Proper Oil & Grease Procedure: How To Get The Most Accuracyby Energy Technical Services on 05/26/16
Written by Eric Williams
According to EPA 1664 B, the O&G-HEM test (or Oil and Grease n-Hexane Extractable Material) is an extensive test that requires various amounts of equipment and time. However, the test does not have to be difficult as long as the necessary precautions are taken. This method is used to determine the amount of n-hexane extractable materials that is retained on a filter from an aqueous matrix. After the sample is pH adjusted, we pour it through a specially designed filter to collect the material. The leftover material is to be extracted, dried and weighed to give a final value.
1. A Filtration Manifold (to separate the HEM material from the non-HEM material.)
2. QuickVap plate with Nitrogen purge (To place the wet pans for drying)
3. Desiccator- with color changing indicator (For the final drying stage)
4. Graduated Cylinders
5. Aluminum (Al) weighing pans (To collect the HEM material.)
6. Calibrated Balance (to weigh the pans)
7. Waste Flasks (to place on the manifold to create a vacuum when depositing the non-HEM material)
8. Collection Flasks (to place on the manifold to collect the HEM material)
9. Foil (to cover the collection flasks so that the Hexane does not evaporate)
10. Disposable Filter Cups (to connect to the top of the manifold to act as a funnel)
12. Filter Funnels
1. Lab Water
2. n-Hexane- 85% minimum purity, 99.0% minimum saturated C6 isomers (FLAMMABLE! Also very poisonous)
3. Methanol- ACS grade (FLAMMABLE!)
4. Sulfuric Acid (6 normality)
5. Sodium Sulfate- granular anhydrous (dry at 200-250C for at least 8 hours and stored in a tightly sealed container)
6. Silica Gel
MAKE SURE THIS TEST IS PERFORMED UNDER A HOOD!! THE FUMES ARE POISONOUS!!!
Stage 1: PREPARATION
1. It is best to bring all samples to room temperature before the test analysis begins. Basic chemistry tells us that the liquid samples will contract when they are exposed to cold temperatures, bringing them to room temperature insures that the samples have the same volume as they were when originally sampled. Remember to pH check the samples too, they should still be below a <2 from the preservation of the sample. If the sample is not below <2, add a drop of sulfuric acid (6N) to the sample to bring the pH down.
2. Set up the manifold system. Refer to your manifold’s manufacturer instructions.
3. Place one disposable filter between the orange fluorosilicate gasket and manifold. (Only use one filter per sample!) Connect the clamp to the cup and manifold to insure there is no leak. Attach waste flask (make sure it is not the collection flask) in the collection position.
4. Most importantly, mark all the sample containers with a marker to show sample level. Later we will reuse this line to measure how much sample was used.
5. Turn on the vacuum pump! Time to collect some Oil and Grease.
Stage 2: HEM SEPARATION
6. Rinse the filter cup with hexane. The hexane should drip into the waste flask. Before continuing, make sure all hexane has dried in the waste flask. This should take roughly 2 minutes. Turn the stopcock to the off position. Now that we are sure there are no contaminants in the filter from manufacturing, we can begin conditioning the filter. First, add 10 milliliters of fresh methanol to the filter. Then, quickly allow a small amount to go through the deposit filter line. (On most manifolds, the stopcock will go to the back, or clockwise) We do this to make sure that the methanol has coated both the top and bottom of the filter. Let the methanol sit for about 2 to 3 minutes.
7. This is the trickiest part of the test. While depositing the rest of the methanol by turning the stopcock to the waste position, add the sample to replace the methanol until there is none of the chemical left. The filter cannot be left empty at all! If for any reason that the filter becomes empty and dry before the sample is complete, the conditioning process must be redone. Add the sample to the filter until there is no sample left. Allow all of the sample to pass through the filter. Let it air dry for 5 minutes after. (TIP: Do not stir solids, if so crud can cause a slight blockage that will make filtering last for hours.)
8. After the 5 minutes has passed, turn the stopcock to the off position. Replace the waste flask with the fresh collection flask. Remember to mark the flask with a number or letter to tell the other flasks apart.
9. Take the original sample container, add 16 milliliters of hexane, then swirl it around with the lid tightly on. Make sure that the lid is opened periodically while swirling as the hexane will cause pressure to build. Pour the hexane into the filter cup. Turn the stopcock to the collection position (forward or counterclockwise for most manifolds) and allow a small portion to go through, then back to the stop position. Wait 1 minute to allow the hexane to soak the filter. After 1 minute has passed, allow the hexane to drip into the collection flask completely.
10. Repeat ‘Step 9’ two more times, rinsing the sample container with 15 milliliters of hexane each time.
11. Rinse the sides of the filter cup with hexane to make sure all Oil and Grease has been collected. When finished allow the filter to air dry for 2 minutes.
12. Now that the separation stage is at its end, we can turn the manifold vacuum off and disconnect the collection flask. Make sure to cover the flask with foil for the future evaporation stage.
13. To clean up, remove and discard the filter cup. Rinse all glass filter parts with hexane. Set up new filter cups and continue filtering samples as in ‘Steps 6 through 12’ until all samples are finished.
14. Take the original sample containers and fill them with regular water to the marked line that was made earlier. Measure the water with a large graduated cylinder (at least 1000 milliliters) to give you the exact volume of the sample.
Stage 3: EVAPORATION
15. Number and weight out the aluminum pans initial weight. (Make sure the weight of the pan was received after it was numbered, the ink will cause a difference in weight if weighed before.)
16. Set up the new filter funnel with 5 grams of sodium sulfate inside the filter paper. Place the weighed number pan below each funnel. Carefully pour the sample from corresponding collection flask through the filter into numbered pan. Squirt and swirl the collection flask with hexane 3 times. The sodium sulfate will collect all moisture from the flask, leaving only the hexane and O&G in the pan. Make sure to rinse the powdered sodium sulfate.
17. Put the QuikVap plate under the hood with the samples. Turn the heat to 65C° and adjust the nitrogen purge. (Nitrogen and heat will accelerate the hexane’s evaporation). Then transfer the aluminum pan full of hexane onto the QuikVap. Near the end of the evaporation, there should be crystals, drops of oil, and other crud in the pan. This means that you have successfully collected the sample. Congratulations! After the pans are completely dried, place them in the desiccator. It is best to leave the pans in the desiccator for at least an hour, but can be left overnight.
18. After the pans are finished cooling in the desiccator, weigh them for their final weight and record it.
19. The final calculation should be: =((Final Weight – Initial weight) x 1000) / (Volume/1000)
Sample calculation ((6.670-6.598) x 1000)/(895 /1000)= 80.45 mg/l
If the test requires SGT-HEM (or Silica Gel Treated-n-Hexane Extractable Material) rather than Oil and Grease there is only one step that needs to be included. Silica Gel extracts a special oil that the original test would have measured. After Step 18, the recording of the final weight, add a sufficient amount of n-hexane to the aluminum pan to dissolve the residue. Then add 3 grams of silica gel for every 10 mg of HEM in the sample. (This can be achieved by doing the calculation on Step 19.) Make note of any silica gel addition greater than 3 grams. After the silica gel absorbs the residue, dry and reweigh the pans in the same fashion from Step 17 and 18.