Evaluation of Organic Impurities in Lithium Carbonate by TOC Analysis

Evaluation of Organic Impurities in Lithium Carbonate by TOC Analysis

Importance of the topic

Lithium carbonate (Li2CO3) is a key precursor in the lithium‑ion battery supply chain used for cathode material synthesis, electrolyte salts, solid electrolytes and recovered lithium in recycling. Organic contaminants—originating from solvents, processing aids, manufacturing residues or handling—can impair downstream syntheses, electrode performance and recycling efficiency. Rapid, reliable quantification of total organic carbon (TOC) in lithium salts is therefore essential for raw‑material quality control, process optimization and regulatory compliance.

Objectives and study overview

This study evaluates the performance of a Shimadzu TOC‑L analyzer for measuring organic impurities in a representative lithium carbonate matrix. Key aims were to: verify method accuracy via spike‑recovery tests; assess precision and stability during extended runs with high‑salt samples; and identify practical measures to avoid instrument damage and biased results when analyzing salt‑rich solutions.

Sample preparation

- A 10 g/L lithium carbonate solution was prepared by dissolving 1.00 g reagent‑grade Li2CO3 in water and adding 3.0% (v/v) of 4.5 mol/L sulfuric acid to ensure complete dissolution; mixing and sonication were used as required.
- A blank was prepared identically but without Li2CO3.
- Spike‑recovery samples were made by adding potassium hydrogen phthalate to obtain nominal TOC levels of 1, 2 and 5 mgC/L.

Methodology

- NPOC (non‑purgeable organic carbon) was used rather than TC–IC because lithium carbonate solutions contain large amounts of inorganic carbon (IC) that make TC–IC subtraction prone to error. NPOC involves acidifying to pH <3, sparging to remove volatile IC, and measuring the remaining TC as TOC.
- Sparging parameters used: 80 mL/min gas flow for 90 seconds. Acid addition during NPOC was limited to 0.5% (using 4.5 mol/L H2SO4) to minimize secondary salt effects.
- Sulfuric acid was chosen over hydrochloric acid because LiCl melts near 610 °C (below the TOC combustion temperature of 680 °C), risking molten salt carry‑over and combustion tube devitrification. Li2SO4 melts at ≈859 °C, avoiding this problem.
- For high‑salt matrices a combustion tube designed for high‑salt samples was used (larger diameter and larger catalyst particles) to reduce salt accumulation, catalyst clogging and maintenance frequency.
- Calibration: 4‑point NPOC calibration with potassium hydrogen phthalate standards at 0, 2, 5 and 10 mgC/L; origin shift applied to correct base water carbon. Correlation coefficient r = 0.9999.

Used Instrumentation

• Shimadzu TOC‑LCPH total organic carbon analyzer (TOC‑L series).
• Combustion tube and dedicated TOC catalyst for high‑salt samples.
• B‑type halogen scrubber.
• Optional ASI‑L autosampler for automated sample throughput.

Main results and discussion

- Measured TOC: blank (pure water) = 0.09899 mgC/L; 10 g/L Li2CO3 solution (S1) = 0.2912 mgC/L. Converted to a solid basis this corresponds to 19.2 mgC/kg Li2CO3 (after blank correction and sample mass/volume factors).
- Spike recovery: for spikes at 1, 2 and 5 mgC/L the recoveries were 98.8%, 101% and 99.2%, respectively, demonstrating accurate quantification and absence of major matrix suppression or enhancement.
- Long‑term consistency: 47 consecutive injections of S1 over ≈7 hours yielded mean TOC 1.278 mgC/L with a coefficient of variation (CV) of 2.6% and normalized values within ±10% of the mean, indicating stable performance for high‑salt continuous runs.
- Key practical point: incomplete removal of inorganic carbon during NPOC pretreatment leads to overestimated TOC; optimization of sparge time and gas flow is essential for matrices with high bicarbonate/carbonic acid content.
- Use of the high‑salt combustion tube and sulfuric acid acidification mitigates combustion tube fouling, molten salt formation and potential instrument damage, improving sensitivity and repeatability for salt‑rich lithium samples.

Benefits and practical applications of the method

• Robust, validated quantification of organic impurities in lithium carbonate with high recovery and precision suitable for QC labs.
• Reduced maintenance and extended consumable life when using the high‑salt combustion tube and catalyst.
• Automated sample handling with the ASI‑L autosampler improves throughput for routine analysis and stability studies.
• Method applicability extends beyond Li2CO3 to lithium hydroxide solutions, water‑extractable TOC in recycled black mass, and streams from direct lithium extraction (DLE) and refining processes.

Future trends and potential uses

- Growing battery demand will increase the need for routine, high‑throughput TOC monitoring across upstream raw materials, recycling and refining operations to assure product quality and process efficiency.
- Integration of TOC measurements with complementary analytics (ion chromatography, GC/MS for speciation, ICP for elemental profiling) will offer broader impurity fingerprints and root‑cause analysis.
- Instrument and method developments may focus on more robust combustion components for aggressive matrices, improved automated pretreatment for complex recycling streams, and on‑line or at‑line TOC monitoring in process streams.
- Standardization of sample preparation and sparging protocols for diverse lithium matrices will improve inter‑laboratory comparability and regulatory acceptance.

Conclusion

The Shimadzu TOC‑L analyzer, operated in NPOC mode with sulfuric acid acidification and a high‑salt combustion tube, provides accurate, precise and stable measurement of organic carbon in lithium carbonate solutions. Careful selection of acid, optimization of IC removal by sparging, and use of high‑salt hardware minimize analytical bias and instrument wear. The method supports quality control and process monitoring across the lithium battery value chain and can be adapted for related lithium matrices and recycling streams.

Reference

• Application News: Determination of Water‑Extractable Total Organic Carbon Content in Recycled Black Mass, Application News No. 01‑00919‑en.
• Application News: TOC and IC Measurements for Lithium Refining Processes, Application News No. 01‑01018‑en.
• Shimadzu Corporation, Application Note 01‑01098‑EN, First Edition Mar. 2026; TOC‑L Series Total Organic Carbon Analyzer documentation.