ASTM D4928 for water in crude oils by Coulometric Karl Fischer titration
11. Procedure
11.1 Mass Determination of Sample Size:
11.1.1 Add fresh solvents to the anode and cathode compartments of the titration vessel and bring the solvent to end-point conditions as described in Section 9.
11.1.2 Add an aliquot of the crude oil test specimen to the titration vessel immediately after the mixing step described in 8.4 using the following method.
11.1.2.1 Starting with a clean, dry syringe of suitable capacity (see Table 1 and Note 4), withdraw at least three portions of the sample and discard to waste. Immediately withdraw a further portion of sample, clean the needle with a paper tissue, and weigh the syringe and contents to the nearest 0.1 mg. Insert the needle through the inlet port septum, start the titration and with the tip of the needle just below the liquid surface, inject the sample. Withdraw the syringe and reweigh the syringe to the nearest 0.1 mg. After the end-point is reached, record the titrated water from the digital readout on the instrument.
NOTE 4 - If the concentration of water in the sample is completely unknown, it is advisable to start with a small trial portion of sample to avoid excessive titration time and depletion of the reagents. Further adjustment of the aliquot size can then be made as necessary.
11.1.2.2 When the background current or titration rate returns to a stable reading at the end of the titration as discussed in 9.5, additional samples can be added in accordance with 11.1.2.1.
11.1.3 Replace the solutions when one of the following occurs and then repeat the preparation of the apparatus as in Section 9.
11.1.3.1 Persistently high and unstable background current.
11.1.3.2 Phase separation in the anode compartment or crude oil coating the electrodes.
11.1.3.3 The total crude content added to the titration vessel exceeds one quarter of the volume of solution in the anode compartment.
11.1.3.4 The solutions in the titration vessel are greater than one week old.
11.1.3.5 The instrument displays error messages that directly or indirectly suggest replacement of the electrodes - see the instrument operating manual.
11.1.3.6 The result from a 10-µL injection of water is outside 10 000 more or less 200 µg.
11.1.4 Thoroughly clean the anode and cathode compartments with xylene if the vessel becomes contaminated with crude. Never use acetone or similar ketones.
NOTE 5 - Clogging of the frit separating the vessel compartments will also cause instrument malfunction.
11.1.5 For crudes too viscous to draw into a syringe, add the sample to a clean, dry dropper bottle and weigh the bottle and crude. Quickly transfer the required amount of sample to the titration vessel with the dropper. Reweigh the bottle. Titrate the sample as in 11.2.
11.2 Volume Determination of Sample Size:
11.2.1 This procedure is applicable only when conditions warrant, that is, when the vapor pressure and viscosity of the crude permit an accurate determination of the volume of the crude oil. Viscous crudes are difficult to measure accurately with a syringe.
11.2.2 Take care in filling the syringe to reduce the formation of gas bubbles.
NOTE 6 - The presence of gas bubbles in the syringe can be a source of interference. The tendency of the crude to form gas bubbles is a function of the crude type and corresponding vapor pressure.
11.2.3 The referee procedure for determination of water in crude oil by coulometric Karl Fischer titration is the mass measurement of the crude oil in 11.1.
11.2.4 The basic steps are the same as those for mass determination (see 11.1) with the following exception. With a clean, dry syringe of suitable capacity (see Table 1), withdraw at least three portions of sample and discard to waste. Immediately withdraw a further portion of sample, expel any gas in the syringe, clean the needle with a paper tissue, and record the volume in the syringe to the nearest 1 or 10 µL as appropriate (see 6.3.1.2). Insert the needle through the port inlet septum, start the titration, and with the tip of the needle just below the liquid surface, inject the entire contents of the syringe. After the end-point is reached, record from the digital readout on the instrument the micrograms titrated.
