Determination of Hexavalent Chromium in Drinking Water Using Ion Chromatography

Importance of the Topic

Hexavalent chromium (Cr(VI)) is a highly toxic contaminant commonly found in drinking water, groundwater and industrial effluents. Regulatory agencies, such as U.S. EPA and California Department of Health Services, have set stringent public health goals for Cr(VI) at low microgram-per-liter levels. Reliable and sensitive methods are essential to monitor trace levels of Cr(VI) and ensure compliance with health guidelines.

Objectives and Study Overview

This study presents an optimized adaptation of EPA Method 218.6 for the determination of hexavalent chromium in drinking water using ion chromatography (IC) coupled with postcolumn derivatization and UV/Vis detection. The primary objective was to enhance sensitivity and lower detection limits to meet California’s public health goal of 0.2 µg/L Cr(VI). Key modifications included reduced eluent flow rate, increased reaction coil volume, and larger injection volumes.

Methodology and Instrumentation

Reagents and eluents were prepared from analytical-grade chemicals. Samples were adjusted to pH 9.0–9.5 using ammonium sulfate/ammonium hydroxide buffer and filtered through 0.2 µm syringe filters prior to injection. Postcolumn derivatization employed 2 mM diphenylcarbazide in acidic methanol reagent. The optimized IC system comprised:

• IonPac NG1 guard column (4 × 50 mm) and IonPac AS7 analytical column (4 × 250 mm)
• 250 mM ammonium sulfate/100 mM ammonium hydroxide eluent at 1.0 mL/min
• Postcolumn reagent flow at 0.33 mL/min through a 750 µL reaction coil
• UV/Vis absorbance detection at 530 nm
• Partial-loop injection of 1000 µL sample

Main Results and Discussion

Optimization studies demonstrated that increasing the reaction coil from 375 µL to 750 µL doubled chromate peak response, while 1500 µL offered marginal gains. Enlarging injection volume from 250 µL to 1000 µL produced a linear rise in signal without compromising chromatography. The modified method achieved a method detection limit (MDL) of 0.02 µg/L Cr(VI) and a minimum quantitation limit of 0.06 µg/L, surpassing requirements for California’s public health goal. Calibration was linear from 0.1 to 10 µg/L (R² ≥ 0.9997). Common anions (chloride up to 1000 mg/L, sulfate up to 2000 mg/L) caused less than 20% signal suppression, demonstrating method robustness in real water matrices.

Benefits and Practical Applications of the Method

This optimized IC method offers:

• High sensitivity and low detection limits for regulatory compliance
• Rapid four-minute separation of Cr(VI) from Cr(III)
• Quantitative recoveries in drinking water samples
• Compatibility with automated sampling and large-volume injections

It can be applied in municipal and industrial laboratories for routine monitoring of Cr(VI) in environmental waters.

Future Trends and Potential Applications

Further advancements may include coupling with mass spectrometry for confirmatory analysis, miniaturized flow cells to reduce reagent consumption, and on-line sample preconcentration to lower detection limits further. Integration of real-time monitoring platforms and remote data reporting will support advanced water quality surveillance and rapid response to contamination events.

Conclusion

By modifying eluent flow, reaction coil size, and injection volume, the sensitivity of EPA Method 218.6 for hexavalent chromium detection was improved more than tenfold. The resulting MDL of 0.02 µg/L enables reliable measurement at levels well below current public health goals. This robust, high-throughput method is suitable for routine monitoring of Cr(VI) in drinking water regulatory programs.

Reference

1. U.S. Environmental Protection Agency. Method 218.6: Determination of Dissolved Hexavalent Chromium by Ion Chromatography, 1991.
2. Dionex Corporation. Technical Note 26: Determination of Cr(VI) in Water, 2001.
3. California Department of Health Services. Chromium-6 in Drinking Water Update, 2001.