ASTM D1552 method for sulfur in petroleum products (High-temperature method)
13. Procedure with Induction-Type Furnace
13.1 Sample Preparation - Add a 3.2 to 4.8-mm (1/8 to 3/16-in.) layer of alundum or magnesium oxide to a sample crucible. Make a depression in the bed with the end of a stirring rod. Weigh the crucible to 0.1 mg. Weigh into the depression the proper amount of sample according to Table 1 (13.1.1) (Note 2). Cover the sample with a separator disk (Note 3). Place on the separator disk the predetermined amount of iron chips necessary to obtain the required temperature (6.1.1.2). This is usually between 1.2 and 2.0 g, but should be held constant with +/- 0.05 g. Sprinkle about 0.1 g of tin on the iron. Cover the crucible with a lid and place on the furnace pedestal.

13.1.1 Under no conditions shall an organic sample larger than 100 mg be burned in an induction-type furnace.

NOTE 2 - More concentrated KIO3 solutions, such as the 0.06238 N solution, may be found more convenient for samples containing more than 10 % sulfur. The sample size and KIO3 concentration should be chosen so that not more than 25 mL of titrant are needed.

NOTE 3 - The use of the separator disk is optional.

13.2 Combustion and Titration - Turn on the plate current switch. After about 1 min for warm-up, raise the pedestal and lock into position. The plate current will fluctuate for a few seconds and should gradually rise to a maximum value. Add the appropriate standard KIO3 solution (Table 1) to the absorber to maintain the blue color. Should the absorber solution become completely colorless, discard the determination. Make KIO3 additions as the rate of evolution of SO2 diminishes such that, when combustion is completed, the intensity of the blue color is the same as the initial intensity. Combustion is complete when this color remains for at least 1 min and the plate current has dropped considerably. Record the volume of KIO3 solution required to titrate the SO2 evolved.

13.3 Blank Determination - Make a blank determination whenever a new supply of crucibles, materials, or reagents is used. Follow the preceding procedure, but omit the sample.

14. Procedure with Resistance-Type Furnace
14.1 Sample Preparation - Weigh into a combustion boat the proper amount of sample according to Table 2. Add 100 +/- 5 mg of vanadium pentoxide and completely cover the mixture with Alundum.

14.2 Combustion and Titration - Place the boat in the cool portion of the combustion tube, near the entrance. To proceed with the combustion, push the boat containing the sample progressively into the hotter zone of the combustion tube using the equipment provided by the manufacturers. The boat should be advanced as rapidly as possible consistent with the rate of evolution of SO2. Add the appropriate standard KIO3 solution (Table 2) to the absorber to maintain the blue color. Should the absorber solution become completely colorless, discard the determination. Make KIO3 additions as the rate of evolution of SO2 diminishes such that, when combustion is completed, the intensity of the blue color is the same as the initial intensity. Combustion is complete when this color remains for at least 1 min. Record the volume of KIO3 solution required to titrate the SO2 evolved.

14.3 Blank Determination - Make a blank determination whenever a new supply of combustion boats, materials, or reagents is used. Follow the above procedure, but omit the sample.

15. Procedure with Resistance Furnace - IR Detection 15.1 Allow the system to warm up and the furnace to reach operating temperature.

15.2 After homogeneity of the sample is assured, select the sample size as follows: for liquid samples, take up to 0.13 g for analysis and for solid samples, take up to 0.4 g for analysis. In each case mass percent sulfur times weight of sample must be less than or equal to four in the case of the SC32 instrument, and two in the case of the SC132 instrument. For other instruments, consult the manufacturer's instructions.

15.3 Determine and store the system blank value.

15.4 Weigh solid samples into combustion boats and record the net weights. For liquid samples, follow 15.4.1 - 15.4.4. It is possible to weigh and store several weights in the microprocessor before beginning a series of burns.

15.4.1 Fill the combustion boat to one-third capacity with evenly spread MgO powder.

15.4.2 Form a slight trench in the MgO powder with a scoop.

15.4.3 Place the combustion boat on the balance and weigh an appropriate amount of the sample into the trench in the MgO powder. Record and enter the weight.

15.4.4 Remove the combustion boat from the balance and add MgO powder until the combustion boat is filled to two-thirds capacity. (Warning - V2O5 can cause deterioration of the furnace ceramics so use it with care.)

NOTE 4 - If unacceptable repeatability is encountered for particular oil samples, combustion promoter such as V2O5 or the LECO product Com-Aid can be substituted for the MgO.

15.5 Initiate oxygen flow and load boat into furnace.

15.6 When the analysis is complete, read the result from the microprocessor.

15.7 Remove the expended combustion boat from the furnace.

15.8 Make repeated runs until two results differ by less than the repeatability of the method.

16. Calculation
16.1 Calculation for Iodate Detection - Calculate the sulfur content of the sample as follows:
Sulfur, mass % = (100 (V - Vb) x Fs x C)/W
where:
V = standard KIO3 solution, mL, used in the analysis,
Vb = standard KIO3 solution, mL, used in the blank determination,
Fs = standardization factor (see 10.1.2),
C = sulfur equivalent of the standard KIO3 solution used in the analysis, mg/mL, and
W = milligrams of sample used in the analysis.

16.2 Calculation for IR Detection:
16.2.1 Report all results using the microprocessor.

16.2.2 Report the average of two results.