Catalyzed and Uncatalyzed Decomposition of Hypochlorite in Dilute Solutions

Abstract

Hypochlorite decomposition has been investigated by the combined measurement of aqueous concentrations of total hypochlorite, chlorate, and chloride, as well as that of evolved oxygen. In all experiments, the initial concentrations of NaOCl and NaCl were 80 mM, and the temperature was 80 °C. The pH was kept constant in the range 5−10.5. The uncatalyzed decomposition of hypochlorite and the formation of chlorate and oxygen were all found to be third order of the form r i = k i [HOCl] 2 [OCl − ], and k O 2 was determined to be 0.046 M −2 s −1 . A reaction mechanism in which oxygen and chlorate formation share an intermediate is proposed. Several compounds were tested for catalytic effects. The addition of chloride salts of cobalt and iridium showed catalytic effects on oxygen formation. The addition of iridium chloride also catalyzed the formation of chlorate with increasing selectivity for chlorate with increasing pH.

The catalyzing effect of chromate in the chlorate formation reaction

“…(16) gives a rate coefficient of 2.51 (mol/L) −2 s −1 in line with previously reported values of Sandin et al (2015) at 2.39 (mol/L) −2 s −1 respectively Knibbs and Palefreeman (1920) at 2.68 (mol/L) −2 s −1 . However, the activation energy of 56.1 kJ/mol is somewhat lower than the 68 kJ/mol reported by Adam et al (1992), D'Ans and Freund (1957), Yokoyama and Takayasu (1967).…”

“…It was recently shown by Sandin et al (2015) that the kinetics of homogeneous oxygen formation according to Reaction (7) at 80 • C, pH 4-9 and ionic strengths up to 2.7 mol/L satisfies…”

The electrocatalytic properties of doped TiO2

“…The low pH of the solution used by Bendtsen et al [27] should prevent most HOCl formation, but the ratio between Cl 2 and HOCl does reach 50% already at about pH 3 [93]. Co has been found to be active for decomposition of HOCl to form oxygen at pH 3 [90]. We believe further verification of the results of Bendtsen et al [27] is warranted.…”

“…We agree that a chemical reaction could be the cause, but suggest that the behavior could be related to hydrolysis of electrochemically formed Cl 2 , followed by catalyzed decomposition of formed HOCl. This decomposition would be accelerated by the Co or Ni dopants, as the ability of both Co [82][83][84][85][86][87][88][89][90] and Ni [82,91,83,84,88,92] species (in salt, oxide and metallic form) to catalyze the chemical decomposition of hypochlorous acid species to form oxygen gas is well-known. This is not mentioned in the papers [80,27].…”

Suppressed oxygen evolution during chlorate formation from hypochlorite in the presence of chromium(VI)

“…Its rate is pH dependent, having its maximum at the same pH as the chlorate‐forming reaction. It was therefore proposed that these reactions share an intermediate, which can decompose to either oxygen or chlorate . In the absence of catalytic species in the solution, the preferred reaction path is the chlorate formation, and only a minor portion of hypochlorite decomposes to oxygen.…”