TOC and TIC Analysis in Hydroxide Solutions Using High-Temperature Catalytic Combustion

Significance of the topic

The accurate determination of total organic carbon (TOC) and total inorganic carbon (TIC) in strong hydroxide solutions such as NaOH and KOH is critical for quality control in chemical manufacturing, environmental monitoring and emerging green hydrogen production. These bases often absorb CO₂ and may contain trace organic impurities that can affect downstream processes, catalyst performance and product purity. Reliable TOC/TIC measurement supports process efficiency, compliance with regulatory limits and the longevity of sensitive analytical equipment.

Objectives and overview

This study demonstrates a robust analytical workflow for simultaneous TOC and TIC analysis of high-salt hydroxide solutions using high-temperature catalytic combustion on a Shimadzu TOC-L analyzer. Key aims include minimizing sample preparation effort, reducing acid consumption through internal acidification, extending catalyst life under saline load and achieving low quantification limits across a range of alkali concentrations.

Methodology

The direct non-purgeable organic carbon (NPOC) method was selected to avoid catalyst damage associated with total carbon minus TIC calculations. Samples are acidified in-situ with 4 mol/L H₂SO₄ at a 12 % volume ratio to lower pH below 2, converting inorganic carbon to CO₂ for purge-off prior to TOC measurement. For TIC, aliquots are reacted with phosphoric acid in an IC reactor and measured by NDIR. Automatic dilution and multiple injection functions adapt the measurement range for unknown sample concentrations, with calibration conducted at five points from 0.1 to 10 mg/L carbon for both TOC and TIC.

Instrumentation used

• Shimadzu TOC-L CPH analyzer equipped for NPOC and IC analysis
• High-salt combustion tube kit (680 °C) with SO₃ mist catcher and enlarged catalyst bed
• OCT-L 8-port autosampler for inert handling of corrosive samples
• LabSolutions TOC software for method control, automatic dilution and result conversion

Results and discussion

NPOC analysis of 2 % KOH samples spiked with 2 and 8 mg/L TOC yielded recoveries near 100 % (mean 2.06 mg/L ± 0.02 and 8.34 mg/L ± 0.07), with a LOQ of 0.25 mg/L. Undiluted 2 % NaOH samples were below LOQ. TIC measurements of 2 % KOH and NaOH showed reproducible values (e.g., 12.45 mg/L ± 0.04 for KOH), with a LOQ of 0.06 mg/L. Automatic re-dilution prevented overload beyond the calibration range. Bound CO₂ can be derived by multiplying TIC by 3.664. Method robustness is evidenced by stable SD values and minimal matrix effects.

Benefits and practical applications

• Reduced acid consumption and waste through internal acidification
• Extended catalyst and combustion tube service life under high salt loads
• Automated dilution and injection routines for high throughput and minimal hands-on time
• Reliable QC for industrial alkali and green hydrogen electrolytes
• Inline bound CO₂ calculation to monitor carbonate uptake

Future trends and potential applications

As the demand for green hydrogen and sustainable chemical processes grows, inline TOC/TIC analyzers with advanced automation and remote monitoring will become integral to process control. Development of more salt-tolerant catalyst materials and miniaturized sensor modules could enable real-time carbon monitoring in continuous flow reactors and large-scale electrolyzers. Integration with Industry 4.0 platforms will facilitate data analytics and predictive maintenance.

Conclusion

The evaluated high-temperature catalytic combustion method on the TOC-L platform provides an efficient, accurate and durable solution for measuring TOC and TIC in high-salt hydroxide matrices. Internal acidification, automatic dilution and specialized hardware modifications ensure low quantification limits, minimal maintenance and high data reliability, supporting quality assurance in chemical, environmental and energy applications.