Sulfur determination in ammonia gas applying Combustion IC

Significance of the topic

Sulfur impurities in ammonia gas, even at trace levels, can provoke high-temperature sulfidation of metal components, form corrosive complexes, and interfere with subsequent chemical processes. Ensuring sulfur content remains below critical thresholds (typically 0.5 mg/kg) is essential for maintaining equipment integrity and process efficiency in industrial settings. Accurate, sensitive analysis of sulfur in gaseous ammonia therefore underpins quality control and safety in sectors such as fertilizer production, petrochemicals, and chemical synthesis.

Objectives and study overview

This application note demonstrates a robust method for quantifying sulfur species in ammonia gas. The approach combines pyrohydrolysis to convert all sulfur compounds into sulfate, followed by ion chromatography (IC) for separation and conductivity detection. The primary goals are to achieve detection limits below the critical level of 0.5 mg/kg and to validate the system’s capability against its own blank level.

Methodology and instrumentation

The analysis workflow consists of:

• Direct injection of 200 µL ammonia gas into a combustion oven at 1050 °C under a stream of argon (100 mL/min) and oxygen (300 mL/min).
• Pyrohydrolysis of sulfur species, with the resulting gases absorbed in a hydrogen peroxide solution.
• Partial loop injection with inline matrix elimination into an ion chromatograph equipped with a Metrosep A Supp 5 column.
• Conductivity detection after sequential suppression to quantify sulfate.

Key operating parameters:

• Absorber solution feed: 0.2 mL/min
• Post-combustion time: 60 s; post-cooling time: 420 s
• IC flow rate: 0.7 mL/min; injection volume: 200 µL
• Eluent: 3.2 mmol/L sodium carbonate and 1.0 mmol/L sodium hydrogen carbonate
• Suppressor regenerant: 500 mmol/L sulfuric acid

Main results and discussion

Analysis of the absorber solution yielded a sulfate concentration of 0.003 mg/L, corresponding to 0.2 mg S/kg in the original ammonia gas. This result lies well below the 0.5 mg/kg critical threshold. Although this value approaches the system blank of the Combustion IC, the method reliably demonstrates that sulfur content remains under the allowable limit. The chromatogram shows a clear sulfate peak with minimal interference, confirming the efficacy of the inline matrix elimination and suppression strategy.

Benefits and practical applications

This method offers several advantages:

• High sensitivity near regulatory limits, ensuring compliance in QA/QC routines.
• Comprehensive conversion of all sulfur species to sulfate, delivering total sulfur quantification.
• Automated sample introduction and partial loop injection for reproducibility and throughput.
• Applicability to other gaseous matrices requiring trace sulfur analysis.

Future trends and potential applications

Advances likely to enhance this approach include coupling with mass spectrometric detection for speciation of individual sulfur compounds, miniaturized pyrolysis reactors for reduced sample consumption, and online, real-time monitoring of process gas streams. Integration of machine-learning algorithms for automated peak identification and quality control may further streamline high-throughput industrial analytics.

Conclusion

The presented Combustion IC method effectively determines total sulfur in ammonia gas at levels down to 0.2 mg/kg, well below the critical 0.5 mg/kg limit. Its combination of pyrohydrolysis, inline suppression, and conductivity detection yields reliable, interference-free results, making it a valuable tool for industrial quality assurance and regulatory compliance.

References

Application Note CIC–031, Metrohm Combustion IC for Sulfur Determination in Ammonia Gas, Version 1, April 2020