Based on: Standard Test Method for Sulfur in Petroleum Products (High-Temperature Method)
Sulfur with the chemical symbol S is the chemical element that is in the sixth and third groups of the periodic table of elements. Sulfur is a non-metallic, multi-volume, most commonly known as yellow crystals in sulfide and sulfate minerals.
This test method is ASTM D1552 and covers three procedures for the determination of total sulfur in petroleum products including lubricating oils containing additives, and in additive concentrates( This standard method is used to determine the amount of sulfur in Gilsonite.). This test method is applicable to samples boiling above 177°C (350°F) and containing not less than 0.06 mass % sulfur.
Two of the three procedures use iodate detection; one employing an induction furnace for pyrolysis, the other a resistance furnace. The third procedure uses IR detection following pyrolysis in a resistance furnace. 1.2 Petroleum coke containing up to 8 mass % sulfur can be analyzed.
The sample is burned in a stream of oxygen at a sufficiently high temperature to convert about 97 % of the sulfur to sulfur dioxide.
A standardization factor is employed to obtain accurate results. The combustion products are passed into an absorber containing an acid solution of potassium iodide and starch indicator. A faint blue color is developed in the absorber solution by the addition of standard potassium iodate solution. As combustion proceeds, bleaching the blue color, more iodate is added.
The amount of standard iodate consumed during the combustion is a measure of the sulfur content of the sample.
The sample is weighed into a special ceramic boat which is then placed into a combustion furnace at 1371°C (2500°F) in an oxygen atmosphere. Most sulfur present is combusted to SO2 which is then measured with an infrared detector after moisture and dust are removed by traps.
A microprocessor calculates the mass percent sulfur from the sample weight, the integrated detector signal and a predetermined calibration factor. Both the sample identification number and mass percent sulfur are then printed out. The calibration factor is determined using standards approximating the material to be analyzed.
For the iodate systems, chlorine in concentrations less than 1 mass % does not interfere. The IR system can tolerate somewhat higher concentrations. Nitrogen when present in excess of 0.1 mass % may interfere with the iodate systems; the extent of such interference may be dependent on the type of nitrogen compound as well as the combustion conditions. Nitrogen does not interfere with the IR system. The alkali and alkaline earth metals, as well as zinc, phosphorus, and lead, do not interfere with either system.
Furnaces:Two major types are available, the primary difference being the manner in which the necessary high temperatures are obtained. These two types are as follows:
The furnace work coil should have a minimum output of 500 W; the minimum input rating of the furnace must be 1000 W. With the correct amount of iron chips, weighed to
60.05 g, the maximum plate current will be between 350 and
Absorber, as described in Test Method D 1266.
Beret, standard 25-mL or automatic types available from the manufacturers of the specific combustion units, are suitable .
comprised of automatic balance, oxygen flow controls, drying tubes, combustion furnace, infrared detector and microprocessor. The furnace shall be capable of maintaining a nominal operating temperature of 1350°C (2460°F).
Specific combustion assemblies require additional equipment such as crucibles, combustion boats, crucible lids, boat pushers, separator disks, combustion tubes, sample inverters, oxygen flow indicator, and oxygen drying trains.
The additional equipment required is dependent on the type of furnace used and is available from the manufacturer of the specific combustion unit. To attain the lower sulfur concentration given in Section 1, the ceramics used with the induction furnace assembly shall be ignited in a
muffle furnace at 1371°C (2500°F) for at least 4 h before use.
Sieve: 60-mesh (250-mm).
Take samples in accordance with the instructions in Practice D 4057.
Assemble the apparatus according to the instructions furnished by the manufacturer. Purify the oxygen by passing it through (1) H2SO4 (relative density 1.84), (2) Ascarite, and (3) magnesium perchlorate (Mg(ClO4)2) or phosphorus pentoxide (P2O5) .Connect a rotameter between the purifying train and the furnace. Insert a small glass-wool plug in the upper end of the glass tubing connecting the furnace with the absorber to catch oxides of tin. Connect the exit end of the combustion tube to the absorber with glass tubing, using gum rubber tubing to make connections. Position the absorber so as to make this delivery line as short as possible. Fig. 2 illustrates schematically the assembled apparatus. Adjust the oxygen flow to 1 6 0.05 L/min. Add 65 mL of HCl (3 + 197) and 2 mL of starch-iodide solution to the absorber. Add a sufficient amount of the appropriate standard KIO3 solution (Table 1) to produce a faint blue color. This color will serve as the end point for the titration. Adjust the buret to zero. Turn on the furnace filament switch and allow at least 1 min warm-up before running samples .
Assemble the apparatus according to the instructions furnished by the manufacturer. Purify the oxygen by passing it through (1) H2SO4 (relative
density 1.84), (2 ) Ascarite, and (3) Mg(ClO4)2 or P2O5 . Connect a rotameter between the purifying train and the furnace. Fig. 3 illustrates schematically the
assembled apparatus. Turn on the current and adjust the furnace control to maintain a constant temperature of 13166 14°C (2400 6 25°F). Adjust the oxygen flow rate to 2 6 0.1 L/min. Add 65 mL of HCl (3 6 197) and 2 mL of starch-iodide solution to the absorber. Add a few drops of the appropriate standard KIO3 solution (Table 2) to produce a faint blue color. Adjust the buret to zero.
9.3 Resistance–Type Furnace–IR Detection—Assemble and adjust apparatus according to manufacturer’s instructions. Initialize microprocessor, check power supplies, set oxygen pressure and flows and set furnace temperature to 1371°C
Determination of Alum Factor:
Because these rapid combustion methods involve the reversible reaction 2SO2 + O2 = 2SO3, it is not possible to evolve all the sulfur as SO2. The equilibrium of the reaction is temperature dependent and, in an oxygen atmosphere above 1316°C, about 97 % of the sulfur is present as SO2. To assure that the furnace is in proper adjustment and that its operation produces acceptably high temperature, potassium alum is employed for standardizing the apparatus. Depending on the type of combustion equipment used, proceed as described in Sections 10-13 to determine the alum factor. Use 15 mg weighed to 60.1 mg of potassium alum for this determination. Use the same materials in the determination of the alum and standardization factors as for the unknown samples. For example, V2O5 has a definite effect and should be included if
used for unknowns as recommended in the procedure with the resistance-type furnace.
Calculate the alum factor as follows:
Alum Factor (AF) = (SA ×WA)/ (100(Va – Vb) × C1)
SA= mass percent sulfur in potassium alum used,
WA= milligrams of potassium alum used,
Va= milliliters of standard KIO3 solution used in determining the alum factor,
Vb= milliliters of standard KIO3 solution used in the blank determination,
C1 = sulfur equivalent of the standard KIO3 solution used in determining the alum factor, mg/mL.
Note! The alum factor should be in the range from 1.02 to 1.08. If values smaller than 1.02 are observed, confirm independently the sulfur content of the alum and the sulfur equivalent of the KIO3 solution before repeating the alum factor determination. If values larger than 1.08 are observed, make adjustments in the equipment in accordance with the manufacturer’s recommendation and repeat the alum factor determination.
Determination of Standardization Factor:
Because effects such as sample volatility can also affect the relative recovery as SO2 of the sulfur originally present in the sample, it is necessary to determine a standardization factor. Proceed as described in Sections 10-13, using an oil sample of similar type to the unknown sample and of accurately known sulfur content.
For IR detection, determine and load the microprocessor with the calibration factor for the particular type of sample to be analyzed (lubricating oil, petroleum coke, residual fuel) as recommended by the manufacturer.9
Calculate the standardization factor as follows:
Alum Factor (FS) = (SS ×WS)/ (100(VS – Vb) × C)
Ss= mass percent sulfur in standardization sample used
Ws= milligrams of standardization sample used
Vb= millilitres of standard KIO3 solution used in the blank determination
Vs= millilitres of standard KIO 3 solution used in determining the standardization factor
C = sulfur equivalent of the standard KIO3 solution usedin determining the standardization factor, mg/mL
Run a suitable analytical quality control sample several times daily. When the observed value lies between acceptable limits on a quality control chart, proceed with sample determinations.