Determination of anionic impurities in sulfuric acid using ion chromatography

Significance of the topic

Concentrated sulfuric acid is widely used in chemical manufacturing and semiconductor fabrication. Trace anionic impurities can impact process efficiency, yield, and reliability. Traditional titration methods are time consuming and require hazardous reagents, while ion chromatography offers a promising, automated alternative for low-level anion analysis.

Objectives and Study Overview

This study presents two direct injection ion chromatography methods to determine anionic impurities in concentrated sulfuric acid:

• Method 1 employs a Dionex IonPac AS19 column with hydroxide eluent to quantify fluoride, chloride, nitrite, bromide, and nitrate.
• Method 2 uses a Dionex IonPac AS23-4µm column with carbonate bicarbonate eluent to measure phosphate.

Both approaches use sample dilution to avoid column overloading by sulfate and automated eluent generation for reproducibility.

Methodology

A 0.018 M sulfuric acid sample was prepared by diluting 100 µL of concentrated acid into 99.9 mL of deionized water. Calibration standards ranged from 0.5 to 500 µg/L. Limits of detection (LOD) and quantitation (LOQ) were determined using signal to noise ratios of 3 and 10, respectively. Recovery and precision experiments assessed method accuracy and repeatability.

Used Instrumentation

• Thermo Scientific Dionex ICS-5000+ HPIC system with DP pump, eluent generator, degasser, and conductivity detector
• Dionex AS AP Autosampler
• Dionex IonPac AS19 and AS23-4µm analytical and guard columns
• Electrolytically regenerated suppressors (AERS 500) and carbonate removal device (CRD 300)
• Chromeleon chromatography data system

Key Results and Discussion

• Method 1 achieved baseline separation of five anions with correlation coefficients above 0.9998. LODs ranged from 0.05 to 1.5 µg/L in the diluted sample, corresponding to 5 to 153 µg/L in 10% w/w acid.
• Method 2 provided a phosphate LOD of 12.6 µg/L (42 mg/L in concentrated acid) with r² = 0.9994; other anions coeluted with sulfate and were not quantified.
• Recovery tests yielded 86 to 115% for all targets, with slightly lower nitrite recovery attributed to oxidation in acid.
• Precision studies showed retention time RSD below 0.1% and peak area RSD below 0.7%.

Benefits and Practical Applications of the Method

• Direct injection minimizes sample preparation steps.
• Automated eluent generation improves reproducibility and reduces manual errors.
• High sensitivity and low detection limits support quality control in semiconductor and chemical production.

Future Trends and Potential Applications

Further integration with online sampling, miniaturized systems, and coupling with mass spectrometry could expand multi anion detection capabilities. Adaptation for other aggressive matrices and simultaneous trace contaminant profiling represents a promising direction.

Conclusion

The two ion chromatography methods offer accurate, sensitive, and automated analysis of anionic impurities in concentrated sulfuric acid. Method 1 covers fluoride, chloride, nitrite, bromide, and nitrate, while Method 2 provides phosphate determination. Implementation of these techniques can enhance process control and product quality in relevant industries.

References

1. Thermo Scientific Technical Note 44: determination of trace anions in concentrated phosphoric acid.
2. Thermo Scientific Technical Note 45: determination of trace anions in hydrofluoric acid and related etchants.
3. Thermo Scientific Application Note 137: trace anions in high-nitrate matrices by ion chromatography.
4. Thermo Scientific Dionex IonPac AS19 Columns Product Manual.
5. Thermo Scientific Dionex IonPac AS23-4µm Column Product Manual.
6. Thermo Scientific Dionex CRD 300 Carbonate Removal Device Product Manual.
7. Thermo Scientific Dionex VP Vacuum Pump Installation Instructions.
8. Thermo Scientific Application Note 72209: trace oxyhalides and bromide in water.
9. United States Pharmacopeia General Chapter 1225: validation of compendial methods.
10. Thermo Scientific Dionex ICS-5000+ System Operator’s Manual.