Hanna Instruments HI 903 - Manual (Page 201)

9
TITRATION THEORY
2.2 Titrations According to The Reaction Type
2.2.1 Karl Fischer Titrations
This method is based on a well-defined chemical reaction between water and the Karl Fischer
reagent. The chemistry provides excellent specificity for water determination. The method
can be used to determine free and bound water in a sample matrix. The Karl Fischer method
is widely considered to produce the most rapid, accurate and reproducible results and has
the largest detectable concentration range spanning 1 ppm to 100%.
The determination of water content is one of the most commonly practiced methods in
laboratories around the world. Knowledge of water content is critical to understanding chemical
and physical properties of materials and ascertaining product quality. Water content
determination is conducted on many sample types including pharmaceuticals and cosmetics,
foods and natural products, organic and inorganic compounds, chemicals, solvents and
gases, petroleum and plastic products as well as paints and adhesives. The KF method is
verifiable and can be fully documented. As a result, Karl Fischer titration is the standard
method for analysis of water in a multitude of samples as specified by numerous organizations
including the Association of Official Analytical Chemists, the United States and European
Pharmacopoeia, ASTM, American Petroleum Institute, British Standards and DIN.
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.
I2 + SO2 + 2 H2O 2 HI + H2SO4
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 H2O + SO2 • (C5H5N)2 + I2 + 2 C5H5N (C5H5N)2 • H2SO4 + 2 C5H5N • HI
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 H2O, I2, SO2 and
RN species react in a 1:1:1:3 stoichiometry.
ROH + SO2 + RN (RNH)•SO3R
(RNH)•SO3R + I2 + H2O (RNH)•SO4R + 2(RNH)I
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