Determination of formic and acetic acids in petroleum products by ion chromatography

Importance of the Topic

Organic acids such as formic and acetic acids present in crude oil and refined products pose significant corrosion risks to pipelines, refinery units, and automotive engines. Traditional bulk acidity measurements (TAN) fail to account for the specific identity and concentration of low molecular weight acids, which are often the most aggressive corrosive species. A reliable, sensitive, and automated method to directly quantify these acids is therefore essential for industrial quality control and corrosion prevention.

Objectives and Study Overview

This study aims to develop and validate an automated ion chromatography (IC) method for the direct determination of formic and acetic acids in diesel, motor oil, and diesel–oil mixtures. Key objectives include:

• Establishing an in-line matrix elimination technique to handle non-aqueous samples.
• Optimizing chromatographic conditions (eluent gradient, suppressor, detection).
• Assessing method performance (linearity, detection limits, recovery, repeatability).
• Demonstrating applicability to real-world samples, including engine-stressed diesel.

Methodology and Instrumentation

Sample Preparation and Matrix Elimination:

• Diesel or diesel–oil mixtures diluted with 1-butanol; standard addition for calibration and recovery studies.
• In-line extraction performed on a polymeric anion concentrator column (UTAC-LP1) using 2-propanol to remove hydrophobic matrix while retaining organic acids.
• Rinse step with water to eliminate solvent before injection.

Chromatographic Conditions:

• Column: Dionex IonPac AS11-HC (2 × 250 mm) with AG11-HC guard.
• Eluent: KOH gradient generated by EGC module (100 → 20 mM over 20 min).
• Flow rate: 0.38 mL/min; injection volume: 10 μL; temperature: 30 °C.
• Detection: suppressed conductivity using AERS 500 suppressor in self-regenerating mode.

Instrumentation Used

• Thermo Scientific Dionex ICS-2100 (now Integrion HPIC) with degasser and auxiliary valve.
• UltiMate LPG-3400 SD pump and WPS-3000 RS autoinjector.
• Dionex EGC KOH Eluent Generator Cartridge with CR ATC II.
• Dionex RFIC eluent and suppressor modules.
• Chromeleon CDS, Version 7.2.

Main Results and Discussion

Calibration and Sensitivity:

• Linear calibration for formic acid (r² = 0.9996) and quadratic for acetic acid (r² = 0.9992) due to partial dissociation effects.
• Detection limits: 1.5 mg/L (formic) and 3.8 mg/L (acetic); quantification limits: 2.7 mg/L and 6.3 mg/L, respectively.

Accuracy and Precision:

• Intra-day repeatability ≤ 2%; recoveries ~105–107% (10 mg/L spike in diesel/oil mix).
• Inter-day repeatability 4% (acetic) and 9% (formic); recoveries 101% ± 5% (acetic) and 90% ± 9% (formic) over four days.
• Organic diluent (2-propanol) matrix effects <1% on calibration response.

Application to Real Samples:

• Diesel with up to 10% motor oil showed negligible impact on method performance; acetic acid spontaneously present in synthetic oil at ~278 mg/L.
• Engine-stressed diesel contained 8.4 mg/L acetic and 1.8 mg/L formic acids; additional unknown low-MW acids observed, indicating potential for further MS identification.

Benefits and Practical Applications

This IC method provides

• Direct, automated quantification of low molecular weight organic acids in hydrophobic matrices without extensive sample cleanup.
• High sensitivity and selectivity suitable for corrosion monitoring in refining and engine maintenance.
• Robustness across varying oil contents and rapid analysis compatible with routine quality control.

Future Trends and Opportunities

Integration with mass spectrometric detection could enable structural elucidation of unknown acids. Further advances may include real-time online monitoring of acid concentrations during refining processes, expansion to other corrosive species, and coupling with multi-dimensional separations for broader petrochemical profiling.

Conclusion

The developed IC approach achieves reliable, low-limit detection of formic and acetic acids in diesel and oil-containing samples via in-line matrix elimination and suppressed conductivity detection. Excellent precision, accuracy, and robustness support its adoption in industrial laboratories for corrosion risk assessment and fuel quality assurance.

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