Fluoride is sometimes found in high concentrations in ground and surface waters.
ID: 822492 • Letter: F
Question
Fluoride is sometimes found in high concentrations in ground and surface waters. Fluoride, in low concentrations, is often added to drinking water to minimize dental cavities, but in higher concentrations fluoride can pose a health risk. Two minerals that may control the concentration of fluoride ion in solution are fluorite (CaF2) and villiaumite (NaF).
a) Write the expressions for and calculate the solubility products of these two minerals at 25 degrees celcius.
b) The following table gives the composition of the relevant species in fluoride-rich ground waters from Ghana:
Species Concentration (mg/L)
Ca2+ 25.3
Na+ 23.9
F- 2.3
Could either of these minerals be present in the aquifer system and therefore contribute to the fluoride concentration of the groundwater at 25 degrees Celcius? What about at 15 degrees Celcius?
Explanation / Answer
For CaF
solubility product is given as, Ksp = [Ca2+][F-] = 25.3 X 2.3 (mg/L)^2 = 58.19 (mg/L)^2
For NaF
solubility product is given as, Ksp = [Na+][F-] = 23.9 X 2.3 (mg/L)^2 = 54.97 (mg/L)^2
As with decreasing temperature, solubility desreases,hence Ksp decreases.