Recently, we showed that the phosphorus hyperfine coupling constant a_Pβ of persistent cyclic nitroxides decreased with the normalized polarity Reichardt's constant E^N_T. Thus, we investigated the changes in a_Pβ in binary mixtures of solvents. The sensitivity of a_Pβ to the solvent was high enough to allow us to perform water titration in THF, 1,4-dioxane, and acetonitrile by EPR. Accuracies of a few percent were achieved.