Cyanuric acids

3 Alternative Ammonia Precursor Compounds 16.3.1 Cyanuric Acid 

Historically, cyanuric acid can be considered one of the first ammonia precursor compounds. As far back as 1977, a Japanese patent mentions the possibility of replacing NH3 in power plant exhaust gas aftertreatment with inorganic ammonium salts, urea or cyanuric acid granules of 0.1-10 mm diameter [59]. However, 

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Case-based reasoning CA. See Cyanuric acid Case-hardened steels Case hardening... 

At atmospheric pressure and at its melting point, urea decomposes to ammonia, biuret (1), cyanuric acid (qv) (2), ammelide (3), and triuret (4). Biuret is the main and least desirable by-product present in commercial urea. An excessive amount (>wt%) of biuret in fertiliser-grade urea is detrimental to plant growth. 

Residual monomers in the latex are avoided either by effectively reacting the monomers to polymer or by physical or chemical removal. The use of tert-huty peroxypivalate as a second initiator toward the end of the polymeri2ation or the use of mixed initiator systems of K2S20g and tert-huty peroxyben2oate (56) effectively increases final conversion and decreases residual monomer levels. Spray devolatili2ation of hot latex under reduced pressure has been claimed to be effective (56). Residual acrylonitrile also can be reduced by postreaction with a number of agents such as monoamines (57) and dialkylamines (58), ammonium—alkali metal sulfites (59), unsaturated fatty acids or their glycerides (60,61), their aldehydes, esters of olefinic alcohols, cyanuric acid (62,63), andmyrcene (64). 

Cyanuric fluoride is readily hydrolyzed to 2,4,6-thhydroxy-l,3,5-triaziae [108-80-5] (cyanuric acid). Cyanuric fluoride reacts faster with nucleophilic agents such as ammonia and amines than cyanuric chloride. 

Ammonia reacts vigorously with phosgene. The products are urea, biuret, ammeUde (a polymer of urea), cyanuric acid, and sometimes cyameUde (a polymer of cyanic acid). The secondary products probably arise through the very reactive intermediate carbamyl chloride [463-72-9] NH2COCI (see... 

ChlorinatedIsocya.nura.tes. The cyanuric acid-based sanitizers, introduced for pool use in 1958, are stable crystalline compounds with moderate-to-high available CI2. Sodium dichloroisocyanurate (Dichlor), sold in granular form, dissolves rapidly, whereas trichloroisocyanuric acid (Trichlor) dissolves very slowly and is widely used in the form of tablets or sticks in feeders, floating devices, or in the pool skimmer. 

In reahty the chemistry of breakpoint chlorination is much more complex and has been modeled by computer (21). Conversion of NH/ to monochloramine is rapid and causes an essentially linear increase in CAC with chlorine dosage. Further addition of chlorine results in formation of unstable dichloramine which decomposes to N2 thereby causing a reduction in CAC (22). At breakpoint, the process is essentially complete, and further addition of chlorine causes an equivalent linear increase in free available chlorine. Small concentrations of combined chlorine remaining beyond breakpoint are due primarily to organic chloramines. Breakpoint occurs slightly above the theoretical C1 N ratio (1.75 vs 1.5) because of competitive oxidation of NH/ to nitrate ion. Organic matter consumes chlorine and its oxidation also increases the breakpoint chlorine demand. Cyanuric acid does not interfere with breakpoint chlorination (23). 

Superchlorination typically refers to a dding FAC equal to 10 x ppm CAC, whereas shock treatment generally involves addition of 10 ppm FAC. The frequency of superchlorination or shock treatment depends on bather load and temperature. Calcium hypochlorite, because of its convenience, is widely used for superchlorination and shock treatment. Sodium hypochlorite, LiOCl, or chlorine gas are also used. Chloroisocyanurates are not recommended since their use would result in excessive cyanuric acid concentrations. 

Although cyanuric acid is a tribasic acid, only the first ionization is important at normal swimming pool pHs. The concentration of cyanurate is given by [H2Cy ] = Oc[, where is the total concentration of cyanuric acid and cyanurate ion and [Pg.299]

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. 

A plot of species distribution for the cyanuric acid-available chlorine system at 25°C is shown in Figure 4. In the presence of excess cyanuric acid, the predominant chlorinated specie is the monochloroisocyanurate ion, HClCy where Cy represents the triisocyanurate anion. Therefore, the only significant equilibria in pool water are... 

At 25°C, pH 7.5, 1.5 ppm FAC, and 25 ppm cyanuric acid, the calculated HOCl concentration is only 0.01 ppm. Although the monochloroisocyanurate ion hydrolyzes to only a small extent, it serves as a reservoir of HOCl because of rapid hydrolysis. Indeed, this reaction is so fast that HClCy behaves like FAC in all wet methods of analysis. Furthermore, since HClCy absorbs uv only below 250 nm, which is filtered out of solar radiation by the earth s atmosphere, it is more resistant to decomposition than the photoactive C10 , which absorbs sunlight at 250—350 nm and represents the principal mode of chlorine loss in unstabilized pools (30). As Httie as 5 ppm of bromide ion prevents stabilization of FAC by cyanuric acid (23) (see also Cyanuric and ISOCYANURIC acids). 

Based on the above equilibria, the concentration of HOCl in the normal pH range varies inversely with the total concentration of cyanurate. Increased concentration of cyanuric acid, therefore, should decrease the biocidal effectiveness of FAC. This has been confirmed by laboratory studies in buffered distilled water which showed 99% kill times of S.faecalis at 20°C increasing linearly with increasing cyanuric acid concentration at constant av. Cl at pH 7 and 9 (45). Other studies in distilled water have found a similar effect of cyanuric acid on kill times of bacteria (46—48). Calculations based on the data from Ref. 45 show that the kill times are highly correlated to the HOCl concentration and poorly to the concentration of the various chloroisocyanurates, indicating that HOCl is the active bactericide in stabilized pools (49). 

The ideal recommended cyanuric acid concentration is 30—50 ppm (Table 2). Although this range can be readily maintained when using hypochlorite sanitizers, it cannot be maintained when using chloroisocyanurates since they increase the cyanuric acid concentration. The NSPI recommends a maximum of 150 ppm cyanuric acid. Many health departments limit cyanuric acid to 100 ppm. No significant increase in stabilization occurs beyond 50—100 ppm, and since high levels of cyanuric acid slow down the rate of disinfection, the pool water should be partially drained and replaced with fresh water to reduce the cyanuric acid to below recommended maximum levels. Cyanuric acid is determined turbidimetricaHy after precipitation as melamine cyanurate. 

Pseudomonas that cause skin infections, within 30 s by 2 ppm FAC at pH 7.4 (62). In a series of tests under stabilized conditions (50—100 ppm cyanuric acid), an initial concentration of 3 ppm FAC could not provide water that met the NSPI standards for bacteria or the maintenance of at least a 2 ppm FAC residual. Increasing the initial FAC to 4 ppm resulted in the bacterial criteria being met 100% of the time even when the FAC residual fell below 1 ppm. For heavy bather loads an initial FAC > 4 ppm is recommended. 

Toxicity of Chlorine Sanitizers. Chlorine-based swimming-pool and spa and hot-tub sanitizers irritate eyes, skin, and mucous membranes and must be handled with extreme care. The toxicities are as follows for chlorine gas, TLV = 1 ppm acute inhalation LC q = 137 ppm for 1 h (mouse) (75). The acute oral LD q (rats) for the Hquid and soHd chlorine sanitizers are NaOCl (100% basis) 8.9 g/kg (76), 65% Ca(OCl)2 850 mg/kg, sodium dichloroisocyanurate dihydrate 735 mg/kg, and trichloroisocyanuric acid 490 mg/kg. Cyanuric acid is essentially nontoxic based on an oral LD q > 20 g/kg in rabbits. Although, it is mildly irritating to the eye, it is not a skin irritant. A review of the toxicological studies on cyanuric acid and its chlorinated derivatives is given in ref. 77. 

Although pH determines the ratio of hypohalous acid to hypohaUte ion, the fraction of the total available halogen present as HOX is dependent on of the halamine as well as the concentration of excess amine. In the case of chloroisocyanurates, which are the most widely used /V-ch1oramine disinfectants in swimming pools and spas, the extent of hydrolysis at 1 ppm av CI2 (as monochloroisocyanurate) is - 34% but only - 1% when 25 ppm cyanuric acid is added (4). Nevertheless, effective disinfection can stiU occur with chloroisocyanurates if a sufficient FAC is maintained, eg, 1—3 ppm. The observed reduction in disinfection rate because of cyanuric acid (6) has been shown to be direcdy related to the concentration of HOCl formed by hydrolysis of chloroisocyanurates (10). 

Dichlorine monoxide, generated in situ in the presence of CCl by reaction of CO2 and NaOCl, has been used in preparation of substituted hydra2ines (48). Dichlorine monoxide reacts with finely divided cyanuric acid in a fluidized bed forming dichloro- and trichloroisocyanuric acids (49) and with sodium cyanurate monohydrate yielding sodium dichloroisocyanurate monohydrate (50) (see Cyanuric and isocyanuric acids). 

Stable A/-chloro compounds are formed by reaction of hypochlorous acid and appropriate N—H compounds. For example, HOCl, formed in situ via chlorine hydrolysis, converts di- or trisodium cyanurates to dichloro- and trichloroiso-cyanuric acids, respectively (114). Chloroisocyanurates can also be prepared from isocyanuric acid or monosodium cyanurate and preformed HOCl (115—117). Hydrolysis of chloroisocyanurates provide HOCl for use in swimming pool disinfection and in bleaching appHcations. 

Hypochlorous acid, preformed or generated in situ from chlorine and water, is employed in the manufacture of chlorohydrins (qv) from olefins, en route to epoxides, and in the production of chloramines (qv), especially chloroisocyanurates from cyanuric acid (see Cyanuric and isocyanuric acids). 

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