Methods optimization, 2 dosing parameters – Hanna Instruments HI 903 User Manual
Page 132
9-4
METHODS OPTIMIZATION
The table that follows correlates endpoint potential ranges for each of the possible polarization
current settings of the HI 903. 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.
Additionally, the duration of a titration is proportional to the polarization current. Thus,
titration time can be reduced by increasing the polarization current. While the default Ipol
value of 20 mA results in the faster titration than smaller 1, 2, 5, 10, and 15 mA options a
further increase to 30 or 40 mA does not significantly shorten a titration. However, the
choice of higher polarization currents will speed contamination of the electrode and potentially
degrade samples using special solvent systems.
9.1.1.2 Dosing Parameters
The HI 903 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.
9.1.1.2.1
Minimum Dose
Decreasing the minimum dose increases precision but lengthens the titration time. The only
exception is when stability time has been selected as the termination criteria and there is a
high drift rate. Under these circumstances, the minimum dose must be large enough to
maintain the endpoint potential by reacting all of the water due to the drift rate over the
course of the chosen time period. Increasing the minimun dose shortens titration time but
reduces precision and increases the chance of overtitration.
9.1.1.2.2
Maximum Dose
The maximum dose volume should be adapted according to the formulation and concentration
of the titrant. The maximum dose volume should be set as high as possible without exceeding
the reaction rate of the reagent system. The table below provides suggested maximum doses
for popular reagent systems based on their relative reaction rates.
The most effective way to optimize the maximum dose volume is to consider the titration
duration and to examine the shape of the titration curve. In the case where the maximum
dose volume is too high, the iodine will be added faster than the titration reaction rate. This
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