Chlorite and chlorate

In response to these needs Dietrich et al. [108] used flow injection analysis with iodometric detection and ion chromatography with conductiometric detection in two methods for the determination of chlorite and chlorate in chlorinated and chloroaminated potable water. 

---

In solution, chlorine dioxide decomposes very slowly at ambient temperatures in the dark. The primary decomposition process is hydrolysis of chlorine dioxide into chlorite and chlorate ions. The hydrolysis rate is a function of the concentration of hydroxyl ions and temperature, proceeding rapidly at solution pH values above 10 ... 

Pulp bleaching with chlorine dioxide is most often performed at an acidic pH, so that the final pH of the bleach Hquor is in the range of 2—5. Under these conditions, the residual concentration of chlorite and chlorate ions in the bleach Hquor are minimized and chloride ion is the predominant chlorine species in the spent bleach (77). In addition to direct addition to pulp in bleaching, chlorine dioxide also finds use in wastewater treatment from pulp mill operations as a means to remove effluent color (85). 

Reagents similai to those used in the analysis of chloiine are commonly employed in the quantitation of gaseous and aqueous chloiine dioxide as well as its reaction coproducts chlorine, chlorite, and chlorate. The volatihty of the gas from aqueous solutions as well as its reactivity to light must be considered for accurate analysis. Other interferences that must be taken into account include other oxidizers such as chloramine, hydrogen peroxide, permanganate, and metal impurities such as ferrous and ferric iron. 

A secondary competing reaction can occur where chlorine dioxide disproportionates in the alkaline solution, producing sodium chlorite and chlorate ... 

By contrast, alkaline solutions hydrolyse vigorously to a mixture of chlorite and chlorate (see scheme overleaO-... 

In basic solution, C102 disproportionates to produce chlorite and chlorate ions. 

Bromate has also been measured using 1C with conductivity detection. For example, EPA Method 302.0 uses two-dimensional 1C with suppressed conductivity detection to measure bromate at 0.12 pg/L detection limits [166]. Bromate, chlorite, and chlorate can also be measured by an earlier EPA Method (Method 300.1), which uses 1C with conductivity detection [167]. Method detection limits ranging from 0.45 to 1.28 pg/L can be achieved. 

Baribeau H, Prevost M, Desjardins R, Lafrance P, Gates DJ (2002) Chlorite and chlorate ion variability in distribution systems. J Am Water Works Assoc 94(7) 96-105... 

Occurrence of Chlorite and Chlorate Ions in Finished Water From Utilities That Use Chlorine Dioxide... 

Chlorine dioxide is a very reactive compound and will not exist in the environment for long periods of time. In air, sunlight will quickly break apart chlorine dioxide into chlorine gas and oxygen. In water, chlorine dioxide will react quickly to form chlorite ions. In water treatment systems, chlorine dioxide will not form certain harmful compounds (e.g., trihalomethanes) when it reacts with dissolved organic compounds. Chlorine dioxide does form other disinfection byproducts, such as chlorite and chlorate ions. 

Chlorate and chlorite ions are disinfection by-products (DBPs) from water treatment using chlorine dioxide. Table 6-2 contains data from four water treatment facilities in the United States that use chlorine dioxide as a disinfectant. Source water samples were also analyzed from each facility and no chlorite or chlorate ions were detected. In all water treatment plants, water taken from the distribution system (i.e., water sampled at water treatment plant) had measurable concentrations of both chlorite and chlorate ions. The ranges of concentrations were 15-740 and 21-330 pg/L for chlorite and chlorate, respectively (Bolyard et al. 1993). 

Source water was analyzed for chlorite and chlorate, and none was detected above the 10 g/L reporting limit. 

Chlorine dioxide does not hydrolyze to any appreciable extent between pH 2 and 10 but remains in solution. Dilute neutral or acidic aqueous solutions are stable if kept cool, well sealed, and protected from sunlight. In the absence of oxidizable substances and in the presence of hydroxide ions, chlorine dioxide will dissolve in water and then decompose with the slow formation of chlorite and chlorate ions (e g., 2CIO2 + 20H" C102 + CIOs + H2O). At chlorine dioxide concentrations in the 5-10 mg/L range at pH 12, the decomposition half-life of chlorine dioxide in solution ranges from 20 to 180 minutes (Aieta and Berg 1986 Stevens 1982 WHO 2000). 

Abdel-Rahman MS, Couri D, Bull RJ. 1984a. The kinetics of chlorite and chlorate in the rat. J Am Coll Toxicol 3(4) 261-267. 

Beitler MK, Chin HB. 1995. Improved determination of chlorite and chlorate in rinse water from carrots and green beans by liquid chromatography and amperometric and conductivity detection. J AO AC Int 78(3) 878-883. 

Couri D, Abdel-Rahman MS, Bull RJ. 1982a. Toxicological effects of chlorine dioxide, chlorite and chlorate. Environ Health Perspect 46 13-17. 

Dietrich AM, Ledder TD, Gallagher DL, et al. 1992. Determination of chlorite and chlorate in chlorinated and chloraminated drinking water by flow injection analysis and ion chromatography. Anal Chem 64 498-502. 

EPA. 1994. Final draft for the drinking water criteria document on chlorine dioxide, chlorite and chlorate. Washington, DC U.S. Environmental Protection Agency, Office of Science and Technology, Office of Water. EPA 68-C2-0139. 

Lubbers JR, Chauhan S, Bianchine JR. 1981. Controlled clinical evaluations of chlorine dioxide, chlorite and chlorate in man. Fundam Appl Toxicol 1 334-338. 

Notice in Table 6-1 that all the common polyatomic ions except ammonium have a negative charge ranging between -1 and -3. You also see a number of -ite/-ate pairs, such as chlorite and chlorate, phosphite and phosphate, and nitrite and nitrate. If you look closely at these pairs, you notice that the only difference between them is the number of oxygen atoms in each ion. Specifically, the -ate ion always has one more oxygen atom than the -ite ion but has the same overall charge. 

Salts of these acids (chlorites and chlorates) may be formed by neutralizing the resulting solution with an appropriate base, and the two salts may then be separated. 

The known and doubtful salts are shown in Figures 7.1 to 7.3. Although more elements form perchlorates than chlorites and chlorates, the chlorates seem underrepresented. Any metal that forms a chlorite should also form a chlorate. One would also expect the lanthanides to form all three classes of salts. 

These two products, chlorite and chlorate, can be separated by crystallisation. Fractional crystallisation can be performed easily if chlorine dioxide is absorbed in a solution of equivalent amounts of sodium and potassium hydroxide. After absorption has been completed, a solution of sodium chlorite and potassium chlorate is obtained. These two salts can easily be separated as sodium chlorite dissolves far more readily in water than potassium chlorate. 

Figure 7.13 shows the good perchlorate formation potential starting from chlorite and chlorate and forming intermediates (exponential concentration increase). Surprisingly, the addition of sulphate improves the formation starting from chlorite. 

Adam, L.C. and Gordon, G. (1995) Direct and sequential potentiometric determination of hypochlorite, chlorite, and chlorate ions when hypochlorite ion is present in large excess. Anal. Chem. 67, 535-540. 

Chlorine dioxide breaks down to leave the inorganic chemicals chlorite and chlorate. These are best managed by controlling the dose of chlorine dioxide applied to the water. Chlorate can also be found in hypochlorite solution that has been allowed to age. There is no guideline value for chlorate because of limited data on its toxicology, but this chemical has been shown to be less toxic than chlorite and is present at lower concentrations. Controlling chlorite will generally also adequately control chlorate. 

Chlorine dioxide Verification together with monitoring of chlorite and chlorate Controlled through dose optimization... 

When chlorine dioxide is used as a disinfectant, chlorite and chlorate are formed as by-products. These are sometimes monitored, but control can be achieved by control of the dose of chlorine dioxide applied, Chlorate may also form in significant quantities in hypochlorite that is stored for an extended period, particularly at higher ambient temperatures again, it is best controlled by management procedures. 

The direct reduction of pollutants also includes other inorganic compounds such as chromates, oxy-chlorinated species (e.g., chlorites and chlorates) and oxynitrogenated ions (nitrates and nitrites), as well as organic compounds (e.g., the dehalogena-tion of chlorinated hydrocarbons and the reduction of organic acids to the corresponding alcohols or phenols). 

---