How Are You Analyzing Anions in Concentrated Caustic Soda

A key challenge in chlor-alkali production processes is the need to monitor the formation of contaminants and by-products in concentrated caustics. Ion chromatography with suppressed conductivity detection is the preferred method for determining ionic species in aqueous media, especially at ppm and ppb concentrations. (Check out this downloadable White Paper: Conductivity Detection in Cation Chromatography—Advantages and Disadvantages of Suppression, for more details on suppressed conductivity detection.)Here, I want to share with you a new, single-pass ion chromatography method developed in collaboration with my colleague Aaron Rose, Product Applications Specialist at our Bannockburn Center of Excellence, that utilizes the latest generation electrolytic suppressor to improve the efficiency of the technique over more traditional techniques. Direct injections of concentrated base samples overload the column, resulting in poor chromatography and quantification. While dilution sacrifices anion determination at desired concentrations. Autoneutralization, introduced by Dionex Corporation in the mid 1990s, eliminated the need to dilute the concentrated base but involved a tedious two-step process requiring either a double pass or a park and neutralize step. With the introduction of the new anion electrolytically regenerated suppressor (Thermo Scientific Dionex AERS 500 Suppressor) base neutralization can now be performed with a single pass.

Figure 1 below shows results from the analysis of a 50% sodium hydroxide sample produced from a diaphram cell process that are within trace contaminant specifications for chlor-alkali producers.

Figure 2 below depicts the two-valve configuration for single-pass neutralization with either one of our high-pressure ion chromatography systems (Thermo Scientific Dionex ICS-5000+ Reagent-Free HPIC system) or one of our integrated ion chromatography systems (Thermo Scientific Dionex ICS 2100 Integrated IC System with Electrolytic Eluent Generation and Sample Preparation).

Figure 3 below depicts the configuration for double-pass neutralization with either an integrated or a high-pressure ion chromatography system (links to systems above). Here, the caustic sample is first loaded into a 25μL sample loop. The sample is then switched in line and the concentrated base from the sample loop is directed to the self-regenerating neutralizer (Thermo Scientific Dionex SRN Self-Regenerating Neutralizer), where the sample is partially neutralized, then transferred to the 5 mL loop. The recycle valve is then actuated to direct the sample through the neutralizer a second time to complete the neutralization. Following the double-pass neutralization process, the sample anions can be focused onto a concentrator column before eluting them onto an analytical column.

The single-pass neutralization pretreatment with the new neutralizer provides a more efficient alternative to the double-pass neutralization for the analysis of anions in concentrated caustics. Results by the single pass method with ion chromatography are shown to satisfy purity objectives in chlor-alkali processes by measuring trace levels of chlorate, chloride, and sulfate in a faster more effective manner.

Sample Preparation Conditions

Sample	Diaphram Caustic 1:500 dilution with DI water
Eluent	DI Water with Anion Trap Column (9×75 mm)
Flow Rate	1 mL/min
Injection Volume	25 μL
Temperature	30 0C
Neutralization	Dionex AERS 500, 4 mm Suppressor (P/N 082542)
Concentrator	UTAC-XLP1 Anion Concentrator Column

Analytical Conditions

Column	Thermo Scientific Dionex IonPac AS19 (2×250 mm)
Eluent	10 mM KOH 0-12 min, 10-45 mM KOH 12-25 min
Eluent Source	Thermo Scientific Dionex EGC 500 KOH Cartridge
Flow Rate	0.5 mL/min
Temperature	30 0C
Detection	Suppressed conductivity Dionex AERS 500, 2 mm Suppressor (P/N 082541) AutoSuppression, recyle mode

Additional Resources

Do check out our Chemical Analysis and Manufacturing web page which is an informative resource dedicated to our Chemicals and Polymers customers and features the latest on-demand webinars, videos, application notes, and more.

Is chlor-alkali raw material testing, process analysis, and QA by IC, HPLC, and ICP-OES of interest to your laboratory? If so, I would like to hear your thoughts and experiences.

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