KF Coulometry

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METTLER TOLEDOVolumetric / Coulometric Titration

Volumetric Karl FischerIodine is added by burette during titration.Water as a major component: 100 ppm - 100 %

Coulometric Karl FischerIodine is generated electrochemically during titration.Water in trace amounts: 1 ppm - 5 %

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METTLER TOLEDO

Anolyte

• Anolyte:sulfur dioxide, imidazol, iodide, different solvent for different applications:methonol or ethanol with chloroform, octanol, etylenglycol

Titration Cell

Sensor electrodedouble platinum pin electrode

• Catholyte:similar (or modified) solution

Catholyte

AnodeCathode+–

Generator electrode

Diaphragm

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METTLER TOLEDOCoulometry

• Based on the same reaction as volumetric Karl Fischer Titration:

H2O + I2 + SO2 + RN + ROH (RNH)SO4R + 2(RNH)I

• But iodine will be produced just in time from iodide:

2 I- I2 + 2 e-

Anodic Oxidation

- Iodine reacts with water 1:1- The solvent methanol is involved in the reaction.- A suitable base keeps the pH 5 - 7

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METTLER TOLEDOIodine Production

+–

Side reaction:Reduction of sulfur components.After 1 - 2 weeks, smells like mercaptans.

Change catholyte every week!

• CathodeHydrogen production by reduction

H2 2 H+ + 2 e-

H+

-H

I--

I

2 I- I2 + 2 e-

• AnodeIodine production by oxidation

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METTLER TOLEDOCoulometry Theory

DefinitionOne 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

Charles Augustin de Coulomb14.6.1736 - 23.8.1806

To produce one mol of a chemical compound, using one electron, 96484 C are required.

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METTLER TOLEDOCoulometry Theory

DefinitionOne 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.

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 Cor

10.72 C / mg water.

Absolute method, no standardization!

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METTLER TOLEDOIodine Production Speed

The iodine production speed depends on:

Normal conductivity high current = 400 mA 2100 µg H2O/min

Warning: Low conductivity

+–

I- -I

H+

-H

-

--

+

-+

• surface of the electrode• voltage at the generator electrode• the conductivity of the electrolyte

Very low conductivity low current = 200 mA 1050 µg H2O/min

Influence to conductivity:Samples andadditional solvent as chloroform, etc.

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METTLER TOLEDOResolution and Detection Limit

Resolution: 0.1 µg water

Detection limit: 5 µg waterfor 5 g sample 1 ppm

Measuring range:10 µg - 100 mg water/sample1 ppm - 5 % water

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METTLER TOLEDORepeatability

srel > 5 %

srel 5 - 0.5 %

srel < 0.5 %

Not suitable for coulometry

coulometry

Not suitable for volumetry

srel > 5 %

srel 5 - 0.5 %

srel < 0.5 %

volumetry

1 ppm

10 ppm

100 ppm

1000 ppm

1 %

10 %

100 %

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METTLER TOLEDOFilling Titration Cell

Anolyte:Fill in ~ 100 mL anolyte

Catholyte:Fill in 5 mL catholyte.

Anode+–

Cathode

Catholyte

Anolyte

The level of the anolyte should be 3 - 5 mm higher than the level of catholyte sothat the flow is from the anolyte compartment to catholyte compartment.

Low drift valueWith stirring the level difference of anolyte and catholyte will be stable.

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METTLER TOLEDOFilling Titration Cell

Anode+–

Cathode

Catholyte

Anolyte

Catholyte always contains water!

If the catholyte level ishigher or at the same levelas the anolyte,there is a flow of moisture into the anolyte compartment.

High drift value

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METTLER TOLEDOWith or Without Diaphragm

What are the differences?

+– +–

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METTLER TOLEDO

With Diaphragm

+–

Without Diaphragm

I- -I

Iodine is only in the anodecompartment and reacts with water.

With or Without Diaphragm

+–

I- -I

-I I-

It is possible that iodine can go to the cathodeand convert to iodide.

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METTLER TOLEDOWithout Diaphragm

+–

I- -I

Prevention:

– bigger sample error has no effect

– high stirrer speed iodine reacts faster with water

– high iodine production speed hydrogen protects cathode

-

H+ H

It is possible that iodine can go to the cathode and convert to iodide.

– Small cathode surface less chance to contact iodine

Only a little less accurate for samples with very low water content.

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METTLER TOLEDOWithout Diaphragm

+–

I- -I

-H+ H

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.

wrong result (too high value)

Not recommended for easily reducible samples: e.g. nitrobenzene, unsaturated fatty acids, etc.

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METTLER TOLEDOWithout Diaphragm

– 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.

+ Titration cell easier to clean.+ Long-term drift value more stable.+ Only one reagent.+ Automation of emptying and refilling electrolyte.

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METTLER TOLEDOApplication

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

With and Without Diaphragm

Examples: Transformer oil

Mean n srel µg water /sample with or without diaphragm

16.3 ppm 6 1.5 % 34 - 40 with diaphragm19.6 ppm 6 5.7 % 39 - 43 without diaphragm

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METTLER TOLEDOWithout Diaphragm

• Hydrocarbons• Halogenated hydrocarbons• Alcohols• Esters• Ethers• Acetamides• Mineral oils• Edible oils• Ethereal oils

Titration cell without diaphragm is ideal for:

For this applications the titration cell without diaphragm is recommended.

+–

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METTLER TOLEDO

Different anolyte for different applications

For a complete water determination the sample must be completely dissolved in the anolyte.

Anolyte

+-

Sample not dissolved, emulsion: Too low result

Analyte

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METTLER TOLEDOAnalyte 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)

Anolyte

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METTLER TOLEDO

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

Anolyte

+-

Analyte for samples not easy to dissolve

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METTLER TOLEDOConductivity

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.

with diaphragm addition of max. 40 % to

CombiCoulomat frit higher robustness

without diaphragmaddition of max. 20 % toCombiCoulomat fritless

high current at generator electrode

limit 5 - 6 mS/cm before current breaks down

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METTLER TOLEDO

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

Analyte for samples difficult to dissolve

Anolyte

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METTLER TOLEDO

For cell with diaphragm(HYDRANAL Coulomat AK and CG-K) with a long chain alcohol instead of methanol

ketones and aldehydesreact with methanol ketal and acetal formation + 1 H2O

For cell without diaphragm(HYDRANAL Coulomat AK)

special reagent for ketones

Caution with aldehydes!Short chain aldehydes (for example acetaldehyde) will be oxidized at the anode. + 1 H2OLong chain aldehydes (for example benzaldehyde) are no problem!

Analyte for Ketons and Aldehydes

Anolyte

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