Hanna Instruments HI 903 - Manual (Page 202)

10
TITRATION THEORY
of reaction decreases and titration endpoint become increasingly difficult to reach. If the pH
exceeds 8, side reactions begin to occur between iodine and hydroxide or methylate ions,
changing the titration stoichiometry.
While solvents not containing alcohols can be used for Karl Fischer analysis, they also have
an effect on reaction stoichiometry. When alcohols are not present, the reaction resembles
the Bunsen reaction stoichiometry where the consumption ratio of water to iodine is 2:1. In
solvents containing higher alcohols, uneven ratios can be observed due to the relative abilities
of higher alcohols to form the sulfite ester that reacts with water. Issues resulting from
solvent-induced variation in stoichiometry are not typically encountered during routine analysis
for two reasons. First, titrant standardization and sample analysis are carried out in the same
titration medium and under the same conditions, effectively compensating for any variation
in reaction behavior.
Second, most Karl Fischer reagent system are formulated to support
standard KF reaction stoichiometry.
2.2.1.2 Visual Indication of Karl Fischer Titrations
Visual methods, originally used by Karl Fischer, are limited in application, require a high
degree of skill and have been made obsolete by electrometric indication. For successful
visual indication, titration samples must be colorless. Additionally, the solution coloration
varies between polar and non-polar titration media.
After the titration equivalence point all of the water in the titration solution has been reacted.
The next drop of titrant added to the solution after the equivalence point contains iodine that
will remain in the titration solution. Thereafter, the concentration of iodine in the titration
solution increases and the solution develops a yellow, and eventually brown, color. It is
difficult, even for an experienced analyst, to generate reproducible endpoint coloration between
successive titrations.
2.2.1.3 Electrometric Indication of Karl Fischer Titrations
Biamperometric and bivoltametric indication are the two types of electrometric detection
methods commonly used for indication of Karl Fischer titrations. Both methods use either a
double platinum pin or a double platinum ring electrode to detect excess iodine in a titration
solution. After the titration equivalence point, all of the water in the titration solution has
been reacted. The next dose of titrant added to the solution contains iodine, which reacts at
the electrode according to the reactions below.
At the cathode: I2 + 2e- 2I-
At the anode: 2I- I2 + 2e-
The excess iodine is easily reduced at the cathode, and the resulting iodide is oxidized at the
anode.
Both electrometric methods of indication rely on electrons (current) being carried through a
titration solution by the oxidation-reduction reactions described above.
Biamperometric indication involves monitoring the flow of current through the titration solution
while a constant voltage is applied across the platinum elements of the electrode. When
water is present in the titration solution and there is no excess iodine, only a minimal current
flows between the electrode elements. After the equivalence point, when iodine is present,
the current flow increases to a few
µ
A.