Online Determination of Anions in 50% NaOH and 50% KOH by Ion Chromatography According to ASTM E1787-16

Importance of the Topic

The accurate monitoring of anionic impurities in highly concentrated caustic soda (NaOH) and caustic potash (KOH) streams is essential for ensuring product quality, protecting downstream processes, and extending the lifetime of membrane cells in chlor-alkali production. Inline analysis delivers real-time data, enabling timely interventions to avoid membrane fouling, reduce maintenance costs, and meet stringent industry purity requirements.

Objectives and Study Overview

This application note evaluates the implementation of online ion chromatography according to ASTM E1787-16 for continuous determination of seven anions—bromide, chlorate, chloride, fluoride, nitrate, phosphate, and sulfate—in 50 wt % NaOH and KOH. The primary focus is on chloride, chlorate, and sulfate, which are the most critical contaminants in caustic streams.

Methodology

The method involves automated sampling of the caustic liquor, dilution (1:10), and inline neutralization before injection into the Process Ion Chromatograph. Metrohm Inline Sample Preparation (MISP) modules handle acidification, anion trapping, and eluent mixing. Conductivity detection follows chromatographic separation on a high-capacity anion exchange column. Periodic automated calibration maintains low detection limits and high reproducibility. For sulfate analysis, perchloric acid is used for inline neutralization, and an anion trap is placed in the transfer line to ensure matrix removal. For chloride concentrations above 2 mg/L, a potentiometric titration option is available.

Used Instrumentation

• Process Ion Chromatograph from Metrohm Process Analytics
• Inline Sample Preparation modules (MISP) for dilution, neutralization, and trapping
• Built-in eluent generation module
• Optional PURELAB® flex 5/6 ultrapure water supply from ELGA®
• Conductivity and potentiometric detectors

Main Results and Discussion

Continuous monitoring of multiple caustic streams (up to 20) was demonstrated in a membrane-cell chlor-alkali environment. The Process IC delivered clear separation of all seven anions with detection limits suitable for industrial QC (<0.1 mg/L). Chromatograms of 50 wt % KOH samples showed baseline separation of chloride, chlorate, and sulfate peaks. The system’s alarm functionality alerted operators when impurity levels approached preset thresholds, allowing preventive maintenance and avoiding costly downtime.

Benefits and Practical Applications

• Real-time quality control of caustic soda and potash streams
• Reduced manual sampling and chemical handling
• Improved membrane cell lifetime by preventing fouling
• Compliance with ASTM E1787-16 for industrial standards
• Centralized monitoring of multiple production cells

Future Trends and Applications

Advances in digital integration will allow seamless data flow to SCADA systems and cloud-based analytics. Emerging miniaturized IC modules and AI-driven peak recognition will further reduce maintenance and enhance sensitivity. Expansion to other caustic-process anions and cation analysis will broaden applicability. Predictive maintenance algorithms will leverage continuous impurity profiles to forecast membrane cell replacement schedules.

Conclusion

Implementing online ion chromatography per ASTM E1787-16 using Metrohm’s Process IC platform offers a robust, automated solution for tracking anionic impurities in 50 wt % NaOH and KOH. High reproducibility, low detection limits, and integrated alarms enable improved process control, cost savings, and compliance with industrial standards.

Reference

1. IC Application Note S–243
2. IC Application Note S–303
3. AW IC CH6-1108-082012