Hanna Instruments HI 903 - Manual (Page 131)

9-3
METHODS OPTIMIZATION
9 OPTIMIZATION
9.1 Titration Settings
The default settings included with the standard methods have been developed by Hanna
Instruments in order to provide accurate results for the majority of samples without requiring
additional analyst input or method fine-tuning. However, in order to suit a wider variety of
sample types and matrices, all of the HI 903 titration parameters are customizable.
This section provides the descriptions of critical titration parameters necessary for an analyst
to modify a standard method or develop a titration method from scratch.
HI 903 methods can be modified and customized based on the requirements of the sample,
sample matrix and the Karl Fischer reagent formulation. The user changeable settings are
separated into two categories: Control Parameters, which set critical functions that determine
the course of a titration and set the way in which titrations are terminated, and Method
Options, which control lesser features not directly affecting measurements and primarily
allow advanced users to shorten titration times.
9.1.1 Control Parameters
9.1.1.1 Endpoint Potential and Polarization Current
The HI 903 uses the polarized electrode system known as bivoltametric indication. The
titrator monitors the voltage required to maintain a constant polarization current (Ipol) between
the pins of a dual platinum-pin Karl Fischer electrode during the course of a titration.
During a titration, no excess iodine is present. In order to maintain the set polarization
current the HI 903 must apply a relatively large voltage across the pins of the electrode.
At the endpoint of the titration, the amount of iodine added is equal to the amount of water
from the sample. When an excess of titrant has been added, iodine is present in the solution.
The excess iodine is easily reduced, and the resulting iodide is easily oxidized in electrode
reactions at the cathode and anode respectively. The ease of these reactions make maintaining
the constant polarization current possible at a much lower electrode potential.
In theory, a large shift in the electrode potential indicates the endpoint. In practice, a
titration endpoint is reached when the electrode potential drops below a value defined by the
user and the chosen termination criteria is met.
The choice of endpoint potential should be based, foremost, on the polarization current and,
to a lesser extent, on the composition of the Karl Fischer solvent and the sample matrix. If
the polarization current is changed, the endpoint potential must also be changed. In addition,
there are pitfalls to be avoided when choosing an endpoint potential. Selecting endpoints which
are both ‘too high’ or ‘too low’ will result in long titration times and poor reproducibility. Endpoints
which are ‘too high’ are those which result in endpoints that either precede or coincide with
equivalence point such that the concentration of excess iodine is not reliably detected. Endpoint
potentials are considered ‘too low’ when they correspond to a large excess of iodine in the
titration cell.