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Determination of Water by the Karl Fischer Titration: Theory

Determination of Water by the Karl Fischer Titration: Theory. Program. Motivation Volumetric KF titration one an two-component reagents resolution and detection limits Coulometric KF titration cell with or without diaphragm resolution and detection limits

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Determination of Water by the Karl Fischer Titration: Theory

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  1. Determination of Water by theKarl Fischer Titration: Theory

  2. Program • Motivation • Volumetric KF titrationone an two-component reagentsresolution and detection limits • Coulometric KF titrationcell with or without diaphragmresolution and detection limits • Indication, control algorithm, termination parameters • KF titration: important points • Support

  3. Why measure water or moisture? Sugar: too much moisture will not flow Flour: too little moisture dust explosion Butter: max 16.5% water content by law Drugs: too much moisture decomposition Compact Disc: too much moisture bad music quality Brake Fluid: too much water brake do not work Kerosene: too much water blocked tubing

  4. Methods for the Determination of Water Drying oven Balance with IR /Halogen / Microwave heater Thermogravimetry / DSC Spectroscopy (IR, MS) Chromatography Karl Fischer Titration

  5. Karl Fischer Titration: Why? • Fast (e.g. 1...2 minutes) • Selective for water • Accurate and precise (0.3% srel) • Wide measuring range : ppm to % Coulometric KF Volumetric KF

  6. Karl Fischer German petrochemist,1901 – 1958 Publication:1935 Bunsen reaction: 2 H2O + SO2 + I2 = H2SO4 + 2 HIPyridine happened to be around in the Lab

  7. KF Titration • KF Reaction SO2 + RN + ROH ------> (RNH)SO3R a sulfite compound (RNH)SO3R + H2O + I2 + 2RN ------> (RNH)SO4R + 2(RNH)I a sulfate compound • Summary H2O + I2 + SO2 + 3RN + ROH ----->(RNH)SO4R + 2(RNH)I • The solvent (generally methanol) is involved in the reaction • A suitable base keeps the pH 5 - 7

  8. Solvent log K 4 side reactions optimal 2 0 slow pH 2 4 6 8 10 optimal pH 5 - 7 pH range buffer needed

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

  10. Volumetric KF Titration • Iodine is added by burette during titration. • Water as a major component: 100 ppm - 100 %

  11. Volumetric KF Titration • One - component reagent • Titrant:I2 , SO2, imidazole, methanol and diethylene glycol monoethyleter • Solvent:Methanol • Two - component reagent • Titrant:I2 and Methanol • Solvent:SO2, Imidazole, Methanol -> fast reaction, chemically stable, higher cost

  12. Volumetric KF Reagents • Titrant Concentration • 1-2-5 mg H2O/mL • Titer stability -----> Check by Standardization • Standardization materials Water 100% Sodium tartrate 15.66% Standard solution 5 mg/mL Water Standard 1% (10 mg/g)

  13. Air Humidity Air humidity: 0.5 - 3 mg water / 10 mL air Tropical countries: Air conditioning Well sealed titration cell Conditioning of the titration stand Protect titration stand, titrant and solvent from ingress of water.

  14. Drift determination The titration stand is not 100 % tight against air humidity. Drift determination The drift is the amount of water entering into the titration stand per minute. 1 - 20 µg H20 / minute Automatic drift compensation in the result calculation.

  15. Resolution and Detection Limit Volumetric Karl Fischer Titration Resolution of burette: 10,000 steps Detection limit : 50 x Resolution Burette size: 5 mL Titrant: 5 mg H20/mL Resolution: 2.5 µg H20/step Detection limit: 125 µg H20 For 5 g sample: 25 ppm Titrant: 2 mg H20/mL Resolution: 1 µg H20/step Detection limit: 50 µg H20 For 5 g sample: 10 ppm

  16. Coulometric KF Titration - + • Iodine is generated electrochemically during titration • Water in trace amounts: 1 ppm - 5 %

  17. Coulometric KF Titration Cathode Anode – + • Titration cell and reagents Generator electrode Double platinum pin electrode Anolyte (sulfur dioxide, imidazole, iodide, different solvent for different application - methanol, ethanol with chloroform, octanol, ethyleneglycol ) Catholyte (similar or modified solution) Diaphragm

  18. Coulometric KF Titration + – AnodeIodine production by oxidation 2 I- I2 + 2 e- Cathode H2 2 H+ + 2 e- H+ H - Side reaction:Reduction of sulfur components. After 1 - 2 weeks, smells like mercaptans Change catholyte every week! - I I- • Same reaction as volumetric KF Titration • but Iodine is produced just in time from iodide

  19. Coulometry Theory Charles Augustin de Coulomb 14.6.1736 - 23.8.1806 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 sAbsolute method, no standardization! • To produce one mol of a chemical compound, using one electron, 96484 C are required. • 2 I- ions react to form I2 which in turn reacts with water • 1 mol of water (18g) is equivalent to 2 x 96484 C or 10.72 C/mg water.

  20. Filling the Titration Cell 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. Catholyte  Low drift value With stirring the level difference of anolyte and catholyte will be stable. Anolyte Catholyte:Fill in 5 mL catholyte. Anolyte: Fill in ~ 100 mL anolyte

  21. Filling the Titration Cell 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. Catholyte always contains water! Catholyte  High drift value Anolyte

  22. With or Without Diaphragm What are the differences?

  23. With Diaphragm – – + + 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 Without Diaphragm

  24. Without Diaphragm H+ H I I- - - Iodine I2 can go to the cathode and convert to iodide. – + Prevention: • Small cathode surface less chance to contact iodine • high stirrer speed iodine reacts faster with water • high iodine production speed hydrogen protects cathode Only a little less accuratefor samples with very low water content. • bigger sample error has no effect

  25. Without Diaphragm – + R-NH2 + H2O R-NO2 H+ H I I- - - The hydrogen produced at the cathode is a very good reducing agent. Easily reducible samples (nitrocompounds) get reduced, which produces water.  too high result Not recommended for easily reducible samples: e.g. nitrobenzene, unsaturated fatty acids, etc.

  26. Without 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. Titration cell without diaphragm is the standard set-up for:Hydrocarbons, halogenated hydrocarbons, alcohols, esters, ethers, acetamides, mineral oils, edible oils, ethereal oils

  27. Resolution and Detection Limit + - Coulometric Karl Fischer Titration 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

  28. Coulometry versus Volumetry 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 %

  29. KF Indication Principle (1/2) Ipol = 20µA U = 650mV 2 • Bivoltametric indicationconstant current at the double platinum pin electrode ==> polarization current (Ipol) • During titration: • I2 reacts with water • no free I2 in the solution • high potential

  30. KF Indication Principle (2/2) Ipol = 20µA U = 84mV 2 e e + - I2 I2 2I- I2 + 2e- -> 2 I- 2 I- -> I2 + 2e- • At endpoint • all water has reacted with I2 • After the endpoint • free I2 in the solution • I2 is reduced to I- at the cathode • ionic conductivity occurs and the measured potential drops • potential change = endpoint

  31. KF Control: Titrator Algorithm KF Classical E/mV Control range EP V/mL KF Fuzzy logic E/mV EP V/mL • Karl Fischer Fuzzy Logic Control DL31/38 • No control band required (typical 300 mV) • The titrant addition rate depends on: • the distance to the endpoint EP • the potential change/increment • Advantages: • Simpler control: Only two control parameters Vmin , Vmax (smallest/largest increment) • Faster, more accurate, and better precision even at low water content(toluene: n = 5, 115 ppm, srel 0.17% )

  32. KF Control: Termination Parameters (1/3) • Delay time • the actual potential is lower than the EP for a defined time after the last titrant increment • typical delay : 15 - 20 sec • Note:Adapt the smallest increment to the drift and to the concentration of the titrant E (mV) EP 15 s t(s)

  33. KF Control: Termination Parameters (2/3) Drift (µg/min) EP abs. drift stop = 30 µg/min • Absolute drift stop • the actual drift is less then the predefined value • typical value : 30 g/min • Note:Adapt the value to the initial drift t(s)

  34. KF Control: Termination Parameters (3/3) Drift (µg/min) Rel. Drift stop = 20 µg/min Initial drift t(s) • Relative drift stop • the sum of the initial and the relative drift has been reached • typical value : 15 g/min • independent from the initial drift and of titrant concentration • ideal with side reactions that cannot be suppressed otherwise

  35. Karl Fischer Titration : Checks • Relevant points to be checked • System tightness : Check carefully • Ambient moisture : Drift determination • Stability of titrant : Standardisation • Side reactions : Check literature • Sample handling : Accuracy, precision • Free water only : Sample preparation

  36. Complete Solution : Solutions and Support • Application brochures • Internet databases www.titration.net

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