The Mechanism of Chromatographic Retention - The Association of Methanol with Water > Page 37

In simpler terms, the program identifies those values of (k) and (ni) that provide the minimum error between the calculated values and the experiential values for the refractive index of the mixtures. The results obtained are given in figure 13 which show both experimental values and calculated values on the same graph.. Excellent agreement between experimental and calculated values of the refractive indices was obtained for the different solvent mixtures. Values for the equilibrium constant (k) range from 0.00443 to 0.00565 with an average value of 0.00504.

Figure 14. Graph of Refractive Index against Solvent Composition for Methanol/Water Mixtures

Employing this average value for the equilibrium constant, the concentrations of the different components of a methanol water mixture were calculated for concentrations ranging from zero to 100%v/v. The curves they obtained are shown in figure 15. There are clearly three distinct ranges of methanol concentration where the solvent mixture will have very different interactive properties. At initial methanol concentrations between zero to 40%v/v, the solvent mixture will behave as though it were a binary mixture of water andmethanolassociatedwithwater. However, at methanol concentrations ranging from 40%v/v to 80%v/v, the solvent will behave as though it were a ternary mixture of water, methanol and water associated with methanol. Finally, at concentrations ranging from 80%v/v to 100%v/v of methanol the solvent will again behave as though it were again a binary mixture but this time a mixture of methanol and water associated with methanol.