ASTM D3242 Standard Test Method for Acidity in Aviation Turbine Fuel
8. Procedure
8.1 Introduce 100 g +/- 5 g of the sample weighed to the nearest 0.5 g, into a 500 mL wide-mouth Erlenmeyer flask. (One type of suitable modified flask is shown in Fig. 1.) Add 100 mL of the titration solvent and 0.1 mL of the indicator solution. Introduce nitrogen through a 6 mm to 8 mm outside diameter glass tube to a point within 5 mm of the flask bottom at a rate of 600 mL/min to 800 mL/min. Bubble the solution for 3 min +/- 30 s with occasional mixing.
8.1.1 The vapor from this treatment contains toluene and should be removed with adequate ventilation.
8.2 Continue the nitrogen addition and titrate without delay at a temperature below 30 °C. Add 0.01 N KOH solution in increments and swirl to disperse. When the solution first turns green, reduce the increment size to dropwise (manual buret) or between 0.01 mL and 0.05 mL (automated buret). Continue until a persistent green end point is reached and held for a minimum of 15 s after the addition of the last increment.
NOTE 4 - The temperature can be measured by any suitable temperature measuring device.
8.3 Blank - Perform a blank titration on 100 mL of the titration solvent and 0.1 mL of the indicator solution, introducing the nitrogen in the same manner and titrating to the same end point as above.
9. Quality Control Checks
9.1 Confirm the performance of the equipment or the procedure each day it is in use, by analyzing a quality control (QC) sample. It is advisable to analyze additional QC samples as appropriate, such as at the end of a batch of samples or after a fixed number of samples to ensure the quality of the results. Analysis of result(s) from these QC samples can be carried out using control chart techniques. When the QC sample result causes the laboratory to be in an out-of-control situation, such as exceeding the laboratory's control limits, instrument recalibration may be required. An ample supply of QC sample material shall be available for the intended period of use, and shall be homogeneous and stable under the anticipated storage conditions. If possible, the QC sample shall be representative of samples typically analyzed and the average value and control limits of the QC sample shall be determined prior to monitoring the measurement process. The QC sample precision shall be checked against the ASTM method precision to ensure data quality.
NOTE 5 - Because the acid number can vary while the QC sample is in storage, when an out-of-control situation arises, the stability of the QC sample can be a source of the error.
10. Calculations
10.1 Calculate the acid number as follows:
Acid number, mg of KOH/g = [(A - B)N x 56.1]/W
where:
A = KOH solution required for titration of the sample (8.2), mL,
B = KOH solution required for titration of the blank (8.3), mL,
N = normality of the KOH solution, and
W = sample used, g.
