THE BASICS OF KARL FISCHER TITRATION

Karl Fischer titration is an analytical technique to measure the amount of water contained
in various samples, which may be solid, liquid or gaseous. This method was originally developed
in the 1930s by German chemist Karl Fischer.

Karl Fischer titration is based on a 1:1 reaction between iodine and water in the presence
of sulfur dioxide and a base. Below is the 2-step mechanism of the reaction:

CHOH
CH OH++ SO
SO2 + RH [RNH]SO3CH3
33 2 + RN

[RNH]SO3CH3 + I2 + H2O + 2 RN [RNH]SO4CH3 + 2 [RNH]I

From the reaction mechanism, it's evident that the reagent used for the titration needs
to contain the following:
- Alcohol - methanol was used originally, but in different variations of the reagent
can be replaced by ethyl carbitol, methoxypropanol, ethanol, etc.
- Sulfur dioxide
- Iodine
- Base - pyridine was used initially, but has since been replaced by imidazole,
which assures a higher rate of reaction and is less toxic

One of the most important developments in Karl Fischer titration chemistry was the replacement
of pyridine with imidazole. Pyridine is a relatively weak base and the equilibrium of the reaction
is not completely shifted toward the products. As a result, use of pyridine-based reagents typically
resulted in slow reactions and unstable endpoints. This, in turn, negatively impacted reproducibility
of measurements.

Imidazole is a stronger base and shifts the reaction equilibrium completely to the right, which
results in a faster titration rate and more stable endpoints.
TYPES OF KARL FISCHER TITRATION

Karl Fischer titration is based on the stoichiometric reaction of iodine with water.
There are two methods of introducing iodine into the sample - addition of molecular iodine in
a solution with a volume of a reagent (volumetric titration) and generation of iodine
electrochemically within the titration cell (coulometric titration).

Volumetric titration has the benefit of speed and generally lower cost, while coulometric
titration allows for accurate analysis of samples with low moisture levels.

The two main methods have their own variations and both make use of calibration standards
to determine titer of volumetric reagents and verify accuracy of measurements.

KARL FISCHER TITRATION

VOLUMETRIC TITRATION COULOMETRIC TITRATION

ONE-COMPONENT WITH DIAPHRAGM

TWO-COMPONENT WITHOUT DIAPHRAGM

CALIBRATION STANDARDS
VOLUMETRIC KARL FISCHER TITRATION

Volumetric determination of water can be done with one-component or two-component titration.

In one-component titration, all of the reactants needed for the reaction are present in a single
solution. Two-component titration uses two solutions - one of them is a solvent that contains
sulfur dioxide and the base and the other is the titrant, which is a solution of iodine in methanol
or another suitable solvent.

In both cases, the sample is dissolved in the solvent/

working medium and is contained in the titration cell.

The iodine-containing titrant is added, typically by

a dosing pump. The water content in the sample is
then calculated based on the volume of the reagent.

Commercial titrators normally automate the process

and integrate all components of the instrument.

WaveTrace™ line of reagents covers both one- and two-component titration methods.
Composit T5/T2/T1

Composit K5/K2

Methanol Sharp
Titrant K5/K2
Titrant T5/T2

Solvent CX
Solvent KT

Solvent D
Solvent C

Solvent F
Solvent

General one-component titration

General two-component titration
Aldehydes & ketones
Crude oil & petrochemicals
Biofuels
Oils, fats and long-chain hydrocarbons
Carbohydrates
VOLUMETRIC KF TITRATION

One-component titration is the most common, due to its simplicity and convenience.

Two-component titration may still be selected for its high rate of reaction, which produces
very sharp endpoints and highly reproducible results. In addition, Titrant T5 and T2 have
a virtually unlimited shelf life.
The titrants for two-component analysis must be used with WaveTrace™ Solvent to produce
the complete reaction medium.

Methanol is not the only solvent that can be used as working medium for one-component
titration, however the choices are limited. The solvent needs to be sufficiently polar and protic,
thus restricting selection to compounds with hydroxyl groups and high overall polarity.
In addition, they need to dissolve a variety of samples, which further limits available options.
As an example, methoxypropanol can be used as a solvent and it is the basis for Solvent KT.
However, the reaction rate is lower than in methanol and the endpoint is not as well defined.

These considerations need to be taken into account when attempting to modify the standard
solvents to work with difficult to dissolve samples. For instance, chloroform, long-chain alcohols
or aromatic solvents may be added. But the fraction of methanol should never be lower
than 40% by volume. If the concentration of methanol or similar solvent is too low, reaction
stoichiometry may change and produce incorrect results.

Composit T5/T2/T1 reagents can be used with dry methanol, but WaveTrace™ Methanol Sharp
is the preferred working medium. It has been formulated to accelerate the reaction rate and
produce exceptionally sharp titration endpoints.
VOLUMETRIC KF REAGENTS: TITER AND REAGENT CALIBRATION

Determination of water content using volumetric Karl Fischer titration is based on the
stoichiometric reaction between the components of the reagent and moisture contained in
the sample. Capacity of the reagent to react with water is expressed in terms of a titer,
or water equivalent (WE).
Titer indicates the amount of water that can be consumed by 1 ml of a given reagent.

Titer (WE) = Weight of water (mg) / Volume of reagent (ml)

For example, if a Composit T2 reagent has a titer of 2.1 it indicates that 1 ml of this reagent will
neutralize 2.1 mg of water present in a sample. When the titer is known, water content in
the sample can be easily calculated from the quantity of reagent used to achieve the endpoint.

WaveTrace™ Karl Fischer reagents are manufactured with several standard titers.
Composit series may have titers of 1, 2 and 5, which are indicated by the name of the specific
reagent (T1, T2 and T5, respectively). The general rule is that the greater the anticipated water
content in the samples, the higher titer reagent should be used. While theoretically Composit T5
could be used for any water determination, in practice it may be too coarse for accurate analysis
of samples with low moisture content.

The titration vessel must be kept as airtight as possible to avoid reagent reacting with ambient
moisture contained in the air, which reduces the titer and may cause fading of the endpoint
(especially for low titer reagents). It is also recommended that the titration system is flushed
with fresh reagent before the analysis, in order to eliminate the accumulated moisture in
the tubes and other components of the system.

Titer stability and reagent calibration

WaveTrace™ reagents are formulated for maximum stability and maintain their titer within
specification for long periods of time. However, once the bottle is opened and the reagent
is being used, ambient moisture diffuses into the bulk of the solution and consumes some
of the reagent. This process reduces the effective titer and requires a recalibration before use,
in order to assure accurate results.
There are two main methods of calibration, both based on providing a standard sample with
a known water content. The first method uses a water sample dissolved in an organic solvent
at a known concentration. The second method makes use of sodium tartrate dihydrate crystals
that contain 15.66% of water by weight.
TITER DETERMINATION WITH STANDARDS

Calibration of WaveTrace™ reagents using sodium tartrate dihydrate

Sodium tartrate dihydrate (STD) forms a stable complex with water at a known saturation
concentration (15.66%) and retains this level even at temperatures over 120 °C. Since STD
exists as crystals, it is easier to handle and can be weighed with high accuracy using standard
analytical balances. The quantity of STD normally depends on the anticipated titer of the reagent,
with greater amounts used for higher titers. For example, we recommend using 150 – 200 mg
of STD to calibrate Composit T5 reagent. Similarly, 75 – 100 mg of sodium tartrate dihydrate
would be more appropriate for calibration of Composit T2.

Once the crystals are weighed, they are added to the titration vessel. Care must be taken
to ensure that the crystals are fully dissolved before beginning the addition of the reagent.
At this point, the reagent is added as in a regular titration until the endpoint is reached.
The titer (water equivalent) is the calculated as follows:

Titer (WE) = Weight of STD (mg) * 0.1566 / Volume of KF reagent (ml)

Calibration of WaveTrace reagents using liquid water standards

Liquid water standards offer the same features as the sodium tartrate dihydrate. The STD
crystals can be weighed easily and do not require any special handling. However, sodium
tartrate dihydrate may not fully dissolve in some reagents, such as WaveTrace™ Solvent KT
and some third-party ethanol-based reagents. To avoid this issue, liquid water standards
represent a more reliable alternative.

Liquid water standards are typically available in sealed glass ampoules and have a known
water content, typically 10 mg per 1 g of the standard. The exact value is specified on
the Certificate of Analysis supplied with the standard.
The standard solution is added to the titration vessel using a syringe in exactly weighed 1 g
portions. Then the reagent is added and titration is carried out to the endpoint. The titer
(water equivalent) is calculated as follows:

Titer (WE) = Weight of sample (g) * Water content in CoA (mg/g)/Volume of KF reagent (ml)

The choice between the two types of standards should be made based on the reagents
being calibrated (solubility) and by experimentally comparing results from both methods.
Ideally, titer should be verified by titrating a standard water solution (such as SRM 2890
available from NIST) to make sure the it has been determined correctly.
VOLUMETRIC KF TITRATION WITH COMPOSIT T5/T2/T1

Composition
WaveTrace™ Composit is a line of one-component Karl Fischer reagents for volumetric
determination of moisture. The reagents include Composit T5, Composit T2 and Composit T1,
which differ by titer.
Any WaveTrace™ Composit reagent contains all components required for a stoichiometric
reaction of iodine with water in the sample. The components include sulfur dioxide, imidazole
and iodine dissolved in diethylene glycol monoethyl ether.
WaveTrace™ Composit reagents are formulated to retain their titer within specification for
at least 5 years. The typical loss of titer is less than 3% per year.

Recommended Solvents
WaveTrace™ Composit reagents can be used with any methanol-based solvent for a
stoichiometric reaction with reproducible results. For best results, WaveTrace™ Methanol Sharp
should be used, since it produces an exceptionally stable and easily detectable titration end point.
In some cases, a specialized solvent is required. For titration of aldehydes and ketones,
use WaveTrace™ Solvent KT. For titration of petrochemicals, biofuels and other samples
poorly soluble in methanol, use WaveTrace™ Solvent CX, Solvent C, Solvent D or Solvent F.
For specific selection criteria please refer to the table at the beginning of the volumetric titration
section.

Testing Procedure
The titration cell must be dry and free of residue from previous measurements.
1. Add WaveTrace™ Methanol Sharp or another suitable solvent to the titration cell.
The exact volume varies with the model of the instrument and ranges from 30 to 100 ml.
2. Fill the burette or connect the bottle with WaveTrace Composit reagent to the dosing pump.
3. Add WaveTrace™ Composit reagent until the end point is reached and the solution
in the titration cell contains no water.
4. Determine the exact titer of the reagent using a water or STD standard. Reach the endpoint.
5. Add a sample of known weight.
6. Titrate the water content with WaveTrace™ Composit T5/T2/T1 (depending on the
anticipated water content and application).
VOLUMETRIC TITRATION OF ALDEHYDES AND KETONES

WaveTrace™ K Series reagent line is formulated for analysis of samples containing aldehydes
and ketones. The distinctive characteristic of the K Series is the absence of methanol,
which is commonly used in other Karl Fischer reagents. The composition of the reagents
is formulated to prevent side reactions that occur in solutions of compounds with carbonyl
groups that can undergo a nucleophilic reaction with alcohols with the formation of
acetals (Figure 1). One of the products of this reaction is water, which interferes with the results
of analysis and causes instability of titration endpoints.

C H+
+ 2 CH3OH C OCH3 + H2O
O
OCH3
Figure 1. Formation of ketals in the reaction of acetone with methanol

WaveTrace™ K Series reagents are also designed to prevent bisulphite addition to

aldehydes (Figure 2). This second side reaction consumes a portion of the water content in
the neat specimen and results in erroneously low water content measurements.
WaveTrace™ Composite K5 is a variant of WaveTrace™ Composite T5, with a formulation for
a slightly slower reaction rate to minimize interference from the bisulphite addition side reaction.
H
R H
C
+ SO2 + H2O + NR+ R C SO3 HNR+
O
OH
Figure 2. Bisulphite addition to aldehydes that consumes water. NR+ is a base.

WaveTrace™ K Series of reagents for aldehydes and ketones includes solution systems for
both one-component and two-component KF titration:
• One-component system includes Composit K5 and Composit K2 reagents, as well as
Solvent KT. They are methanol-free and include components that inhibit side reactions.
• Two-component system includes Titrant K5 and Titrant K2, with Solvent KT suitable for
use with these reagents. This titration method allows for a more accurate moisture
determination with slower side reactions and a more stable titration end point.
TITRATION OF OILS, BIOFUELS AND OTHER SPECIAL SAMPLES

Methanol is the solvent of choice in the majority of situations. However, many samples have
low solubility in methanol, thus requiring a different solvent for volumetric titration. In order to
handle these situation, Quveon has developed a number of solvents that are ready for use
out of the box.
The naming convention adopted by Quveon helps easily identify the solvent required for a
particular application and is based on the first letter of the primary solvent(s) added to the
formulation.

WaveTrace™ Solvent C contains chloroform and should be used for titration of fuels and general
organic compounds with low methanol solubility.

WaveTrace™ Solvent C contains chloroform and xylene and is the best choice for crude oil,
heavy hydrocarbons, transformer oils and other similar samples.

WaveTrace™ Solvent D contains 1-decanol and is best suited for titration of oils, fats and long-
chain hydrocarbons. The most common applications are analysis of biofuels, plant oils and
fatty acids.

WaveTrace™ Solvent F contains formamide and is formulated for analysis of carbohydrates,

including glucose, starch, dextrin and other sugars.
IMPORTANCE OF pH

Karl Fischer reaction has an optimum pH range between 5 and 7.5. If pH is below 2, the reaction
does not occur at all. In very basic conditions, iodine is consumed in side reactions and leads
to false high values.

Since increased acidity leads to a drop in the reaction rate, acidic samples need to be neutralized.
Weak bases, such as imidazole, can be used to bring pH into a more neutral range.

In case of basic samples, weak organic acids are recommended. Benzoic or salicylic acids
are good choices. The exact amount will vary depending on the pH of the initial solution,
but can be between 5 and 15 g per 60 ml of solvent.
COULOMETRIC KARL FISCHER TITRATION

In coulometric KF titration, iodine is generated electrochemically and its amount is proportional

to the amount of electricity passed through the solution. Once the titration endpoint is achieved,
the current is turned off and the final amount of electricity is used to calculate how much iodine
was required for the titration.

Coulometric titration can be carried out in two types of

titration cells - with or without a diaphragm. Cells with
a diaphragm were developed first and used a semi-
permeable glass diaphragm to separate the volume
of the cell into two chambers containing the cathode
and the anode, respectively. The separation was used
to isolate the individual electrode processes and avoid
reduction of iodine back to iodide.

Cells without a diaphragm were developed in late 1980s. They allow for a simpler setup and
only require one reagent solution. In addition, a diaphragm introduces additional electrical
resistance, which can lead to overheating of the solution and limit the maximum current.
The specific material of the diaphragm can also impact the drift in the anode and cathode
chambers, thus adding a source of error.

Currently, cells without a diaphragm are more common. However, cells with a diaphragm have
the advantage of separation from the anode chamber where the samples are added. When
multiple samples have been added, the composition of the anolyte changes and can impact
reproducibility of consecutive measurements. In cells without a diaphragm, the anolyte reagent
has to be changed more frequently to avoid this issue. Cells with a diaphragm help alleviate this
problem for samples with very low moisture content that need to be added in large amounts.
REAGENTS FOR COULOMETRIC TITRATION

The basic reagent used in most of the coulometric titrations is WaveTrace™ Coulometric AGR,
which is the general purpose anolyte solution for titration with or without a diaphragm.
Different types of samples require different variants of the reagent, mainly to address solubility
issues.
In cells with a diaphragm, WaveTrace™ Coulometric CGR is required and can be used
with any of the specialized coulometric reagents, as listed in the table below.

Coulometric AGR-Oven

Coulometric CGR-AF7
Coulometric AGR-AF7
Coulometric AGR-Oil
Coulometric AGR-C
Coulometric AGR

Coulometric CGR
Coulometric AHR
General titration with a diaphragm
General titration without a diaphragm
Titration of biofuels and oils
Titration of crude oil and petrochemicals
Titration with oven
Titration of oils, fats and long-chain hydrocarbons
Titration with Orion AF7

The Coulometric CGR reagent has an increased viscosity compared to most existing reagents,
which leads to a much lower interference with the anolyte content across the diaphragm.
This feature translates into higher accuracy with a lower relative error of measurement.
WaveTrace™ Coulometric CGR is formulated to be non-hygroscopic and allows packaging
in 110 ml bottles instead of the traditional single-use ampoules.
COULOMETRIC TITRATION OF PETROCHEMICALS AND BIOFUELS

Samples containing oils, biofuels and petrochemicals usually have limited solubility in methanol.
Specialized reagents have been formulated to dissolve these samples and retain stoichiometry
of the reaction. For example, Coulometric AGR-C is an anolyte solution with added chloroform,
which is suitable for most oils and biofuels. It can be used with Coulometric CGR in cells
with a diaphragm.

In cases of crude oil or petroleum products, it is recommended to use Coulometric AGR-Oil,

which contains chloroform and xylene.

It should be noted that addition of non-polar solvents also increases electrical resistance
of the solution. Higher electrical resistance may lead to overheating of the solution when
titrations take a long time. Therefore, it’s important to pay attention to the temperature of
the cell and periodically allow it to cool in order to avoid excessive evaporation of the solution.

Samples containing crude oil, petrochemicals and biofuels most frequently are titrated in
cells with a diaphragm. It prevents the samples from contaminating the cathode and maintains
constant composition of the cathode solution. In cells without a diaphragm, these samples can
dramatically alter electrochemistry of the cell and negatively impact accuracy. This is especially
true for low moisture samples that have to be added in large amounts (up to 10 ml). After only
several measurements, the solution becomes so diluted by non-polar components that the
current efficiency at the cathode can substantially decrease and lower its performance.

Samples containing long-chain hydrocarbons can in some cases be dissolved in AGR-C and
AGR-Oil reagents. However, the best choice in this situation is Coulometric AHR, which is based
on decanol as the solubilizing agent. It ensures correct stoichiometry and optimal solubility
of the sample along with high current efficiency.
COULOMETRIC TITRATION WITH OVEN

In some cases, samples contain components that do not dissolve in any of the available
reagents or cause side reactions that are difficult to prevent. In this situations, titration
with oven can provide a good alternative.
A drying oven can also be used for samples that only release water at elevated temperatures,
such as plastics and resins.

The oven is used to elevate the temperature of the sample

and evaporate the moisture, which is then delivered to the
titration cell with a carrier gas. Since the moisture is delivered
in gas phase and at an elevated temperature, the reagent
must be able to dissolve it without boiling.

WaveTrace™ Coulometric AGR-Oven has been designed specifically for this purpose and
contains propylene glycol to control the boiling point of the reagent.
 TECHNICAL SUPPORT BY QUVEON

If you have technical questions about use of WaveTrace reagents or selection of the appropriate
reagent for your application, please contact our Technical Support team:

Phone: 1-800-471-3575
Email: support@quveon.com
Web: www.quveon.com

We can help you with any of the following:

- advice on reagent/titrator application
- custom reagents
- method development
- technical literature
- Safety Data Sheets
- regulatory questions

© 2014 All rights reserved. Quveon, Inc.