Hanna Instruments HI934 Bedienungsanleitung Seite 119

2.2.1.1. HISTORY OF KARL FISCHER TITRATIONS
Water determination by Karl Fischer titration is based on the reaction described by Bunsen in 1853 in which sulfur
dioxide is oxidized by iodine in the presence of water.
I + SO + 2 H O → 2 HI + H
2 2 2
In Karl Fischer's 1935 article, "a new procedure for the titration of water", he presented a modified form of the Bunsen
reaction adapted for use in determining the water content of non-aqueous solutions. His titrations were conducted in
methanol in the presence of excess sulfur dioxide and pyridine in order to neutralize the acidic reaction products and
drive the reaction to completion.
2 H O + SO • (C H
2 2 5
Two key developments have since lead to the currently accepted description of the Karl Fischer reaction. First, pyridine
acts as a pH buffer and does not play a direct role in the reaction. This has allowed reagent formulators to replace
pyridine with bases which are both less toxic and result in pH ranges that facilitate faster and more accurate titrations.
Second, the species that reacts with water is not sulfur dioxide but the monomethyl sulfite ion resulting from the reaction
between sulfur dioxide and methanol. Subsequently, researchers showed that higher alcohols can be used in place of
methanol. The Karl Fischer reaction can therefore be described by the following generalized reaction sequence in which
the H O, I , SO and RN species react in a 1:1:1:3 stoichiometry.
2 2 2
ROH + SO + RN → (RNH)•SO
2
(RNH)•SO R + I2 + H
3
The maximum rate of the Karl Fischer reaction is reached between the pH range of 5.5 to 8 where all of the sulfur
dioxide is available as methyl sulfite. If the pH drops below 5, the rate 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 systems are formulated
to support standard KF reaction stoichiometry.
2.2.1.2. VOLUMETRIC KARL FISCHER TITRATIONS
In volumetric Karl Fischer titrations, the iodine for the Karl Fischer reaction is introduced via the titrant. This method is
suitable for higher water contents, 100 ppm to 100%. The other reaction components (sulfur dioxide, base, alcohol) can
either be introduced by the titrant (one-component system) or by the solvent (two-component system). One-component
reagent systems can utilize a custom solvent or solvent mixture since all of the Karl Fischer reaction components are
in the titrant. However, one-component reagents are not very stable, they have a short shelf life and slower titration
speeds. Two-component reagent systems are stable, have long shelf lifes and faster titration speeds.
2.2.1.3. COULOMETRIC KARL FISCHER TITRATIONS
In coulometric Karl Fischer titrations, the iodine for the Karl Fischer reaction is generated electrolytically inside the
titration vessel. This method is suitable for lower water contents, 1 ppm to 5%. The generator consists of two electrodes:
an anode and a cathode. The reaction that occurs at each can be summarized as follows:
SO
2 4
N) + I + 2 C H N → (C H
5 2 2 5 5 5
R
3
O → (RNH)•SO R + 2(RNH)I
2 4
N) • H SO + 2 C H N • HI
5 2 2 4 5 5
4
4-5