of the
detector and therefore lower the detector’s mV readings. To counteract this change, lower the
endpoint mV value, or raise the imposed current under Method Options. The platinum-iodine
complex will reform after several titrations.
9.1.1.2 Titration Speed
The HI 904 predicts the approaching endpoint and reduces the volumes of titrant added
until the endpoint is reached. This is a software controlled process known as dynamic dosing.
Dynamic dosing prevents the addition of titrant beyond the endpoint and provides enhanced
data density in the vicinity of the endpoint resulting in accurate endpoint determination and
faster titrations. The minimum and maximum dose volume must be set appropriately by the
user for dynamic dosing to be effective.
The Titration Speed setting controls the rate of iodine generation. Faster titration speeds will
reduce the titration time, but will increase the chance of over-titration. Slower titration speeds
will allow greater endpoint accuracy, but will lengthen the titration time. The recommended
titration speed for each sample depends on the amount of water introduced by the sample:
If Automatic is selected, the HI 904 will determine the appropriate titration speed based on
the estimated water content and the amount of sample added to the vessel for each titration.
If over-titration frequently occurs, select a slower titration speed. If a shorter titration duration
is desired, select a faster titration speed.
9.1.1.3 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).
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