THE WORKING MEDIUM (KARL FISCHER METHOD)

The solvent
The working medium (i.e. the solvent required), can be freely chosen by the user
depending on the dissolution properties of the sample to be investigated. For
methanol-based reagents, the stoichiometry 1:1 of the Karl Fischer reaction is only
fulfilled if there is more than 25% methanol in the reaction mixture. A methanol-free
working medium can be used, however it is important to determine the titre of the
KF reagent in the same working medium.
The modern solvents available today present a high buffer and dissolution capacity.
These solvents consist of sulphur dioxide, a base and methanol or ethanol.
The main advantages of these solvents are:
• A more rapid titration due to better reaction kinetics; an advantage especially
for the titration of large amounts of water.
• A better reproducibility, because the reaction environment is stable. The pH
and the sulphur dioxide concentration remain constant.
For the titration of samples producing side reactions (aldehydes, ketones and
silanols), it is necessary to use an appropriate solvent. Most reagent manufacturers
include the letter K in the commercial name of such solvents.
Note:
If you are using an ethanol-based Karl Fischer solvent (example: E-Solvent) and
you have difficulties balancing your cell, you may need to immerse the Pt-Pt
electrode for 2 min. in a 10% v/v TritonX-100 solution once a week. Then rinse with
dry methanol and gently wipe. This treatment allows the electrode to recover full
efficiency after a few minutes operation. 10% v/v TritonX-100 is available from
reagent manufacturers or can be prepared by diluting 10 ml of TritonX-100 in 100
ml of deionised water.
The titrant
The titrant consists of iodine dissolved in methanol or ethanol. We often find that
the titrant has three titres 1, 2 and 5 mg of water per ml titrant. Even if it is possible
to perform a titration with more than one stroke of the burette piston, it should be
avoided by an appropriate reagent titre and choice of sample size. This allows the
titration time to be reduced and therefore improves the reproducibility.
The titration of samples producing side reactions (aldehydes, ketones and silanols)
requires an appropriate solvent.
The following table gives the recommended maximum speeds for given reagents
and solvents. However, conditions may be modified with respect to the additives,
solvents e.g. chloroform, or samples added.
If the titrator indicates an excess of iodine at the end of the titration, the burette
speed should be halved (a speed below 5 ml/min is rarely used). It should be
remembered that the titration time is not necessarily proportional to the rate of
reagent addition. It is recommended to adapt the addition rate so that it is proportional
to the speed of the Karl Fischer chemical reaction. Increasing the speed may lead
to a momentary excess of non reacted iodine which puts a stop to the reagent
addition. The titrator must therefore wait until this excess has been consumed
before continuing the reagent addition.
Water determination using Radiometer Analytical titrators
1) Filling the burette with titrant
Place the reagent bottle in the bottle holder (if mounted) and connect the
suction tubing from titrant bottle to stopcock. Add desiccant to the absorption chamber
mounted on the bottle. To prepare the titrant, use the titrator burette functions “Bottle
exchange” or “New titrant”.
Note: replace the desiccant when saturated.
2) Filling the KF cell with solvent
Place the solvent bottle in the bottle holder (if mounted) and connect the tubing
from the solvent bottle to the KF cell. Fill the desiccant tubes for KF cell and KF
pump with an appropriate desiccant, e.g. silica gel. Start the pump then using the
titrator solvent button, add between 30 and 40 ml of solvent to the titration cell.
Radiometer Analytical has marked the KF cell to indicate the minimum level to
which solvent must be added.
3) Mounting the waste bottle
Label and identify the waste bottle and place the bottle at the rear of the titrator.
Connect the tubings, KF cell to waste bottle and pump to waste
bottle. Make sure that the tubing is correctly connected to the pump module. A bad
connection could release liquid into the pneumatic module and cause severe
damage. Start the pump then use the waste button to empty the cell.
Reagent Solvent Manufacturer Recommended speed
Recommended
current
HYDRANAL®-Composit 5 Methanol Riedel-de-Hahn 150 %/min = 15 ml/min AC 50 μA
HYDRANAL®-Composit 5K HYDRANAL®-Solvent K Riedel-de-Hahn 50 %/min = 15 ml/min AC 50 μA
HYDRANAL®-Titrant 1 HYDRANAL®-Solvent Riedel-de-Hahn 150 %/min = 15 ml/min AC 50 μA
HYDRANAL®-Titrant 2 HYDRANAL®-Solvent Riedel-de-Hahn 150 %/min = 15 ml/min AC 50 μA
HYDRANAL®-Titrant 5 HYDRANAL®-Solvent Riedel-de-Hahn 150 %/min = 15 ml/min AC 50 μA
HYDRANAL®-Titrant 5 HYDRANAL®-Solvent CM Riedel-de-Hahn 150 %/min = 15 ml/min AC 50 μA
HYDRANAL®-Titrant 5 E HYDRANAL®-Solvent E Riedel-de-Hahn 15 ml/min AC 20 μA
HYDRANAL®-Titrant 2 E HYDRANAL®-Solvent E Riedel-de-Hahn 15 ml/min AC 20 μA
HYDRANAL-Composolver E Ethanol Riedel-de-Hahn
Karl Fischer Réagent T Karl Fischer reagent S Merck 150 %/min = 15 ml/min AC 50 μA
Karl Fischer Reagent 2.5 Karl Fischer reagent S Merck 150 %/min = 15 ml/min AC 50 μA
Karl Fischer Reagent 5 Pyridine Merck 50 %/min = 5 ml/min AC 50 μA

4) Pre-titration
Pre-titration allows the removal of traces of water introduced with the solvent. It is
only necessary if the stand by function is not used.
5) Sample introduction
The titrator is ready to start titrating when the message "Introduce sample" appears
on the display.
In general, at least 5 mg/ml is allowed. With a newly filled KF cell, it is possible to
titrate 5 x 35 = 175 mg of water.
6) Titrating the water
If "Autostart" has been activated, the titration will start as soon as the water in the
sample is detected. Otherwise the titration will start as soon as the ✓ key is pressed.
The titrator constantly determines the speed of titrant addition which is adapted to
the titration. The introduction or simple confirmation of the sample addition can be
carried out by the operator in his own time. In fact, the titration may have finished
before the sample amount is introduced. The titrator waits for the input of the sample
amount before calculating the final result.
7) Result calculation
The titrator calculates the water content of the sample. The drift measured from
titration start and if necessary the quantity of water introduced by the blank, the
dilution factor etc.are also taken into account during calculations.
At the same time, the titrator will determine whether or not the result falls within the
acceptance range specified by the user during programming. This allows the user
to determine if the water content conforms to the specifications and if the result can
be used for statistical purposes.
8) Solvent renewal
It is possible to perform successive titrations in the same solvent. However, it is
important to ensure that the methanol concentration is above 25% and that pH is
maintained within the range 5 to 7.
Although it is advisable to renew the solvent after each analysis, successive titrations
may be performed using the same solvent. Due to the fact that the quantity of
sulphur dioxide present in the cell is limited, care must be taken to respect the
quantity of water that can be analysed using the volume of solvent present in the
KF cell. For further information, consult the reagent manufacturer's instructions for
use.

9) Restarting the titration using a new aliquot or return to the start of
a menu
The KF titration cell will remain permanently on stand by i.e. ready for immediate
use.
10) At the end of a series of titrations, the following statistical
calculations are performed:
• Mean.
• Standard deviation.
Use of an oven
The oven is necessary when:
• The solvent does not allow a sufficient dissolution of the sample.
• The sample interacts with the working medium.
• The sample inhibits the response of the indicating electrodes.
The preparation steps 1 to 4 for the titration are identical to the conventional method.
Use a specific method based on the preprogrammed "Oven KF method".
Remember to pre-titrate the cell after having turned on the gas flow.
1. The titrator prompts you to weigh an "advised" amount of aliquot. The
approximate value of this aliquot has been entered in the titrator beforehand.
2. Introduce the sample in the oven's cold zone.
3. Enter the "exact" amount of sample weighed.
4. The titrator will determine the drift value.
5. Move the sample to the oven's hot zone.
6. The titration will start automatically if the option “Autostart” has been selected.
The titrator will display the result until the end of the titration.
7. Withdraw the sample from the oven.
8. Start a new titration with another aliquot or return to the menu. The cell will
remain in stand by condition, i.e. ready for a new titration.
9. At the end of a series of titrations statistical calculations are performed.
The user is guided through all the stages in the titration by the titrator's, clear and
concise messages. In this way the quality of the analyses is optimised.