ASTM D2896 for base number of petroleum products
ASTM D2896 for base number of petroleum products by potentiometric perchloric acid titration
16. Back Titration, Procedure A (120 mL)
16.1 Some used oils give no inflection point or only a very poor one with the test method described in Section 11. 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.
16.2 Accurately weigh the amount of sample specified in 11.1 into the titration beaker. (See Note 17.)
NOTE 17 - The sample size for the back titration modification does not exceed 5 g. When, with a 5-g sample, no inflection point is found, reduce the sample size to 3 g and repeat the analysis. Reducing the sample size generally improves the clarity of the inflection point. However, it should be noted that the cooperative work that led to the precision statement (see 19.2) employed a sample size of 5 g maximum.
16.3 Dissolve the sample in 80 mL of chlorobenzene and add 40 mL of acetic acid.
16.4 Use a volumetric buret or pipet to add accurately 8.00 mL of standard 0.1 N HClO4 solution to the beaker. (The standard HClO4 solution must be in excess. If necessary, add more than 8.00 mL and correct accordingly (16.7).)
16.5 Stir the contents of the beaker for 2 min.
16.6 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 11. For the back titration, the starting point will be in the range from 0 to 100 mV.
16.7 Instead of weighing out a separate sample and proceeding through 16.6, the back titration procedure can be used on a sample being titrated as in 11.1 to 11.6.2, provided that the sample size did not exceed 5 g (see Note 17). 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 16.6. The standardization (8.2.1) should be modified to coincide with the volume of standard HClO4 solution.
16.8 Calculate the base number, BN, as follows:
BN, mg KOH/g = [(G–H) • N(B)• 56.1]/S
where:
G = volume of standard sodium acetate used in the standardization, mL (see 8.2.1 or 8.2.2),
N(B) = normality of the sodium acetate solution,
H = volume of standard sodium acetate used in the sample back titration, mL, and
S= weight of sample, g.
