5.3.1.2 Determination of the speed of sound

Figure 5.7. Speed of sound in nitromethane–pentanol measured using the sound-velocity cell, compared with values obtained from resonator measurements.

Another important quantity for characterizing the solution under investigation is the speed of sound. As described in Chapter 4, 1-butanol was again used as the reference liquid. As with the density measurement, this measurement does not exhibit the typical critical variation in the vicinity of the critical point. Considering the relationship between the speed of sound cₛ, density ρ and adiabatic compressibility κₛ given by Eq. (3.13):

It follows that the previously discussed increase in density fluctuations and compressibility must directly affect the speed of sound. The absence of a clearly visible critical behavior in this measurement is consistent with the density results.

By contrast, the transition to the heterogeneous state of the mixture is clearly visible. This can be explained by the way in which the electronics determine the center frequencies. As shown in Fig. 5.8, the sound velocities of the two components are recorded alternately.