ASTM D3230 standard test method for salts in crude oil (Electrometric Method)
8. Sampling
8.1 Obtain a sample and test specimen in accordance with Test Methods D4928. Ensure that the sample is completely homogenized with a suitable mixer. See Annex A1 of Test Methods D4928 for suitable apparatus and proving.
8.2 Samples of very viscous materials may be warmed until they are reasonably fluid before they are sampled; however, no sample shall be heated more than is necessary to lower the viscosity to a manageable level.
8.3 Samples of crude oil contain water and sediment and are inhomogeneous by nature. The presence of water and sediment will influence the conductivity of the sample. The utmost care shall be taken in obtaining homogenized representative samples.
9. Preparation of Apparatus
9.1 Support the apparatus on a level, steady surface, such as a table.
9.2 Prepare the apparatus for operation in accordance with the manufacturer's instructions for calibrating, checking, and operating the equipment. (Warning - The voltage applied to the electrodes can be as great as 250 V ac, and hazardous.)
9.3 Thoroughly clean and dry all parts of the test beaker, the electrodes, and its accessories before starting the test, being sure to remove any solvent that had been used to clean the apparatus.
10. Calibration
10.1 The conductivity of solutions is affected by the temperature of the specimen when measurements are made. The temperature of the test specimen at the time of measurement shall be within 3°C of the temperature at which the calibration curves were made.
10.2 Establish a blank measurement by following the procedure in 10.3 and 10.4, omitting the mixed salts solution. When the indicated electrode current is greater than 0.25 mA at 125 V ac, water or another conductive impurity is present and its source must be found and eliminated before calibration can be completed. Determine a blank measurement each time fresh xylene or mixed solvent is used.
10.3 Into a dry, 100-mL graduated, glass-stoppered mixing cylinder, add 15 mL of xylene. From a pipet (total delivery), add 10 mL of neutral oil. Rinse the pipet with xylene until free of oil. Make up to 50 mL with xylene. Stopper and shake the cylinder vigorously for approximately 60 s to effect solution. Add a quantity of dilute mixed salts solution, in accordance with Table 1, that is appropriate to the range of salt contents to be measured. Dilute to 100 mL with mixed alcohol solvent. Again shake the cylinder vigorously for approximately 30 s to effect solution, and allow the solution to stand approximately 5 min. Pour the solution into a dry test beaker.
10.4 Immediately place the electrodes into the solution in the beaker, making sure that the upper edge of the electrode plates are below the surface of the solution. Adjust the indicated electrode voltage to a series of values, for example 25, 50, 125, 200, and 250 V ac. At each voltage, note the current reading and record the voltage displayed and the current to the nearest 0.01 mA. Remove the electrodes from the solution, rinse with xylene followed by naphtha, and allow them to dry.
NOTE 5 - With some apparatus, the detailed settings will not be required since the electronics are built-in for auto-ranging. Determination of the blank and the calibration standard responses are the same.
10.5 Repeat the procedure in 10.3, using other volumes of mixed salts solution (dilute solution) as needed to cover the range of chloride contents of interest.
10.6 Subtract the value obtained for the blank measurement from the indicated current readings of each standard sample, and plot the chloride content (ordinate) against net current (mA) readings (abscissa) for each voltage on 3 by 3 cycle log-log paper, or other suitable format.
NOTE 6 - Some apparatus are capable of internally recording the current readings, standard concentration, and blank, and they provide an output in direct concentration units.
NOTE 7 - The apparatus are calibrated against standard solutions of neutral oil and mixed chloride solutions in xylene because of the extreme difficulties in keeping crude oil-brine mixtures homogeneous. The calibration may be confirmed, if desired, by careful replicate analysis of crude-oil samples by exhaustive extraction of salts with hot water, followed by titration of the chlorides in the extract.
NOTE 8 - In calibrating over a wide range of chloride concentrations, it may be necessary to apply several voltages to obtain current readings within the limit of the apparatus current level display (0 to 10 mA). Higher voltages are applied for low concentrations and lower voltages are applied for high concentrations.
