💧 Moisture — general Karl Fischer method

Physicochemistry PN-ISO 760

Overview

STATUS (as of 06.11.2026). PN-ISO 760:2001 — Polish version, published on 13.08.2001, implements ISO 760:1978 [IDT]; the PKN card has no withdrawal note — the standard is current. ISO 760:1978 (first edition of 01.12.1978, which replaced Recommendation ISO/R 760:1968) is current in the ISO catalogue, under periodical review (stage 90.20).

SCOPE (clause 1). The standard specifies a general method, known as the Karl Fischer method, for determining free water or water of crystallization in most solid or liquid chemical products, both organic and inorganic. In certain cases precautions are necessary, specified in the standards for the individual products. Two methods of titration are specified depending on how the end point is detected: visual — used when no electrometric apparatus is available, applicable only to colourless solutions and always a direct titration — and electrometric, by direct titration or back-titration. The electrometric method is the more accurate and is therefore recommended. Coulometric titration is not covered by the standard.

PRINCIPLE AND REACTIONS (clauses 2–3). Water present in the test portion reacts with a solution of iodine and sulfur dioxide in a pyridine/methanol mixture (Karl Fischer reagent), previously standardized by titration with an exactly known mass of water. Reactions according to the standard: H₂O + I₂ + SO₂ + 3 C₅H₅N → 2 C₅H₅N·HI + C₅H₅N·SO₃ and C₅H₅N·SO₃ + ROH → C₅H₅NH·OSO₂OR. Methanol may be replaced by 2-methoxyethanol — it gives a more constant titration volume and allows water to be determined in the presence of aldehydes and ketones without a special technique.

REAGENTS (clause 4). Methanol, 2-methoxyethanol and pyridine with a water content of not more than 0.05 % (m/m) (dried by distillation if necessary; methanol over magnesium turnings activated with iodine). Sample solvent: 4 parts by volume of methanol or 2-methoxyethanol and 1 part of pyridine; in special cases e.g. acetic acid, pyridine or methanol with chloroform 1:3. Karl Fischer reagent: 670 ml of methanol or 2-methoxyethanol, about 85 g of iodine, about 270 ml of pyridine and 65 g of sulfur dioxide (60–70 g acceptable) introduced from a cylinder with cooling so that the liquid does not exceed 20 °C; it is left for at least 24 h before use. The water equivalent of the reagent is between 3.5 mg/ml and 4.5 mg/ml; it is determined daily if methanol is used and less often with 2-methoxyethanol. The reagent is stored in the dark, protected from moisture. For standardization: sodium tartrate dihydrate or water; standard water/methanol solutions of 10 mg/ml and 2 mg/ml. Desiccant: anhydrous aluminium sodium silicate in 1.7 mm granules (regenerated at 350 °C for at least 48 h) or activated silica gel; silicone grease for the joints.

APPARATUS (clause 5). All glassware is dried for 30 min at about 130 °C and stored in a desiccator. For direct titration: automatic burette of 25 ml graduated in 0.05 ml protected by a guard tube with desiccant, titration vessel of 100 ml effective capacity with two side tubes (for platinum electrodes and for a cap through which liquid test portions are introduced by syringe), electromagnetic stirrer at 150–300 min⁻¹, reagent bottle of about 3 l of brown glass, device for electrometric end-point detection (Figure 3 in Annex B), syringes of calibrated volume and a tube for weighing solid samples. For back-titration — two automatic burettes (reagent and 2 mg/ml standard water solution) in a closed circuit with a drying tube (Annex C).

VISUAL TITRATION (clause 6). The end point is the development of colour on addition of the first drop of excess reagent coloured with iodine. Standardization: 25 ml of methanol is introduced into the vessel by syringe, the stirrer is switched on and reagent is added until a brown colour is obtained (this removes the traces of water in the methanol); then about 0.250 g of sodium tartrate weighed to 0.0001 g (or about 0.040 g of water from a dropping bottle) is introduced and titrated to the same brown colour. The determination proceeds in the same way: solvent (usually 25 ml or the volume specified in the procedure for the product), titration of water traces, introduction of the test portion by syringe or weighed to 0.0001 g.

SECOND SOURCE — ASTM E203-24. That standard covers volumetric Karl Fischer titration with pyridine-free reagents in automatic titrators with potentiometric end-point detection (visual end point in Appendix X1); with a proper choice of sample size, reagent concentration and apparatus it covers parts per million to pure water. According to it, coulometric titration (ASTM E1064), which is more accurate for very low levels, should be considered when samples routinely contain 500 mg/kg of water or less.

WHAT WE DO NOT GIVE HERE. The further course of the determination, electrometric titration (clauses 7–8), calculation formulae and the test report (clause 9) — the available text of the standard ends at 6.2.2. Determination ranges and accuracies for particular titrators or trade names of reagents — the standard does not give them.

Method principle

Water present in the test portion reacts stoichiometrically with iodine and sulfur dioxide in a pyridine/methanol medium (Karl Fischer reagent). The reagent is standardized with an exactly known mass of water — sodium tartrate dihydrate, water or a standard water/methanol solution — and then used to titrate the test portion dissolved in a previously dehydrated solvent. The end point is detected visually (appearance of the colour of excess iodine, colourless solutions only) or electrometrically with platinum electrodes, by direct titration or back-titration.

Applications

• Free water and water of crystallization in most solid or liquid chemical products, organic and inorganic (clause 1)
• General method referred to by the standards for individual products together with their precautions (clause 1)
• Products containing aldehydes and ketones — with a 2-methoxyethanol-based solvent (clause 2 note, 4.4)
• Coloured solutions — electrometric end-point detection only (clause 1)

Key parameters

Standard

Standard number

PN-ISO 760:2001 (Polish version; adopts ISO 760:1978 [IDT])

Title (PL)

Oznaczanie wody — Metoda Karla Fischera (Metoda ogólna)

Title (EN)

Determination of water — Karl Fischer method (General method)

Step-by-step procedure

1. Preparing the reagent

   Prepare the Karl Fischer reagent according to 4.5 with cooling (temperature not above 20 °C) and leave it for at least 24 h; store in the dark, protected from moisture.

2. Preparing the glassware

   Dry the glassware for 30 min at about 130 °C and store it in a desiccator with desiccant; lubricate the joints with silicone grease (clause 5, 4.10).

   🌡 Temperature: about 130 °C

3. Dehydrating the solvent in the vessel

   Introduce 25 ml of methanol by syringe through the cap, switch on the stirrer and add reagent until a brown colour is obtained (6.2.1.1).

4. Standardizing the reagent

   Introduce about 0.250 g of sodium tartrate dihydrate weighed to 0.0001 g (or about 0.040 g of water) and titrate to the same brown colour, recording the volume; with methanol standardize daily (6.2.1.2, 4.5).

5. Preparing for the determination

   Empty the vessel, introduce 25 ml of solvent (or the volume in the product procedure) and titrate the water traces to a brown colour (6.2.2).

6. Introducing the test portion and titration

   Introduce the test portion by syringe (liquid) or weighed to 0.0001 g and titrate with the reagent to the end point — visually or electrometrically (6.2.2, clauses 7–8).

7. Calculation and report

   Calculate the water content from the reagent volume and its water equivalent according to the standard and prepare the test report (clause 9).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Methanol — harmonised CLP classification: H225, H301, H311, H331, H370
• Pyridine — H225, H302, H312, H332; iodine — H312, H332, H400; sulfur dioxide — H314, H331, H370 (harmonised CLP classification); prepare the reagent under a fume hood, introducing SO₂ from the cylinder with cooling (4.5)
• 2-Methoxyethanol — including H360FD (may damage fertility and the unborn child), H226 (harmonised CLP classification)
• Chloroform (solvent in special cases, 4.4) — including H351, H331 (harmonised CLP classification)

Frequently asked questions