ASTM D2896 for base number of petroleum products by potentiometric perchloric acid titration
12. Procedure B (60 mL)
12.1 Calculate the quantity of sample required from its expected base number as follows:
Approximate weight of sample, g = 10/expected BN
NOTE 13 - For the back titration procedure (see 17.2) it may be necessary to use a smaller sample weight.
12.1.1 Weigh the sample into the titration beaker, applying the limits shown as follows. A maximum of 10 g should be taken for analysis.
NOTE 14 - It is especially important for Procedure B that great care be exercised in obtaining accurate weights particularly for the high base number samples which require small sample weights.
12.2 Add 60 mL of titration solvent to the sample.
12.3 Place the sample on the titration stand and stir the solution until the sample is dissolved.
NOTE 15 - If the solution of the sample proves difficult, dissolve it in 40 mL of chlorobenzene in the titration beaker, then add 20 mL of glacial acetic acid.
12.4 Prepare the electrodes as directed in 10.1, 10.2, and 10.3. Position the electrodes in the solution so that they are immersed as far as possible. Continue stirring throughout the determination at a rate sufficient to produce vigorous agitation without spattering and without stirring air into the solution. Adjust the meter so that it reads in the upper part of the millivolt scale; for example, 700 mV. For simple meters without this adjustment, it may be necessary to incorporate a source of potential in series with the electrode. A 1.5-V dry cell and potential divider is suitable.
12.5 Fill the buret with 0.1 N HClO4 solution and place the buret in position in the titration assembly, taking care that the tip is immersed below the level of the surface of the liquid in the beaker. Record the initial buret and meter (cell potential) readings.
12.6 Titration:
12.6.1 Manual Titration - Add suitable small portions of titrant and after waiting until a constant potential has been established (Note 12), record the buret and meter readings. At the start of the titration and in any subsequent regions (inflections) where 0.1 mL of titrant consistently produces a total change of more than 0.03 V (corresponding to 0.5 pH scale unit) in the cell potential, add 0.05-mL portions. In the intermediate regions (plateaus) where 0.1 mL increments change the potential by less than 0.03 V, add large portions sufficient to produce a total potential change approximately equal to, but not greater than, 0.03 V. Titrate in this manner until the potential changes less than 0.005 V (corresponding to 0.1 pH scale unit) per 0.1 mL.
12.6.2 Automatic Recording Titration - Adjust the instrument in accordance with the manufacturer's instructions and set the titration speed at 1.0 mL/min maximum.
12.7 On completion of the titration, remove the beaker and rinse the electrodes and buret tip with titration solvent, then with water, then again with titration solvent (see 10.3). Store the electrodes in water when not in use (see 10.1).
12.8 For each set of samples make a blank on 60 mL of titration solvent. For a manual titration add 0.1 N HClO4 solution in 0.05-mL increments, waiting between each addition until a constant cell potential is established. Record meter and buret readings after each increment. Follow the procedure in 12.6.2 for an automatic titration.