ASTM D3246 Standard Test Method for Sulfur in Petroleum Gas by Oxidative Microcoulometry
8. Sampling
8.1 Supply samples to the laboratory in high-pressure sample cylinders, obtained using the procedures described in Practice D1265 and Practice F307.

8.2 Because of the reactivity of most sulfur compounds, it has been found desirable to use TFE-fluorocarbon-coated cylinders or other specially treated sample containers. Test samples as soon as possible after receipt.

9. Preparation of Apparatus
9.1 Carefully insert the quartz pyrolysis tube in the pyrolysis furnace and connect the reactant and carrier gas lines.

9.2 Add the electrolyte solution to the titration cell and flush several times. Maintain an electrolyte level of 1/8 to 1/4 in. (3.2 to 6.4 mm) above the platinum electrodes.

9.3 Place the heating tape on the inlet of the titration cell.

9.4 Place an exit tube insert packed loosely with about 1 in. (25 mm) of quartz wool into the exit end of the pyrolysis tube. The quartz wool end of the exit tube should be in the hot zone of the pyrolysis tube.

9.5 Depending upon the instrumentation used, set up the titration cell to allow for adequate mixing of its contents and connect the cell inlet to the outlet end of the pyrolysis tube. Position the platinum foil electrodes (mounted on the movable cell head) so that the gas inlet flow is parallel to the electrodes with the generator anode adjacent to the generator cathode. Assemble and connect the coulometer and recorder (integrator optional) as designed or in accordance with the manufacturer's instructions. Fig. X1.2 illustrates the typical assembly and gas flow through a coulometric apparatus.
9.5.1 Turn the heating tape on.

9.6 Adjust the flow of the gases, the pyrolysis furnace temperature, titration cell, and the coulometer to the desired operating conditions. Typical operational conditions are given in Table 1.

10. Calibration and Standardization
10.1 Prepare a series of calibration standards covering the range of sulfur concentration expected. Follow instructions in 7.13, 7.14, or dilute to appropriate level with isooctane.

10.2 Adjust the operational parameters (9.5).

NOTE 8 - A ratio of 80 % oxygen to 20 % inert gas gives an acceptable recovery, and permits the use ofa larger sample and a more rapid-charging rate.

10.3 The sample size can be determined either volumetrically or by mass. The sample size should be 80 % or less of the syringe capacity.
10.3.1 Volumetric measurement can be obtained by filling the syringe with about 8 µL or less of sample, being careful to eliminate bubbles, retracting the plunger so that the lower liquid meniscus falls on the 1-µL mark, and recording the volume of liquid in the syringe. After the sample has been injected, again retract the plunger so that the lower liquid meniscus falls on the 1-µL mark, and record the volume of liquid in the syringe. The difference between the two volume readings is the volume of sample injected.

10.3.2 Alternatively, the sample injection device can be weighed before and after the injection to determine the amount of sample injected. This test method provides greater precision than the volume delivery method, provided a balance with a precision of +/- 0.01 g is used.

10.4 Insert the syringe needle through the inlet septum up to the syringe barrel and inject the sample or standard at an even rate not to exceed 0.1 to 0.2 µL/s. When a microlitre syringe is used with an automatic injection adapter, the injection rate (volume/pulse) should be calibrated to deliver 0.1 to 0.2 µL/s.

10.5 Repeat the measurement of each calibration standard at least three times.

NOTE 9 - Not all of the sulfur in the sample comes through the furnace as titratable SO2. In the strongly oxidative conditions of the pyrolysis tube some of the sulfur is also converted to SO3 which does not react with the titrant. Accordingly, sulfur standards of n-butyl sulfide in isooctane or sulfur standards appropriate to sample boiling range and sulfur type and sulfur concentration should be prepared to guarantee adequate standardization. Recoveries less than 75 % are to be considered suspect. Low recoveries are an indication to the operator that he should check his parameters, his operating techniques, and his coulometric system. If the instrument is being operated properly, recoveries between 75 and 90 % are to be expected.

10.6 Calculate the percent sulfur found by the coulometer. For a 1-mV (span) recorder with a sensitivity of 0.1 mV/in. and a speed of 0.5 in./min:
Sulfur recovered, % = [(A x 1.99)/(R x So x VL/1000)] x 100
where:
A = area, cm2,
R = coulometer range setting, Ω,
So = known concentration of sulfur in the standard blend, µg/mL, and
VL = volume standard blend charged, µL.

10.6.1 For a disk integrator:
Sulfur recovered, % = [(C x 1.99 x 10(-3)/(R x So x VL/1000)] x 100
where:
C = 100 x number of integrator pen full scale excursions.

Derivation of equations is given in Appendix X1.

10.6.2 For an electronic integrator:
Sulfur Recovered, % = A/B x 100
(using consistent sample sizes)
where:
A = integrator result, mg/kg, and
B = known concentration of sulfur in standard blend, mg/kg.

NOTE 10 - For further explanation of the derivation of the calculation, see Test Method D3120.

10.6.3 For Instruments Equipped with a Microprocessor or Computer - Associated instrument software may be used to do the calculations automatically.

10.7 If the fraction of sulfur converted to SO2 drops below 75 % of the standard solutions, fresh standards should be prepared. If a low conversion factor persists, procedural details should be reviewed.

10.8 Calculate the average calibration factor, F, µg S/cm2, as follows:
F = (So x VL/1000)/A

11. Quality Assurance (QA)
11.1 Calibration Check Sample(s) - A sample of known sulfur content shall be run after each calibration. The sample can also be analyzed periodically throughout a series of analyses to check the functioning of the instrument and the validity of the calibration curve.

11.2 Quality Control (QC) Sample(s) - Confirm the performance of the instrument or the test procedure by analyzing a QC sample (see 7.16).
11.2.1 When QC/QA protocols are already established in the testing facility, they may be used when confirming the reliability of the test result.

11.2.2 When there is no QC/QA protocol established in the testing facility, Appendix X2 can be used as the QC/QA system.