Nascent Oxygen?–What is Really Killing Bacteria in Water?

by Michael J. McGuire

Oxidation-Reduction Reactions

Before we understood that oxidation-reduction reactions involved electron transfers, chemists theorized that oxygen existed in a “nascent state.” This state made it possible for oxidation reactions to take place. Such an outmoded chemistry concept is relevant to a discussion of the history of chlorination in the U.S.

Nascent Oxygen Theory

The first continuous use of chlorine to disinfect a U.S. water supply occurred at Boonton Reservoir—the water supply for Jersey City, New Jersey. As recounted in a forthcoming book (The Chlorine Revolution), two trials defined the need for disinfection and documented how it happened. In the second Jersey City trial, Dr. John L. Leal claimed that chlorine was not responsible for killing bacteria. Instead, he put forth the long-standing theory that chlorine when added to water liberated something called nascent oxygen, and it was the nascent oxygen was responsible for disinfection. (McGuire 2013)

The concept of nascent oxygen originated with James Watt, who described the importance of liberated oxygen in the bleaching process. An equation suggested by Watt (Race 1918) showed chlorine producing oxygen when it was dissolved in water:

Cl₂ + H₂O = 2HCl + O

In which Cl₂ = chlorine, H₂O = water, HCl = hydrochloric acid, and O = nascent oxygen.

In a later, well-known publication, Albert D. Hooker stated the theory most clearly: “It should be well understood that chloride of lime, in its industrial application of bleaching, deodorizing, or disinfecting, does not act by its chlorine, but by its oxygen.” (Emphasis in original.) (Hooker 1913)

In 1918, Joseph Race described the controversy surrounding chlorine’s mode of action in water. Race stated that Fischer and Proskauer (1884) believed that chlorine was not directly toxic. Warouzoff, Winograoff, and Kolessnikoff (1886) found that chlorine gas killed airborne tetanus spores. Interestingly, Race quoted at length John L. Leal’s second-trial testimony supporting the theory of disinfection by nascent or potential oxygen. However, Race’s laboratory work in 1915–17 appeared to convince him that disinfection was caused by the direct toxic action of chlorine and not by nascent oxygen. (Race 1918)

Other publications reflected the confusion over chlorine’s mechanism of action. In his 1917 textbook, Ellms (who would testify in the second Jersey City trial) presented equations showing the formation of hypochlorous acid (HOCl) when chlorine was added to water. At this point in his discussion, he was correct. However, he then stated “The HOCL is decomposed into HCl and oxygen, which latter acts upon any oxidizable matter that may be present.” (Ellms 1917)

2HOCL à 2HCl + O₂

In this case, HOCl = hypochlorous acid and O₂  = oxygen.

“The energy liberated by the decomposition of the hypochlorous acid, as previously stated, explains the powerful oxidizing action of the evolved oxygen, and the destructive effect upon the microorganisms. Chlorine or the hypochlorites are therefore, merely agents for the production of oxygen under conditions which render it extremely active.” (Ellms 1917)

Abel Wolman and I.H. Enslow tried to put a stop to the nascent oxygen theory in 1919, but it persisted long after that. (Fair and Geyer 1954) We know now that HOCl exists in water in equilibrium with the dissociated hypochlorite ion and that the degree of dissociation is a function of the water’s pH.

HOCL ↔ OCl^– + H⁺

For this equation, OCl^– = hypochlorite ion and H⁺ = hydrogen ion.

In a textbook published in 1924, authors F.E. Turneaure and H.L. Russell tried to straddle the issue:

“The reaction of both hypochlorite and liquid chlorine in sterilization of water is substantially the same. The accepted theory is that the chlorine forms hypochlorous acid with the water setting free nascent oxygen which is considered the effective sterilization agent. Some authorities, however, contend that the chlorine itself has a toxic effect upon the bacteria.” (Turneaure and Russell 1924)

A 1935 rewrite of Sedgwick’s famous book on sanitary science favored the direct action of chlorine theory but did not totally discount the action by nascent oxygen.

“The mechanism by which chlorine brings about germicidal action is still undetermined. It is believed by some that the bacteria are destroyed because of the direct toxic effect of the chlorine. Others maintain that the introduction of chlorine into water results in the formation of hypochlorous acid—an unstable compound—which breaks up and liberates nascent oxygen and hydrochloric acid, the supposition being that the bacteria are destroyed by the nascent oxygen. . . . Since chlorine compounds can destroy bacteria even when oxygen is not liberated it would seem that those mechanisms that explain the germicidal action of chlorine without hypothesizing the formation of nascent oxygen have a more sound scientific basis.” (Prescott and Horwood 1935)

A 1944 publication by S.L. Chang appeared to put the controversy to rest: “The action of chlorine and chloramine compounds on cysts was attributed to the active chlorine which may oxidize or chlorinate the proteins in the protoplasm. The possibility of action by nascent oxygen liberated by HOCl was indirectly studied, and the evidence strongly indicated that this was unlikely to occur.” (Chang 1944) Since Chang’s publication, nascent oxygen has not been mentioned in professional publications except as a historical curiosity.

In their classic 1954 textbook on water and wastewater engineering, Gordon M. Fair and John C. Geyer addressed the historically curious concept and stated categorically that oxygen did not accomplish disinfection. It was chlorine in its various forms in water that was toxic to bacteria. (Fair and Geyer 1954) Like many a scientific theory that conveniently explained a troubling public relations problem, it took a lot of time to kill the nascent oxygen idea.

References:

• Chang, S.L. 1944. “Destruction of Micro-Organisms.” Journal AWWA. 36:11 1192-1207.
• Davies, Edward. 1878. “The Nascent State as Affecting Chemical Action.” The Pharmaceutical Journal and Transactions. 8: 485-6.
• Ellms, Joseph W. 1917. Water Purification. New York City, N.Y.: McGraw-Hill.
• Fair, Gordon M., and John C. Geyer. 1954. Water Supply and Waste-water Disposal. New York City, N.Y.: John Wiley & Sons, Inc.
• Hooker, Albert D. 1913. Chloride of Lime in Sanitation. New York City, N.Y.: John Wiley & Sons.
• McGuire, Michael J. The Chlorine Revolution:  Water Disinfection and the Fight to Save Lives. Denver:American Water Works Association, 2013.
• Prescott, Samuel C. and Murray P. Horwood. 1935. Sedgwick’s Principles of Sanitary Science and the Public Health:  Rewritten and Enlarged. New York:McMillan.
• Race, Joseph. 1918. Chlorination of Water. New York City, N.Y.: John Wiley & Sons.
• Turneaure, F.E., and H.L. Russell. 1924. Public Water-Supplies: Requirements, Resources, and the Construction of Works. 3rd Edition. New York City, N.Y.: John Wiley & Sons, Inc.