Ion Chromatography Analysis: Determining Anion Identity

[Physter]

In our analytical lab we were given an unknown solution containing up to 4 anions - fluoride, chloride, nitrate, sulphate and phosphate. It was diluted, run through the chromatograph and a chromatogram was printed for 3 trials. Then we printed chromatograms for two different standard solutions containing the same ions but in different concentrations that were known. Finally we tested tap water. Now what I have in front of me are a bunch of graphs of conductivity as a function of retention time. What does all that mean though? The sulphate ion has the highest conductivity whereas chloride has the lowest. That's all very nice but what on Earth does that tell me? Essentially what I mean to say is I have no idea what the computer did.

I was reading about ion chromatography in our textbook and in a nutshell, all they really say is its "generally the method of choice for anion analysis". That doesn't help.

Oh and something else. We're asked to "determine the identity of each observable component in the unknown from the retention times given in the computer output." On the output itself, the computer's already labeled which peak corresponds to which ion as well as the retention times of when they occur. If the computer's already identified all that, what am I doing? You know how sometimes you get so confused that you don't even know what you don't know anymore? That's certainly how I'm feeling atm :S.

But yah, any help would be greatly appreciated.

 

[chemisttree]

As the ions pass by the detector, they and their counterion, form a weak salt solution briefly. Thus the conductivity increases briefly. The different anions are retained differently and will elute at different times. The intensity of the 'peak' of conductance is a function of both the concentration (how much) and the charge of the ions. The identity of the ions is determined by its specific elution time.