[Omega Glory]

Hey all!

For a recent lab, we were required to do an analysis on the fluoride content of toothpaste and mouthwash using a fluoride ion selective electrode.  I'm a little stumped when it comes to the standard addition part!  The spiked and unspiked potentials go like this:

10mL unknown + 10mL TISAB: 10.95mV
10mL unknown + 10mL TISAB + 1mL 10ppm SPIKE: 8.9mV

When I plot these values with response (mV) on the ordinate and +spiked concentration on the abcissa, I get two points whose slope is opposite what a standard addition plot should be.  The slope is negative, not positive, so that y increases as x becomes more and more negative.  I know I need to find y=0 and therefore the concentration of fluoride in the unknown, but how can I do this with this graph?  

[enahs]

If your instructor ever tells you to do some sort of line fitting with just 2 data points tell them they are a horrible scientists!

For this, you are just use a much simpler method. So, the only different in the two samples is that one sample has some extra, known amount, added fluoride. So you know what the detector response for just that extra added amount.

So then knowing the detector response per unit amount, how much do you have in the original samples?


Note, this is probably the worst way you could be doing "standard addition" there is.