Intracellular production of hypochlorous acid (HOCl) results from myeloperoxidase activity in neutrophil granulocytes during the ‘oxidative burst’ when these cells respond to invasive organisms. While short-lived <i>in vivo</i> due to its high reactivity with local tissue substrates, the use of HOCl <i>in vitro</i> as a powerful oxidative antimicrobial decontaminant has been limited by its inherent instability in solution. This is true of HOCl generated either chemically from NaOCl, or by electrolysis of brine. The proposed decomposition of HOCl follows a 3rd order reaction leading to formation of chlorate, reaching a maximum at pH 7. In previous studies on HOCl decomposition  phosphate buffer, chloride, and sodium perchlorate were added to keep the pH and/or ionic strength constant.  We used Raman Spectroscopy, iodometric titrations, UV-visible spectrophotometry, and ORP measurements to characterize an electrolytically-generated, solution of pure HOCl (pH 4) made from only NaCl and water. This unbuffered solution was unexpectedly stable, even at elevated temperatures, sealed in a variety of vessels. There were no detectable changes in oxidative Cl levels (ppm), ORP (+mv), or pH in HOCl solutions maintained in glass containers at 52 °C for 38 days.  After 30 days at 70 °C in glass, active Cl declined from 190 ppm to 151 ppm, but ORP remained constant, while pH rose to 4.3. In comparison, in HDPE at 52 °C, the active chlorine decreased by 53 ppm over 38 days and the pH rose to 5.3, though the ORP remained constant. No oxidative aqueous Cl species other than HOCl were detected in any stored samples. The results demonstrate that long-lived HOCl solutions can be prepared that undergo little or no change on storage, even at high temperatures. The findings open up prospects for wider use of HOCl in environmental disinfection, with avoidance of on-site production, or concerns about the efficacy of stored solutions.

Purity and stability of an electrolytically-generated hypochlorous acid solution

254th National Meeting (2017)

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