At a frequency of 0.5 THz the crystalline phase exhibits negligible absorption (Figure ) and α ≈ α liquid . The vast majority of crystalline systems do not exhibit phonon modes at frequencies below a few hundred gigahertz and hence this method is not restricted to the MgSO 4 system but generally applicable as long as the spectral characteristics have been established by a simple transmission measurement of the crystalline phase. As a general rule it can be assumed that the higher the frequency that is chosen for the analysis the more likely it would be affected by the crystalline phase when calculating the equivalent concentration.