ISO 12937 Petroleum products - Determination of water - Coulometric Karl Fischer titration method
6 Sampling and sample preparation (see annex A)
6.1 Sampling
Samples shall be drawn in accordance with ISO 3170, ISO 3171, or an equivalent national standard.
If sampling is carried out manually, use a clear borosilicate glass bottle. If an automatic technique is employed, either collect a separate sample for water determination, or treat the whole sample collected in accordance with 6.2.4.
6.2 Sample preparation
6.2.1 If the sample is not in a container suitable for visual inspection, or is opaque, then it should be treated as if it were not clear and bright (see 6.2.4). It should not be transferred to another container.
6.2.2 Immediately prior to analysis, shake the sample vigorously by hand for 30 s and then, when free from bubbles, visually inspect it. Hold the sample up to the light and examine it for haze or lack of clarity and then swirl the sample to produce a vortex and examine both the bottom of the vortex and the bottom of the sample container for water droplets and particulate matter. Record the visual clarity as clear and bright or not clear and bright. Record whether water droplets or particulate matter were, or were not, observed on swirling.
6.2.3 If the sample is both clear and bright, free from water droplets and particulate matter, proceed in accordance with clause 9.
6.2.4 If the sample is not clear and bright, or if water droplets or particulate matter were observed on swirling, proceed in accordance with 6.2.5 to 6.2.8.
NOTE The precision of this method for samples which are not clear and bright is critically dependent upon the effectiveness of the homogenization stage which is proved periodically, see normative annex A.
6.2.5 Use a clean, dry 10 ml syringe (5.3) to add a volume of sodium dioctylsulfosuccinate solution (4.4.1) as established by the procedure specified in annex A.
NOTE Correction of the sample water content for the water content of the sodium dioctylsulfosuccinate solution is not required because the latter is negligible.
6.2.6 Record the temperature of the sample in degrees Celsius immediately before mixing.
6.2.7 To ensure homogeneity, mix the sample in the original container immediately prior to analysis. The mixing time, mixing power (speed) and mixer position relative to the bottom of the container, shall be that found to be satisfactory for the material and sample size as established by the procedure given in A.3. The sample volume and water content of the sample shall not exceed the maxima validated in A.3.
6.2.8 Record the temperature of the laboratory sample in degrees Celsius immediately after mixing. The rise in temperature between this reading and the reading in 6.2.6 shall not exceed 2 °C, otherwise loss of sample light ends or loss of water may occur. If this criterion cannot be met, the sample shall be placed in a cooling bath (5.8) prior to carrying out the procedure in 6.2.6.
7 Apparatus preparation
7.1 Due to the known reaction of acetone and other ketones with Karl Fischer reagent, the use of such solvents to dry apparatus, sample syringes, homogenizers and sample receivers, is not permitted.
7.2 Follow the manufacturer's directions for preparation and operation of the titration apparatus.
7.3 Seal all joints and connections to the titration cell to prevent atmospheric moisture from entering.
7.4 If a single Karl Fischer solution (4.3.3) is used, add this solution to the cell.
7.5 If separate electrolyte solutions are used, add the anolyte (4.3.1) to the outer compartment of the titration cell to the level recommended by the manufacturer. Add the catholyte (4.3.2) to the inner compartment of the titration cell to a level 2 mm to 3 mm below the level of the anolyte.
7.6 Turn on the titrator and stirrer. Allow the residual moisture in the titration cell to be titrated until the end-point is reached. Do not proceed beyond this stage until the background current (or background titration rate) is constant and less than the maximum recommended by the manufacturer of the instrument (see the note below).
NOTE High background current for a prolonged period may be attributable to moisture on the inside walls of the titration cell. Gentle swirling of the cell will wash the inside with electrolyte. Also check all fittings to ensure atmospheric moisture does not enter the cell. It is recommended that the titrator be permanently switched on to stabilize to a low background current.
7.7 If the vessel becomes contaminated with a petroleum product, thoroughly clean the anode and cathode compartments with xylene (4.2). If the frit becomes blocked, thoroughly clean with xylene (4.2). Do not use acetone or other ketones for cleaning or drying the apparatus.
8 Apparatus check test
The water titrated is a direct function of the coulombs of electricity consumed. However, reagent performance deteriorates with use and shall be regularly monitored by accurately injecting 10 μl of water (4.5). Suggested monitoring intervals are initially when fresh reagents are used and then after every 10 injections. Replace the anolyte (4.3.1) and catholyte (4.3.2) solutions, or the single Karl Fischer solution (4.3.3), whenever one of the following occurs:
a) the result from a 10 μl injection of water is outside 10000 μg +/- 200 μg, or persistently high;
b) unstable background current, or phase separation in the outer compartment, or petroleum products coating the electrodes;
c) the total petroleum product content of the titration cell exceeds one third of the volume of the anolyte;
d) the titrator displays error messages which suggest replacement of the electrolytes (see the manufacturer's instructions).
