# The Mechanism of Chromatographic Retention - The Association of Methanol with Water > Page 36

Thus, if (n_{i})
is the refractive index of a mixture and (v_{i}) is the equivalent
molar volume of the mixture,

Now, (24)

Substituting
for (v_{i}) from (41), for (X_{M}),
(X_{W}) and (X_{MW}) from (20), (21) and (22); and
simplifying,

(25)

Thus, (26)

Accepting
published values for the molar refractivities of water and methanol, and
assuming a range of values for the equilibrium constant (k) and the refractive
index (n_{i}) of the methanol/water associate, the actual values that
fit the equation for these two parameters can be identified, using a simple
iterative program that compares the calculated values for (n_{i}) to those
experimentally determined.