ASTM D2784 Test Method for Sulfur in Liquefied Petroleum Gases (Oxy-Hydrogen Burner or Lamp)
TURBIDIMETRIC FINISH
12. Apparatus
12.1 Photometer - Preferably a spectrophotometer having an effective band width of about 50 nm, and equipped with a blue-sensitive phototube for use at 450 nm, or alternatively a filter photometer equipped with a color filter having a maximum transmission at approximately 450 nm.
12.2 Absorption Cells, having an optical path length of 5 cm. With use the cells may become coated with a film. To remove this film, wash the cells with a detergent, using a soft brush. After cleaning, rinse thoroughly with water.
NOTE 11 - The procedure as written assumes an absorbance change of about 0.10 for each 100 µg of sulfur in 50 mL of solution measured in a 5-cm cell. Photometers employing cells of shorter optical paths will not give the precision of measurement stated in this method.
12.3 Scoop, capable of dispensing 0.30 +/- 0.01 g of barium chloride dihydrate as specified in 13.2.
13. Reagents
13.1 Alcohol-Glycerin Mixture - Mix 2 volumes of denatured ethyl alcohol conforming to Formula No. 3A of the U.S. Bureau of Internal Revenue or ethyl alcohol (99 % by volume) with 1 volume of glycerin.
13.2 Barium Chloride Dihydrate(BaCl2·2H2O) - Crystals passing a 20-mesh (850-µm) sieve and retained on a 30-mesh (600-µm) sieve conforming to Specification E11.
NOTE 12 - The crystal size of the BaCl2·2H2O is an important variable that affects the development of turbidity.
13.3 Hydrochloric Acid (1 + 12) - Add 77 mL of concentrated hydrochloric acid (HCl, relative density 1.19) to a 1-L volumetric flask and dilute to the mark with water.
14. Calibration
14.1 Only by the most scrupulous care and attention to details can reliable results be obtained by this method. Before using new glassware and thereafter as required, clean the glassware with concentrated nitric acid. Rinse three times with tap water and follow with three rinses of deionized water. Reserve the glassware for use in this method alone.
14.2 Into 50-mL volumetric flasks introduce, by means of the buret, 0.25, 0.50, 0.75, 1.00, 1.50, 2.00, 3.00, and 5.00 mL of standard sulfate solution (1 mL = 100 µg S). See 6.3. Add 3.0 mL of HCl (1 + 12) to each flask, dilute to volume with water, and mix thoroughly. Prepare a reagent blank standard in a similar way, omitting the standard sulfate.
14.3 Pour the entire contents of each flask into a 100-mL beaker. Add by means of a pipet 10 +/- 0.1 mL of alcohol-glycerin mixture and mix for 3 min on the magnetic stirrer. Select a stirring speed just below that which might cause loss of sample through splashing. Maintain this speed throughout the entire procedure.
14.4 Allow the solution to stand undisturbed for 4 min. Transfer to an absorption cell and measure the initial absorbance, using water as reference.
14.5 Return the solution to the beaker and add 0.30 +/- 0.01 g of BaCl2·2H2O crystals, either by weighing this amount or by using the scoop. Stir with the magnetic stirrer for exactly 3 min. Allow to stand for an additional 4 min, transfer to the cell, and again measure the absorbance relative to water.
14.6 Following steps described in 14.3, 14.4, and 14.5, obtain a reagent blank reading by subtracting the initial absorbance of the reagent blank standard from that obtained after addition of BaCl2·2H2O. This reading should not exceed 0.005.
14.7 Obtain the net absorbance for each standard by subtracting the initial absorbance and reagent blank reading from the absorbance obtained in accordance with 14.5. Plot the net absorbance of each standard against micrograms of sulfur contained in 50 mL of solution and draw a smooth curve through the points.
14.8 To detect possible shifts, check the calibration curve daily by making single determinations.
15. Procedure for Analysis of Absorber Solutions
15.1 Drain the absorber solution into a 250-mL beaker and quantitatively rinse the absorber collecting the rinsings in the beaker.
15.2 Reduce the volume of the absorber solutions to about 25 mL by evaporation on a hot plate. Quantitatively transfer the resultant solution to a 50-mL volumetric flask, rinsing the beaker with several small portions of water. Add 3 mL of HCl (1 + 12) to the flask, make up to volume with water, and mix thoroughly.
15.3 Into a 100-mL beaker pour the entire contents of the 50-mL volumetric flask containing the solution to be analyzed. Proceed as directed in 14.3, 14.4, and 14.5.
NOTE 13 - Should the blank reading exceed 0.020, the precision obtainable will be impaired. If so, make an analysis of the reagents alone to determine whether the atmosphere or reagents are at fault. Place 30 mL of the H2O2 (1.5 %) in the 50-mL volumetric flask, dilute to the mark with HCl (1 + 215), and proceed as described in 14.6. If this reagent blank reading exceeds 0.010, results should not be considered reliable.
15.4 Obtain the net absorbance of the analysis solution by subtracting the initial absorbance and the net absorbance for the oxy-hydrogen combustion blank or the lamp combustion (depending upon the apparatus used for combustion) from that obtained after the addition of BaCl2·2H2O.
15.5 Convert net absorbance to micrograms of sulfur by using the calibration curve.
