Karl Fischer titration principle apparatus titration types Endpoint detection Hybrid titrations

1. Karl Fischer titration
Titration method in analytical chemistry that
uses coulometric or volumetric titration to
determine trace amounts of water in a sample.
Invented in 1935 by the German chemist Karl
Fischer.

2. The Karl Fischer Titration is based on an iodine / iodide reaction: The water
reacts with iodine. The endpoint of the titration is reached when all the water
is consumed
The process uses an organic base (B), sulphur dioxide, iodine and an
alcohol. The original Karl Fischer method used pyridine as organic base
and methanol as alcohol. Since then the reagents have been improved.
Nowadays Karl Fischer reagents are available which are less toxic (no more
pyridine but imidazole as organic base, ethanol instead of methanol) and
which provide a faster reaction.
During the titration, iodine is added to sample and the amount of iodine used
to consume all the water contained in the sample is measured.
karl fischer titration principle

3.  Karl fischer titration apparatus

4.  Coulometric titration
 Volumetric titration
Karl fischer titration
 Volumetric Karl Fischer Titration, a solution with an
exactly known concentration of iodine is added to sample
by means of an electric burette. The amount of iodine
added to the sample is calculated from the volume of
iodine solution used.

5. In the Coulometric Karl Fischer Titration the iodine is
electrolytically generated. The amount of idodine added to the sample is
determined by measuring the current needed for the electrochemical generation
of the iodine. When reacting with water, the brown iodine is reduced to the
colourless iodide.
 Hybrid Karl Fischer titration
It is basically a combination of both, the coulometric and the volumetric method:
The iodine is electrolytically generated and –if the moisture content of the
sample exceeds a certain level – a solution with an exactly know concentration of
iodine is added at the same time.

6. SO2 + CH3OH + B ↔ CH3SO3-+ HB+ CH3SO3-+ H2O + I2 + 2B →
CH3SO4-+ 2HB++ 2I-
 Karl Fischer titration Reaction
 Base(B):-Pyridine
Imidazole
 Alcohol:-Methanol
Ethanol

7.  Coulometric Titration
The main compartment of the titration cell contains the anode solution plus the
analyte. The anode solution consists of an alcohol (ROH), a base (B), SO2 and I2.
A typical alcohol that may be used is ethanol or diethylene glycol monoethyl
ether, and a common base is imidazole.
The titration cell also consists of a smaller compartment with a cathode immersed
in the anode solution of the main compartment. The two compartments are
separated by an ion-permeable membrane.
The Pt anode generates I2 when current is provided through the electric circuit.
The net reaction as shown below is oxidation of SO2 by I2. One mole of I2 is
consumed for each mole of H2O. In other words, 2 moles of electrons are
consumed per mole of water.
B·I2 + B·SO2 + B + H2O → 2BH+I− + BSO3
BSO3 + ROH → BH+ROSO3
−
The end point is detected most commonly by a bipotentiometric method. A
second pair of Pt electrodes are immersed in the anode solution. The detector
circuit maintains a constant current between the two detector electrodes during
titration. Prior to the equivalence point, the solution contains I− but little I2. At the
equivalence point, excess I2 appears and an abrupt voltage drop marks the end
point. The amount of charge needed to generate I2 and reach the end point can
then be used to calculate the amount of water in the original sample.

8.  Volumetric titration
The volumetric titration is based on the same principles as
the coulometric titration except that the anode
solution above now is used as the titrant
solution. The titrant consists of an alcohol (ROH), base
(B), SO2 and a known concentration of I2. Pyridine has been
used as the base in this case.
One mole of I2 is consumed for each mole of H2O. The
titration reaction proceeds as above, and the end point may
be detected by a bipotentiometric method as described
above.

9.  Endpoint detection
When reacting with water, the brown iodine is reduced to the colourless
iodide. At the endpoint of the titration when all the water is consumed the
colour of the solution turns increasingly from yellow to brown. As there is
no sharp colour change and the coloration differs in nonpolar solvents
(such as DMF) and polar solvents (as e.g. methanol) , it is not easy to
determine the endpoint of the titration visually.
For this reason, the endpoint of the titration is usually determined
electrometrically with a double platinum wire electrode.
There are two ways to electrometrically detect the
endpoint:
• Biamperometric indication
• Bivoltametric indication

10.  Biamperometric indication
A constant voltage of approximately 500 mV is applied to the wires of the
electrode and the resulting current is measured. As long as there is water in
the sample, no free iodine is present in the solution. When the endpoint of
the titration has been reached, the following reactions occur at the wires of
the electrode:
Cathode: I2 + 2e- → 2I-
Anode: 2I-- 2e- → I2
When the endpoint has been reached the current thus increases from
almost nil to a few μA.

11.  Bivoltametric indication
A small current (normally in the range of 1 … 50 μA) is applied between the
electrodes and the voltage required to maintain this current is measured. Normally
alternating current is used (AC) as it yields a higher sensitivity of the electrode
than direct current (DC). The voltage required to maintain the current is in the
range of several 100 mV as long as an excess of water is present in the sample.
When the endpoint of the titration is reached, free iodine is available in the solution
and the voltage drops to 100 mV or less. Normally, the endpoint potential level
(switch off voltage) must be selected according to the type of solvent being used
and/or the type of sample being titrated. The ideal switch off voltage depends on
the type of sample and solvent used. With normal Karl Fischer Titrators it must be
determined experimentally:
• If the switch off voltage is too low, too much iodine is added before
the endpoint is detected, the water contents yielded are too high.
• If the switch off voltage is too high, the titration does not start
automatically as no free iodine is required for this voltage to be
achieved.

12. Which method should be used,
coulometric or volumetric?
In general it can be said that the method has to be
chosen depending on the water content of the samples
to be measured:
• the coulometric method is suitable for samples with
a low water content (10 μg … 100 mg)
• the volumetric method is suitable for samples with
a higher water content (0.1 … 500 mg).

13. Which sample size should be used?
The suitable sample size depends on the desired degree of accuracy
the method used (coulometric, volumetric or hybrid)
the titer (water equivalent) or the Karl Fischer Reagent (for volumetric and
hybrid titrations)
When working with a Hybrid Titrator, the recommended sample
sizes are
the same as for coulometric titrations for low water contents (0.001 … 0.1 %)
the same as for volumetric titraitons for higher water contents (> 0.01%), as it
is advisable to work with bigger sample sizes in order to avoid weighing
errors.

14. Water content Volumetric Coulometric
[%] [ppm] WE = 2 WE = 5
0.001 10 > 25 g not recommen
ded
5 … 10 g
0.01 100 > 20 g not recommen
ded
1 … 5 g
0.1 1000 2 … 9 g 5 … 22.5 g 100 mg … 1 g
1 10000 0.2 … 0.9 g 0.5 … 2.25 g 10 mg … 100 mg
5 50000 40 … 180 mg 100 … 450 mg < 50 mg
10 100000 20 … 90 mg 50 … 225 mg < 50 mg
50 500000 not recommen
ded
< 50 mg not recommended
The table below gives a rough indication for the sample size to be used
in volumetric and coulometric titrations based on its estimated water
content.

15. • Volumetric titrations
To ensure maximum accuracy of the results the amount of titrant used should
(ideally) be between 20% and 90% of the burette volume. All volumetric Karl
Fischer Titrators from KEM are equipped with 10 mL burettes.
 The samples should thus ideally contain between 4 and 18 mg of H2O if the titer
of the reagent is 2 mg/mL or between 10 and 45 mg of water it the titer of the
reagent is 5 mg/ml.
• Coulometric titrations
The amount of water contained in the samples should be small for two reasons:
Many samples can be titrated without needing to replace the Karl Fischer
reagent.
Short analysis times.
For these reasons, the sample should ideally contain between 100 and 1000 μg of
H2O. To ensure reliable results, the samples should contain at least 50 μg of H2O
as always small traces of water can enter into the titration cell.

16. • Hybrid titrations
The sample size should be such that the samples contain between
50 μg and 18 mg of H2O if the titer of the reagent is 2 mg/L
50 μg and 45 mg of H2O if the titer of the reagent is 5 mg/L
Reagents for hybrid titrations
For hybrid titrations, three reagents are required:
• Anolyte (solvent in the titration cell)
• Catholyte (cathode compartment of the generator electrode)
• Volumetric titration reagent

17. • Reagents for volumetric titrations
Single component reagents
This type of reagent contains all reactants (iodine, sulfur dioxide, imidazole
and diethyleneglycol monoethyl ether). Since methanol has been replaced by
diethyleneglycol monoethyl ether, the titer of one component reagents became
quite stable. The loss of titer is approximately 5% per year. This type of reagent
is suitable for most volumetric Karl Fischer titrations, including the analysis of
ketones
Two component reagents
The reactants are in two separate solutions. The solvent is normally a solution of
sulphur dioxide and imidazole in methanol. The titrant is a solution of iodine
dissolved in a suitable solvent.
Faster titration speed.
More accurate results for samples with low water contents.
Exact titre with a high stability.
Higher buffer capacity.
Two component reagents contain methanol and are thus not suitable to analyse
samples which contain aldehydes and ketones with short chain lengths. Aldehydes
and Ketones react with the methanol contained in the Karl Fischer reagents: They
form acetals and water which leads to erroneously high results.
.

18. Special reagents for aldehydes and ketones
During the titration of aldehydes, a side reaction, the so called bisulfite
addition can occur. This side reaction consumes water and leads to erroneously
low results. For the determination of water in aldhydes and in certain reactive
ketones it is thus required to use this type of reagent.
There are two types of coulometric Karl Fischer reagents: Anolytes and
Catholytes. When working with a coulometer equipped with a generator
electrode with diaphragm (1) , the Anolyte (3) is filled into the titration
vessel whereas the Catholyte (2) is filled into the cathode compartment of
the generator electrode . For coulometers with a diphragmless generator
electrode, only one single reagent is required. It is important to make sure
that only reagents intended for diaphragmless electrodes are used.
 Reagents for coulometric titrations

19. CALCULATION FORMULA
%Water(w/w)=B.R. F. 100
Wt.100
B.R=KFR reagent vol. consumed in ml
F= KFR factor in mg/ml
Wt.=weight of sample in mg