1. KF Coulometry

2. Volumetric / Coulometric Titration
Volumetric Karl Fischer Iodine is added by burette during titration.
Water as a major component: 100 ppm %
Coulometric Karl Fischer Iodine is generated electrochemically during titration.
Water in trace amounts: 1 ppm - 5 % + -
2018/11/9 AA

3. Titration Cell + – Generator electrode Diaphragm Anolyte
Anode
Cathode + –
Generator electrode
Diaphragm
Sensor electrode double platinum pin electrode
Anolyte
Anolyte: sulfur dioxide, imidazol, iodide, different solvent for different applications: methonol or ethanol with chloroform, octanol, etylenglycol
Catholyte: similar (or modified) solution
Catholyte
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4. 2 I- I2 + 2 e- Coulometry H O + I + SO + RN + ROH Ù (RNH)SO
Based on the same reaction as volumetric Karl Fischer Titration: H 2
O + I
+ SO
+ RN + ROH Ù
(RNH)SO 4
R + 2(RNH)I
- Iodine reacts with water 1:1
- The solvent methanol is involved in the reaction.
- A suitable base keeps the pH 5 - 7
But iodine will be produced just in time from iodide:
2 I-
I e-
Anodic Oxidation
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5. 2 I- I2 + 2 e- H2 2 H+ + 2 e- Iodine Production + – I- I
Anode Iodine production by oxidation
Cathode Hydrogen production by reduction H2
2 H e- H+ - H
Side reaction: Reduction of sulfur components.
After weeks, smells like mercaptans.
 Change catholyte every week!
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6. Coulometry Theory Definition
One Coulomb C is the quantity of charge transported by an electric current of one Ampere (A) during one second (s).
1 C = 1 A • 1 s
To produce one mol of a chemical compound, using one electron, C are required.
Charles Augustin de Coulomb
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7. Absolute method, no standardization!
Coulometry Theory
Definition
One Coulomb C is the quantity of charge transported by an electric current of one Ampere (A) during one second (s).
1 C = 1 A • 1 s
Two iodide ions react to iodine, which in turn reacts with water:
2 I–  I2 with H2O
Therefore 1 mol water (18 g) is equivalent to 2 x C or
10.72 C / mg water.
To produce one mol of a chemical compound, using one electron, C are required.
Absolute method, no standardization!
2018/11/9 AA

8. Iodine Production Speed
The iodine production speed depends on: + – I- - I H+ H
surface of the electrode
voltage at the generator electrode
the conductivity of the electrolyte
Influence to conductivity:
Samples and
additional solvent as chloroform, etc.
Warning: Low conductivity
Normal conductivity  high current = 400 mA  µg H2O/min
Very low conductivity  low current = 200 mA  µg H2O/min
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9. Resolution and Detection Limit+ -
Resolution: 0.1 µg water
Detection limit: 5 µg water
for 5 g sample  1 ppm
Measuring range:
10 µg mg water/sample
1 ppm - 5 % water
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10. Repeatability coulometry volumetry srel < 0.5 % srel < 0.5 %
Not suitable
for volumetry
srel > 5 %
srel %
srel < 0.5 %
volumetry
1 ppm
10 ppm
100 ppm
1000 ppm
1 %
10 %
100 %
Not suitable
for coulometry
srel < 0.5 %
srel %
srel > 5 %
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11. Filling Titration Cell
Anode + –
Cathode
Catholyte: Fill in 5 mL catholyte.
Anolyte:
Fill in ~ 100 mL anolyte
The level of the anolyte should be
3 - 5 mm higher than the level of catholyte so
that the flow is from the anolyte compartment
to catholyte compartment.
 Low drift value
Catholyte
With stirring the level difference of anolyte
and catholyte will be stable.
Anolyte
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12. Filling Titration Cell
Anode + –
Cathode
Catholyte always contains water!
If the catholyte level is
higher or at the same level
as the anolyte,
there is a flow of moisture
into the anolyte compartment.
Catholyte
 High drift value
Anolyte
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13. With or Without Diaphragm+ – + –
What are the differences?
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14. + – + – With or Without Diaphragm With Diaphragm Without Diaphragm I-
It is possible that iodine can go to the cathode
and convert to iodide. I- - I
Iodine is only in the anode
compartment and reacts
with water.
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15. Without Diaphragm
It is possible that iodine can go to the cathode and convert to iodide. – +
Prevention:
Small cathode surface  less chance to contact iodine - H+ H
high iodine production speed  hydrogen protects cathode
high stirrer speed  iodine reacts faster with water I- - I
Only a little less accurate for samples
with very low water content.
bigger sample  error has no effect
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16. – + Without Diaphragm The hydrogen produced at the cathode
is a very good reducing agent. – +
R-NO2
R-NH2 + H2O
Easily reducible samples (nitro compounds) get reduced, which produces water. H+ H - I- - I
 wrong result (too high value)
Not recommended for easily reducible samples: e.g. nitrobenzene, unsaturated fatty acids, etc.
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17. Without Diaphragm Titration cell easier to clean.
Long-term drift value more stable.
Only one reagent.
Automation of emptying and refilling electrolyte.
A little bit less accuracy for very small water content (< 50 µg/sample)
Not recommended:
for easily reducible samples: nitro compounds, unsaturated fatty acids, etc.
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18. With and Without Diaphragm
Application
With and Without Diaphragm
With out diaphragm:
a little bit less accuracy for very small water content (< 50 µg/sample)
Examples: Transformer oil
Mean n srel µg water /sample with or without diaphragm
16.3 ppm % with diaphragm
19.6 ppm % without diaphragm
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19. – + Without Diaphragm Titration cell without diaphragm is ideal for:
Hydrocarbons
Halogenated hydrocarbons
Alcohols
Esters
Ethers
Acetamides
Mineral oils
Edible oils
Ethereal oils + –
For this applications the titration cell without diaphragm is recommended.
2018/11/9 AA

20. Different anolyte for different applications
Analyte
For a complete water determination
the sample must be completely dissolved
in the anolyte.
Anolyte + -
Sample not dissolved, emulsion:
 Too low result
Different anolyte for different applications
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21. Analyte for samples easy to dissolve
alcohols, ethers, esters, hydrocarbons,
halogenated hydrocarbons, nitro components, etc.
For cell with diaphragm
 with methanol (HYDRANAL Coulomat AG) (apura - combiCoulomat frit)
 with ethanol (HYDARANAL Coulomat E)
Anolyte + -
For cell without diaphragm
 with methanol (HYDRANAL Coulomat AD)
(apura - combiCoulomat fritless)
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22. Analyte for samples not easy to dissolve
edible oils, ethereal oils, ointments, etc.
 with methanol and octanol
For cell with diaphragm
(HYDRANAL Coulomat AG-H) (with 20 % hexanol)
(apura - combiCoulomat fritless)
add up to 40 % octanol or decanol
Anolyte + -
For cell without diaphragm
(HYDRANAL Coulomat AD)
(apura - combiCoulomat fritless)
add up to 20 % octanol or decanol
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23. Conductivity influences
Generation of iodine
Indication of the endpoint
Conductivity of electrolyte decreases during determination
long chained alcohols (hexanol, octanol, decanol), xylene or chloroform can be added.
without diaphragm
addition of max. 20 % to
CombiCoulomat fritless
high current at generator electrode
limit mS/cm before current breaks down
with diaphragm
addition of max. 40 % to CombiCoulomat frit
higher robustness
2018/11/9 AA

24. Analyte for samples difficult to dissolve
mineral oils, transformer oil, silicon oils, etc
 with methanol and chloroform
For cell with diaphragm
(HYDRANAL Coulomat A) (with 20 % chloroform)
(HYDRANAL Coulomat AG) (without chloroform)
(apura - combiCoulomat frit) (without chloroform)
add chloroform (maximum 50 %)
Anolyte + -
For cell without diaphragm
(HYDRANAL Coulomat AD)
(apura - combiCoulomat fritless)
add up to 30 % chloroform
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25. Analyte for Ketons and Aldehydes
ketones and aldehydes
react with methanol
ketal and acetal formation H2O
Anolyte + -
 special reagent for ketones
For cell with diaphragm
(HYDRANAL Coulomat AK and CG-K) with a long chain alcohol instead of methanol
For cell without diaphragm
(HYDRANAL Coulomat AK)
Caution with aldehydes!
Short chain aldehydes (for example acetaldehyde) will be oxidized at the anode H2O
Long chain aldehydes (for example benzaldehyde) are no problem!
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