Hypochlorites decomposition

C16-0049. Draw molecular pictures that illustrate the reversibility of the reactions involved in the hypochlorite decomposition given in Problem. (See Figure 16-1 for examples.)... 

The process concept is shown in Fig. 26.7 where the recycle loop of the caustic scrubbing liquor passes through a fixed-bed reactor and then through the normal cooler. The blow-down of spent caustic and make-up with fresh caustic can be carried on in the same fashion as without the in-loop hypochlorite decomposition. Consideration of the optimum locations for removal and addition may, however, be slightly different. 

Fig. 26.7 Catalytic hypochlorite decomposition integrated into the scrubbing process.

Fig. 26.9 Batch concentration profiles with integrated hypochlorite decomposition.

Oil, charcoal, other organic materials powdered metals reducing agents strong acids alkyl esters hypochlorites Decomposition/combustion oxides of nitrogen, ammonia May detonate with strong shock or if heated A confined... 

Assuming all alkoxy radicals abstract hydrogen from cyclohexane at the same rate, and that there is no interference by chlorine atom chains in the hypochlorite decompositions. See Reference c. 

In other studies it was found that a maximum of HOC1 decomposition exists at pH = 6.89. For the third-order reaction (7.14), catalytic activity of the chloride ion was suggested for hypochlorite decomposition and the stabilising effect of higher pH was quantified in the pH range 9-14 (Adam and Gordon 1999). 

The choice of metals is highly restricted. The only metals found satisfactory and in wide use are titanium, tantalum, and precious metals. Of these, only titanium is used in quantity for the construction of equipment. Its application is straightforward, and commercially pure metal (Grade 2) is satisfactory without alloying. The other metals listed above are used primarily in instruments and other small items. The ban on the large majority of metals extends to their use in auxiliary systems. Copper, for example, is used in many industrial water systems. It should not be used with hleach dilution water, however, because of its catalysis of hypochlorite decomposition reactions. 

Nickel, monel, and nickel-copper alloys have better corrosion resistance, but they are not used to avoid adding nickel and copper, which catalyze hypochlorite decomposition. ... 

Attempts to develop an activated cathode for chlorate cells have not yet been successful, and a material for the application faces many constraints. Some important properties for a chlorate cathode are (a) low overpotential for hydrogen evolution, (b) high stability during hydrogen evolution (resistant to the mechanical stress from gas bubbles and no detrimental hydride formation), (c) resistant during shut downs (low corrosion rate at open circuit in chlorate electrolyte), (d) low activity for hypochlorite decomposition, (e) low activity for reduction of hypochlorite and chlorate in the presence and in the absence of the chromium hydroxide film (the latter a step in the search for a chromate-free process), (f) relatively resistant to impurities in the electrolyte, (g) easy to manufacture, (h) easy to install in existing cell concepts, and (i) cost-effective. 

As shown by reactions (9)-(ll), the end products of hypochlorite decomposition in the original container are chloride and chlorate ions in addition to oxygen gas. Disregarding the relatively small amount of chloride produced by reaction (11), the final solution would contain NaCl and NaClOa in a 2 1 molar ratio. The overall rates and amounts of chlorate formation at pH s above about 11 can be estimated for any given initial hypochlorite solution by means of the equations in Ref. 5. Data on actual amounts of chlorate found in various hypochlorite solutions used in water disinfection have been assembled and analyzed [14,15]. 

A similar decomposition of the chlorate(I) (hypochlorite) ion, OCl. catalysed by both light and cobalt(II) ions, is less commonly used ... 

A number of chemiluminescent reactions may proceed through unstable dioxetane intermediates (12,43). For example, the classical chemiluminescent reactions of lophine [484-47-9] (18), lucigenin [2315-97-7] (20), and transannular peroxide decomposition. Classical chemiluminescence from lophine (18), where R = CgH, is derived from its reaction with oxygen in aqueous alkaline dimethyl sulfoxide or by reaction with hydrogen peroxide and a cooxidant such as sodium hypochlorite or potassium ferricyanide (44). The hydroperoxide (19) has been isolated and independentiy emits light in basic ethanol (45). 

Sta.bilizers. Cyanuric acid is used to stabilize available chlorine derived from chlorine gas, hypochlorites or chloroisocyanurates against decomposition by sunlight. Cyanuric acid and its chlorinated derivatives form a complex ionic and hydrolytic equilibrium system consisting of ten isocyanurate species. The 12 isocyanurate equilibrium constants have been determined by potentiometric and spectrophotometric techniques (30). Other measurements of two of the equilibrium constants important in swimming-pool water report significantly different and/or less precise results than the above study (41—43). A critical review of these measurements is given in Reference 44. 

The solutions are most stable above pH 11 where the decomposition rate is nearly independent of pH. In this region, the decomposition rate has a second-order dependence on the concentration of hypochlorite. It also increases with increa sing ionic strength. Thus concentrated solutions decompose much faster than dilute solutions. Because of an unusually high activation energy, the decomposition rate increases greatiy with temperature. Nevertheless, solutions with less than about 6% available chlorine and a pH above 11 have acceptable long-term stabiUty below about 30°C. 

Even very small amounts of transition-metal ions like cobalt, nickel, and copper cause rapid decomposition. They form reactive intermediates that can decrease the stabiUty of oxidizable compounds in the bleach solution and increase the damage to substrates. Hypochlorite is also decomposed by uv light (24,25). Acidic solutions also lose available chlorine by the reverse of equations 1 and 2. 

Dilute (1—3%), chloride-containing solutions of either HOCl, hypochlorite, or aqueous base, can be stripped in a column against a current of CI2, steam, and air at 95—100°C and the vapors condensed giving virtually chloride-free HOCl solutions of higher concentration in yields as high as 90% (122—124). Distillation of more concentrated solutions requires reduced pressure, lower temperature, and shorter residence times to offset the increased decomposition rates. 

In Solution. Although hypochlorite solutions ate much mote stable than HOCl, they ate subject to decomposition, which is influenced by concentration, ionic strength, pH, temperature, light, and impurities. Decomposition occurs in two ways ... 

The rate-conttolling step to chlorate is the bimoleculat formation of chlorite, which reacts rapidly with hypochlorite. The temperature dependence of the rate constants is expressed by the equations = 2.1 x 10 g-io3.8/i T 3 2 x 10 g-87.o/i T L/(mol-s) (144). The uncataly2ed decomposition to... 

Solid Sta.te. The stabiHty of neutral calcium hypochlorite is primarily a function of moisture, lime, impurities, and temperature. Product containing - 7% water may lose 2—3% av CI2 during the first year when stored in warehouses without temperature control in moderate climates. Decomposition produces CaCl2, Ca(C102)2, and O2. 

Dibasic magnesium hypochlorite is more thermally stable than neutral or dibasic calcium hypochlorite. In addition, its decomposition, which starts at - 325° C, is endothermic rather than exothermic as in the case of the Ca compounds. 

By contrast, decomposition of dibasic calcium hypochlorite begins at 265° C to give Ca(OH)2, CaCl, and O2. Dibasic magnesium hypochlorite exhibits a high degree of stabiUty to moisture as shown by the following relative available chlorine losses at 24°C and 80% rh for 60 d Mg(OCl)2 2Mg(OH)2 2%,... 

Calcium Hypochlorite. High assay calcium hypochlorite [7778-54-3] was first commercialized in the United States in 1928 by Mathieson Alkali Works, Inc. (now Olin Corp.) under the trade name HTH. It is now produced by two additional manufacturers in North America (Table 5). Historically, it usually contained about 1% water and 70—74% av CI2, so-called anhydrous product, but in 1970, a hydrated product was introduced (234). It is similar in composition to anhydrous Ca(OCl)2 except for its higher water content of about 6—12% and a slightly lower available chlorine content. This product has improved resistance to accidental initiation of self-sustained decomposition by a Ht match, a Ht cigarette, or a small amount of organic contamination. U.S. production in the 1990s consists primarily of partially hydrated Ca(OCl)2, which is sold as a 65% av CI2 product mainly for swimming pool use. Calcium hypochlorite is also sold as a 50% av CI2 product as a sanitizer used by dairy and food industries and in the home, and as a 32% product for mildew control. 

In contrast to the alkyl hypochlorites, the fluoroalkyl hypochlorites are extremely susceptible to hydrolysis but are much more thermally stable. Trifluoromethyl hypochlorite, eg, showed no decomposition when heated for several days at 100°C. When decomposition does occur, several products are formed C2F OCl gives COF2, CF Cl, CF COF, and GIF, whereas (GF2)3GOGl gives (GF2)2GO, GI2, GF Gl, and G2F (40). 

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