Determination of Sulfate and Chloride in Ethanol Using Ion Chromatography

Importance of the Topic

Ethanol used as a gasoline blending component must comply with stringent chloride and sulfate limits specified in ASTM D4806 to avoid corrosion, spark plug fouling and engine performance issues. Ion chromatography provides a precise, sensitive and reproducible approach for quantifying these anions at milligram-per-liter levels in ethanol matrices.

Objectives and Study Overview

This work updates earlier direct-injection IC methods by addressing two main drawbacks: baseline drift under electrolytic suppression and rapid sulfate signal loss at higher concentrations. The revised protocol employs a trace anion concentrator to trap and purge ethanol prior to analysis, thereby enhancing sensitivity, stability and ease of operation while maintaining alignment with ASTM collaborative study requirements.

Methodology and Instrumentation

The ethanol sample is injected directly using an AS Autosampler operating in concentrate mode onto a TAC-ULP1 Ultra Low Pressure Trace Anion Concentrator. Following ethanol removal with reagent water, separation is achieved on an IonPac AS22 column (4×250 mm) with AG22 guard using a 4.5 mM Na2CO3 / 1.4 mM NaHCO3 eluent at 1.2 mL/min, 30 °C. Detection is performed by suppressed conductivity in recycle mode (ASRS ULTRA II at 31 mA), yielding stable baselines without chemical regenerants. Calibration standards (1–20 mg/L) are prepared by spiking ethanol (with 5 % gasoline when required) and daily validated.

Used Instrumentation

• Dionex IC system (ICS-3000, ICS-2000, ICS-1500, ICS-1000 or ICS-90)
• AS Autosampler with TAC-ULP1 Ultra Low Pressure Trace Anion Concentrator
• IonPac AS22 analytical column with AG22 guard
• Anion Self-Regenerating Suppressor (ASRS ULTRA II, recycle mode)
• Chromeleon Chromatography Management Software ver. 6.8

Main Results and Discussion

The updated method demonstrated flat, interference-free baselines in ethanol spiked with 5 % gasoline. Chloride and sulfate at 4 mg/L were clearly separated within 15 min with no baseline elevations. Sensitivity improved roughly eightfold relative to earlier AS14A-based methods, enabling reliable quantification down to 1 mg/L. Critically, 20 mg/L sulfate in gasoline-denatured ethanol exhibited excellent signal stability over 99 consecutive injections (<2 % area drift), contrasting with the 20 % loss reported previously.

Benefits and Practical Applications

The revised IC approach offers simplified operation by eliminating acid regenerants, enhanced analyst safety, and robust performance under routine QC conditions. Its high sensitivity and stability make it ideally suited for fuel laboratories verifying compliance with ASTM D4806.

Future Trends and Applications

The method can be further automated via RFIC systems with EluGen II carbonate cartridges and electrolyte mixers, reducing manual eluent preparation. For trace-level anion monitoring (<µg/L), coupling this concentrator approach with microbore columns or post-column derivatization may extend detection capabilities. Integration into inline fuel monitoring platforms presents an opportunity for real-time quality assurance.

Conclusion

This application update delivers a rugged, high-throughput IC method for chloride and sulfate determination in gasoline-denatured ethanol. By combining trace anion concentration with electrolytic suppression, it achieves superior sensitivity, stable baselines and practical ease of use, fully addressing limitations of previous protocols.

References

• Dionex Application Update 161, Determination of Sulfate and Chloride in Ethanol Using Ion Chromatography, February 2007
• Dionex Application Note 175, Direct Injection Ion Chromatography Methods for Sulfate and Chloride in Ethanol