I need to quantitate NaF with a Pulse Electrochemical Detector from Waters (model 464). I know the best way is to use a conductivity detector but no money to purchase. Any and all suggestions. Thank You

I hardly know where to begin. Since this is a chromatography forum I presume you intend to separated something. You can spearate Na+ or F- but not both, at least in an aqueous solution. So, you will need to decide which. Next is the problem of what you intend to oxidize to get a PAD signal. Na+ to Na++ does not look promising, nor does F- to F. Perhaps you should consider that not only is conductivity the best way, it may be the only way. Once that conclusion is reached then one might consider a cheap ion selective electrode approach for F- or flame emission for Na+.

Bill,

You might find this amusing: you can use an electrochemical detector to detect electrochemically inert analytes. Under certain conditions you can use it as an overpriced pH meter, detecting based on pH. One reference on the topic is: J. G. Tarter, Anal. Chem., 56, 1264-1268 (1984). There are more references to this method in a book edited by Tarter: Ion Chromatography, 1987, page 245 but this book might be a bit hard to find these days. But the point is that you can do it although the sensitivity isn't as good as conductivity detection. If you are interested, I can post the other references.

A few thoughts.

I am thinking with the right complexing agent you may be able to do this with "regular old" UV detection.

With the right additive you may also be able to use a UV detector in 'negative peak mode' (I forget the proper term). Waters used to advertise this approach for their Capillary Ion Analysis system.

There is a simple electrode that can be purchased for this determination (it measures iodide) but it isn't great. Tends to be a fair amount of drift in the response. Not the best if this is to be run in a quality environment. Probably sufficient otherwise.

Chris, you should not do things like this to an electrochemist. I have been wondering all day how to get a quantitative response out of a PAD detector for an electrochemically inert species. (Well, it helps pass the time while mowing.) I can see how one might get a qualitative response but not a quantitative one. I will get to a library tomorrow and follow up on this tantalizing tidbit to satisfy my curiosity. If its good I will try it on Dennis J. as a trivia quiz to see if he remembers any PAD.

Anon: you are thinking about "indirect detection". One adds a chromaphore to the mobile phase and detects its displacement by the nonabsorbing analyte. Good idea as it could work.

I can well imagine that an iodide electrode would not do well for fluoride. However, the Orion fluoride electrode can be a good choice for this application.

Bill

You are correct on both counts.

The 'Indirect detection' approach, as you described it, is exactly what I had in mind.

Regarding the statement about an iodide electrode: my mistake. I was thinking of NaI (probably because we recently had to deal with that in my lab). But as you point out a F- electrode would be appropriate here. Come to think of it, I remember reading somewhere years ago, that a F- electrode was one of the most specific of all detection systems. Hence it may work very well even if the measurement is being done in the presence of other stuff.