ASTM D808 Chlorine in New and Used Petroleum Products (Bomb Method)
8. Procedure
8.1 Preparation of Bomb and Sample - Cut a piece of firing wire approximately 100 mm in length. Coil the middle section (about 20 mm) and attach the free ends to the terminals. Arrange the coil so that it will be above and to one side of the sample cup. Insert into the coil a nylon thread, or wisp of cotton, of such length that one end will extend into the sample cup. Place about 5 mL of Na2CO3 solution in the bomb and by means of a rubber policeman, wet the interior surface of the bomb, including the head, as thoroughly as possible. Introduce into the sample cup the quantities of sample and white oil (Note 1) (Warning - Do not use more than 1 g total of sample and white oil or other chlorine free combustible material.) specified in Table 1 (Warning - Do not add oxygen or ignite the sample if the bomb has been jarred, dropped, or tilted.), weighing the sample to the nearest 0.2 mg. (When white oil is used, stir the mixture with a short length of quartz rod and allow the rod to remain in the sample cup during the combustion.)
8.1.1 After repeated use of the bomb for chlorine determination, a film may be noticed on the inner surface. This dullness can be removed by periodic polishing of the bomb. A satisfactory method for doing this is to rotate the bomb in a lathe at about 300 rpm and polish the inside with Grit No. 2/0 or equivalent paper coated with a light machine oil to prevent cutting, and then with a paste of grit-free chromic oxide and water. This procedure will remove all but very deep pits and put a high polish on the surface. Before using the bomb wash it with soap and water to remove oil or paste left from the polishing operation. Bombs with porous or pitted surfaces should never be used because of the tendency to retain chlorine from sample to sample.
8.1.2 When the sample is not readily miscible with white oil, some other nonvolatile, chlorine-free combustible diluent may be employed in place of white oil. However, the combined weight of sample and nonvolatile diluent shall not exceed 1 g. Some solid additives are relatively insoluble, but may be satisfactorily burned when covered with a layer of white oil. (Warning - Do not use more than 1 g total of sample and white oil or other chlorine-free combustible material.)
NOTE 1 - The practice of running alternately high and low samples in chlorine content shall be avoided whenever possible. It is difficult to rinse the last traces of chlorine from the walls of the bomb and the tendency for residual chlorine to carry over from sample to sample has been observed in a number of laboratories. When a sample high in chlorine has preceded one low in chlorine content, the test on the low-chlorine sample shall be repeated and one or both of the low values thus obtained can be considered suspect if they do not agree within the limits of repeatability of this method.
8.2 Addition of Oxygen - Place the sample cup in position and arrange the nylon thread, or wisp of cotton, so that the end dips into the sample. Assemble the bomb and tighten the cover securely. Admit oxygen (Warning - Do not add oxygen or ignite the sample if the bomb has been jarred, dropped, or tilted.) slowly (to avoid blowing the oil from the cup) until a pressure is reached as indicated in Table 2.
8.3 Combustion - Immerse the bomb in a cold water bath. Connect the terminals to the open electrical circuit. Close the circuit to ignite the sample. Remove the bomb from the bath after immersion for at least 10 min. Release the pressure at a slow, uniform rate such that the operation requires not less than 1 min. Open the bomb and examine the contents. If traces of unburned oil or sooty deposits are found, discard the determination, and thoroughly clean the bomb before again putting it in use (8.1.1).
8.4 Collection of Chlorine Solution - Rinse the interior of the bomb, the sample cup, and the inner surface of the bomb cover with a fine jet of water, and collect the washings in a 600-mL beaker. Scrub the interior of the bomb and the inner surface of the bomb cover with a rubber policeman. Wash the base of the terminals until the washings are neutral to the indicator methyl red. (The volume of the washings is normally in excess of 300 mL.) Take special care not to lose any wash water.
8.5 Determination of Chlorine - Acidify the solution by adding HNO3(1 + 1) drop by drop until acid to methyl red. Add an excess of 2 mL of the HNO3 solution. Filter through a qualitative paper (if the solution is cloudy, the presence of lead chloride (PbCl2) is indicated and the solution should be brought to a boil before filtering) and collect in a second 600-mL beaker. Heat the solution to about 60°C (140°F) and, while protecting the solution from strong light, add gradually, while stirring, 5 mL of AgNO3 solution. Heat to incipient boiling and retain at this temperature until the supernatant liquid becomes clear. Test to ensure complete precipitation by adding a few drops of the AgNO3 solution. If more precipitation takes place, repeat the above steps which have involved heating, stirring, and addition of AgNO3, as often as necessary, until the additional drops of AgNO3 produce no turbidity in the clear, supernatant liquid. Allow the beaker and contents to stand in a dark place for at least an hour. Filter the precipitate by suction on a weighed fritted-glass filter crucible. Wash the precipitate with water containing 2 mL of HNO3(1 + 1)/L. Dry the crucible and precipitate at 110°C for 1 h. Cool in a desiccator, and weigh.
8.6 Blank - Make a blank determination with 0.7 to 0.8 g of white oil by following the normal procedure but omitting the sample (Note 2 and Note 9 ). Repeat this blank whenever new batches of reagents or white oil are used. The blank must not exceed 0.03 % chlorine based upon the weight of the white oil.
NOTE 2 - This procedure measures chlorine in the white oil and in the reagents used, as well as that introduced from contamination.
9. Calculation
9.1 Calculate the chlorine content of the sample as follows:
Chlorine, mass % = [(P - B) x 24.74]/W
where:
P = grams of AgCl obtained from the sample,
B = grams of AgCl obtained from the blank, and
W = grams of sample used.
