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KF Coulometry

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

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KF Coulometry

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  1. KF Coulometry

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

  3. Generator electrode Sensor electrodedouble platinum pin electrode Cathode Anode – + • Anolyte:sulfur dioxide, imidazol, iodide, different solvent for different applications:methonol or ethanol with chloroform, octanol, etylenglycol Catholyte Diaphragm • Catholyte:similar (or modified) solution Anolyte Titration Cell

  4. H O + I + SO + RN + ROH (RNH)SO R + 2(RNH)I Ù 2 2 2 4 Coulometry • Based on the same reaction as volumetric Karl Fischer Titration: - 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- I2 + 2 e- Anodic Oxidation

  5. + • AnodeIodine production by oxidation – 2 I- I2 + 2 e- • CathodeHydrogen production by reduction H2 2 H+ + 2 e- H+ H Side reaction:Reduction of sulfur components. After 1 - 2 weeks, smells like mercaptans. - - I I-  Change catholyte every week! Iodine Production

  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, 96484 C are required. Charles Augustin de Coulomb 14.6.1736 - 23.8.1806

  7. 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!

  8. – + - H+ H + + - - I I- - - - 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

  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 - 100 mg water/sample 1 ppm - 5 % water

  10. volumetry 100 % srel < 0.5 % Not suitable for coulometry 10 % 1 % 1000 ppm srel 5 - 0.5 % 100 ppm srel > 5 % 10 ppm Not suitable for volumetry 1 ppm Repeatability coulometry srel < 0.5 % srel 5 - 0.5 % srel > 5 %

  11. Cathode Anode – + 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 With stirring the level difference of anolyte and catholyte will be stable. Filling Titration Cell Catholyte:Fill in 5 mL catholyte. Anolyte: Fill in ~ 100 mL anolyte Catholyte Anolyte

  12. Cathode Anode – + If the catholyte level is higher or at the same level as the anolyte, there is a flow of moisture into the anolyte compartment. Filling Titration Cell Catholyte always contains water! Catholyte  High drift value Anolyte

  13. – – + + With or Without Diaphragm What are the differences?

  14. – – + + I- I I I Iodine is only in the anode compartment and reacts with water. It is possible that iodine can go to the cathode and convert to iodide. I- I- - - - With or Without Diaphragm With Diaphragm Without Diaphragm

  15. H+ H • high stirrer speed iodine reacts faster with water • high iodine production speed hydrogen protects cathode I I- - - It is possible that iodine can go to the cathode and convert to iodide. Without Diaphragm – 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.

  16. Easily reducible samples (nitrocompounds) get reduced, which produces water. R-NH2 + H2O R-NO2 I I- - - 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.

  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.

  18. 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 6 1.5 % 34 - 40 with diaphragm 19.6 ppm 6 5.7 % 39 - 43 without diaphragm

  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.

  20. + - Anolyte 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

  21. + - Anolyte 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)

  22. + - Anolyte 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

  23. 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

  24. + - Anolyte 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

  25. + - Anolyte 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!

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