ASTM D3227 Sulfur in Gasoline, Kerosine, Aviation Turbine, and Distillate Fuels
ASTM D3227 Standard Test Method for (Thiol Mercaptan) Sulfur in Gasoline, Kerosine, Aviation Turbine, and Distillate Fuels (Potentiometric Method)
11. Interpretation of Results
11.1 Treatment of Data - Plot the cumulative volumes of 0.01 M AgNO3 solution added against the corresponding cell potentials. Select the end point at the inflection point of the steepest portion of each "break" in the titration curve as shown in Fig. 1. The shape of the titration curve may change with different instruments. However, the above interpretation of the end point should be followed.
11.1.1 Mercaptans Only - If mercaptans alone are present in the sample, the titration produces a curve of the first type shown in Fig. 1, having an initial plateau at a potential equal to or more negative than -250 mV, and an end point when a potential change of less than 6 mV/min is reached and the change in mV/min of titrant is reduced with each incremental addition.
11.1.2 Mercaptans and Elemental Sulfur - When elemental sulfur and mercaptans are both present in the sample, a chemical interaction occurs which, in the titration solvent used, precipitates silver sulfide (Ag2S) during the titration.
11.1.3 When mercaptans are present in excess, the end of the Ag2S precipitation occurs at about -550 to -350 mV, and is followed by the precipitation of the silver mercaptide to the +300-mV end point. This situation is shown in the middle curve of Fig. 1. Since all of the Ag2S originates from an equivalent amount of mercaptan, the total titration to the mercaptide end point must be used to calculate the amount of mercaptan sulfur.
11.1.4 When elemental sulfur is present in excess, the end of the Ag2S precipitation is taken in the same region (+300 mV) as in the case of silver mercaptide, and is calculated as mercaptan sulfur.
11.1.5 When samples of light gasolines containing methanethiol (methyl mercaptan) or heavier thiols (mercaptans) appear to give erratic results, it may be necessary to cool and maintain the test apparatus below 4°C prior to proceeding, as detailed in 9.2-9.4 inclusive. For these samples, this step may be necessary for more reproducible test results.
12. Quality Assurance/Quality Control
12.1 Confirm the performance of the instrument and the test procedure by analyzing a control (QC) sample.
12.1.1 When QA/QC protocols are already established in the testing facility, these may be used when they confirm the reliability of test result.
12.1.2 When there is no QA/QC protocol established in the testing facility, Appendix X2 may be used as the QA/QC system.
12.2 Users of this test method are advised that in contractual agreements, one or more of the contracting parties can and may make Appendix X2 a mandatory practice.
13. Precision and Bias
13.1 Precision - The precision of this test method as determined by statistical examination of interlaboratory results is as follows:
13.1.1 Repeatability - The difference between two successive test results, obtained by the same operator with the same apparatus under constant operating conditions on identical test material, would in the long run, in the normal and correct operation of the test method, exceed the following values only in one case in twenty:
Repeatability: 0.00007 + 0.027x (see Note 5)
where:
x = average mercaptan sulfur, mass %.
NOTE 5 - This amount is shown graphically in Fig. 2.
13.1.2 Reproducibility - The difference between two single and independent results obtained by different operators working in different laboratories on identical test material would, in the long run, in the normal and correct operation of the test method, exceed the following values only in one case in twenty:
Reproducibility: 0.00031 + 0.042x (see Note 5)
where:
x = average mercaptan sulfur, mass %.
13.2 Bias - The bias for the procedure in this test method has not been determined.
14. Keywords
14.1 mercaptan; potentiometric; sulfur
