The Determination of Trace Anions in Concentrated Phosphoric Acid

Significance of the Topic

The accurate determination of trace anions such as chloride, nitrate, and sulfate in highly concentrated phosphoric acid (85% w/w) is critical for quality control in chemicals, fertilizers, and semiconductor industries. High levels of phosphate interfere with conventional ion chromatography, necessitating specialized sample pretreatment to achieve low detection limits and robust quantification.

Aims and Study Overview

This study presents an improved analytical workflow to overcome matrix suppression by phosphate. The objectives are to (1) isolate trace anions from the dominant phosphate matrix via ion-exclusion, (2) concentrate the isolated fraction, and (3) separate and detect the target anions at sub-mg/L levels using suppressed conductivity ion chromatography.

Used Instrumentation

• Dionex DX-500 Ion Chromatography system with GP50 Gradient Pump, CD20 Conductivity Detector (temperature-controlled), LC20 enclosure, and Dionex RP-1 piston pump.
• Pressurizable reservoir for sample loading and PEEK tubing for fluid connections.
• Rheodyne injection valves (sample and concentrator switching).
• IonPac columns: ICE-AS6 (ion-exclusion), AG10 (trap), AG11-HC (guard & concentrator), AS11-HC (analytical).
• Anion Self-Regenerating Suppressor (ASRS™) in external water mode.

Methodology and Procedure

The protocol combines ion-exclusion (IE) pretreatment with microbore ion-exchange chromatography:

1. Sample Loading: A pressurized helium stream delivers 85% phosphoric acid into a 200 µL PEEK loop; four loop volumes ensure reproducible sampling.
2. IE Separation: The sample is injected onto a 9 × 250 mm ICE-AS6 column with deionized water eluent (0.50 mL/min). The first 7 minutes of effluent (phosphate-rich) is discarded.
3. Fraction Collection: From 7.0–13.0 minutes, analyte-rich effluent is directed to a 4 mm AG11-HC concentrator column.
4. Analytical Chromatography: The concentrator is switched inline with a 2 mm AG11-HC/AS11-HC column set. Analytes are eluted isocratically at 20 mM NaOH (0.38 mL/min) and detected by suppressed conductivity.
5. Column Cleanup: After phosphate elution, a 5-minute step to 200 mM NaOH removes residual phosphate, followed by re-equilibration to 20 mM NaOH.
6. Calibration: Standard additions (0.1% dilution) minimize matrix effects. Working standards (1 mg/L) are prepared in deionized water and spiked into 20 mL acid aliquots.
7. Trap Column Regeneration: Monthly regeneration of AG10 with 200 mM NaOH (50 min) and water rinse maintains low background.

Main Results and Discussion

Method detection limits (MDLs) calculated at 99.5% confidence are 0.15 µg/L for chloride, 2.5 µg/L for nitrate, and 31 µg/L for sulfate, well below SEMI purity specifications. Seven replicate analyses of high-purity acid yielded RSDs of <6% for chloride and sulfate; nitrate displayed higher variability (21% RSD) due to partial co-elution. Spike recoveries (50–1000 µg/L) ranged from 84–111% and produced linear calibration (r²>0.99). Consistent RP-1 flow (0.50±0.02 mL/min) and precise switching times are essential for reproducible fraction capture.

Benefits and Practical Applications

• Reliable quantification of trace anions in challenging high-phosphate matrices.
• Sub-ppb detection supports stringent industrial quality requirements.
• Microbore format reduces eluent consumption and waste generation.
• Adaptable to routine QA/QC in semiconductor, fertilizer, and chemical manufacturing.

Future Trends and Potential Applications

Advances may include coupling ion chromatography to mass spectrometry for enhanced selectivity, miniaturized high-pressure systems for even lower sample and reagent consumption, and automated inline sample preparation modules for high-throughput environmental and process monitoring.

Conclusion

The two-stage ion-exclusion and ion-exchange method effectively separates trace anions from a high phosphate background in 85% phosphoric acid, achieving low detection limits and reliable quantification. The workflow demonstrates robust performance for routine industrial analysis, with simple column regeneration and cleanup steps ensuring long-term system stability.

Reference

1. Watanabe, K. Poster 66, International Ion Chromatography Symposium, Dallas, 1995.
2. Wu, M.; Chen, J. Micro. 15(10), 1997, 74.
3. Bader, M. J. Chem Educ. 57, 1980, 730.
4. Weiss, J. Ion Chromatography, VCH, 1995, 209–210.
5. Troubleshooting Guide for HPLC Injection Problems, Rheodyne, 1992.
6. SEMI International Standards, SEMI C7.11-93, 1997.