https://doi.org/10.1351/goldbook.15362
For a solvent \(\ce{A}\), volume fraction of \(\ce{A}\) divided by the sum of the volume fractions of solvent in a solution containing both \(s\) solutes (\(i = 1, 2, \ldots, s\)) and \(p\) solvents (\(i = s + 1, s + 2, \ldots, p\)) \[\varphi_{{\rm{v}},{\ce{A}}} = \varphi_{\ce{A}}/\sum \limits_{i=s+1}^{p} \varphi_{i}\] where the summation is over the \(p\) solvent components only.
Note: Used to express solvent composition for a system containing a solute in a mixed solvent.