ASTM D3104 Standard Test Method for Softening Point of Pitches (Mettler Softening Point Method)
7. Calibration of the METTLER TOLEDO Apparatus
7.1 This step, required only occasionally, is designed to establish that the temperature indicated by the instrument is in agreement with a known standard. A special cup with a bottom orifice of 2.8 mm is used instead of the one prescribed for the testing of pitch.

7.2 Reagent - Use either analytical reagent or primary standard grade benzoic acid for the calibration. As this material is hygroscopic it must be stored in a tightly sealed container, and replaced with fresh material from a newly opened supply if hydration or other contamination is suspected.

7.3 Procedure - Old Instruments:
7.3.1 Filling the Sample Cup - Place the cup on a clean, flat surface. Add a small amount of benzoic acid crystals and press down with a rod (approximately 4.5 mm in diameter). Check that the bottom orifice is completely filled. Refill and repeat the pressing step until the cup is filled with benzoic acid. Remove any crystals from the exterior of the cup.

7.3.2 Heating - Preheat the Mettler furnace to 121 °C, and maintain it at that temperature. Place the cartridge assembly containing the benzoic acid in position in the furnace, taking care that the slits for the light beam are properly positioned. Wait for temperature equilibration, that is, the furnace and the sample are in equilibrium at the preset temperature, but not less than the 30 s after inserting the cartridge, start the automatic heating cycle at 0.2 °C/min. The temperature will rise steadily at the correct rate until the drop point is reached, and then remain steady on the readout.

7.3.3 Cleaning - Immediately remove the cartridge assembly. Check to determine that the sample has passed through the light beam and no pre-triggering has occurred. If a malfunction is suspected, the entire procedure must be repeated. Inspect the apparatus carefully to ensure that no residue remains. Use a spatula shaped to the contour of the cup to remove most of the remaining acid from the cup and from the bottom of the cartridge. Wash the cup and cartridge in xylene, or other suitable solvent, to remove the last traces of the residue.

7.4 Interpretation - See 7.6.

7.5 Procedure - New Instruments:
7.5.1 Filling the Sample Cup - Place the cup on a clean, flat surface. Optionally, the sample preparation tool may be used. See Fig. 2. Add a small amount of benzoic acid crystals and press down with a rod (approximately 4.5 mm in diameter). Check that the bottom orifice is completely filled. Refill and repeat the pressing step until the cup is filled with benzoic acid. Remove any crystals from the exterior of the cup. Place a glass collector below the cup and a cup cap on top, and place in the sample carrier. Two cup assemblies may be placed in the sample carrier.

7.5.2 Heating - Preheat the furnace to 121 °C, and maintain it at that temperature. Place the sample carrier containing the benzoic acid in position in the furnace. Wait for temperature equilibration, that is, the furnace and the sample are in equilibrium at the preset temperature, but not less than the 30 s after inserting the cartridge, start the automatic heating cycle at 0.2 °C/min. The temperature will rise steadily at the correct rate until the drop point is reached, and then remain steady on the readout. If there are two samples, heating will continue until both have dropped.

7.5.3 Cleaning - Immediately remove the cartridge assembly. Check to determine that the sample has dropped and no pre-triggering has occurred. The video may be viewed to make sure the drop was detected correctly. If a malfunction is suspected, the entire procedure must be repeated. Inspect the apparatus carefully to ensure that no residue remains. Use a spatula shaped to the contour of the cup to remove most of the remaining acid from the cup. Wash the cup and glass collector in xylene, or other suitable solvent, to remove the last traces of the residue. Glass collectors may be treated as disposable items. Aluminum cups may be used which may also be considered to be disposable.

7.6 Interpretation - If the result is not 123.5 °C +/- 0.5 °C, repeat the test. If the second value is 0.6 °C above or below 123.5 °C, measure the dropping point of a fresh sample of benzoic acid. If the deviation exceeds 0.6 °C, the instrument requires recalibration or repair. Consult the manufacturer's instruction manual.

8. Sampling
8.1 Samples from shipments shall be taken in accordance with Practice D4296, and shall be free of foreign substances. Thoroughly mix the sample before removing a representative portion for the determination or for dehydration.

8.2 If a solid bulk sample contains free water, air-dry a representative portion in a forced draft oven at 50 °C.

8.3 If the sample is liquid, check for any surface foam which may indicate the presence of water. If foam is found, maintain the sample at a temperature of about 125 °C in an open container until the surface is free of foam. Take care not to overheat. Remove from the heat source as soon as the foam has subsided. Proceed as directed in 8.4.2.

8.4 Filling the Sample Cup:
8.4.1 Crush air-dry lumps of pitch to a size of 6 mm to 12 mm. Add the material to a container, having a height equal to or exceeding its width, and having a volume of not less than 50 mL, until it is about half full. Place the container on a hot plate, or in an oven or a bath. Do not use an open flame to melt pitch. After melting is complete, stir gently but thoroughly, avoiding incorporation of air bubbles. The maximum temperature of the molten pitch should not exceed the expected softening point by more than 50 °C. Any foam that forms must be skimmed off.

8.4.2 Prepare the sample by slightly overfilling the sample cup, which rests on a brass plate. Optionally, the sample preparation tool may be used (see Fig. 2). Keep the lip of the container close to the top of the cup to minimize entrainment of air bubbles. Underpouring past the blade of a spatula is helpful in this respect. Cool the pitch in the cup until firm, under cold water if necessary. Trim the sample flush with the top of the cup by pressing with a heated knife or spatula while drawing it toward you. Do not trim by cutting across the top with a forward or sideward motion as this tends to lift the sample out of the mold, thus creating an internal void which is not easily detected, and which significantly alters the result.

8.4.3 An alternative method which avoids possible loss of volatiles in remelting, is to press the cold, solid sample into the sample cup. Select clean, dry, dust-free lumps from the representative portion of the sample and crush to a size of less than 2 mm. Place a sample cup in the mold (see Figs. 4-6). Place the assembled mold, with the knockout pin in position, on the bottom plate of a hydraulic press. Pour about 2 g of the crushed pitch into the top of the mold and insert the top ram. Compress the pitch into the cup with a total force on the ram of 22.24 kN (5000 lb). Hold this pressure for a minimum of 15 s. Excessive molding pressure can cause the sample cup to fail by shearing. Remove the mold assembly from the press and rotate the upper half while keeping the lower half of the mold stationary. This action causes the pitch to shear horizontally at the top of the cup. Separate the two parts of the mold and remove the sample cup from the lower half. It may be necessary to dislodge it by pressing against the knockout pin. A small-diameter rod mounted in the base plate may be used for this. Clean the mold of all residual pitch to avoid misalignment or binding during subsequent use. This procedure does not work well for pitches having softening points below 80 °C unless the mold is precooled below room temperature.