[aftereleven]

hello, I'm currently trying to learn the working theory behind ion selective electrodes. The problem is that i dont have a chemistry background(I only know the really basic stuff) and the principles of ISE seem to be really complex. I have been trying to understand the various explanations around the net but theres somethings i just cant get my head around.

Specifically it has to do with the Nernst equation: E = Eo -(RT/nF)ln(y x Ct)/(y x Ci). the last part is sometimes written simply as ln Q, but i dont understand what that means exactly and so prefer this version where y is the activity coefficient, Ct is the test ion concentration and Ci is the known ion concentration in the internal filling solution of the measuring electrode.

I have read that in indirect ISE measurements, the sample is diluted with a high ionic strength buffer that keeps the activity coefficient constant. My questions are:

1. How does diluting the sample in this manner make the activity coefficient constant? I know the activity coefficient is based on ionic strength. i dont know if this sounds dumb but does increasing the ionic strentgh by a large enough known amount make the ionic strentgh of the sample negligible? i.e. the activity coefficent becomes equal to that of the buffer?

2. Why does diluting the sample make it an indirect method? Is it because the activity coefficient is not being measured?

3. How does this differ from a direct method?

4. Why does the activity coefficient need to be constant or even known about at all since it cancels itself out in the above equation anyway?

alot of my info is from this article: http://www.clinchem.org/cgi/reprint/27/8/1435.pdf

I know this is alot to ask, but ive been searching the net for days and i cant find an answer i can understand. I would really appreciate any help

thanks  

[Borek]

Specifically it has to do with the Nernst equation: E = Eo -(RT/nF)ln(y x Ct)/(y x Ci). the last part is sometimes written simply as ln Q, but i dont understand what that means exactly and so prefer this version where y is the activity coefficient, Ct is the test ion concentration and Ci is the known ion concentration in the internal filling solution of the measuring electrode.

Q is a http://en.wikipedia.org/wiki/Reaction_quotient

1. How does diluting the sample in this manner make the activity coefficient constant? I know the activity coefficient is based on ionic strength. i dont know if this sounds dumb but does increasing the ionic strentgh by a large enough known amount make the ionic strentgh of the sample negligible? i.e. the activity coefficent becomes equal to that of the buffer?

Ionic strength buffer is basically solution of known (and relatively high) ionic strength. When you add small volume of your sample, change of concentration of ions that were in the buffer is negligible, so ionic strength change is negligible as well.

4. Why does the activity coefficient need to be constant or even known about at all since it cancels itself out in the above equation anyway?

It cancels out only if you have identical ionic strength in both solutions.

[aftereleven]

Thanks for the reply!

ok so does this mean that the ionic strength/activity coefficent of the internal filling solution that bathes the electrode is a known value and the activity coefficient of the diluent is at the same value so when it is added to the sample, the activity coefficient of the two solutions can cancel out?

and is this what makes this an indirect method?