Determination of Total Organic Carbon in Soil, Rock, and Shale by Acid Digestion and Combustion

Importance of the Topic

Determining total organic carbon (TOC) in geological and soil samples underpins hydrocarbon exploration by identifying potential source rocks for petroleum and natural gas. In agriculture, TOC content reflects soil fertility and nutrient cycling, while in construction materials such as cement, it serves as a critical quality control marker.

Objectives and Study Overview

This application note describes a validated procedure for quantifying TOC in soil, rock, and shale using acid digestion followed by high-temperature combustion on the LECO C832 carbon determinator. The protocol aligns with ISO 10694, ensuring removal of inorganic carbon and accurate measurement of the residual organic fraction.

Methodology and Instrumentation

Sample Preparation:

• Dry and finely pulverize representative samples.
• Divide material into portions for total carbon and TOC assays.
• Treat TOC portions with dilute HCl (1:3 HCl:H₂O) to effervesce carbonates, repeating until effervescence ceases.
• Dry acid-treated samples at 105 °C before analysis.

Instrument Configuration:

• LECO C832 macro-combustion furnace at 1350 °C under pure oxygen.
• Moisture removal via anhydrone tubes; CO₂ detection by non-dispersive infrared (NDIR) sensor.

Accessories and Reagents:

• Ceramic combustion boats (pre-baked at 1000 °C).
• Halogen trap kit for TOC measurements to remove chlorine.
• LECONAL surfactant for hydrophobic samples.

Calibration and Quality Control:

• Instrument blank, acid blank and drift corrections performed using certified reference materials (LCRM®, LRM®, NIST).
• Calibration range established with synthetic carbon and calcium carbonate standards.

Main Results and Discussion

Typical data demonstrate high accuracy and precision across a range of carbon contents:

• High-carbon soil samples (~13–23 % total C) yielded TOC values within 1–2 % absolute of expected values, with standard deviations below 0.1 %.
• Low-carbon samples (~3 % total C) showed TOC reproducibility within ±0.02 %.

These results confirm the robustness of acid digestion combined with combustion for reliable TOC analysis in diverse matrices.

Benefits and Practical Applications

The described method offers:

• Accurate differentiation between organic and inorganic carbon fractions.
• High sample throughput and reproducibility for exploration geochemistry.
• Essential soil quality assessment in agronomy.
• Quality control of raw materials and final products in cement manufacturing.

Future Trends and Applications

Emerging directions include:

• Integration of automated sample handling and data analytics for real-time decision support.
• Miniaturized, field-deployable TOC analyzers for on-site screening.
• Coupling TOC analysis with isotope ratio and molecular speciation techniques.
• Advanced chemometric approaches to correlate TOC with geochemical and agronomic indicators.

Conclusion

The acid digestion and combustion procedure on the LECO C832 provides a reliable, standardized approach for TOC determination in soils, rocks, and shale. Compliance with ISO 10694 and rigorous quality controls ensure data accuracy and reproducibility critical for hydrocarbon exploration, environmental assessment, and industrial quality assurance.

References

• ISO 10694: Soil Quality – Determination of Organic and Total Carbon after Dry Combustion.
• LECO Corporation, C832 Series Operator's Manual.