ASTM D4377 Standard Test Method for Water in Crude Oils by Potentiometric Karl Fischer Titration
8. Sampling and Test Samples
8.1 Sampling, is defined as all the steps required to obtain an aliquot representative of the contents of any pipe, tank, or other system, and to place the sample into the laboratory test container. The laboratory test container and sample volume shall be of sufficient dimensions and volume to allow mixing as described in 8.1.2.1.
8.1.1 Laboratory Sample - Only representative samples obtained as specified in Practice D4057 (API MPMS Chapter 8.1) and Practice D4177 (API MPMS Chapter 8.2) shall be used for this test method.
8.1.2 Test Samples - The following sample handling procedure shall apply in addition to those covered in 8.1.1.
8.1.2.1 Mix the test sample of crude oil immediately (within 15 min) before analysis to insure complete homogeneity. Mix the test sample at room temperature (25°C) in the original container.
NOTE 1 - The sample should be mixed at room temperature (25°C) or less. Mixing of the sample should not increase the temperature of the sample more than 10°C, or a loss of water may occur. The type of mixer depends on the quantity of crude. Before any unknown mixer is used, the specifications for the homogenization test, Annex A1, must be met. The mixer must be re-evaluated for any changes in the type of crude, quantity of crude, or shape of the sample container.
(1) For small sample volumes, 50 to 500 mL, a non-aerating, high speed, shear mixer is required. Use the mixing time, mixing speed, and height above the bottom of the container found to be satisfactory to Annex A1. Clean and dry the mixer between samples.
8.1.2.2 The test sample size is selected as indicated in Table 1 based on the expected water content.
9. Calibration and Standardization
9.1 Standardize the Karl Fischer reagent at least once daily.
9.2 Add enough solvent to the clean, dry titration vessel to cover the electrodes. The volume of solvent depends on the size of the titration vessel. Seal all openings to the vessel and start the magnetic stirrer for a smooth stirring action. Turn on the indicating circuit and adjust the potentiometer to give a reference point with approximately 1 µA of current flowing. Add Karl Fischer reagent in suitable amounts to the solvent to cause the needle to deflect from the reference point. At first the needle will deflect due to local concentration of the unreacted reagent about the electrodes but will fall back to near the reference point. As the end-point is approached, the needle will fall back more slowly after each addition of Karl Fischer reagent. The end-point is reached when, after the addition of a single drop of reagent, the needle remains deflected at least 1 µA from the reference point for at least 30 s. Swirl the titration vessel to dry the inside walls of the vessel. Add more Karl Fischer reagent, if needed, until a steady end-point is reached for at least 30 s.
9.3 Standardize the Karl Fischer reagent with distilled water by one of the following methods:
9.3.1 From a water filled weighing pipet or syringe previously weighed to the nearest 0.1 mg, add 1 drop of distilled water (about 20 mg) to the sample solvent at end-point conditions and reweigh the syringe. Record the weight of the water added. Titrate the water with Karl Fischer reagent added from the buret until a steady end point is reached for at least 30 s. Record to the nearest 0.01 mL the volume of the Karl Fischer reagent needed to reach the end-point.
NOTE 2 - After adding water do not shake the cell.
NOTE 3 - When wiping the needle exercise care, so not to siphon liquid through the tip of the needle.
9.3.2 Fill a 10-µL syringe with water taking care to eliminate air bubbles, wipe the needle with a paper tissue to remove any residual water from the needle and accurately determine the weight of syringe plus water to 0.1 mg. Add the contents of the syringe to the sample solvent in the cell which has been adjusted to the end point ensuring that the tip of the needle is below the surface of the sample solvent. Reseal the vessel immediately. Remove any solvent from the needle by wiping with a paper tissue and reweigh the syringe to 0.1 mg. Titrate the water with Karl Fischer reagent as in 9.3.1.
9.4 Calculate the water equivalence of the Karl Fischer reagent as follows:
F = W/T
where:
F = water equivalence of the Karl Fischer reagent, mg/mL,
W = water added, mg, and
T = reagent required for titration of the added water, mL.
9.5 Duplicate values of water equivalence should agree within 2 % relative. If the variation between the two titrations is greater than 2 % relative, discard the contents of the titration vessel. Introduce a further portion of sample solvent into the vessel and repeat the standardization procedure. If the titrations for two further portions of distilled water still vary by more than 2 %, it is likely that either the Karl Fischer reagent or the sample solvent, or both, have aged. Replace these with fresh reagents and repeat the procedure for calibration and standardization.
9.6 Determine and record the mean water equivalence value.
