A gas-free electrodialytic pH modifier for ion chromatography

We for the first time describe a gas-free electrodialytic pH modifier deployed at the pump outlet side to manipulate carbonate eluent used for ion chromatography (IC). It is in sandwich configured, in which the central eluent channel is spatially isolated from two outer regenerant chambers by a stacked cation exchange membranes (sCEM) and a bipolar membrane (BPM) plus a stacked anion exchange membranes (sAEM) (BPM-sAEM), the cation exchange side of BPM is facing the central channel (the cathode direction). One electrode is put in each regenerant chamber and the sCEM side is cathode with respect to the anode of BPM-sAEM side. When a potassium carbonate eluent is pumped into the central channel and a DC current is applied, a controlled amount of potassium ions will be removed by migrating across sCEM into cathodic chamber. Meanwhile, hydronium ions generated from enhanced water splitting at the intermediate layer of BPM will electromigrate into central channel to form bicarbonate. By controlling the current, the potassium carbonate eluent can be manipulated to obtain a mixed eluent with different ratio of carbonate and bicarbonate. At least 10 mM K2CO3 eluent can be online changed modified into 10 mM KHCO3 with near-ideal Faradaic efficiency (similar to 92%). The device demonstrated good reproducibility, as indicated by retention time of relative standard deviation (RSD) < 0.43% and the peak area of RSD <0.93%.

Automated summary

This study describes a gas-free electrodialytic pH modifier for manipulating carbonate eluent in ion chromatography, achieving mixed eluents with different carbonate and bicarbonate ratios by controlling the migration of potassium ions and water splitting. The device can online convert at least 10 mM K2CO3 eluent into 10 mM KHCO3 with near-ideal Faradaic efficiency. Good reproducibility was demonstrated with RSD values for retention time and peak area below 0.43% and 0.93% respectively.