PowerPointPrsentation

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KF Coulometry
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Volumetric / Coulometric Titration
Volumetric Karl FischerIodine is added by
burette during titration. Water as a major
component 100 ppm - 100
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Titration Cell
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Coulometry

• Based on the same reaction as volumetric Karl
  Fischer Titration

- Iodine reacts with water 11 - 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-
I2 2 e-
Anodic Oxidation
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Iodine Production
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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, 96484 C are required.
Charles Augustin de Coulomb
14.6.1736 - 23.8.1806
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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 96484 C or 10.72 C / mg water.
To produce one mol of a chemical compound, using
one electron, 96484 C are required.
Absolute method, no standardization!
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Iodine Production Speed
The iodine production speed depends on

• 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 ? 2100 µg H2O/min
Very low conductivity ? low current 200
mA ? 1050 µg H2O/min
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Resolution and Detection Limit
Resolution 0.1 µg water
Detection limit 5 µg water for 5 g sample ? 1
ppm
Measuring range 10 µg - 100 mg water/sample 1
ppm - 5 water
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Repeatability
coulometry
srel lt 0.5
srel 5 - 0.5
srel gt 5
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Filling Titration Cell
CatholyteFill in 5 mL catholyte.
Anolyte Fill in 100 mL anolyte
Catholyte
Anolyte
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Filling Titration Cell
Catholyte always contains water!
Catholyte
? High drift value
Anolyte
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With or Without Diaphragm
What are the differences?
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With Diaphragm
With or Without Diaphragm
Without Diaphragm
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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

• bigger sample ? error has no effect

Only a little less accurate for samples with
very low water content.
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Without Diaphragm
The hydrogen produced at the cathode is a very
good reducing agent.


H
H
? wrong result (too high value)
Not recommended for easily reducible samples
e.g. nitrobenzene, unsaturated fatty acids, etc.
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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 (lt 50 µg/sample)
• Not recommended
• for easily reducible samples nitro compounds,
  unsaturated fatty acids, etc.

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Application
With and Without Diaphragm

• With out diaphragm
• a little bit less accuracy for very small water
  content (lt 50 µg/sample)

Examples Transformer oil
Mean n srel µg water /sample with or without
diaphragm 16.3 ppm 6 1.5 34 - 40 with
diaphragm 19.6 ppm 6 5.7 39 - 43 without
diaphragm
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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.
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Analyte
For a complete water determination the sample
must be completely dissolved in the anolyte.
Sample not dissolved, emulsion ? Too low result
Different anolyte for different applications
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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)
For cell without diaphragm ? with methanol
(HYDRANAL Coulomat AD) (apura - combiCoulomat
fritless)
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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
For cell without diaphragm (HYDRANAL Coulomat
AD) (apura - combiCoulomat fritless) add up to
20 octanol or decanol
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Conductivity

• 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 5 - 6 mS/cm before
current breaks down
with diaphragm addition of max. 40 to
CombiCoulomat frit higher robustness
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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 )
For cell without diaphragm (HYDRANAL Coulomat
AD) (apura - combiCoulomat fritless) add up to
30 chloroform
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Analyte for Ketons and Aldehydes
ketones and aldehydes react with methanol ketal
and acetal formation 1 H2O
? 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. 1 H2O Long chain aldehydes (for
example benzaldehyde) are no problem!