4.4.2 Measurement procedure (continued)
The second measurement can then be carried out with the sample substance and a suitable reference substance. For organic or nonaqueous solutions, the choice of reference must be made with particular care. It is not sufficient for the sound velocities to be similar at a single temperature, for example room temperature; their temperature dependences must also be similar.
This requirement follows from the measurement principle. If a resonance of a given order is selected for both the sample and the reference, an unsuitable reference liquid can cause the two resonances to move too far apart or even change their relative ordering. Further remarks concerning the nitromethane–pentanol system are given in Section 4.8.
4.4.3 Sources of error in the sound-velocity measuring cell
- As with the resonators discussed in Section 4.2, errors during filling can strongly affect measurement accuracy.
- Any temperature instability has a substantial influence and may adversely affect the measurement. The thermostating system used maintains temperature to within 0.001 °C.
On the basis of empirical information and an extensive series of water measurements, a relative accuracy of 10−7% is stated for the sound-velocity apparatus.
4.5 Overview of the experimental equipment
This section provides an overall view of the available apparatus and its capabilities. The measuring cells described above represent only part of the complete experimental setup.
As noted in the introduction, no chemical rate processes occur in the frequency range of interest here. Consequently, measurements across every available frequency decade are not required. For intermolecular interactions or molecular conformational changes occurring over a broad range of timescales, from 1 ps to 1 s, however, the entire measurement spectrum illustrated in Figure 4.13 must be used.
Figure 4.13 clearly shows the overlap between the different methods, comparable to the overlapping frequency ranges of the resonators and pulse cells themselves.
Original mathematical notation and figures are preserved in extracted image assets.