Applying the above method, it was possible to determine the local concentration of solute in the liquid phase during the crystallisation process. Compared to the case of measuring the solute concentration in the neat solution outlined above in the presence of a solid phase, i.e. the crystals, the total absorption (α) is the sum of the absorption from the liquid phase α liquid and the solid phase α solid , here crystalline MgSO 4 heptahydrate. To identify the local concentration in solution the contribution from α solid can be minimised by measuring the absorption at the lower frequency end of the available bandwidth for the calibration curve.