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 Indication, control algorithm, termination parameters KF titration: important points Support

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

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

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

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

H2O + I2 + SO2 + 3RN + ROH ----->(RNH)SO4R + 2(RNH)I 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

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

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

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

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

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)

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.

Drift determination Drift determination 1 - 20 µg H20 / minute 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.

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

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

Coulometric KF Titration Titration cell and reagents Anode Cathode + – 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

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

1 C = 1 A • 1 s Absolute method, no standardization! Coulometry Theory 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 Absolute method, no standardization! Charles Augustin de Coulomb 14.6.1736 - 23.8.1806 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.

Filling the Titration Cell Catholyte: Fill in 5 mL catholyte. Anode + – Cathode Catholyte Anolyte Anolyte: Fill in ~ 100 mL anolyte 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 the Titration Cell Anode + – Cathode Catholyte always contains water! 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  High drift value Anolyte

With or Without Diaphragm What are the differences?

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

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

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

Titration cell easier to clean. Without Diaphragm 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 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.

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

Coulometry versus Volumetry Repeatability 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 % Not suitable for coulometry srel < 0.5 % srel 5 - 0.5 % srel > 5 %

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

KF Indication Principle (2/2) Ipol = 20µA U = 84mV 2 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 + - e I2 I2 + 2e- -> 2 I- 2 I- -> I2 + 2e- 2I-

KF Control: Titrator Algorithm 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% ) V/mL E/mV Control range EP KF Classical V/mL E/mV EP KF Fuzzy logic

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)

KF Control: Termination Parameters (2/3) Drift (µg/min) abs. drift stop = 30 µg/min EP 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)

KF Control: Termination Parameters (3/3) Drift (µg/min) t(s) Initial drift Rel. Drift stop = 20 µg/min 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

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

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