💧 Water Content by Karl Fischer Method
Determination of water in petroleum products by coulometric Karl Fischer titration method in range 0.003–0.100% (m/m).
Overview
Water is one of the most undesirable contaminants in petroleum products. Even small amounts of water (several dozen ppm) can cause corrosion of metal components, degradation of performance additives, increased microbiological growth, deterioration of oil lubricity and damage to precision elements of engine injection systems.
The Karl Fischer method is the gold standard for water determination in petroleum products. In the coulometric variant, the iodine needed for reaction with water is generated electrochemically at the anode — it is not added from a burette as in the volumetric variant. This makes the coulometric method much more sensitive and suitable for determining very small amounts of water (from 10 µg, i.e., approx. 10 ppm).
The method basis is the Karl Fischer reaction: I₂ + SO₂ + H₂O → 2HI + SO₃, in presence of base (imidazole) and alcohol (methanol). One mole of iodine reacts stoichiometrically with one mole of water. Iodine is generated coulometrically according to Faraday's law — water amount is proportional to total electrical charge (1 mg H₂O = 10.71 coulombs). End of titration is detected electrometrically (biamperometric detection of excess free iodine).
The method is not suitable for products containing ketones (react with KF reagent) or heavy residual fuels. Interferences may be caused by hydrogen sulfide and mercaptans, but at concentrations below 0.003% (m/m) as sulfur they do not significantly affect the result.
Method principle
Weighed sample is introduced into reaction vessel containing Karl Fischer anolyte reagent (solution of SO₂, imidazole and methanol). Iodine is generated electrochemically at platinum anode. Generated iodine immediately reacts with water present in sample per reaction: I₂ + SO₂ + 3C₅H₅N + CH₃OH + H₂O → 2C₅H₅NH·I + C₅H₅NH·SO₄CH₃. When all water is bound, excess free iodine appears, detected by biamperometric end-point detector (double platinum electrode). Water amount is calculated from total electrical charge used to generate iodine, according to Faraday's law.
Applications
- Quality control of diesel — max 200 mg/kg water (PN-EN 590)
- Testing lubricating oils — new oil typically <100 ppm water
- Monitoring hydraulic and transformer oils in service
- Quality control of jet fuel Jet A-1 — max 100 mg/kg
- Testing base oils and performance additives
- Quality control of gasolines and fatty acid methyl esters (FAME)
- Fuel receipts at storage depots and terminals
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0.003–0.100% (m/m), i.e., 30–1000 mg/kg |
| Sensitivity | ~10 µg H₂O (0.001 mg) |
| Repeatability | ±0.0005% (m/m) or ±5 mg/kg |
| Reproducibility | ±0.0020% (m/m) or ±20 mg/kg |
| Sample volume | 0.1–5 g (dependent on water content) |
| Determination time | 3–10 min per sample |
Standard
- Standard number
- PN-EN ISO 12937:2005
- Title (PL)
- Przetwory naftowe — Oznaczanie wody — Metoda miareczkowania kulometrycznego Karla Fischera
- Title (EN)
- Petroleum products — Determination of water — Coulometric Karl Fischer titration method
Step-by-step procedure
1. Apparatus preparation
Fill measurement cell with fresh anolyte reagent (and catholyte if cell with diaphragm). Turn on generator and detector. Pre-dry reagent to anhydrous state.
2. Conditioning
Start conditioning — instrument automatically generates iodine to remove residual water from reagent. Wait for drift stabilization (<10 µg/min).
3. Verification with standard
Inject known amount of water standard (e.g., HYDRANAL Water Standard 1.0). Recovery should be 98–102%. If not — replace reagents or electrodes.
4. Sample preparation
Weigh sample on analytical balance (to 0.1 mg accuracy). Typical sample size: 0.5–5 g, dependent on expected water content. Record mass.
5. Sample introduction
Inject sample into measurement cell through septum using syringe. Avoid contact with moist air. Syringe weight before and after = sample mass.
6. Titration
Instrument automatically generates iodine and titrates water. End of titration detected biamperometrically. Time depends on water amount — typically 3–10 min.
7. Result readout
Instrument displays result in µg water. Convert to mg/kg or % (m/m) accounting for sample mass: W [mg/kg] = (µg H₂O) / (sample mass [g]).
8. Repeat determination
Perform minimum two determinations. Check agreement of results with standard repeatability.
9. Quality control
Every 10 samples (or per series) measure water standard. Maintain Shewhart control chart for QC results.
10. Cleaning and maintenance
After series completion replace reagent if exhausted (color change). Clean cell with anhydrous methanol. Store electrodes in anhydrous methanol.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Karl Fischer coulometer | Metrohm 899 Coulometer, Mettler Toledo C30S | 25 000–55 000 PLN |
| Coulometer with evaporation oven | Metrohm 860 KF Thermoprep + 899, Mettler Toledo C30S + DO308 | 60 000–100 000 PLN |
| Titration vessel with electrodes | Generator cell with/without diaphragm, double Pt electrode | 2 000–5 000 PLN |
| Sample dosing syringe | Glass syringe 1–10 mL with Luer valve, Hamilton | 300–800 PLN |
| Analytical balance | Mettler Toledo XPR205, Sartorius Quintix 224, accuracy 0.1 mg | 8 000–25 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Anolyte reagent (anolyte) | — | HYDRANAL Coulomat AG (methanol-based) or HYDRANAL Coulomat AG-H (for long-chain hydrocarbons), Honeywell/Fluka, 500 mL bottle |
| Catholyte reagent (catholyte) | — | HYDRANAL Coulomat CG, for cathode chamber (in cells with diaphragm), Honeywell/Fluka, 500 mL bottle |
| Water standard (CRM) | — | HYDRANAL Water Standard 1.0 or 0.1, certified water content for equipment verification |
| Anhydrous methanol | 67-56-1 | Purity ≥99.8%, max 0.01% water, for cell cleaning and as solvent |
| Chloroform (optional) | 67-66-3 | For dissolving oil samples poorly miscible with methanol |
| Drying gas (N₂ or dry air) | 7727-37-9 | For protecting cell from atmospheric moisture, dew point <-40°C |
Health and safety (OHS)
- Methanol — toxic (inhalation, skin contact, ingestion), flammable, work in fume hood
- Chloroform — harmful, suspected carcinogenicity, fume hood mandatory
- KF reagents contain SO₂ and imidazole — irritating, avoid inhalation
- Petroleum samples — flammable, store away from ignition sources
- Safety glasses, nitrile gloves, laboratory coat mandatory
- Iodine generated in cell — stains skin, irritating