💧 Water Content by Karl Fischer Method

Petrochemistry PN-EN ISO 12937

Overview

STATUS (as of 30.10.2026). PN-EN ISO 12937:2005 — Polish version, implements EN ISO 12937:2000 and ISO 12937:2000 [IDT]; published on 06.01.2005, it replaced PN-EN ISO 12937:2002 (English version); in the PKN catalogue it has no withdrawal note. A national amendment PN-EN ISO 12937:2005/Ap1:2021-11 (approved on 29.10.2021) corrected a typing error in 6.2.5 of the Polish text (a syringe of “10 m” replaced by a syringe with a capacity of “10 ml”). ISO 12937:2000 is the first and only edition of the standard (published on 16.11.2000), last confirmed by ISO on 26.06.2024.

SCOPE (clause 1). Direct determination of water in petroleum products boiling below 390 °C, over the mass fraction range from 0.003 % (m/m) to 0.100 % (m/m). The standard does not apply to products containing ketones or to residual fuel oils; it may be applicable to lubricating base oils, but the precision has not been established for them. The precision data (clause 12) come from dual-cell, dual-electrolyte instruments. In petroleum products the most common interferences are hydrogen sulfide and mercaptan sulfur, but below 0.003 % (m/m) expressed as sulfur they do not cause significant interference over this range; sulfides, disulfides and thiophenes do not interfere. Informative Annex B gives a variant of the determination as a volume fraction, from 0.003 % (V/V) to 0.100 % (V/V). According to the corresponding American standard ASTM D6304 (5.1–5.2), knowing the water content is important in the manufacture, purchase, sale and transfer of petroleum products, and moisture can lead to premature corrosion and wear, an increased debris load with diminished lubrication and premature plugging of filters, impaired action of additives and the growth of harmful bacteria.

PRINCIPLE (clause 3). The sample is inspected visually: when it is clear and bright, free from water droplets and particulate matter on swirling, a weighed portion is injected into the titration vessel of a coulometric instrument in which the iodine for the Karl Fischer reaction is generated coulometrically at the anode. When all the water has reacted, the excess iodine is detected by an electrometric end-point detector and the titration is terminated. One mole of iodine reacts with one mole of water, so the quantity of water is proportional to the total electric charge (Faraday’s law). A non-homogeneous sample — hazy, with water droplets or particulate matter — receives a solution of sodium dioctylsulfosuccinate before homogenization with a mixer.

REAGENTS AND APPARATUS (clauses 4–5). Anolyte: 6 parts by volume of commercial Karl Fischer anode solution and 4 parts of dry xylene (other proportions are acceptable if they meet the requirements of clause 8); catholyte — commercial cathode solution; in cells with or without a diaphragm a single Karl Fischer solution can be used. Xylene is dried overnight with 4A molecular sieve (about 100 g per 2 l; sieve activated for 4 h at 200 °C to 250 °C). Sodium dioctylsulfosuccinate solution: 10 g of the salt dried for 4 h at 105 °C to 110 °C is dissolved in dry xylene to 100 ml, and the water content of the solution must be less than 0.010 % (m/m); another anionic surfactant meeting Annex A may be used instead of this salt. Water of grade 3 according to ISO 3696. Apparatus: automatic coulometric Karl Fischer titrator, non-aerating mixer (A.3), glass syringes with a needle reaching under the anolyte surface (needle bores from 0.5 mm to 0.8 mm have been found suitable; recommended syringes: 10 µl for checking the titrator, 1 ml or 2 ml for samples, 10 ml for the surfactant solution), balance weighing to ±0.1 mg, ovens, cooling bath and a thermometer reading to 1 °C.

COURSE OF THE TEST (clauses 6–11). Samples are taken according to ISO 3170 or ISO 3171; for manual sampling into a clear borosilicate glass bottle. Immediately before analysis the sample is shaken vigorously by hand for 30 s, inspected against the light and after swirling (bottom of the vortex and bottom of the container), and the result of the inspection is recorded. A clear sample is analysed straight away; the others — also those in an opaque container, without transferring — receive the surfactant solution in an amount established according to Annex A and are mixed in the original container under the conditions of A.3; the temperature may rise by no more than 2 °C, otherwise the sample is cooled in a bath. Acetone and other ketones must not be used for drying or cleaning the apparatus (they react with the Karl Fischer reagent); a cell contaminated with product is cleaned with xylene. The catholyte is filled to a level 2 mm to 3 mm below the anolyte, and the residual moisture in the cell is titrated until the background current is constant and lower than the maximum recommended by the manufacturer. The titrator is checked by injecting exactly 10 µl of water — initially with fresh reagents, then after every 10 injections; the electrolytes are replaced when the result is outside 10 000 µg ± 200 µg or persistently high, when the background current is unstable, phases separate or product coats the electrodes, when the product in the cell exceeds one third of the anolyte volume, or when the instrument calls for replacement. Determination: at least three portions of the sample are drawn into the syringe and discarded, then a 1–2 ml test portion is drawn, the syringe is weighed to the nearest 0.1 mg, the whole portion is injected just below the anolyte surface and the syringe is weighed again. Result: w = m₂ / (m₁ · 10⁴) in % (m/m), where m₁ is the mass of the test portion in g and m₂ is the mass of water displayed by the titrator in µg; if required, the volume fraction φ = w · ρ / 1 000 (ρ in kg/m³ at 15 °C). The result is reported to the nearest 0.001 % (m/m).

WHAT WE DO NOT GIVE HERE. The precision values — repeatability and reproducibility (clause 12), the way of establishing the amount of surfactant and checking the effectiveness of homogenization (Annex A) or the course of the volume variant (Annex B) — we only know the titles of these parts of the standard.

Method principle

The iodine for the Karl Fischer reaction is generated coulometrically at the anode in the titration vessel; one mole of iodine reacts with one mole of water. When all the water from the injected, weighed portion of the sample has reacted, the excess iodine is detected by an electrometric end-point detector.

The quantity of water is proportional to the total electric charge used to generate the iodine (Faraday’s law). Non-homogeneous samples are homogenized before the determination with a solution of sodium dioctylsulfosuccinate added.

Applications

• Petroleum products boiling below 390 °C — water from 0.003 % to 0.100 % (m/m) (clause 1)
• Diesel fuel — requirement of the Polish regulation on quality requirements for liquid fuels: at most 0.020 % (m/m) water (Dz.U. 2024 item 1018, Annex 3 No 10)
• Lubricating base oils — possible application, without established precision (clause 1)
• Samples that are hazy or contain water droplets or particulate matter — after homogenization with sodium dioctylsulfosuccinate (6.2.4–6.2.8)
• Volume fraction of water from 0.003 % to 0.100 % (V/V) — informative Annex B

Key parameters

Standard

Standard number

PN-EN ISO 12937:2005

Title (PL)

Przetwory naftowe — Oznaczanie wody — Miareczkowanie kulometryczne metodą Karla Fischera

Title (EN)

Petroleum products — Determination of water — Coulometric Karl Fischer titration method

Step-by-step procedure

1. Visual inspection of the sample

   Shaking by hand for 30 s, inspection against the light and after swirling; record: sample clear and bright or not, water droplets or particulate matter — yes or no (6.2.2).

2. Homogenizing a non-homogeneous sample

   Addition of the sodium dioctylsulfosuccinate solution in the amount from Annex A and mixing in the original container under the conditions of A.3; temperature rise of no more than 2 °C (6.2.4–6.2.8).

3. Preparing and checking the titrator

   Sealing the joints, filling with electrolytes, titrating the moisture to a constant background current below the manufacturer’s maximum; check with 10 µl of water — 10 000 µg ± 200 µg (clauses 7–8).

4. Determination

   Three portions discarded, a 1–2 ml test portion weighed before and after injection to the nearest 0.1 mg, injection just below the anolyte surface, reading of the mass of water after the end point (clause 9).

5. Calculation and result

   w = m₂ / (m₁ · 10⁴) in % (m/m); if required φ = w · ρ / 1 000 in % (V/V); result to the nearest 0.001 % (m/m) (clauses 10–11).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The standard does not address all hazards associated with its use — safety measures are established by the user (warning of the standard)
• The molecular sieve is activated in an oven at 200 °C to 250 °C — risk of burns when transferring it to the desiccator (4.1)

Frequently asked questions