Sodium hydroxide (NaOH, also known as caustic soda or soda lye) is a strong, harmful base made of solid white crystals. It is used in many industries (e.g., paper manufacturing, petrochemistry, soap manufacturing [1]) with an annual production of about 60 million tons worldwide [2]. Caustic soda is very hygroscopic and also absorbs carbon dioxide (CO₂) from the atmosphere. This increases the carbonate content while reducing the hydroxide concentration of the product. Therefore, it should be stored under both water-free and CO₂-free conditions, with minimal atmospheric contact. Depending on the application and its purity requirements, caustic soda as a raw product must fulfill the respective quality criteria.

Quality control analysis comprises the determination of contaminants such as alkalinity, trace metals, anionic impurities (e.g., nitrate and sulfate), as well as carbonate. The analysis of carbonate by ion chromatography (IC) is quite tricky. Suppressed anion IC with hydroxide eluents does not work as suppression converts carbonate back to carbonic acid (H₂CO₃) with almost no sensitivity. In this Application Note carbonate determination in NaOH is demonstrated by applying ion-exclusion chromatography with conductivity detection after inverse suppression – a way for automated, reliable, and secure quantification.

The analysis is performed on a Compact IC Flex system, fully automated with a 858 Professional Sample Processor and the Metrohm intelligent Partial Loop Injection Technique (MiPT) (Figure 2). Carbonate (as carbonic acid) is separated from other analytes in the sample on the Metrosep Organic Acids - 250/7.8 analytical column with a sulfuric acid eluent (Figure 1). The column is operated at 30 °C, providing thermal stability for reproducible results.

With direct conductivity detection, carbonate (as carbonic acid) cannot be quantified. Thus, after separation, inverse suppression is applied prior to conductivity detection. In contrast to classical anion suppression, inverse suppression replaces the H⁺ ion with Li⁺. This step neutralizes the sulfuric acid eluent and converts weak acids into their salts. In this way, the eluent background conductivity is reduced, and the weak acids are dissociated. This improves the conductivity response of e.g., organic acids or carbonic acid.

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The calibration ranged from 5 to 100 mg/L for carbonate (Figure 1). The calibration curve is built by injecting different volumes of a single standard solution as a fully automated process with MiPT (Figure 1).

Two sample bottles of 50% NaOH were tested in this application example. The first was supplied externally, and the second bottle was sourced from the testing laboratory. Table 1 shows the concentration of carbonate in sodium hydroxide from these two samples.