ASTM D4928 for water in crude oils by Coulometric Karl Fischer titration
8. Sampling and Test Specimens
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 a container for analysis by a laboratory or test facility. The laboratory sample container and sample volume shall be of sufficient dimensions and volume to allow mixing as described in 8.4.
8.2 Laboratory Sample - The sample of crude oil presented to the laboratory or test facility for analysis by this test method. 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 to obtain the laboratory sample.
NOTE 2 - Examples of laboratory samples include sample bottles from manual sampling, receptacles from automatic crude oil samplers, and storage containers holding a crude oil from a previous analysis.
8.3 Test Specimen - The sample aliquot obtained from the laboratory sample for analysis by this test method. Once drawn, the entire portion of the test specimen will be used in the analysis. Mix the laboratory sample properly as described in 8.4 before drawing the test specimen.
8.4 Mix the laboratory sample of crude oil immediately (within 15 min) before drawing the test specimen to ensure complete homogeneity. Mix the sample at room temperature (15 to 25°C) or less in the laboratory sample container and record the temperature of the sample in degrees Celsius immediately before mixing. The type of mixer depends on the quantity of crude oil in the laboratory sample container. Before any unknown mixer is used, the specifications for the homogenization test, Practice D5854 (API MPMS Chapter 8.3), shall be met. Reevaluate the mixer for any changes in the type of crude, volume of crude in the container, the shape of the container, or the mixing conditions (such as mixing speed and time of mixing).
8.5 For small laboratory sample containers and volumes, 50 to 500 mL, a non-aerating, high-speed, shear mixer is required. Use the mixing time, mixing speed, and height of the mixer probe above the bottom of the container found to be satisfactory in Practice D5854 (API MPMS Chapter 8.3). For larger containers and volumes, appropriate mixing conditions shall be defined by following a set of procedures similar to those outlined in Practice D5854 (API MPMS Chapter 8.3) and Practice D4177 (API MPMS Chapter 8.2) but modified for application to the larger containers and volumes. Clean and dry the mixer between samples.
8.6 Record the temperature of the sample in degrees Celsius immediately after homogenization. The rise in temperature between this reading and the initial reading before mixing (8.4) is not to exceed 10°C, otherwise loss of water can occur or the emulsion can become unstable.
8.7 Select the test specimen size as indicated in Table 1 based on the expected water content.
9. Preparation of Apparatus
9.1 Follow the manufacturer's directions for preparation and operation of the titration apparatus.
9.2 Seal all joints and connections to the vessel to prevent atmospheric moisture from entering the apparatus.
9.3 Add to the anode (outer) compartment the mixture of xylene and Karl Fischer anode solutions which has been found suitable for the particular reagent and apparatus being used. Add the solutions to the level recommended by the manufacturer.
9.4 Add to the cathode (inner) compartment the Karl Fischer cathode solution to a level 2 to 3 mm below the level of the solution in the anode compartment.
9.5 Turn on the apparatus and start the magnetic stirrer for a smooth stirring action. Allow the residual moisture in the titration vessel to be titrated until the end-point is reached.
NOTE 3 - High background current for a prolonged period can be due to moisture on the inside walls of the titration vessel. Gentle shaking of the vessel (or more vigorous stirring action) will wash the inside with electrolyte. Keep the titrator on to allow stabilization to a low background current.
10. Standardization
10.1 In principle, standardization is not necessary since 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 pure water. Suggested intervals are initially with fresh reagent and then after every ten determinations (see Section 11.1.3). If the result is outside 10 000 6 200 µg, replace both the anode and cathode solutions.
