QUANTIFICATION OF AMMONIUM CATIONS IN INDUSTRIAL WASTEWATER USING ION CHROMATOGRAPHY
Ion monitoring in wastewater using a non-suppressed IC system for cation analysis Introduction
Ammonia, as present in industrial wastewater, poses a toxic threat to aquatic life and ultimately to humans, when converted to nitrate. The National Pollutant Discharge Elimination System (NPDES) permit program, as authorized by the Clean Water Act (US), requires information on the amount of ammonia dissolved in water by monitoring of ammonium cations.
Ion chromatography with conductivity detection is the ideal tool for the selective and sensitive determination of inorganic ions. However, since the eluent also contains ions that generate an increased background noise level, an ion suppressor located between the column and detector can be used to neutralize the eluent by means of ion exchange.
When using a suppressor, a minimum of six calibration points are required, as the responsiveness to conductivity decreases with increasing concentration of ammonium ions, causing poor linearity of the calibration curve. An IC system without cation suppression does not show this effect, resulting in accurate quantitation even with only a few calibration points. This article compares calibration curves created using a suppressor versus a non-suppressor system and introduces an example of measuring cations contained in industrial wastewater using no cation suppression.
Calibration curve with and without ion suppression
Figures 1 and 2 show calibration curves for quantifi cation of ammonium ions over the concentration range of 0.5 to 20 mg/L by ion chromatography with and without ion suppression respectively.
created accurately, the non-suppressor system provides accurate quantitation with less calibration points due to the linear calibration curve which allows for reliable linear approximation.
Table 2: Analytical conditions for IC analysis of industrial wastewater.
Analysis of industrial wastewater
Figure 2: NH4 calibration curve obtained using an IC system without eluent suppression (First-order approximation).
Both graphs were created using the same analytical conditions as listed in table 1.
Table 1: Analytical conditions for NH4 standard determination.
Industrial wastewater was fi ltered through a membrane fi lter with a pore size of 0.2 µm dedicated to ion chromatography, and then analyzed using an ion chromatography system without ion suppressor. As can be seen from the analytical conditions described in table 2, 2 mmol/L of 18-Crown-6 was added to the eluent to ensure adequate separation of sodium and ammonium ions. Quantitative results for cations found in the sample are listed in table 3; fi gure 3 shows the corresponding chromatogram. Good separation and accurate quantifi cation were achieved even for samples containing signifi cantly more sodium than ammonium ions.
Table 3: Quantitative results of cation analysis in industrial wastewater.
As can be seen from the two fi gures, the non-suppressor system (fi g. 2) provided good linearity over the calibration range, while the curve obtained using ion suppression was non-linear (fi g. 1).
When using the ion exchange suppressor, the eluent is converted into water with no pH buffering capacity, resulting in suppressed dissociation of target ions with increasing pH of the eluent fl owing into the conductivity detector, along with an increase of ammonium ions. In other words, the equilibrium relationship expressed in equation 1 shifts to the left [1].
Hence, while the concentration of ammonium ions in the sample solution is increasing, the response of the conductivity detector is reduced, resulting in a non-linear calibration curve. This effect cannot be seen in the system without eluent suppression.
Figure 1: NH4 calibration curve obtained using an IC system with eluent suppression (Second-order approximation).
Although quantitative values of the same level can be obtained with or without a suppressor as long as the calibration curve is
IET NOVEMBER / DECEMBER 2021
WWW.ENVIROTECH-ONLINE.COM Figure 3: Typical chromatogram of IC analysis of industrial wastewater.
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