Hanna Instruments HI 904 - Manual (Page 159)

11
TITRATION THEORY
The amount of current that passes through the generator can easily and accurately be measured
by the electronics of the titrator. Therefore, 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 made typically of porous frit
glass. The diaphragm serves to prevent the iodine generated at the anode from being reduced
at the cathode, which would 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.