2. Resonators

A 1.1 MHz plano-concave resonator was used in this work. First, the measuring volume had to be adapted to the properties of nitromethane. Because nitromethane is a powerful solvent, a Teflon boundary was used instead of a PVC boundary. The sealing rings were likewise made of Teflon. Naturally, these modifications alter the measuring characteristics of the apparatus. Numerous trial measurements were therefore necessary before the final measurements in order to characterize the resonator.

The inlet and outlet tubes presented another problem. Originally made of silicone, they are incompatible with nitromethane. Even when the cell is filled in the homogeneous phase, absorption of nitromethane by the tubing causes a dramatic change in the liquid composition. Here, too, Teflon tubing proved to be a good solution. In critical mixtures, attenuation rises sharply as the demixing temperature is approached, requiring an additional electrical amplifier. Consequently, the number of usable resonances decreases because strong electrical crosstalk distorts the resonance curves.

3. Pulse Cells

As noted earlier, the low- and high-frequency ultrasonic transmission cells were used to determine the B value and supplement the measured frequency spectrum. Filling procedures were analogous to those used for the resonators. Since the measuring chambers were made of stainless steel in the low-frequency cell and glass in the high-frequency cell, the aggressiveness of nitromethane was not a major problem.

Care was required with the Viton seals, however: they dissolve in nitromethane, making the cell leak and allowing dissolved seal material to change the concentration of the investigated solution. The measurement itself is more convenient than with resonators. Because attenuation rises so strongly, there is no risk of moving the receiving transducer out of the sample.

4. Speed of Sound

The sound-velocity cell completes the range of ultrasonic methods. All measures intended to protect the solution and apparatus likewise required modifications to the instrument. Since the measurement proceeds automatically, there were no additional difficulties in the measurement procedure itself.

Original mathematical notation is preserved in the extracted image assets.