Online determination of anionic impurities in 50% NaOH and KOH

Significance of the Topic

The control of anionic impurities in concentrated sodium hydroxide (NaOH) and potassium hydroxide (KOH) is critical for downstream industries such as pulp and paper, agrochemicals, and specialty chemical production. Impurities like chloride, chlorate, and sulfate can impair catalyst activity, damage equipment, and reduce product yields. Traditional offline methods are time-consuming and involve hazardous reagents, underscoring the need for real-time, safe, and reliable online monitoring.

Study Objectives and Overview

This application note demonstrates the implementation of continuous anion analysis in 50 wt % NaOH and KOH streams using the 2060 IC Process Analyzer. It follows the ASTM E1787 standard to quantify bromide, chlorate, chloride, fluoride, nitrate, phosphate, and sulfate in caustic solutions, aiming to replace manual laboratory assays with automated inline measurements.

Methodology and Instrumentation

Samples of concentrated caustic are automatically withdrawn, diluted, and neutralized inline prior to ion chromatography. Key components include:

• 2060 IC Process Analyzer platform with modular cabinets and up to 20 inlet streams
• Continuous eluent production module and ultrapure water supply (e.g., PURELAB® flex 5/6)
• Wet part modules for sample conditioning, inline neutralization using perchloric acid for sulfate analysis
• Conductivity detector for anion response
• Software with Modbus/Discrete I/O for automated alarms, diagnostics, and trend monitoring

Main Results and Discussion

The system reliably quantified anionic impurities in the range of 0.1–1000 µg/g. Inline sample preparation ensured consistent recovery and low detection limits. Automated calibration routines maintained reproducibility over extended operation. Real-time data and process alarms enabled immediate corrective actions to prevent membrane fouling and product off-specification.

Benefits and Practical Applications

• Continuous monitoring reduces manual sampling and hazardous reagent handling
• Multiple caustic streams (up to 10) can be analyzed by a single instrument
• Automated alarms and trend analysis support proactive maintenance
• Improved product quality leads to lower downstream waste and operational costs

Future Trends and Applications

Advancements may include integration with machine-learning platforms for predictive maintenance, expansion to cation or organic impurity analysis, miniaturized flow cells for decentralized monitoring, and enhanced remote access for global plant networks. Such developments will further streamline process control and reduce operational risks.

Conclusion

The Metrohm 2060 IC Process Analyzer provides a robust solution for inline determination of anionic impurities in 50 % NaOH and KOH, complying with ASTM E1787. Its modular design, automated sample preparation, and software-driven diagnostics ensure high reliability, safety, and cost savings compared to traditional offline methods.

References

1. Euro Chlor. How Are Chlorine and Caustic Soda Made? Euro Chlor, 2017.
2. ASTM International. Standard Test Method E1787-16: Anions in Caustic Soda and Caustic Potash by Ion Chromatography, 2016.