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Karl Fischer Titration

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Presentation on theme: "Karl Fischer Titration"— Presentation transcript:

1 Karl Fischer Titration
Regina Schlink Metrohm Ltd, Herisau

2 Topics KF reaction Volumetric KF Titration Coulometric KF Titration
Endpoint indication Drift as second endpoint indication Parameters KF Instruments

3 Topics KF reaction Volumetric KF titration Coulometric KF titration
Endpoint indication Drift as second endpoint indication Parameters KF instruments

4 Method for water determination
in technical products (oil, plastics and gases) in cosmetic products in pharmaceutical products in food industry In addition to determination of pH, weighing, acid-base titration, determination of water-content is one of the most frequently used methods in laboratories world wide. In 1935 KF published his “New Method for the Determination of water…”. Many chemists have since been occupied with this reagent. It is used in various industries, from quality control of raw materials to water determination in pharmaceutical products.

5 The KF reaction I. CH3OH + SO2 + RN  [RNH]SO3CH3
II. H2O + I2 + [RNH]SO3CH3 + 2RN  [RNH]SO4CH3 + 2[RNH]I (RN = Base) SO2 + ROH -> ester, which is neutralized by the base. Anion of alkyl sulphurous acid = reacting compound, already present in the KF reagent. Titration: Oxidation SO32- -> SO42- by I2, consumes water. Stoichiometric reaction suitable ROH to esterify SO2 completely Suitable base for complete neutralization pH 5-7 preferred (optimized by imidazole) pH adjustment of prime importance => reaction rate is constant Reaction only takes place as long as sample releases water

6 Basic ingredients of KF reagents
Iodine Sulphur dioxide Buffer Solvent I2 SO2 Imidazole Methanol Reagents needed to carry out KFT Water comes from the sample, other components of reaction from reagent

7 Optimum: pH range between 5 and 7
pH dependency log K < pH 5: log K increases linearly with pH reaction rate to low (low results) 5.5-7: optimum, reaction rate constant >8: reaction rate increases slightly (side reactions probably occur) pH Optimum: pH range between 5 and 7

8 Topics KF reaction Volumetric KF titration Coulometric KF titration
Endpoint indication Drift as second endpoint indication Parameters KF instruments

9 KF titration methods Volumetric KF titration Coulometric KF titration
Volumetric -> classical titration, I2-containing titrant is added via buret Coulometric -> titrant is produced from I- in solution by anodic oxidation H20 calculated out of applied current Both methods: -titration cell as impervious as possible (atmospheric water!) -water adhered to wall of cell -> removed by swirling conditioned solvent Working medium & titrant Iodine is generated in titration cell (anodic oxidation)

10 Volumetric KF titration step by step
Fill titration vessel with solvent Pretitration with KF reagent Add the sample Titrate with KF reagent Volumetric Higher (absolute) water contents Solid or pasty samples can be directly introduced Variety of suitable solvents, specifically adapted Lower detection limit ( ppm) No stable titre -> determination of titre regularly Why pretitration? O-ring and molecular sieve -> change regularly (in general 6 weeks, depends on air-humidity)

11 Volumetric KF reagents
One component reagents Titrant contains iodine, sulphur dioxide, buffer and methanol/ethanol Working medium contains only the methanol/ethanol Disadvantage: the titre decreases 5% per year in the closed bottle! Two component reagents Titrant contains iodine Solvent contains buffer and sulphur dioxide Advantages: pH optimum in the solvent / fast reaction / titre is very stable Two possibilities for working medium and titrant 1-Component: flexibility regarding solvent additives for improving solubility of sample Titrant 5 mg/mL: 5 mg H2O can react with 1 mL Titre determination: Water Std 10 recommended, Na-tartrat not as reproducible (bad solubility in methanol) dest water possible, but very tiny amount, good practice needed

12 Topics KF reaction Volumetric KF titration Coulometric KF titration
Endpoint indication Drift as second endpoint indication Parameters KF instruments

13 Coulometric KF titration step by step
H+ H I I- - + current is our burette iodine is produced from a iodide containing solvent by anodic oxidation generating current is switched off as soon as a slight excess of free iodine is present free iodine is indicated by a double platinum electrode Not a classical titration -> “electronic buret” Same chem. process as volumetric KFT: 1 H2O consumes 1 I2 I2 generated electronically -> absolute method -> no titre Instrument measures time + current flow to EP -> prop. I2 -> prop. H2O Primarily for small amounts of water Metrohm: 10 g – 200 mg H2O, resolution 0.1 g

14 Coulometric KF titration
I2 produced by generator electrode, 2 different types: Cell without diaphragm - easy to handle and clean - only 1 reagent (!!!Use only reagents intended for these electrodes!!!) - electrode rapidly ready for use (no water on diaphragm) - no cathode compartment -> reduction on Pt plate b) Cell with diaphragm - ketones/aldehydes in sample (special reagents available only for this type) - reagents with low conductivity - lower trace range Cell with diaphragm Cell without diaphragm

15 How to fill coulometric cell with diaphragm?
Catholyte 5 mL Reduction: 2 H+ +2 e- = H2 change catholyte weekly! Anolyte: about 100 mL 2 I- = I2 + 2 e- Reagents anolyte -> titration vessel catholyte -> cathode compartment same level of both (prevents pressure compensation) EP: double Pt electrode Absolute -> no titre determination Check from time to time with certified standard

16 Comparison with and without diaphragm
With diaphragm Recommendable for most applications, sample should have a good solubility in alcohol Reagents with low conductivity (the addition of chloroform or xylene > 10%), with ketone reagents absolute water content < 50 ppm Generator I: 400 mA Generator I: auto With diaphragm low water content, conductivity Anode + cathode compartment separated to avoid interferences Allows application of lower current densities of A

17 Coulometric KF reagents
Capacity of more than 1000 mg of water (100 mL KF reagent) Anodic and cathodic reagents Combined reagents Special reagents for ketones 2 reagent solutions necessary for cell with diaphragm: anolyte: modified KF reagent, containing I- instead of I2 KF reaction occurs in anolyte catholyte: enables counterpart cathode reaction which must proceed so that the by-products of the KF-reaction produced at the cathode are not disturbed Combined reagents: Merck reagents and Coulomat E, in both compartments same solution Keton reagents <- side reactions

18 Which is the right method for my application?
Volumetric titration range of application 0.1 % % depends on sample size Coulometric range of application % - 1 % (10 µg mg absolute water content), mainly liquids and gases

19 Topics KF reaction Volumetric KF titration Coulometric KF titration
Endpoint indication Drift as second endpoint indication Parameters KF instruments

20 Endpoint indication Bivoltametry Ipol = 50 uA
Constant current applied to double Pt electrode During titration: Excess H2O  High voltage between Pt wires Ipol: Small current is applied to between the electrodes (AC much more sensitive than DC) and resulting voltage is measured. As long as there is H2O-> high voltage is applied (I2 -> I-) End of titration: reaction turnover is decreasing (small excess of free I2) -> strong decrease of voltage EP is determined by entering switch-off voltage. Reagent addition stops when reached At end of titration: Small excess of free iodine  Voltage decreases sharply

21 Endpoint indication Voltage: High as long as H2O in sample + I2 added
Once reaction turnover decreases, potential decreases End point is not sufficient as criteria (titration would stop at signal at 88 sec) -> Additionally drift as EP criteria Drift <- water entering system

22 Topics KF reaction Volumetric KF titration Coulometric KF titration
Endpoint indication Drift as second endpoint indication Parameters KF instruments

23 permanent consumption of KF reagent
 Drift Aim: constant and low drift Optimal: Volumetry <10 L/min Coulometry g/min Influence on the results  drift correction Drift correction: 20 ug/min drift and 1 min measurement, Sample 10 ug => 30 ug => correction necessary Drift higher than sample -> lower drift recommended for trace analysis Optimal drift is for europe, depends on humidity, air condition might be good in some countries for trace analysis

24 Drift Start drift – acceptable drift value for start of determination (cond. OK) Stop criteria drift – absolute or relative drift value Absolute drift– the entered value is the stop drift Relative drift – the stop drift is calculated from the measured drift value (start) and the entered value

25 Topics KF reaction Volumetric KF titration Coulometric KF titration
Endpoint indication Drift as second endpoint indication Parameters KF instruments

26 Titration parameter Volumetry Coulometry EP at U 250 mV
Dynamics mV Stop criteria drift/time Stop drift 20 uL/min Delay time 10 s I(pol): uA Coulometry EP at U mV Dynamics 70 mV Stop criteria drift/rel drift Stop drift 5 ug/min Rel. Drift 5 ug/min Start drift 20 ug/min I(pol) : uA Standard parameters Dynamics: near to EP -> add smaller amount Dynamics 100: from 350 smaller steps (volumetry), 120 (coulometry) Special parameters: Coulometer: Ketone reagents Volumeter: Ethanol reagents

27 Topics KF reaction Volumetric KF titration Coulometric KF titration
Endpoint indication Drift as second endpoint indication Parameters KF instruments

28 Volumetric KF Titration
Titrandos: 841 Titrando PC Control / tiamo Touch Control Titrinos: 787 KF Titrino 795 KFP Titrino

29 Volumetric KF Titration
Modi 841 Titrando KFT X SET Meas High-end titrator for up to four 800 Dosino dosing systems. Karl Fischer titration (KFT) and endpoint titration (SET), pH measurement (MEAS), liquid handling. With four MSB connections, one measuring interface; USB connection.

30 Only a version with dosing units ?
Advantages: The dosing unit can be totally emptied that means: No crystallisation in cock and tubings Rinsing the buret several times after changing the titrant is no longer necessary Direction downwards -> no air-bubbles dosed Possibility to empty totally -> saves reagents

31 Coulometric KF titration
756 Coulometer with internal printer 831 Coulometer Work relatively precisely Both available with generator electrodes with or without diaphragm Differences: Printer included 756 Optional: Automatic reagent changer (after a defined time)

32 Principle of the oven technique
Many substances release water slowly or at elevated temps -> not suitable for direct KFT Further problem: low solubility of sample in alcohol, side reactions of other substances => Substance heated in oven, locate upstream from cell Released water is transferred by flow of dry carrier gas to titration cell, where KFT takes place => Only water enters cell => No unwanted side reactions + matrix effects and contaminations Coulometric cell Vial placed in the oven

33 832 Thermoprep 774 Oven Sample Processor
Oven for volumetric and coulometric KFT 832 -> stand alone 774 -> automated

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