The concentration of fluoride in toothpastes containing soluble F–
may
be determined with a F–
ion-selective electrode using a calibration curve
prepared with external standards. Although the F–
ISE is very selective
(only OH–
with a KF–/OH– of 0.1 is a significant interferent), Fe3+ and
Al3+ interfere with the analysis because they form soluble fluoride complexes
that do not interact with the ion-selective electrode’s membrane.
This interference is minimized by reacting any Fe3+ and Al3+ with a suitable
complexing agent.