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)
9. Procedure
9.1 Determination of Density - If the sample is to be measured volumetrically, determine the density by Test Method D1298 or Test Method D4052 at the temperature at which the test portion will be taken, either directly or from the density determined at a reference temperature and converted to the transfer temperature by use of the Petroleum Measurement Tables (Guide D1250).
9.2 Removal of Hydrogen Sulfide - Test the sample qualitatively for hydrogen sulfide (H2S) by mixing 5 mL of the sample with 5 mL of the acid CdSO4 solution. If no precipitate appears, proceed with the analysis of the sample as described in 9.3. If a yellow precipitate appears, remove the H2S in the following manner: Place a quantity of the sample, three to four times that required for the analysis, in a separatory funnel containing a volume of the acid CdSO4 solution equal to one half that of the sample and shake vigorously. Draw off and discard the aqueous phase containing the yellow precipitate. Repeat the extraction with another portion of the CdSO4 solution. Again draw off the aqueous phase, and wash the sample with three 25 to 30-mL portions of water, withdrawing the water after each washing. Filter the hydrocarbon through a rapid paper. Test a small portion of the washed sample, such as in a test tube or vial, with a few millilitres of the CdSO4 solution as described previously. If no further precipitate is formed, proceed as directed in 9.3. If a precipitate appears, repeat the extraction with the CdSO4 solution until all of the H2S has been removed. (Warning - Certain straight run gasolines can contain significant amounts of both low molecular weight mercaptans and dissolved elemental sulfur, which, when heated under total reflux conditions, may produce H2S by the inter-reaction of both. This phenomenon is particularly noticed in straight run gasolines produced from some natural gas condensates. Therefore, it is advisable that during the removal of H2S (and after all H2S has been extracted), that no heat should be applied to the sample.)
9.2.1 When the test results obtained are not for referee purposes and Quality Assurance/Quality Control (QA/QC) protocol permits, an alternative test for, and a procedure for the removal of, hydrogen sulfide can be used. This process uses lead acetate paper and sodium hydrogen carbonate and is described in Appendix X1.
9.2.2 There are available automated instruments that eliminate the necessity to remove H2S when present, prior to performing the analysis, since the H2S titration curve does not interfere with the titration curve of the thiol or mercaptan determination. Such equipment may be used when the test is not for referee purposes. The precision statement does not include data obtained from this alternate procedure.
9.2.3 For referee purposes, the protocol, as detailed in 9.2, shall be used. A possible non-referee situation may be the instance of routine refinery control.
9.3 Measure with a pipet (or other suitable transfer device) or weigh 20 to 50 mL of the original or treated sample into an appropriately sized beaker (for example, a 200, 250, or 300 mL size beaker is typically large enough) containing 100 mL of the appropriate titration solvent. Place the beaker on the titration stand or on the auto-sampler of an automatic titration system. If an automatic titration system is used, set up the system to reproduce the experimental conditions specified in 9.3.1, 9.3.2, and 9.3.3. Adjust the position of the titration stand so that the electrodes are about half immersed. Fill the buret with 0.01 mol/L alcoholic AgNO3 solution and position it in the titration assembly so that the tip extends approximately 25 mm (1 in.) below the surface of the liquid in the beaker. Adjust the speed of the stirrer to give vigorous stirring without spattering.
9.3.1 Record the initial buret and cell potential readings. The usual meter readings for mercaptan presence are in the -250 mV to -350 mV range. Add suitable small portions of 0.01 mol/L AgNO3 solution and, after waiting until a constant potential has been established, record the buret and meter readings. Consider the potential constant if it changes less than 6 mV/min.
NOTE 2 - If potential readings obtained with freshly prepared electrodes are erratic, it is possible that the electrodes are not properly conditioned. This difficulty usually disappears in succeeding titrations.
NOTE 3 - When analyzing samples with a relatively high mercaptan concentration, the use of a 20 to 50 mL sample size can require an unreasonably long titration time and a large quantity of the titrant. For such samples, a smaller sample size may be used or the original sample may be diluted with a suitable solvent that is miscible with the sample and free of mercaptans into the 20 to 50 mL sample size range prior to titrating the sample. The precision of samples using <20 mL sample sizes or samples that have been diluted have not been determined in an interlaboratory study.
NOTE 4 - With certain instruments, the algebraic sign of the potentials may appear reversed.
9.3.2 When the potential change is small for each increment of AgNO3 solution, add volumes as large as 0.5 mL. When the change of potential becomes greater than 6 mV/0.1 mL, use 0.05-mL increments of 0.01 mol/L AgNO3 solution. Near the end point of the titration, 5 or 10 min may elapse before a constant potential is obtained. Although it is important to wait for equilibrium conditions, it is also important that the duration of the titration be as short as possible to avoid oxidation of the sulfur compounds by atmospheric oxygen. Once started, a titration must never be interrupted and resumed later.
9.3.3 Continue the titration until the meter reading change of the cell potential per 0.1 mL of 0.01 M AgNO3 solution has become relatively constant. Consider the potential constant if it changes less than 6 mV/min. Remove the titrated solution, rinse the electrodes with alcohol, and wipe with a dry tissue. If an automatic titration system is used, rinse the electrodes well with alcohol, allow the excess alcohol to drain off the electrode and then proceed with the next sample. Between successive determinations (or batches of determinations in the case of automatic titration systems) on the same day, store the electrodes in accordance with 8.1 and 8.2.5.
9.4 As often as is required and preferably, at least daily, when the test equipment is being used, perform a blank titration following 9.3-9.3.3 without adding a test sample.
10. Calculation
10.1 Calculate the mercaptan sulfur content of the sample as follows:
Mercaptan sulfur, mass % = (DM (A1 - A0) x 3.206)/W
or
Mercaptan sulfur, mass % = (DM (A1 - A0) x 3.206)/(d x V)
D = (W + I)/W
D = (V + J)/V
where:
A1 = millilitres of AgNO3 solution required to reach the end point in the vicinity of +300 mV (see Fig. 1), when a sample is being titrated,
A0 = mL of AgNO3 solution required to reach the end point in the vicinity of +300mV (see Fig. 1), when only the blank is titrated. (Note, there are no other changes to the factors in the equation).
d = density of sample at transfer temperature, g/mL,
D = dilution factor (if necessary, see Note 3),
I = grams of diluent used,
J = mL of diluent used,
M = molarity of the AgNO3 solution,
W = grams of sample used,
3.206 = 100 3 g meq weight S in mercaptan, and
V = mL of sample used.
