Hi,
I'm working on an ion exclusion project where I inject different molarity of NaCl into an anion exchanger (positive charged resin bed). Using an isocratic method (WFI water) and a conductivity detector, the salt is excluded from the column resulting in a peak with significant fronting (as expected).
My question here is related to, which ion is used to transmit electrons in a conductivity meter? I assume in water, NaCl dissociates into Na+ and Cl- ions. As the dissociated ions travel down the bed, the positive charge of the resin repells the Na+, which results in a peak with significant fronting. Does the conductivity meter only pick up the Na+ ion and not the Cl- ion (and why)? I'm not too certain about how a conductivity meter works and how it measures the ionic content of the medium. Thanks.

With pure water, the Cl- ion is bound to the anion exchange medium. To elute the anion requires some salt content in the MP to release the chloride. The mobile phase must then pass through an ion suppressor before going to the detector. Check www.dionex.com, applications, search chloride, 2nd application is Determination of Inorganic Anions in Drinking Water by Ion Chromatography, #AN133.pdf (reqires adobe acrobat reader). It uses a sodium bicarbonate/sodium carbonate mobile phase with suppressed conductivity detection.

The question is what is the counterion in your exchanger. It may very well be that your Cl- ion is unretained as well, as you had indicated in your original mail.

The conductivity detector essentially measures the current that flows through the cell (charge per unit time) at a given applied voltage (yes, I know this is an oversimplification, but close enough). If the applied voltage is held constant, the current will depend on two things: the concentration of charged groups in the cell, and the mobility of those charged groups.

As the previous post suggests, what you will see depends on what counterion is originally attached to your anion exchanger (what "form" the exchanger is in). If the counterion is more weakly bound than chloride (e.g., hydroxide), then your chloride will stick to the exchanger, displacing an equivalent amount of hydroxide. What your conductivity detector will see is sodium ions + hydroxide ions.

If the counterions is more strongly bound than chloride (e.g., carbonate), then the chloride will be unretained. If you have an efficient column (high plate number), you might actually see two peaks: first the sodium (excluded from the ion exchanger) as sodium hydroxide, then chloride (unretained, but able to penetrate the ion exchange) as hydrochloric acid. If your column is not particularly efficient, you may see only a single "funny" peak for sodium hyrdroxide shading into sodium chloride shading into hydrochloric acid.

-- Tom Jupille / LC Resources Inc.

Need a suggestion on what process is to be used to buffer an electronics circuit board that has been washed with a 5% solution of NaOH. Acids of any sort cannot be used due to their residual effect over 20 years.