Ion Chromatography Suppressor: how does it work?

[chroma]

In ion chromatography the eluent is a NaHCO3 / Na2CO3 buffer solution. The suppressor replaces all Na+ from the eluent with H+. If there is NaCl in the sample it also replaces those Na+, so there is more H+ in the mobile phase than HCO3-. H+ won't be absorbed by the hydrogen carbonate anymore, but the conductivity of H+ is the biggest. So why is H+ in excess not spoiling the conductivity-measurement of the analysis?


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[Markus Laeubli, Metrohm]

There never is an excess of H+. Every H+ has its counter-anion (electroneutrality).
The eluent is converted to CO2 and water (some of the CO2 stays disolved in water and the minimal dissociation creates a small background conductivity).

Formula: Na2CO3 + 2H+(supp) <-> H2O + CO2 + 2Na+(supp)

H+(supp): added by the suppressor
Na+(supp): removed by the suppressor

If there is NaCl in the sample this is converted to HCl.

Formula: NaCl+ 2H+(supp) <-> HCl + 2Na+(supp)

The measured conductivity in the peak is the sum of the single ion conductivities of H+ and Cl- respectively. The high conductivity of H+ just gives a high response for the "Chloride" peak.

[chroma]

Thank you, I understood it. My mistake was to assume, that different ions are detected by different conductivity. But of course, they're already separated by the column.