July 4th Edition – Mercury in Caustic Soda

July 4th Edition – Mercury in Caustic Soda

Mercury Analysis in Caustic Soda by Cold Vapor Atomic Absorption

Caustic soda (sodium hydroxide) is commonly used in the manufacturing process for several basic chemicals. Caustics are also widely used in the petroleum refining process to produce diesel fuels, heating oils and other petroleum products. Mercury is a common contaminant in certain hydrocarbon streams in a refinery and can readily cause damage to pipes and other equipment. Mercury Chlor-Alkali Cells have been used in the past and are occasionally still used to produce chlorine, hydrogen gas and sodium hydroxide from a brine solution.

Mercury is a potent neurotoxin which can also damage refinery and chemical processing equipment, so it is important to monitor levels of total mercury in a caustic solution. The measurement of mercury in caustic soda can present quite a challenge to the analytical chemist. The American Society for Testing & Materials (ASTM) has published a method for measuring mercury in caustic soda and caustic potash (potassium hydroxide), Method ASTM E538. This method utilizes cold vapor atomic absorption (CVAA) for the final analysis of total mercury. The sample must undergo extensive sample preparation by means of a series of digestion steps at different temperatures in the presence of excess potassium permanganate. It is challenging to perform, requires very careful monitoring of the pH during the process and is very time consuming. Nippon Instruments Corp. (NIC) and AGS Scientific have recently developed a method for the analysis of total mercury in a caustic sample utilizing the NIC Model RA-4300A Reducing Vaporization Mercury Analyzer.

RA-4300A Flow Diagram                    RA-4300A Automation

This method requires very little sample preparation, requires only 5 mL of sample and uses minimal amounts of reagent per sample. Additionally, no heating is required, so the method is much faster than ASTM Method E538, while providing comparable if not improved accuracy and precision.

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