ASTM D3339 Test Method for Acid Number of Petroleum Products
ASTM D3339 Standard Test Method for Acid Number of Petroleum Products by Semi-Micro Color Indicator Titration
9. Procedure
9.1 Sample Titration - Weigh to the nearest 0.1 mg the required amount of sample into a clean, tared titration beaker. Guidelines for determining the amount of sample required are given in Table 1. Place the magnetic stirring bar into the beaker taking care not to splash the sample. Pour 40 mL of titration solvent into the titration beaker, using part of this volume of solvent to wash down any sample that may have splashed onto the sides of the beaker above the solvent-sample solution level. Add 0.100 ± 0.002 mL of p-naphtholbenzein indicator solution to the solvent-sample solution. Immediately assemble the titration beaker with the purging stopper, previously connected with the rotameter and nitrogen purge gas. Insert the buret tip into the beaker through the hole in the stopper. Adjust the position of the buret tip so that the purge gas can freely exit through the hole (Warning - The purge gas must be able to exit freely or otherwise dangerous pressures could develop. Also, the vapor from this treatment contains toluene and should be removed with adequate ventilation.) and the bottom of the buret tip is approximately 10 mm (0.4 in.) above the surface of the solution. Immediately start the purge gas flow through the beaker at a relatively fast rate, for example, about 30 to 40 L/h, to purge the atmosphere in the beaker rapidly, and then start mixing the solution using the magnetic stirrer. Adjust the purge gas flow rate to 10 ± 1 L/h after purging at the faster flow rate for 15 to 30 s. Adjust the stirring rate to mix the solution efficiently but without splashing the solution or mixing purge gas into the solution.
9.1.1 Titrate the sample at a temperature below 30°C with the standardized 0.01 M KOH titrant. The indicator color changes are from the starting orange to a bright yellow and then to green. Take the point at which a stable, green color is first obtained as the end point. Consider the color stable if it does not change back to a yellow or yellow-green color within 15 to 20 s (Note 10). This end point can be anticipated by the initial change to yellow, which occurs very close to the end point. Record the quantity of titrant required to obtain the stable end point.
NOTE 10 - Several drops of titrant can be required to obtain the stable end point color after reaching a yellow-green color with used oil samples having relatively high total acid numbers, for example, an acid number of about 3 or greater.
9.2 Blank Titration - Make a blank titration each day that samples are analyzed. Conduct this titration using the procedure described in 9.1 except omit the sample. Record the quantity of titrant required to obtain the stable green end point.
10. Calculation
10.1 Calculate the acid number of the sample as follows:
Acid number, mg KOH/g = 56.10 M (A - B)/W
where:
A = millilitres of titrant required for titration of the sample (9.1),
B = millilitres of titrant required for titration of the blank (9.2),
M = molarity of the titrant (7.9.2), and
W = grams of sample used (9.1).
