ASTM D2784 Test Method for Sulfur in Liquefied Petroleum Gases (Oxy-Hydrogen Burner or Lamp)
7. Sampling Test Specimens and Test Units
7.1 Obtain the test unit in a container by the method conforming to the recommendations in Practice D1265, or IP Method 181.
7.2 Evacuate a clean, dry 75-mL cylinder and weigh to the nearest 0.05 g. Connect the container to the inverted supply cylinder and introduce 24 to 45 g of the liquefied gas, taking care that the container does not become full of liquid. To prevent this, bleed off a small amount of the liquid phase of the material after filling but before reweighing. Reweigh the cylinder to 0.05 g.
NOTE 2 - The 75-mL, corrosion-resistant metal vessel can be cleaned as follows: Remove the needle valve. Wash the interior of the vessel and valve, first with a sulfur-free hydrocarbon, such as n-pentane, and then wash with acetone. Dry the interior of the vessel with clean compressed air and rinse it with HCl (1 + 10). Rinse the interior with water until the wash water is neutral to a pH test paper. Wash the vessel with acetone and allow to drain at least 10 min. Dry the vessel with a stream of clean, compressed air and reassemble.
NOTE 3 - The corresponding liquid petroleum gas sample density of the material being analyzed by this test method for sulfur will ultimately determine the maximum weight that can be introduced into the 75-mL cylinder without becoming full of liquid. This maximum weight value needs to be compared to the sample sizes in Table 1 needed for the corresponding finishes and expected sulfur concentrations indicated for determining the appropriate sample weight that needs to be introduced into the cylinder in 7.2. As an example, if the sample density is 0.6 g/mL at room temperature, and if the weight of the liquefied petroleum gas is maintained below 45 g in a 75-mL container, the container cannot become full of liquid at room temperature.
8. Procedure for Combustion of Sample
8.1 Connect the sample cylinder with stainless steel tubing to the gas expansion valve. Attach to this another section of stainless steel tubing which runs to the vicinity of the burner. Make the final connection to the burner with sulfur-free rubber tubing. Wrap the expansion valve with heating tape and connect this to a variable transformer. Insert a thermometer between the heating tape and expansion valve so that the thermometer bulb is in contact with the valve body. See Fig. 4.
NOTE 4 - Other temperature measuring devices such as thermocouples or resistance thermometers, may be used when the temperature readings obtained by these devices are determined to produce the same results that are obtained when mercury-in-glass thermometers are used. The precision and bias given in Section 18 may or may not apply in such cases since the published precision is based on an interlaboratory study where only mercury-in-glass thermometers were used. No information on the effect on precision when using alternative temperature measuring devices is available.
8.2 Turn on the variable transformer and allow the expansion valve to reach 43°C (110°F). Alternatively the expansion valve may be placed in a suitable metal beaker and covered with water maintained at 110°F.
8.3 Oxy-Hydrogen Combustion - Assemble the apparatus according to the manufacturer's directions (see also 14.1). Add to the absorber 25 mL of the hydrogen peroxide solution. (Warning - Before attempting subsequent operations, the operator should (1) be aware of the various hazards that can exist through the improper use of hydrogen as a fuel, and (2) have the safety shield in place.)
8.3.1 Light the burner and insert into the combustion chamber. If necessary, readjust gas flows. Open the bottom valve of the sample cylinder. Slowly open the expansion valve until an optimum burning rate is achieved.
8.4 Lamp Combustion - Add to the absorber 25 mL of hydrogen peroxide solution. Assemble the chimney, absorber, and spray trap and connect to the CO2-O2 and vacuum manifold. Make the necessary vacuum adjustments (see 5.1 of Test Method D1266). Set up a control blank absorber as in 5.3 of Test Method D1266.
8.4.1 Open the bottom valve of the sample cylinder. Slowly crack the gas expansion valve. Light the burner with an alcohol lamp, and insert the burner into the combustion chamber (chimney).
8.5 Burn a quantity of sample in accordance with Table 1.
NOTE 5 - In burning materials with sulfur concentrations greater than 50 µg/g, restrict sample sizes to give quantities that will not contain more than 250 µg of sulfur for the turbidimetric finish or more than 150 µg for the barium perchlorate finish. Alternatively, aliquots of the absorber solutions which do not contain more than these maximums can be used.
NOTE 6 - Minor adjustment of the gas flow rates can be necessary to maintain those recommended by the manufacturer.
8.6 After a sufficient quantity of sample has been burned, turn the bottom valve of the cylinder off. Allow the remaining gas in the tubing and gas expansion valve to burn itself out. Turn the heat off on the gas expansion valve. Disconnect the tubing from the sample cylinder and reweigh the sample cylinder to the nearest 0.05 g. Leave the absorber solution in the assembled lamp unit. The same absorber solution will be used for the scavenger-rinse burning. Allow the expansion valve to cool to ambient temperature.
8.7 If the oxy-hydrogen burner permits, flush the tubing and valve with 10 mL of scavenger-rinse and burn without disconnecting the tubing. Otherwise disconnect the tubing and burn in the normal liquid mode. For the lamp burning collect the rinsings in a standard lamp sulfur (see Test Method D1266) flask. Insert a standard burner equipped with a wick into the flask and carry out the combustion as described in Section 7 of Test Method D1266.
8.8 For the oxy-hydrogen burners, when all of the rinsings are consumed, shut down the burner as recommended by the manufacturer.
8.9 After the rinsings have been burned in the lamp, remove it, turn off the CO2-O2 supply, and turn off the vacuum pump.
8.10 For oxy-hydrogen blank determinations burn a hydrocarbon stock with a very low or nondetectable sulfur content. Make at least two of these prior to the analysis of samples with trace sulfur contents to ensure that the blanks are small and constant. Subtract from the total sulfur figures any blank so obtained. The remainder is the net micrograms of sulfur from the sample. Likewise subtract any sulfur obtained in the lamp combustion blank from the total figure.
8.11 Disconnect the spray trap from the vacuum line and thoroughly rinse the spray trap and chimney with about 35 mL of distilled water, collecting the rinsings in the absorber. It is important that any materials clinging to these parts be transferred to the absorber to avoid low values for sulfur content.
