Determination of Total and Potential Sulfate and Total Chloride in Ethanol According to ASTM Method D 7319

Importance of the Topic

Ethanol is a widely used biofuel additive that raises octane and lowers tailpipe emissions but may contain trace sulfate and chloride impurities. These anions promote corrosion and engine deposits. Reliable, rapid quantification of total and potential sulfate and total chloride in fuel-grade ethanol ensures compliance with ASTM specifications and protects downstream fuel systems.

Objectives and Study Overview

This study adapts ASTM Method D 7319 to develop a direct-injection ion chromatography protocol capable of determining both total and potential sulfate plus total chloride in ethanol. Key aims include:

• Measuring total inorganic sulfate and chloride by direct injection.
• Converting all sulfur species to sulfate via oxidation for potential sulfate determination.
• Validating linearity, detection limits, quantification limits, and precision relative to ASTM D 4806 thresholds.

Methodology

Samples of 90% ethanol are spiked with chloride or sulfate for total determinations, and with thiosulfate plus H₂O₂ (1.5% final) for potential sulfate. A 5 μL aliquot is injected onto a Dionex IonPac AS4A-SC guard (2 × 50 mm) and analytical (2 × 250 mm) column. Eluent consists of 1.8 mM sodium carbonate/1.7 mM sodium bicarbonate at 0.5 mL/min, 30 °C. Suppressed conductivity detection is provided by a chemical-regeneration Dionex AMMS 300 suppressor operated with 50 mN sulfuric acid.

Applied Instrumentation

• Dionex ICS-2100 (or ICS-1100/1600/5000) system with isocratic pump, vacuum degasser, high-pressure injector, column heater, conductivity detector.
• Dionex AS autosampler (2 mL tray) or AS-DV (5 mL vials).
• Dionex IonPac AS4A-SC guard and analytical columns.
• Dionex AMMS 300 anion suppressor in chemical regeneration mode.
• Chromeleon CDS software, 0.2 μm nylon filter, helium/nitrogen supply for eluent degassing.

Main Results and Discussion

Retention times for chloride (~2 min) and sulfate (~4 min) were stable over multiple injections. No significant interferences were observed from common anions (nitrite, bromide, nitrate, phosphate). Calibration was linear over 0.3–50 mg/L for chloride (r² > 0.9999) and 0.3–20 mg/L for sulfate (r² > 0.9993). Limits of detection were 5 μg/L (Cl⁻) and 20 μg/L (SO₄²⁻); limits of quantification were 14.7 μg/L and 60 μg/L, respectively. Repeatability (n=7) produced RSDs <1% for all analytes at relevant concentrations. The chemical-regeneration suppressor provided a stable low-noise baseline critical for low-level chloride quantification.

Benefits and Practical Application

• Direct injection requires minimal sample preparation and 10-minute run times.
• Method meets and exceeds ASTM D 4806 impurity limits (4 mg/L sulfate, 40 mg/L chloride).
• Capability to measure potential sulfate distinguishes total inorganic sulfate from oxidizable sulfur species.
• High sensitivity and precision support QC/QA screening in fuel production and blending facilities.

Future Trends and Opportunities

Advances may include integration of inline oxidation modules for potential sulfate, faster suppressor technologies, coupling with mass spectrometry for speciation, automated multiplexing for high throughput, and extension of protocols to other biofuels like butanol or methanol.

Conclusion

The direct-injection suppressed ion chromatography method described provides a robust, rapid, and sensitive approach for simultaneous determination of total and potential sulfate plus total chloride in ethanol. It aligns with ASTM D 7319 requirements, ensures regulatory compliance, and safeguards engine performance.

References

• Antoni D, Zverlov VV, Schwarz WH. Biofuels from Microbes. Appl Microbiol Biotechnol. 2007;77(1):23–35.
• ASTM D 4806. Standard Specification for Fuel Ethanol for Blending with Gasolines for Use as Automotive Spark-Ignition Engine Fuel. ASTM Int., West Conshohocken, PA.
• ASTM D 7319-13. Test Method for Existent and Potential Sulfate and Inorganic Chloride in Fuel Ethanol and Butanol by Direct Injection Suppressed Ion Chromatography. ASTM Int.
• Rohrer J. Determination of Sulfate and Chloride in Ethanol by Ion Chromatography. Thermo Fisher Scientific Application Note 175; 2006.
• Rohrer J. Determination of Sulfate and Chloride in Ethanol Using Ion Chromatography. Thermo Fisher Scientific Application Update 161; 2007.