Hanna Instruments HI934 Bedienungsanleitung Seite 120

4
Anode: 2 I
Cathode: 2 RN-H
The iodine that is generated at the anode reacts with the water from the sample according to the Karl Fischer reaction.
The amount of water that is reacted during a titration can be calculated based on the total charge that has passed
through the generator. According to the Karl Fischer reaction (in protic solvents), 1 mole of water is titrated by 1 mole
of iodine. According to the anodic reaction above, 1 mole of iodine is generated with 2 moles of electrons. Faraday's
Constant states that 1 mole of electrons equates to 96485 coulombs (C) of electricity. Therefore, 96485 coulombs will
cause 0.5 moles of water to be titrated, or 1 coulomb equals 93.36 μg of water:
The amount of current that passes through the generator can easily and accurately be measured by the electronics of the
titrator. Coulometric Karl Fischer titrations are considered absolute, standardization is not necessary. Water standards
can be titrated as a system check to ensure proper system functioning.
2.2.1.3.1. GENERATOR ELECTRODES WITH DIAPHRAGM
The first coulometric Karl Fischer titrators used a diaphragm cell. In this design, the anode and cathode of the generator
are separated by a diaphragm typically made of porous frit glass. The diaphragm prevents the iodine generated at the
anode from being reduced at the cathode, this can cause false high water determinations. The anode compartment
contains the Karl Fischer reaction components (sulfur dioxide, methanol, base) and iodide salts for the generation of
molecular iodine. The cathode compartment contains a source of hydrogen ions, typically ammonium salts.
Diaphragm titrations have some disadvantages. The first disadvantage is the higher drift rates that occur due to moisture
collecting inside the catholyte. Since the Karl Fischer reaction only occurs in the anode compartment, moisture inside
the catholyte cannot be eliminated by pre-titration. Instead of being pre-titrated, the moisture inside that catholyte will
slowly diffuse across the diaphragm during drift analysis and sample analysis, and will add to the apparent drift rate.
The second disadvantage is the risk of diaphragm blockage or contamination. Substances in the sample matrix may clog
the diaphragm, or salts could precipitate inside the diaphragm. A clogged diaphragm will prevent ion migration which,
in severe cases, will block the electrolytic reaction of the generator. The third disadvantage is difficulty in cleaning. The
diaphragm does not absorb or drain fluid quickly, making cleaning very time-consuming. The cathode compartment
itself is also not very accessible for cleaning.
2.2.1.3.2. GENERATOR ELECTRODES WITHOUT DIAPHRAGM
To overcome the drawbacks of diaphragm titrations, diaphragm-less titration systems were made through modification
of the generator's design and modification of the reagent. The cathode's surface is much smaller compared to the anode,
allowing the generated iodine to react before possibly reaching the cathode. The reagent is also modified to prevent
oxidizable sulfur compounds from forming.
Diaphragm-less titration offers very low drift rates and easy cell maintenance, but there are several drawbacks. First, side
reactions are prone to occur particularly at slower titration rates. Therefore, samples with very low water contents may
suffer from false high concentrations. Second, compounds that are easily reduced will react at the cathode and produce
water, causing false high concentrations. These compounds include nitro compounds, unsaturated hydrocarbons, and
certain metals.
4-6
→ I2 + 2 e
- -
+ 2e → H + 2 RN
+ -
2
µ
µ