Hanna Instruments HI934 Bedienungsanleitung Seite 121

2.2.1.4. 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.
2.2.1.5. 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: I + 2e
2
At the anode: 2I → I
-
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 minimal current flows between the electrode
elements. After the equivalence point, when iodine is present, the current flow increases to a few μA.
Bivoltametric indication involves measuring the voltage required to maintain a constant current flow between electrode
elements. A small direct or alternating current called a polarization current (I
rings, and the resulting voltage is measured in order to monitor the titration progress.
L-shaped titration curves (figure 3) are generated for both methods by plotting either the electrode current or voltage
against the volume of titrant added during the titration.
→ 2I
- -
+ 2e
-
2
Figure 3
) is applied between the electrode pins or
pol
4
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