# The Mechanism of Chromatographic Retention - The Association of Methanol with Water > Page 34

(18)

where (R) | is the molar refractivity of the substance, |

(n_{r}) |
is the refractive index of the substance, |

and (V) | is the molar volume of the substance. |

Due to the molar refractivity is additive and constitutive, the molar refractivity of a mixture of (n) solvents will be,

(19)

where (R_{1},_{n}) |
is the molar refractivity of the mixture, |

(R_{1}) |
is the molar refractivity of solvent (1), |

(R_{2}) |
is the molar refractivity of solvent (2), |

(R_{3}) |
is the molar refractivity of solvent (3), |

(R_{n}) |
is the molar refractivity of solvent (n), |

(X_{1}) |
is the molar fraction of solvent (1), |

(X_{2}) |
is the molar fraction of solvent (2), |

(X_{3}) |
is the molar fraction of solvent (3), |

(X_{n}) |
is the molar fraction of solvent (n), |

and (n) | is the number of solvents in the mixture, |