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 (∼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%.”

2. Experimental

2.1. Regents and materials

Analyte solutions were prepared in the form of either sodium or potassium salts. Typically, these were analytical grade chemicals, used as received from the vendors. Milli-Q ultra-pure water was used throughout with a specific resistivity of 18.2 MΩ▪cm. Cation exchange membrane (CEM) (Type FKB), anion exchange membrane (AEM) (Type FAA) and bipolar membrane (BPM) (FBM) were from Fumatch Corp. (Germany).

2.2. Fabrication of the gas-free EPM

The fabrication of the gas-free EPM was similar to a two-membrane configuration EDG described previously with some modifications [7]. In brief, the device consists of three compartments, one central eluent and two outer regenerant (Its schematic diagram is provided in Figure 1). 4 layers of AEM (sAEM) plus a BPM stacked together and 5 layers of CEM (sCEM) stacked together (membrane area is 38 mm long ×12 mm wide) are used to isolate the anodic regenerant chamber and the cathodic regenerant chamber from the central channel, respectively. Prior to use, the CEM, BPM and AEM is suggested to be cleaned by immersing in potassium carbonate solution, aiming to remove possible impurity ions. The CEM side of BPM is facing the eluent channel and the AEM side of BPM is contact with sAEM. Both porous platinum screen electrodes are placed in two regenerant chambers, which are in contact with sCEMs and sAEM via a sulfonate functionalized gasket screen and a quaternary ammonium functionalized gasket screen, respectively. Fine cation exchange resins are packed into the central channel, aiming to reduce the electric resistance of the device. Unless otherwise stated, the effluent from the suppressor is used for feeding both regenerant chambers in one direction (co-current flow). Potassium carbonate eluent driven by a high-pressure pump flows through the eluent channel in the opposite direction with the regenerant solution. Alternatively, potassium carbonate eluent can be produced by a carbonate EDG [3]. Here, the former was chosen to evaluate the EPM. Note: the EPM described here has been demonstrated to modify potassium carbonate eluent, but definitely it can also be applied for sodium carbonate. The former was chosen to be resulting from slightly higher mobility of potassium relative to sodium, and then leading to higher current efficiency. This is also helpful to achieve higher current efficiency of an electrodialytic suppressor downstream.

Figure 1

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Figure 1. Schematic diagram of gas-free EPM. Note: To show clearly, the scale of BPM is not proportional to its real size.

2.3. Chromatographic system

Figure 2 schematically shows the total chromatographic system. An IC equipment (Metrohm 861, Switzerland) was used to evaluate the performance of the device. A high-pressure pump was used to drive potassium carbonate eluent to pass through the EPM, followed by an injector, an analytical column housed in a column oven (35 °C), an anion electrolytic membrane suppressor (AEMS-100, obtained from Minghao Corp., China), and a conductivity detector. The effluent of the detector went to the regenerant chambers of the suppressor, then went to the regenerant chambers of the EPM, finally went to waste.

Figure 2

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Figure 2. Schematic diagram of the whole IC system.

Two kinds of commercial anion columns were also used here, including Metrosep A supp 5 (4.0 mm i.d.×150 mm length, Metrohm Corp., Switzerland) and TSKgel SuperIC-AZ (4.6 mm i.d. ×150 mm length, TOSOH Corp., Japan).

2.4. Calibration of carbonate solution

Since carbonate is a weak base and the fractions of concomitant anions (CO32-, HCO3 and OH) vary in different concentration of potassium carbonate solution. Thus, the concentration calibration of carbonate solution cannot be directly carried out by conductivity measurement. Here it was performed by a suppressed IC method, in which either carbonate or bicarbonate can be normalized to be carbonic acid followed by measuring its conductance to establish the calibration curve of conductance and concentration [8].

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