ASTM D2007 standard test method for characteristic groups in rubber extender and processing Oils and other petroleum-derived oils by the clay-gel absorption chromatographic method
8. Procedure
8.1 Fractionation:
8.1.1 Prepare the adsorption column (Fig. 2) by placing 100 g of clay adsorbent in the upper section of the column and 200 g of silica gel plus 50 g of clay on top of the gel in the lower section (Note 4). Place a piece of glass wool (of about 25-mm loose thickness) over the top surface of the clay in the upper column to prevent agitation of the clay while charging the eluent solvents. Join the columns (clay over gel) after lubricating the joint with hydrocarbon-insoluble grease. It is important that the adsorbents in each column be packed to a constant level. A minimum of ten taps with a soft rubber hammer at different points up and down and 25 taps on top of each column should be employed to achieve constant level. A suitable rubber hammer may be assembled by fastening two No. 7 or 8 rubber stoppers on one end of a small rod about 200-mm long. Use fresh adsorbents for each determination.
8.1.2 If n-pentane insolubles were not determined, select the appropriate sample size in accordance with the following polar content ranges, if the proper range can be anticipated; otherwise, use a 10 +/- 0.5 g sample.
8.1.3 Dilute with 25 mL of n-pentane solvent and mix well to ensure a uniform solution of the sample. The sample should not display precipitate or flocculate at this point. If a precipitate is present asphaltenes may be removed by the procedure of Appendix X1, however, the precision statement no longer applies. It is important that the polar content result obtained be not greater than that for the sample size as specified above, since the capacity of the clay for retaining polar constituents becomes limited at these concentrations. If results exceed this specification, repeat the test using a smaller sample. Partitioning between aromatics and polar compounds is affected by sample size. Results using different sample size may not be equivalent.
NOTE 2 - For viscous oils, dilutions of the sample with 25 mL of cyclohexane is more convenient and does not affect the results. Cyclohexane used in this manner will not detect small quantities of asphaltenes, however.
8.1.4 Add 25 mL of n-pentane to the top of the clay portion of the assembled column and allow to percolate into the clay. As soon as nearly all of the n-pentane has entered the clay, charge to the column the diluted sample of 8.1.3. Wash the sample beaker (or flask) with n-pentane and add the washings to the column. After nearly all of this material has entered the clay, wash the walls of the column above the clay free of sample with n-pentane. The sample and eluent solvent can be added to the column through a 65-mm diameter, wide-stem funnel (the funnel can be left on top of the column). At no time during the run should air be allowed to enter the clay bed.
8.1.5 When nearly all of the washings have entered the clay, charge n-pentane to the column and maintain a head level well above the clay beds (Note 3) to wash the saturate portion of the sample from the adsorbents. Recover 280 +/- 10 mL of the first n-pentane effluent from the column in a graduated, 500-mL wide-mouth conical flask.
NOTE 3 - Columns may be briefly separated, if necessary, to give a solvent head 5- to 10-mm deep in the second (lower) column. Loss of the head will give channeling in the lower column, with inaccurate results.
NOTE 4 - With long use, the frits in the absorption columns become progressively less porous. If the time for percolation doubles over that for a new column, the slow columns are to be discarded.
8.1.6 Disconnect the two sections. Allow the lower section to drain into a receiver. Continue washing the upper clay section with n-pentane. Maintain a moderate liquid head level above the clay during this wash and adjust n-pentane additions so that the level is about 25 mm when 150 mL have been collected in the receiver. Discontinue additions at this point and allow the liquid to essentially drain from the column. The quantity in the receiver should then be about 200 mL. The n-pentane from this step and from the draining of the lower column should be discarded if aromatics are to be determined by difference. This n-pentane should be added to the aromatics solution from the gel column during solvent evaporation (8.3) if aromatics are to be recovered.
NOTE 5 - This extra n-pentane washing of the clay section is necessary in order to ensure complete removal of aromatics from the clay.
8.1.7 After n-pentane effluent has essentially drained from the column, charge a 50 to 50 volume mixture of toluene-acetone. Collect the effluent in a 500-mL separatory funnel. Collect 250 mL of the toluene-acetone (plus n-pentane) effluent or until the effluent is practically colorless (only in exceptional cases will more than 300 mL of effluent be required).
8.1.8 Stopper the separatory funnel containing the toluene-acetone fraction and swirl it a few times to aid in settling the water. Then let it stand for about 5 min. Drain off and discard the lower (aqueous) layer. Add approximately 10 g of anhydrous calcium chloride granules to the fraction remaining in the separatory funnel and shake for about 30 s; vent frequently during the shaking period. Allow the mixture to settle for at least 10 min.
8.1.9 Filter the fraction through a rapid folded filter paper catching the filtrate in a 500-mL conical flask. Rinse the separatory funnel with approximately 25 mL of n-pentane, filter and collect with the mixed solvent fraction. Wash the filter paper with an additional 10 to 15 mL of n-pentane and collect with the mixed solvent fraction. IMPORTANT - Make all transfers of organic solvents from the separatory funnels through the top and avoid transferring any water that may have accumulated around the calcium chloride.
8.2 Desorption of Aromatics:
8.2.1 If it is desired to determine the aromatics by isolation rather than by difference, the gel column (lower column of the clay gel adsorption column of 8.1, Fig. 2), after the 280 +/- 10 mL of n-pentane have been collected, is placed in the extraction assembly of Fig. 3.
8.2.2 Toluene (200 +/- 10 mL) is placed in the 500-mL flask and refluxed at such a rate of 10 +/- 2 mL/min for 2 h.
NOTE 6 - Toluene reflux can be measured by collection for a 1 min period using a graduated cylinder through valve A, Fig. 3.
8.2.3 At the end of this time, the valve (A) is opened and the toluene removed into a waste solvent receiver to a volume of approximately 50-mL in the flask. The solution remaining is then combined with the n-pentane from 8.1.6 for recovery of aromatics. Do not go further by distillation, as oil will be lost, giving inaccurate results.
8.3 Solvent Removal:
8.3.1 Label and weigh for tare the anti-creep beakers that are to be used for the evaporation of solvent (one each for polar compounds, saturates, and aromatics desorbed). Fill the anti-creep beakers approximately half full with the respective solutions, (saturates from 8.1.4, aromatics from 8.1.6 and 8.2.3, polars from 8.1.8), then place them on the controlled hot plate at surface temperature of 100 to 105°C, refilling as this volume is reduced to one-quarter full. A gentle sweep may be used over the surface of the liquid. It should not ruffle the surface nor should this nitrogen jet be placed below the surface. The flasks which contained the fractions should be rinsed with n-pentane, and this n-pentane added to the respective anti-creep beakers. To avoid a potential safety incident, it is recommended that the anti-creep beakers be temporarily removed from the hot plate when transfers are made, such as when the n-pentane rinsings are added, before placing the anti-creep beakers back onto the hot plate.
8.3.2 When essentially all the solvent is evaporated, weigh the beakers at 10 min intervals. Solvents are considered removed when the weight loss between weighings is less than 10 mg.