[bluemoon]

i have run an experiment about concentration of fluoride in tea leaves. This time, i standardize the solvent that i use. adding 50ml distill water(boiled) into 5 different biker with 0.5g of tea leaves respectively, i take out 10ml of tea from 1 of them after 5min, 10min, 20min, 30min,and 40min. then i add the solution with TISAB (10ml), and dilute it to 50ml.then the solutions are being transfer to plastic bottles. After all the samples have been prepared,  i run the ISE. The concentration of fluoride seems to DECREASE with time. How can this happen?i didn't maintain the temperature, i leave the tea in room temperature after adding boiling water into it, is this a possible cause? How can this happen?? thank you.

[enahs]

You need to do a standard addition, as despite the TISAB the fluoride ion can still complex with many things, such as Iron(III) (which I think teas are rich in?).