Karl Fischer Titration:The Principle and Procedure of The Method

Karl Fischer Titration:The Principle and Procedure of The Method

00510013 Xu Kerui



1.Introduction



Karl Fischer Titration is a widely used method for determining the micro amount of
water in a variety of products. Since its invention by the German petroleum chemist
Karl Fischer in the 1930's, the iodometric titration method that bears his name has
become an increasingly popular analytical technique for quantifying water in a variety
of industries. During this time, Karl Fischer titration has evolved from an esoteric
novelty to a widely used instrumental method employed in Research & Development,
Production, and Quality Control. Karl Fischer titration has been included in most key
international Pharmacopeias, as well as in ISO 9000, SOPs, and ASTM guidelines. In
this paper the principle and procedure of Karl Fischer Titration will be primarily
discussed in detail.

The popularity of the Karl Fischer titration is due in large part to several critical
advantages that it holds over other methods of quantifying water, including:



1.High accuracy and precision

2.Selectivity for water

3.Small sample quantities required

4.Easy sample preparation

5.Short analysis duration

6.Nearly unlimited measuring range (1ppm to 100%)

7.Suitability for analyzing solids, liquids, and gases

8.Independence of presence of other volatiles
9.Suitability for automation

2.The principle of Karl Fischer Titration

2.1 Fundamental Reaction



The fundamental principle behind the Karl Fischer Titration is based on the Bunsen


Reaction between iodine and sulfur dioxide in an aqueous medium shown below:



I2+ SO2+2H2O . 2HI+ H2SO4



Karl Fischer discovered that this reaction could be modified to be used for the
determination of water in a non-aqueous system containing an excess of sulfer dioxide.
He used a primary alcohol (methanol) as the solvent, and a base (pyridine) as the
buffering agent. So the reagent changed into:



py ·I2+ py ·SO2+H2O+ py . 2py ·HI+ py ·SO3



2.2 The Function of Pyridine



In classical Karl Fischer Reaction pyridine is used as a basic reagent. And as a
ligand it can complex the I2 and SO2, which can lower the vapor pressure of both I2
and SO2, shifting the equilibrium further to the right of the reaction equation.





2.3The Function of Anhydrous Methanol



During the titration py ·SO3 can reacts with H2O ,which varies the stoichiometry
of H2O and I2 from 1:1 to 2:1:



py+·SO3– +H2O . C5H5NH +SO4H–



To prevent such side reaction, excessive anhydrous methanol is added as it can
react with py +·SO3–, which lower the concentration of py +·SO3–



CH3OH +py+·SO3–.C5H5N(H)SO4CH3



2.4 The advancement of Karl Fischer Reaction



Classical Karl Fischer reagent contained pyridine, a noxious carcinogen, as the base.
The reagents most frequently used today are pyridine-free and contain imidazole or
primary amines instead. And the reactive alcohol methanol can be replaced by
2-(2-Ethoxyethoxy)ethanol or another suitable alcohol. The reaction can be
summarized by this equation:



ROH+SO2+R’N.[R’NH]SO3R+H2O+I2+2R’N.2[R’NH]I+[R’NH]SO4R



In this reaction, the alcohol reacts with sulfur dioxide (SO2) and base to form an


intermediate alkyl sulfite salt, which is then oxidized by iodine to an alkyl sulfate salt.
This oxidization reaction consumes water.





3.Titration procedure



3.1 Preparation for titration



Before the titration being carried out, some preparing work should be done. Mostly
the pH of the sample solution, the standardization of KF reagent and the pre-treatment
of sample is concerned.





3.1.1 Select a proper pH range



Karl Fischer Titration is sensitive to the pH and the rate of the reaction depends on
the pH value of the solvent, or working medium. When pH is between 5 and 8,the
titration proceeds normally. However, when the pH is lower than 5, the titration speed
is very slow, On the other hand, when pH is higher than 8, titration rate is fast, but
only due to an interfering etherification side reaction which produce water, resulting
in an vanishing endpoint. Thus, the optimal pH range for the Karl Fischer Reaction is
from 5 to 8, and highly acidic or basic samples need to be buffered to bring the overall
pH into that range.





3.1.2 Standardization of KF reagent



Karl Fischer reagent decompose on standing. Because decomposition is particularly
rapid immediately after preparation , it is common practice to prepare the reagent a
day or two before it is to be used. Ordinarily, its strength must be standardized against
a standard solution of water in methanol or the solid sample of
(CHOH)2(COONa)2 ·2H2O.



It is obvious that great care must be exercised to keep atmospheric moisture from
contaminating the Karl Fischer reagent and the sample. All glassware must be
carefully dried before use, and the standard solution must be stored out of contact
with air. It is also necessary to minimize contact between the atmosphere and the
solution during the titration.





3.1.3 Pre-treatment of the titrand




As the property of titrand differs from one to another, Karl Fischer Titration can be
directly applied to only part of the materials to be titrated. For the rest there must be
some treatment brought into the method to eliminate the interfering factors. For
example, the hydrosulfide in the sample, causing a higher result of the titration by a
red-ox reaction with I2, must be removed in a addition reaction by adding alkene.



Under most circumstances, the interfering factors includes:

a. The sample itself, not water only, reacts with Karl Fischer reagent.

b. incomplete reaction, usually caused by incomplete extract of the water

in sample.

 c. uncertain reaction.



3.2 Volumetric Karl Fischer Titration method



In volumetric Karl Fischer, iodine is added mechanically to a solvent containing the
sample by the titrator’s burette during the titration. Water is quantified on the basis of
the volume of Karl Fischer reagent consumed.



Volumetric is best suited for determination of water content in the range of 100
ppm to 100%.



There are two main types of Karl Fischer Titration reagent system.



a. In one-component volumetric KF, the titrating reagent (also known as a
CombiTitrant, or a Composite) contains all of the chemicals needed for the
Karl Fischer Reaction, namely iodine, sulfur dioxide, and the base, dissolved
in a suitable alcohol. Methanol is typically used as the working medium in
the titration cell. One-component volumetric reagents are easier to handle,
and are usually less expensive than two-component reagent.



b. In two-component volumetric KF, the titrating agent (usually known as the
Titrant) contains only iodine and methanol, while the Solvent containing the
other Karl Fischer Reaction component is used as the working medium in
the titration cell. Two-component reagents have better long-term stability
and faster titration time than one-component reagents, but are usually more
costly, and have lower solvent capacity.



A volumetric titrator is usually applied in practical work and it performs the
following three key functions:



1. It dispenses KF titrating reagent containing iodine into the cell using the
burette


2. It detects the endpoint of the titration using the double platinum pin indicator

electrode

3. It calculates the end result based on the volume of KF reagent dispensed using

the on-board microprocessor





3.3 Coulometric Karl Fischer Titration method



In coulometric Karl Fischer, iodine is generated electrochemically in situ during the

titration. Water is quantified on the basis of the total charge passed (Q), as measured
by current (amperes) and time (seconds), according to the following relationship:



Q = 1 C (Coulomb) = 1 A x 1 s where 1 mg H2O = 10.72 C



Coulometry is best suited for determination of water content in the range of 1 ppm to
5%.



There are two main types of coulometric KFT reagent systems:



a. In conventional, or fritted-cell, coulometric KF, a diaphragm – or frit –
separates the anode from the cathode that form the electrolytic cell known as
the generator electrode. The purpose of the frit is to prevent the iodine
generated at the anode from being reduced back to iodide at the cathode
instead of reacting with water.



b. In fritless-cell coulometric KF, an innovative cell design is used that through
a combination of factors, but without a frit, makes it nearly impossible for
iodine to reach the cathode and get reduced to iodide instead of reacting with
water.



The advantages of the fritless cell include:

• Uses only one reagent

o Lower reagent cost

• Titration cell much easier to clean

o Reduced downtime

o Lower maintenance cost

• Long-term drift (background) value more stable

o Can use reagent longer without refilling

• Refilling of electrolyte suitable for automation

o Reduced downtime

o Increased lab safety



Also there exists a coulometric titrator which performs the following three key


functions:



1. It generates iodine at the anode of the titration cell, instead of dispensing KF

reagent as in volumetric titration

2. It detects the endpoint of the titration using the double platinum pin indicator

electrode

3. It calculates the end result based on the total charge passed (Q), in Coulombs,

using the on-board microprocessor.



3.4 Acceptable Sample Size



In both KF methods, the amount of sample used depends on the anticipated water
content and the desired degree of accuracy. The following convenient reference table
shows the relation between water content and proper sample size:





References:

[1] Douglas A.Skoog,Fundamentals of Analytical Chemistry.8th Ed.

[2]....-..(Karl-Fisher)............ ...(......
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[3]AQUASTAR® Karl Fischer Titration Basics
Mozahedul islam
PAU Bd