💧 Moisture — general Karl Fischer method
In short
Determination of water content in chemical, pharmaceutical, cosmetic and petrochemical samples by Karl Fischer titration (volumetric or coulometric). The test is performed according to PN-EN ISO 760:2018; Volumetric range: 100 ppm – 100% H₂O. The procedure comprises 9 steps (about 44 min–1 h 2 min in total); it is used for: Pharmaceuticals — water content control in raw materials, active substances, drugs (European Pharmacopoeia), Petrochemicals — water in lubricating oils, fuels, solvents (ASTM D6304, D1744), Chemical industry — quality control of solvents, acids, bases.
At a glance
- Standard: PN-EN ISO 760:2018
- Category: Physicochemistry
- Procedure steps: 9
- Total time of stages: 44 min–1 h 2 min
- Volumetric range: 100 ppm – 100% H₂O
- Coulometric range: 1 ppm – 5% H₂O
- Accuracy: ±0.1–0.3% (volumetric), ±3 µg H₂O (coulometric)
Overview
The Karl Fischer (KF) method is the most common and most selective technique for determining water content in a wide range of materials — from organic solvents and oils, through pharmaceutical and cosmetic products, to food, polymers and building materials. The method developed in 1935 by German chemist Karl Fischer uses selective reaction of water with iodine in presence of sulfur dioxide and base in methanol medium.
Standard PN-EN ISO 760 describes the general Karl Fischer method in volumetric variant — used for samples with water content from 100 ppm to 100%. Coulometric variant (electrochemical iodine generation) is used for samples with very low water content — below 50–100 ppm (0.005–0.01%).
The key advantage of KF method is its selectivity — it reacts only with water, not with other volatile components. It is an absolute (stoichiometric) method, requiring only periodic verification of reagent titer. Modern KF titrators are fully automatic — with reagent dosing, bipotentiometric endpoint detection and calculations.
Analytical laboratories use KF method routinely: pharmaceuticals (quality control of raw materials and drugs), petrochemicals (water determination in oils, fuels, solvents), cosmetics, chemical industry, environmental laboratories (soil moisture, sediments).
Method principle
Karl Fischer reaction: H₂O + I₂ + SO₂ + 3 Base + CH₃OH → [Base·H]⁺·SO₄CH₃⁻ + 2[Base·H]⁺·I⁻. Water reacts stoichiometrically with iodine (1 mol H₂O = 1 mol I₂). In volumetric variant, KF reagent (containing iodine) is dosed from burette into sample dissolved in methanol. Endpoint — moment when small excess iodine persists in solution — is detected bipotentiometrically (double platinum electrode). In coulometric variant, iodine is generated electrochemically in situ from iodide in solution — electric charge is proportional to water amount (Faraday's laws).
Applications
- Pharmaceuticals — water content control in raw materials, active substances, drugs (European Pharmacopoeia)
- Petrochemicals — water in lubricating oils, fuels, solvents (ASTM D6304, D1744)
- Chemical industry — quality control of solvents, acids, bases
- Cosmetics — moisture in creams, emulsions, raw materials
- Food industry — water content in oils, fats, spices
- Polymers and plastics — granulate moisture before processing
- Building materials — moisture in cements, gypsum, mortars
- Electronics — purity of process solvents and reagents
Key parameters
| Parameter | Value |
|---|---|
| Volumetric range | 100 ppm – 100% H₂O |
| Coulometric range | 1 ppm – 5% H₂O |
| Accuracy | ±0.1–0.3% (volumetric), ±3 µg H₂O (coulometric) |
| Sample volume (volumetric) | 0.1–10 mL or 10–500 mg (solids) |
| Determination time | 2–10 min/sample |
| Working temperature | 20–25°C (titration cell) |
Standard
- Standard number
- PN-EN ISO 760:2018
- Title (PL)
- Oznaczanie zawartości wody — Metoda Karla Fischera (metoda ogólna)
- Title (EN)
- Determination of water — Karl Fischer method (General method)
Step-by-step procedure
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Titrator preparation
Turn on titrator, check reagent levels. Fill cell with working solvent (Hydranal Medium K). Run pre-titration to remove residual moisture.
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Titer verification
Inject 10–20 µL water standard (Hydranal Water Standard 10.0). Titrate. Check agreement with certified value ±3%.
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Sample preparation
Liquid samples — draw with syringe, weigh. Solid samples — weigh 50–500 mg, introduce through opening. Hard-to-dissolve samples — use KF oven (120–250°C).
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Sample introduction to cell
Inject liquid sample through septum or introduce solid through opening. Titrator automatically starts titration.
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Titration
Titrator doses Hydranal Composite 5 (volumetric) or generates iodine (coulometric). Magnetic stirring. Bipotentiometric detection.
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Result reading
Titrator automatically calculates water content: % wt. or ppm (mg/kg). Record result with sample identification.
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Duplicate
Repeat determination at least 2 times. Difference between duplicates: ≤10% relative value for low concentrations, ≤2% for high.
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Quality control
Every 10 samples verify titer with standard. Per series determine blank (drift correction). Maintain control chart.
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Cleaning and maintenance
After work empty cell, rinse with methanol. Replace septum. Seal openings. Replace molecular sieves in reagent bottle.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Volumetric KF titrator | Metrohm 888 Titrando, Mettler Toledo V30S, Hanna HI934 | 20 000–60 000 PLN |
| KF coulometer | Metrohm 917 Coulometer, Mettler Toledo C30S | 25 000–70 000 PLN |
| Evaporation oven (KF Oven) | Metrohm 860 KF Thermoprep, Mettler Toledo DO308 | 30 000–80 000 PLN |
| Double platinum electrode | Metrohm 6.0338.100, Mettler Toledo DM143-SC | 500–1 500 PLN |
| Analytical balance 0.01 mg | Mettler Toledo XPR205, Sartorius Cubis II | 15 000–50 000 PLN |
| Dosing syringes (liquid samples) | Hamilton, 1–10 mL, with Luer Lock connection | 200–800 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Hydranal Composite 5 (volumetric titrant) | — | Honeywell/Fluka, one-component, methanol-free, 1 mL = ~5 mg H₂O, 500 mL bottle |
| Hydranal Medium K (working solvent) | — | Honeywell/Fluka, medium for volumetric titration cell, 1 L bottle |
| Hydranal Coulomat AG (coulometric anolyte) | — | Honeywell/Fluka, anode solution for coulometer with diaphragm, 500 mL bottle |
| Hydranal Coulomat CG (catholyte) | — | Honeywell/Fluka, cathode solution for coulometer with diaphragm, 500 mL bottle |
| Water standard (Water Standard) | 7732-18-5 | Hydranal Water Standard 10.0 (10.0 mg H₂O/g), certified CRM for titer verification |
| Anhydrous methanol pure | 67-56-1 | For cell cleaning and as alternative solvent, H₂O content <0.01%, 1 L |
| Iodine (reagent component) | 7553-56-2 | Active component of KF reagent, stoichiometric reaction with H₂O (1:1 mol) |
| Sulfur dioxide (reagent component) | 7446-09-5 | SO₂ in KF solution, participates in Bunsen-Fischer reaction |
Health and safety (OHS)
- Methanol — toxic (inhalation, skin contact)! Work in fume hood, nitrile gloves
- KF reagents contain iodine and SO₂ — irritating, avoid vapor inhalation
- KF oven — temperature up to 300°C, risk of burns, do not touch heating elements
- Syringes with sharp needles — risk of puncture, sharps disposal container
- Safety goggles and laboratory apron mandatory
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 760:2018 — “Determination of water — Karl Fischer method (General method)”.
How does this method work?
Karl Fischer reaction: H₂O + I₂ + SO₂ + 3 Base + CH₃OH → [Base·H]⁺·SO₄CH₃⁻ + 2[Base·H]⁺·I⁻. Water reacts stoichiometrically with iodine (1 mol H₂O = 1 mol I₂).
What is the measuring range and accuracy?
Volumetric range: 100 ppm – 100% H₂O; Coulometric range: 1 ppm – 5% H₂O; Accuracy: ±0.1–0.3% (volumetric), ±3 µg H₂O (coulometric); Sample volume (volumetric): 0.1–10 mL or 10–500 mg (solids).
How long does the test take?
The times given for the individual stages add up to approximately 44 min–1 h 2 min. The procedure comprises 9 steps.
What equipment is required?
Volumetric KF titrator, KF coulometer, Evaporation oven (KF Oven), Double platinum electrode, Analytical balance 0.01 mg, Dosing syringes (liquid samples). Indicative cost of the core instrument (Volumetric KF titrator): 20 000–60 000 PLN.
Where is this test used?
Pharmaceuticals — water content control in raw materials, active substances, drugs (European Pharmacopoeia); Petrochemicals — water in lubricating oils, fuels, solvents (ASTM D6304, D1744); Chemical industry — quality control of solvents, acids, bases; Cosmetics — moisture in creams, emulsions, raw materials; Food industry — water content in oils, fats, spices; Polymers and plastics — granulate moisture before processing; Building materials — moisture in cements, gypsum, mortars; Electronics — purity of process solvents and reagents.
What safety precautions apply?
Methanol — toxic (inhalation, skin contact)! Work in fume hood, nitrile gloves; KF reagents contain iodine and SO₂ — irritating, avoid vapor inhalation; KF oven — temperature up to 300°C, risk of burns, do not touch heating elements; Syringes with sharp needles — risk of puncture, sharps disposal container; Safety goggles and laboratory apron mandatory.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 760.