ASTM D2784 Test Method for Sulfur in Liquefied Petroleum Gases (Oxy-Hydrogen Burner or Lamp)
BARIUM PERCHLORATE TITRATION FINISH
9. Reagents
9.1 Ion-Free Water - Distill deionized water and store in tightly capped, high-density polyethylene bottles.
9.2 Hydrochloric Acid, Standard Alcoholic (0.1 M) - Dilute 20 mL of aqueous 0.5 M HCl with 80 mL of isopropanol.
9.3 Inhibited Thorin-Methylene Blue Mixed Indicator Solution - The indicator is made up as two solutions and these mixed together in equal volumes once per week as follows:
Solution A:
0.8 g thorin, 5,8 0.29 g potassium bromate, water to make 500 mL
Solution B:
0.16 g methylene blue, 0.2 mL of 0.5 M HCl, water to make 500 mL
9.4 Fleisher's Methyl Purple Indicator Solution.
9.5 Barium Perchlorate (0.005 M) - Dissolve 1.95 g of barium perchlorate trihydrate in 200 mL of water and add 800 mL of isopropanol. Adjust the apparent pH to about 3.5 with perchloric acid, using a pH meter.
9.6 Perchloric Acid, 70 %.
9.7 Sodium Hydroxide, Standard Solutions (0.03 M) - Prepare by mixing 7 parts of water with 3 parts of standard 0.1 M sodium hydroxide (NaOH) solution. Concentrate 400 mL of 0.03 N NaOH solution by evaporating to 30 mL, and determine any sulfate present in accordance with Appendix A1, Turbidimetric Procedure for Sulfate of Test Method D1266. If sulfate is found, corrections must be made for any sulfur introduced by the reagent in the alkali titration following combustion.
9.8 Methylene Blue.
10. Preparation of Working Curve
10.1 Into separate 30-mL beakers pipet each of the aliquots of the standard sulfate solution given in Table 2. See 6.3. To each aliquot add sufficient water to make 3.4 mL, 12 mL of isopropanol (total volume 15.4 mL) and 3 drops of mixed thorin-methylene blue indicator solution. Titrate as indicated below. For each sulfur level given in Table 2, titrate three of the corresponding aliquots. Plot the millilitres of titrant used versus micrograms of sulfur. Draw the best straight line through points. Check at least two points on the curved at least every 10 days.
11. Procedure for Analysis of Solutions
11.1 Quantitatively transfer the absorber contents to a 500-mL Erlenmeyer flask, using ion-free water for rinsing. Add 2 drops of Fleisher's methyl purple indicator solution to this solution and titrate to a faint green end point with 0.03 M NaOH solution (Note 8). Add 1 mL more of the 0.03 M NaOH solution to the solution and reduce the volume to 2 to 3 mL by evaporation on a hot plate in sulfate-free environment. (Warning - Do not boil.) DO NOT BOIL DRY. Cool the solution to room temperature and measure its volume in a 10-mL graduate (Note 8). Adjust the volume to 3.0 mL by adding ion-free water.
NOTE 7 - The volume of sodium hydroxide should not exceed 2 mL. More indicates that the sulfur or halogen content is excessive or that there is a serious air leak in the apparatus.
NOTE 8 - For high or completely unknown sulfur contents, the concentrated absorbent can be quantitatively transferred to a 5-mL volumetric flask, adjusted to 5 mL, and aliquots used. Each aliquot is then subsequently made up to 3 mL as in 11.1. Continue as in 11.2.
11.2 Transfer the absorbent to a 30-mL beaker, rinse the graduate and the 500-mL boiling flask successively with two 6-mL portions of isopropanol, and add the rinses to the beaker.
11.3 Pipet 0.40 mL of the standard sulfate solution (40 µg of sulfur) into the beaker. Add 2 drops of the thorin-methylene blue mixed indicator solution. Adjust the resultant gray-green color by adding 0.1 M HCl dropwise to the solution until the color changes to bright green.
11.4 The 2-mL buret containing standard barium perchlorate should have its tip positioned just below the surface of the solution in the beaker. The solution must be stirred by a small bar on a magnetic stirrer or with a small propeller stirrer. A white background and good white light may be helpful in obtaining a precise end point. Add the barium reagent at a steady rate of0.1 mL in 5 (+/- 1) s until the end point is indicated by a rapid, though slight, color change from green to a bluish gray (Note 9). Shut off the buret at the point of greatest rate of color change (Note 10).
NOTE 9 - It is helpful to match end point colors with solutions saved from prior standardization titrations performed within the last 15 min and well stirred to prevent drop-out of the colored barium sulfate precipitate. People having a low red-green color sensitivity find that using the blue light of Test Method D156, sharpens the end point very considerably.
NOTE 10 - The end point can be checked by again adding 40 µg of sulfur (0.4 mL standard sulfuric acid) and retitrating to the end point.
11.5 From the working curve, find the total sulfur titrated to the nearest 1 µg. Subtract the 40 µg added.
11.6 For blank determinations, repeat the operations in 8.3 and 8.7, and burn a hydrocarbon stock with a very low or nondetectable sulfur content. Burn for the same length of time as the sample in the normal liquid mode. Subtract from the sulfur figures in 11.5 any blank so obtained. This is the net micrograms of sulfur from the sample.
