[nj_bartel]

In quant lab, we were given a sample that contained an unknown amount of KI and KCl.  We dissolved and diluted the sample to 100 mL's, then took a 25 mL aliquot.  The aliquot was connected to a silver electrode (indicator) and pH meter (reference).  We titrated with silver nitrate on mL at a time (shorter intervals around determined endpoints) to create a potentiometric graph with 2 endpoints - the first one for the disappearance of iodide, due to the lower solubility of AgI, and the second one for the disappearance of chloride.  I know that we're supposed to use the Nernst Equation to figure out the concentrations of the two salts, but I'm not sure exactly how.  Any assistance would be appreciated.

Attached is an excel of the data/graph - the y axis is in milivolts and the x axis is in mL of 0.0712 M AgNO₃.

Thanks!

http://img27.imageshack.us/my.php?image=quantu.jpg

[ARGOS++]

Dear nj_bartel;

IMHO you can handle it similar a common titration, expect the detection of the endpoint.
See the first link in the Attachment.

I hope to have been of help to you.
Good Luck!
                    ARGOS⁺⁺

[nj_bartel]

Thank you ARGOS++; that was useful for endpoint calculation.  However, I'm still not sure how you relate that to concentration.  I'm honestly not seeing why you have a changing voltage at all now, either.

You start with (y+z)K⁺, yI^-, and zCl^- in solution.  You end with (y+z)NO₃^- and (y+z)K⁺ in solution.  The amount of ions in solution stays the same - why does the conductivity change so dramatically?

[ARGOS++]

Dear nj_bartel;

No! – The number of ions changes during titration, because insoluble salts are build.
More important during precipitation potential-active ions are "eliminated", and so the most dramatic potential "jump" arrives at the Equivalence-Point (EP).

So the used molar amount on silver nitrate till the EP(1) corresponds to the molar amount of iodide in your aliquot. Similar is true for the chloride, but the corresponding amount is going from EP(1) till the EP(2).

That means that you can do the similar calculation as for other titrations.

Hope it is of some help to you.
Good Luck!
                    ARGOS⁺⁺

[nj_bartel]

I accounted for the formation of insoluble salts in my ion concentrations presented, didn't I?

Also - my TA specifically told us that the Nernst equation must be used, and that a calculation based on volume would be wrong.

[ARGOS++]

Dear nj_bartel;

Yes you did, but you forgot that nitrate and potassium are not potential-active ions, so they "don’t count" this way. That’s way you had to use the special electrode combination.

Very interesting! – My reference (in German since 1975) tells me to do it with the volumes.
Your reference must be something very special.

Good Luck!
                    ARGOS⁺⁺

[nj_bartel]

Ah ok, thanks.  Is there a list of potential-active ions or a method of determining whether an ion is a potential-active ion?

[Borek]

However, I'm still not sure how you relate that to concentration.  I'm honestly not seeing why you have a changing voltage at all now, either.

E = E0 + RT/F ln [Ag⁺]

At any given time you have a saturated solution over a precipitate. Can you see how the concentration of Ag⁺ depends on the precipitates present?

You start with (y+z)K⁺, yI^-, and zCl^- in solution.  You end with (y+z)NO₃^- and (y+z)K⁺ in solution.  The amount of ions in solution stays the same - why does the conductivity change so dramatically?

Because each ion has different mobility, so the total conductivity of the solution changes when ions are replaced.

No idea about the volumes/Nernst equation thing - IMHO you should use changing potential to detect equivalence points, but then use volumes for precise calculation of amounts. That's the way you do it. Perhaps you have misunderstood the TA (or s/he misunderstood your question?).

[nj_bartel]

At any given time you have a saturated solution over a precipitate. Can you see how the concentration of Ag+ depends on the precipitates present?

1) Through quantity of AgX formed.
2) Through ionic strength of the solution at the time.

Correct?

[Borek]

Not exactly, although these both have to be taken into account. However, when you titrate, you remove I^- (later Br^-) from the solution. How does their concentration change? How does the Ag⁺ change?

Compare

http://www.titrations.info/precipitation-titration-curve-calculation