To eliminate the interference of background ions ClO4 -and Na + , we also prepared both serial solutions containing Zn 2+ //F -//Na + //SO4 2-by paths 1 and 2 (Fig. ), as well as the serial solutions containing Zn 2+ //F -via directly diluting ZnF2 stock solution (~0.12 mol•kg -1 ) with deionized water (Fig. ). The 19 F NMR spectral difference between both serial solutions containing SO4 2-is analogous with that for the solutions containing ClO4 -above. Meanwhile, 19 F NMR spectra for the Zn 2+ //F -solution also only display a signal when the ZnF2 concentration is less than 0.01 mol•kg -1 , whereas the second peak (at ~-130 ppm) appears when the ZnF2 concentration is up to 0.03 mol•kg -1 . These results indicate that the spectral difference for the solutions prepared by paths 1 and 2 is not derived from the interaction between background ions and F -.