ASTM D2896 for base number of petroleum products
ASTM D2896 for base number of petroleum products by potentiometric perchloric acid titration
17. Back Titration, Procedure B (60 mL)
17.1 Some used oils give no inflection point or only a very poor one with the test method described in Section 12. When this situation is encountered, the following modified test method may be used. In this modified test method, excess standard HClO4 solution is added to the sample, then the excess HClO4 solution is back titrated with standard sodium acetate solution.
17.2 Accurately weigh the amount specified in 12.1 into the titration beaker (see Note 18).
NOTE 18 - The sample size for the back titration modification does not exceed 2.5 g. When, with a 2.5-g sample, no inflection point is found, reduce the sample size to 1.5 g and repeat the analysis. Reducing the sample size generally improves the clarity of the inflection point.
17.3 Dissolve the sample in 40 mL chlorobenzene and add 20 mL of acetic acid.
17.4 Use a volumetric buret or pipet to add accurately 4.00 mL of standard 0.1 N HClO4 solution to the beaker. (The standard HClO4 solution must be in excess. If necessary, add more than 4.00 mL and correct accordingly (17.7).)
17.5 Stir the contents of the beaker for 2 min.
17.6 Titrate the unneutralized HClO4 solution with standard 0.1 N sodium acetate solution. Carry out the titration in the same manner as described in Section 12. For the back titration, the starting point will be in the range from 0 to 100 mV.
17.7 Instead of weighing out a separate sample and proceeding through 17.6, the back titration procedure can be used on a sample being titrated as in 12.1 to 12.6.2, provided that the sample size did not exceed 2.5 g (see Note 18). When it is apparent from the forward titration that a satisfactory inflection is not present, note the volume of standard HClO4 solution used, then proceed with 17.6. The standardization (8.2.2) should be modified to coincide with the volume of standard HClO4 solution used.
17.8 Calculate the base number as described in 16.8.
