ASTM D2503 Standard Test Method for Relative Molecular Mass (Molecular Weight) of Hydrocarbons by Thermoelectric Measurement of Vapor Pressure
6. Sampling
6.1 The sample must be completely soluble in the selected solvent at concentrations of at least 0.10 M, and it must not have an appreciable vapor pressure at the test temperature.
7. Preparation and Calibration of Apparatus
7.1 Prepare standard 0.01 M, 0.02 M, 0.04 M, 0.06 M, 0.08 M, and 0.1 M solutions of the calibrating compound in the solvent selected.
7.2 Remove the upper sample chamber assembly. Rinse the solvent cap with the solvent to be used. Install a vapor wick in the cup and fill with solvent to the bottom of the notches in the inner wick. Place the cup in the chamber base recess, align the vapor wick openings with the viewing tubes, and replace the upper assembly. Take care that the guide pins properly engage matching holes in the thermal block and that the matching surfaces of the base and block are clean. Be careful not to allow the thermistor beads to touch the cup or wicks as they may be bent out of alignment. Turn on the thermostat and allow the temperature of the sample chamber to reach equilibrium at 37 °C.
NOTE 2 - If the block is at room temperature, 2 h to 3 h will be required. To avoid such delay, it is desirable to always leave the thermostat switch in the "on" position. If the chamber is at equilibrium and is opened briefly, 30 min to 45 min will generally be required before temperature stabilization is regained. The exchange or refilling of syringes does not necessitate any waiting period.
7.3 Thoroughly rinse all syringes with the solvent being used and allow to dry.
7.4 Fill the syringes from guide tubes "5" and "6" with the solvent. Fill the syringes for guide tubes "1" through "4" with the standard solutions in order of increasing concentration.
7.5 Insert the syringes into the thermal block, keeping the guide pins pointed away from the probe. Turn on the "Null Detector" switch (Note 3). Set the sensitivity control to sufficient gain so that a 1.0 Ω shift in the "Dekastat" produces one major division shift of the meter needle.
NOTE 3 - No measurements should be attempted until the "Null Detector" switch has been on for at least 30 min.
7.6 Turn on the "Bridge" switch and turn the "T-ΔT" switch to "T". Approximately zero the meter with the "T" potentiometer and observe the drift of the needle. If the solvent chamber is at equilibrium, the needle should not drift more than 1 to 2 mm during one complete heating cycle; a steady drift to the right indicates that the chamber is still warming up; if "T" is stable, switch the selector to the "ΔT" position.
7.7 While observing the thermistors in the viewing mirror, lower the syringe in position "5", by rotating the knurled collar of the holder fully clockwise. With the end of the needle directly above the reference thermistor, turn the feed screw and rinse the thermistor with about 4 drops of solvent. Finally, deposit a drop of solvent on the thermistor bead and raise the syringe by rotating the knurled collar in a counterclockwise direction. Rinse the sample thermistor with solvent from syringe "6" and apply a drop approximately the size of the drop on the reference thermistor. Depress the zero button, and zero the meter with the "Zero" control. Set the decade resistance to zero, and balance the bridge using the "Balance" control. Repeat the balancing of the bridge with fresh drops of solvent on each thermistor to assure a good reference zero.
7.8 Lower syringe "1" and rinse the sample thermistor with 3 to 4 drops of solution, finally applying one drop to the bead. Start the stop watch. Center the meter by means of the decade dials and take readings at 1-min intervals until two successive readings do not differ by more than 0.01 Ω. Record the ΔR value, estimating to the nearest 0.01 Ω from the meter. Record the time required to reach this steady state, and use this time for all subsequent readings for the solvent used.
7.9 Upon completing each series of sample readings, rinse the sample thermistor with solvent, deposit a drop, and recheck the zero point. The meter should reproduce the original indication within 0.5 mm. If the needle shows a negative deflection, the sample thermistor should be rinsed again. If it shows a positive deflection, the drop on the reference thermistor should be replaced.
7.10 Plot the ΔR values for each concentration of standard against the molarity of the standard for the solvent used.
NOTE 4 - The calibration must be repeated for each of the solvents to be employed and separate working curves constructed. Recalibration is necessary each time a new batch of solvent is put into use.
