The recommended concentration of fluoride ion in drinking water is up to $1 \mathrm{ppm}$as fluoride ion is required to make teeth enamel harder by converting

 $[ 3 {\mathrm{Ca}}_3({\mathrm{PO}}_4{)}_2.\mathrm{Ca}(\mathrm{OH}{)}_2 ]$ to $[ 3 {\mathrm{Ca}}_3({\mathrm{PO}}_4{)}_2.{\mathrm{CaF}}_2 ]$.

$[ 3 {\mathrm{Ca}}_3({\mathrm{PO}}_4{)}_2.\mathrm{Ca}(\mathrm{OH}{)}_2 ]$ + $2 {\mathrm{F}}^{-}$ $\to [ 3 {\mathrm{Ca}}_3({\mathrm{PO}}_4{)}_2.{\mathrm{CaF}}_2 ]$