TOC analysis pinpoints water chemistry changes


Direct conductometric TOC analyzer alerts engineers to potentially damaging trihalomethane excursions in pharmaceutical water

By Terry Stange, PhD, and Matthew J. Smith, PhD, Hach Ultra Analytics

Total organic carbon (TOC) analyzers are becoming industry standard for monitoring purified water (PW) and water for injection (WFI) systems. Accuracy and stability are critical parameters for daily process monitoring of water distribution systems. Direct conductometric (DC) technology, self-calibrating conductivity circuits, and dynamic endpoint detection are being used in new-generation instruments to measure on-line TOC. Although the application of membrane conductivity (MC) for TOC measurement emerged in the mid-1980s, this technology was deterred from use in on-line PW and WFI applications due to the inherent instability of membrane-based TOC analyzers. MC-based analyzers suffer from continuous drift and instability owing to the constant change in CO2 transfer rates across the membrane in real-world, on-line applications. Variations in pH, temperature, membrane fouling, rouging, and dissolved gases (e.g., chlorine) all contribute to variations in CO2 transfer rates. These variations require frequent calibration of MC-based TOC analyzers compared to DC-based analyzers, which are also simpler to operate and maintain. DC TOC analyzers are based on the assumption that the only conductive species generated during UV oxidation is CO2–an assumption rarely violated in today’s advanced PW and WFI systems. When water conditions depart from normal ultra-pure water (UPW) levels, DC TOC analyzers can alert facilities engineers to subtle water chemistry changes that are otherwise ignored by MC-based TOC analyzers.

This article lays out the response of a TOC analyzer to hypothetical trihalomethane (THM) excursions in a UPW system. The sensitivity of DC analyzers to the halogen ions created during oxidation of THMs can alert UPW engineers to potential excursions that might harm the water system components or even violate the Environmental Protection Agency’s (EPA’s) requirements for total THMs in water used to manufacture pharmaceutical products. When incoming source water or UPW meets EPA requirements for THM levels, the TOC analyzers studied will never report a false positive TOC value. In the rare occurrence of THM excursions, the analyzer becomes more than just a TOC analyzer–it also becomes a THM event monitor.

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