Ind. Eng. Chem. Res. 1998, 37, 2367-2372
2367
Disproportionation of Chlorous Acid at a Strong Acidity Y. Ni* and G. Yin Dr. Jack McKenzie Limerick Pulp and Paper Research and Education Centre, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 6C2
Commercial chlorine dioxide generators are operated in an about 4.5-mol/L sulfuric acid solution. The chlorous acid disproportionation may be an important pathway to generate chlorine dioxide under such a condition. On the basis of the product determination, we found that, in the absence of a chloride ion, chloric acid, hypochlorous acid, and chlorine, rather than chlorine dioxide, are the products from the chlorous acid disproportionation. However, chloride addition to the chlorous acid solution under otherwise the same condition results in the generation of chlorine dioxide. The underlying mechanism of the chloride effect on the chlorous acid disproportionation is discussed. Introduction Chlorine dioxide-based elemental chlorine free (ECF) technology has become the dominant bleaching process for the production of bleached chemical pulps in the pulp and paper industry (Pryke et al., 1997). Chlorine dioxide is usually generated on the basis of the reduction of sodium chlorate at a high acidity (about 4.5-mol/L sulfuric acid solution) by chemicals, such as chloride, sulfur dioxide, methanol, and hydrogen peroxide. Presently, the methanol-based process is the most widely used system in North America (Stockburger, 1993). Recently, we started a project aiming at a better understanding of the fundamentals of the methanolbased chlorine dioxide generation process (Ni and Wang, 1997). In such a process, hypochlorous acid (chlorine) and chlorous acid were identified as reaction intermediates (Lenzi and Rapson, 1962; Hong et al., 1967; Tenney et al., 1990; Hoq et al., 1992; Ni and Wang, 1997). It is very well-known that the reactions between chlorine (hypochlorous acid) and chlorous acid (Emmenegger and Gordon, 1967; Aeita and Robert, 1986) and the disproportionation of chlorous acid (Barnett, 1935; Kieffer and Gordon, 1968a,b; Hong and Rapson, 1968) will result in the generation of chlorine dioxide under various conditions. The objective of this paper is to examine the chlorine dioxide generation from the chlorous acid disproportionation at a 4.5-mol/L H2SO4 sulfuric acid solution. The disproportionation of chlorous acid under acidic conditions has been extensively studied (Barnett, 1935; Kieffer and Gordon, 1968a,b; Hong and Rapson, 1968; Gordon et al., 1972). Its products are the chlorate ion, chlorine dioxide, and chloride ion. The stoichiometry of the reaction can be approximated (Barnett, 1935; Hong and Rapson, 1968; Gordon et al., 1972) by
4HClO2 f Cl- + 2ClO2 + ClO3- + 2H+ + H2O
(1)
Considerable variation in stoichiometry of the reaction has been reported (Barnett, 1935; Hong and Rapson, 1968) depending on the exact conditions. The acidity has a drastic influence on the disproportionation of chlorous acid. It was found (Gordon et al., * To whom correspondence should be addressed. Tel.: (506) 4534547. Fax: (506) 4534767. E-mail:
[email protected].
1972) that chlorine dioxide is the main product in acetic acid-sodium acetate buffer (pH 2.7), but less chlorine dioxide is formed as the pH decreases; at low acidity and high chlorite concentration, Barnett (1935) reported that reaction 2 predominates:
5HClO2 f 4ClO2 + HCl + 2H2O
(2)
Kieffer and Gordon (1968a) reported that chlorine dioxide is the main product from the chlorous acid disproportionation and that the ClO2 yield decreases as the hydrogen ion concentration is decreased from 2.0 to 0.49 mol/L, but it increases as the hydrogen ion concentration is further decreased to 10-3 mol/L. The chloride ion not only affects the rate but also alters the stoichiometry of the chlorous acid disproportionation (Kieffer and Gordon, 1968a,b; Hong and Rapson, 1968). Kieffer and Gordon (1968b) reported that the chloride ion has an acceleration effect; however, Hong and Rapson (1968) found that it could have both inhibiting and acceleration effects on the chlorous acid disproportionation. The mechanism of the chlorous acid disproportionation was proposed in stepwise reactions by Hong and Rapson (1968) from 4.9 × 10-2 to 0.97 mol/L hydrogen ion concentrations.
2HClO2 f HOCl + HClO3
(3)
ClO2- + HClO2 f HOCl + ClO3-
(4)
HOCl + ClO2- f OH- + Cl-ClO2
(5)
Cl-ClO2 + HClO2 f H+ + Cl- + 2ClO2
(6)
In this mechanism, the reaction is initiated by hypochlorous acid produced simultaneously from the disproportionation of two molecules of chlorous acid or one molecule of chlorous acid with one chlorite ion. Subsequently, hypochlorous acid reacts with chlorite to form dichlorine dioxide, a well-known intermediate proposed by Taube and Dodgen (1949) and Emmenegger and Gordon (1967) of the reactions between chlorine (hypochlorous acid) and chlorite. Presumably, chlorite acts as a nucleophile in the reaction. Dichlorine dioxide then reacts with chlorous acid to produce chlorine dioxide.
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2368 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998
Alternatively, chlorine dioxide may be generated from dichlorine dioxide in a second-order reaction (Taube and Dodgen, 1949; Emmenegger and Gordon, 1967), as shown in reaction 7:
2Cl2O2 f Cl2 + 2ClO2
(7)
In this paper, we will report our recent results from the chlorous acid disproportionation at an acidity similar to that of commercial chlorine dioxide generators. Experimental Section Pure chlorite reagent was prepared by following the fractional crystallization of technical grade sodium chlorite (Perrin et al., 1966). The third fraction of crystals was used for the reactions. Using a ion chromatographic (IC) (Dionex Corporation, CA) analysis, we found no other Cl-containing species except trace Cl-, approximately 0.1% (molar percentage on chlorite), in the purified product. The preparation of pure hypochlorous acid was similar to the procedure used by Adam et al. (1992). The chlorine generated from the reaction of hydrochloric acid and calcium hypochlorite was carried, by nitrogen gas, into a slurry of yellow mercuric oxide. Chlorine monoxide (Cl2O) produced by the reaction of mercuric oxide with chlorine was distilled together with a HgCl2‚HgO slurry under a nitrogen atmosphere and was collected in a 0.1 mol/L of NaOH. With a chloride ion selective electrode (VWR Scientific, PA) in a pH 5 buffer solution, we found that the chloride impurity in this solution was only about 1.2% (molar percentage on hypochlorous acid). The chloride was monitored by a chloride ion selective electrode (ISE) during the reaction. The chloride concentration was calculated by following an established calibration curve. The calibration was carried out after each run at the temperature and acidity identical to those of the reaction system. The chlorine dioxide concentration was determined with a UV spectrophotometer (Model Milton Roy 1001 Plus) at 370 nm. Its extinction coefficient was experimentally determined as 1056 (mol/L)-1‚cm-1. The reactions were performed in the UV cuvette. Water from a constant-temperature bath was circulated through the cuvette chamber to maintain the desired temperature. For the determination of HOCl(Cl2), HClO2, and HClO3, the method involves a multistep iodometric titration similar to the procedure used by Hoq et al. (1992). The reactions were performed in a 100-mL flask submerged in a constant-temperature bath. At a specified time, an aliquot of reaction mixture was pipetted into a flask containing potassium bromide and concentrated hydrochloric acid for the determination of chloric acid; another aliquot of the reaction mixture was pipetted into a flask containing water and a sufficient amount of tetraborate for the titration of hypochlorous acid (chlorine). Potassium iodide was then added into the flasks, and the liberated iodine by the oxidizers in these solutions was titrated with a standard thiosulfate solution. The accuracy of the above iodometric titrations was checked with a mixture of known concentrations of chlorine-containing species. When a solution containing 0.5 mmol/L of hypochlorous acid, 1 mmol/L of chloric acid, and 0.5 mmol/L of chlorine dioxide was prepared in a 4.5-mol/L sulfuric acid solution, the
Table 1. Mass and Charge Balances of the Chlorous Acid Disproportionation in the Absence of Chloride (1.0 × 10-3 mol/L of [HClO2], 4.5 mol/L of H2SO4, 25 °C, 30 min) Cl-containing species
Cl mass balance as Cl
Cl charge balance
HClO2 total
Reactant 1.0 × 10-3 1.0 × 10-3
1.0 × 10-3 × (+3) 3 × 10-3
Cl2 HOCl HClO3 total
Products 2 × 1.7 × 10-4 8.0 × 10-5 6.0 × 10-4 1.02 × 10-3
2 × 1.7 × 10-4 × (0) 8.0 × 10-5 × (+1) 6.0 × 10-4 × (+5) 3.08 × 10-3
titration method gave a result of 0.518, 0.979, and 0.500 mmol/L, respectively. The Cl2 and HOCl concentrations were determined on the basis of separate results from the iodometric titration and chloride analysis with the ion selective electrode (ISE) method. The iodometric titration gives the sum of hypochlorous acid and chlorine. Upon neutralization to pH 6, molecular chlorine releases an equimolar chloride ion. Therefore, the Cl- concentration determined by ISE in the neutralized solution is equivalent to the Cl2 concentration originally present in the strong acidic reaction mixture. The hypochlorous acid concentration can thus be obtained by subtraction. This method was checked with a mixture in a 4.5-mol/L sulfuric acid solution containing 0.5 mmol/L of chlorine and 0.5 mmol/L of hypochlorous acid. By following the above methods, we found that the chlorine and hypochlorous acid concentrations are 0.521 and 0.492 mmol/L, respectively. Therefore, one can conclude that the methods are reliable. Results and Discussion In the Absence of Chloride. In the first experiment, we added purified sodium chlorite to concentrated sulfuric acid at 25 °C. The concentrations of chlorite ion and sulfuric acid were 1 × 10-3 and 4.5 mol/L, respectively. Following the UV spectroscopic method, we did not find any chlorine dioxide generated after 30 min of reaction. This is contrary to the well-known fact that chlorine dioxide is the main product from the chlorous acid disproportionation, as described in reactions 1 and 2. However, it should be pointed out that the present acidity is substantially higher than that used in other studies (Kieffer and Gordon, 1968a,b; Hong and Rapson, 1968; Gordon et al., 1972). Further effort was made to identify the products from the above reaction. By following the methods detailed in the Experimental Section, we found that the reaction solution contained 6.0 × 10-4 mol/L of chloric acid, 1.7 × 10-4 of mol/L chlorine, and 8.0 × 10-5 mol/L of hypochlorous acid. Chloride ions, which were reported as the coproduct during the chlorous acid disproportionation (reactions 1 and 2), were not detected. We performed the Cl mass and charge balances between the reactants and the products, shown in Table 1. The satisfactory results confirmed the accuracy of our experimental techniques. The above conclusion is further supported by the duplicate runs under the same conditions as well as others under similar conditions, shown in Table 2. These results are in agreement with an earlier study by Lenzi and Rapson (1962), who reported that the main reaction product was chlorine, with little or no chlorine dioxide evolved when aqueous chlorite
Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2369 Table 2. Disproportionation of HClO2 at the Initial Concentration of 6 × 10-4 and 2.5 × 10-3 mol/L in the Absence of Chloride (4.5 mol/L of H2SO4, 25 °C, 30 min) Cl-containing products initial HClO2 (×10-3 mol/L) balance (%) conc. (×10-3 mol/L) [HClO3] [Cl2] [HOCl] [ClO2] mass charge 0.6 2.5
0.36 1.45
0.095 0.39
0.07 0.31
0 0
103 102
104 101
was added to concentrated sulfuric acid, with sodium chlorite and sulfuric acid concentrations of 0.005 and 5 mol/L, respectively. The stoichiometry was represented by reaction 8 (Lenzi and Rapson, 1962):
5HClO2 f 3HClO3 + Cl2 + H2O
(8)
It is worth noting that the disproportionation of chlorous acid in a 4.5-mol/L sulfuric acid solution is very fast. This is supported by Lenzi and Rapson’s observation (1962) that the disproportionation of chlorite in a 5-mol/L sulfuric acid solution is completed within 2-3 min. By comparison, Kieffer and Gordon (1968b) reported a much slower second-order kinetics on chlorous acid consumption at a hydrogen ion concentration over the range of (1.2 × 10-3)-2 mol/L. The possible explanation to account for this obvious difference is that the reactions involved between Kieffer and Gordon’s study (1968b) and the present study as well as that by Lenzi and Rapson (1962) are different, as indicated by the fact that in the former chlorine dioxide is the major product while in the latter two cases no chlorine dioxide, only chloric acid and chlorine, is generated. As a consequence, the reaction kinetics will be expected to be different. Also, the impurity effect due to the presence of some transition-metal ions, such as iron, on the disproportionation of chlorous acid, which has been well-documented (Schmitz and Rooze, 1985), may be partly responsible for the difference between these studies. The presence of hypochlorous acid in the reaction products suggests that reaction 3, which was proposed (Taube and Dodgen, 1949; Lenzi and Rapson, 1962) during the chlorous acid disproportionation, takes place in the present reaction system. In addition, chlorine, another product during the course of the reaction, may be the result of a further reaction between hypochlorous acid, generated in reaction 3, and chlorous acid, as shown in reaction 9:
2HOCl + HClO2 f Cl2 + HClO3 + H2O
(9)
This reaction was suggested by Peintler et al. (1990) as one of the possible routes during the HOCl-ClO2reaction at pH 5-6. Since hypochlorous acid is a product from reaction 3 and also a reactant in reaction 9, by using the steady-state hypothesis that the hypochlorous acid concentration in reactions 3 and 9 is 0, one can obtain reaction 8, which was found by Lenzi and Rapson (1962). On the basis of the concentrations of chlorine, hypochlorous acid, and the stoichiometry of reactions 3 and 9, we can calculate that about 83% of initially charged chlorous acid is disproportionated via reaction 3 while the remainder is consumed via reaction 9, as shown in Table 3. A question arises as to why chlorine dioxide, which is a well-known product from the chlorous acid disproportionation, is not generated under the present condition.
Figure 1. Effect of acidity on the chlorine dioxide formation from the chlorous acid disproportionation in the absence of chloride (25 °C, 1 mmol/L of HClO2).
In the proposed mechanism to account for the chlorine dioxide generation from the chlorous acid disproportionation (reactions 3-7), one of the key steps is reaction 5 in which the chlorite ion acts as a nucleophile to give the dichlorine dioxide intermediate. Therefore, the chlorine dioxide yield is directly related to the chlorite ion concentration. It is expected that a lower chlorite, which can be achieved by increasing the acidity, will result in a decreased chlorine dioxide generation. This is confirmed experimentally, as demonstrated in Figure 1. An increase in the sulfuric acid concentration from 0.05 to 0.5 mol/L significantly decreases the chlorine dioxide concentration in the reaction solution while no chlorine dioxide is generated at all in the 4.5-mol/L H2SO4 solution. The accuracy of these experimental results was confirmed in duplicated trials, as shown in Figure 1. It should be pointed out that even in the cases of 0.05- and 0.5-mol/L sulfuric acid solutions where chlorine dioxide is generated, it only accounts for about 9% and 3%, respectively, of the chlorous acid consumed. These results are different from those by Kieffer and Gordon (1968b) that at a hydrogen ion concentration over the range of (1.2 × 10-3)-2 mol/L of chlorine dioxide is the major product. The above may be explained by the difference in the reactant (chlorite) concentration between the two studies, and the possible presence of chloride in Kieffer and Gordon’s study (1968b). At a very high acidity, for example, 4.5 mol/L of H2SO4, one might expect that the presence of the chlorite ion (ClO2-) in solution is negligible and all the chlorite ions are undissociated (i.e., in the form of chlorous acid (HClO2)). As a result, the nucleophilic reaction (reaction 5) may become impossible since chlorous acid (HClO2) is no longer a strong nucleophile. Therefore, no chlorine dioxide can be generated. On the other hand, at a high acidity, Cl2O, the anhydride form of hypochlorous acid, is expected to be present. Further reaction of Cl2O with chlorous acid can lead to the formation of both chlorine and chloric acid. These reactions are formulated in Figure 2. Therefore, reaction 3, as well as those in Figure 2, explains our experimental evidence that, chloric acid, hypochlorous acid, and chlorine, not chlorine dioxide, are the products from the chlorous acid disproportionation in 4.5-mol/L H2SO4 solution. The presence of hypochlorous acid (chlorine), however, not chlorine dioxide during the chlorous acid dispropor-
2370 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 Table 3. Competitive Reactions of the Chlorous Acid Disproportionation in the Absence of Chloride (1.0 × 10-3 mol/L of [HClO2], 4.5 mol/L of H2SO4, 25 °C, 30 min) Cl containing species (×10-3 mol/L) reaction no. (3) (9)
stoichiometry
(×10-3
mol/L)
2HClO2 f HOCl + HClO3 0.5 × 0.83 0.5 × 0.83 0.83 2HOCl + HClO2 f Cl2 + HClO3 + H2O 2 × 0.17 0.17 0.17 0.17
HClO2
HOCl
HClO3
-0.83
+0.42
+0.42
-0.17
-0.34
+0.17
Cl2
+0.17
Table 5. Effect of Chloride on the Chlorous Acid Disproportionation (1.0 × 10-3 mol/L of [HClO2], 4.5 mol/L of H2SO4, 25 °C, 30 min) Cl-containing products initial Cl(×10-3 mol/L) balance (%) (×10-3 mol/L) [HClO3] [Cl2] [HOCl] [ClO2] [Cl-] mass charge 0 5
Figure 2. The formation of chloric acid and chlorine from the chlorous acid disproportionation at strong acidity. Table 4. Mass and Charge Balances of the HClO2-HClO Reaction in the Absence of Chloride (1.0 × 10-3 mol/L of [HClO2], 5.0 × 10-4 mol/L of [HClO], 4.5 mol/L of H2SO4, 25 °C, 30 min) Cl-containing species
Cl mass balance as Cl
Cl charge balance
HOCl HClO2 total
Reactants 5.0 × 10-4 1.0 × 10-3 1.5 × 10-3
5.0 × 10-4 × (+1) 1.0 × 10-3 × (+3) 3.5 × 10-3
Cl2 HOCl HClO3 total
Products 5.6 × 10-4 3.0 × 10-4 6.4 × 10-4 1.5 × 10-3
5.6 × 10-4 × (0) 3.0 × 10-4 × (+1) 6.4 × 10-4 × (+5) 3.5 × 10-3
tionation under the present condition is surprising, based on the well-known fact (Emmenegger and Gordon, 1967; Aeita and Robert, 1984) that the reaction between hypochlorous acid (chlorine) and chlorous acid is very rapid in an acidic solution, producing chlorine dioxide. Therefore, we further studied the HOCl-HClO2 reaction at a strong acidity in the absence of chloride. We did not find any chlorine dioxide in the reaction solution when rapidly adding purified hypochlorite and chlorite solutions to a concentrated sulfuric acid (the concentrations of hypochlorous acid, chlorous acid, and sulfuric acid were 5 × 10-4, 1.0 × 10-3, and 4.5 mol/L, respectively). Instead, the reaction solution contained 2.8 × 10-4 mol/L of chlorine, 3.0 × 10-4 mol/L of hypochlorous acid, and 6.4 × 10-4 mol/L of chloric acid. Both the Cl mass and charge balances shown in Table 4 indicated that there is no other chlorine-containing species present. In addition, we have shown (Yin and Ni, 1998) that the HClO2-HOCl reaction under so high acidity in the absence of chloride can be well-described by reactions 3 and 9. On the basis of the product yields and the stoichiometry of reactions 3 and 9, we calculated that about 72% chlorous acid was disproportionated following reaction 3 while the remainder of chlorous acid was consumed via reaction 9. The increased consumption of chlorous acid in reaction 9 in the HClO2-HOCl system compared to the pure HClO2 system discussed earlier, under otherwise identical conditions, is not
0.6 0.48
0.17 0
0.08 0
0 0.19
0 5.33
102 100
103 94
unexpected. This is because the increased concentration of hypochlorous acid favors reaction 9. In the Presence of Chloride. We also studied the effect of chloride on the chlorous acid disproportionation in 4.5-mol/L H2SO4 solution. The products obtained with the addition of 5 × 10-3 mol/L of chloride were shown in Table 5. As a comparison, included in the table are also those in the absence of chloride under otherwise the same conditions. The Cl mass and charge balances again were satisfactory, confirming the results are reliable. Compared to the case in the absence of chloride, the striking difference is that chlorine dioxide is generated when Cl- is present. This result indicates that the chemistry has changed upon the addition of chloride and supports that chloride has an accelerating effect on the chlorine dioxide generation from the chlorous acid disproportionation, which was reported earlier at less acidic conditions (Kieffer and Gordon, 1968a,b; Hong and Rapson, 1968). As discussed in the previous section, in the absence of chloride and so high acidity, because chlorous acid is not a strong nucleophile, reaction 5, which is the key step for the production of chlorine dioxide, is not possible. Consequently, no chlorine dioxide is generated. However, the presence of chloride changes the chemistry and chloride can catalyze the reaction of chlorous acid with hypochlorous acid. The mechanism was postulated earlier (Yin and Ni, 1998). As suggested, the catalytic effect of chloride in such a process is to convert the weak nucleophile of chlorous acid to a strong nucleophile, so that the subsequent reaction with hypochlorous acid to produce dichlorine dioxide becomes possible. The generation of chlorine dioxide from dichlorine dioxide is a well-established reaction (Taube and Dodgen, 1949; Emmenegger and Gordon, 1967; Hong and Rapson, 1968), as represented in reactions 6 and 7. Also, chloric acid may be formed from dichlorine dioxide (Taube and Dodgen, 1949; Emmenegger and Gordon, 1967) as
Cl2O2 + H2O f HCl + HClO3
(10)
This reaction follows first-order on Cl2O2, while reaction 7, which forms chlorine dioxide, is second-order on Cl2O2. Also a higher concentration of the reaction intermediate, dichlorine dioxide, is expected when the initial chlorous acid concentration is higher. Conse-
Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2371 Table 6. Competitive Reactions of the Chlorous Acid Disproportionation in the Presence of Chloride (1 × 10-3 mol/L of [HClO2], 5 × 10-4 mol/L of [Cl-], 4.5 mol/L H2SO4, 25 °C, 30 min) Cl-containing species (×10-3 mol/L) reaction no. (3) (11) (12)
stoichiometry
(×10-3
mol/L)
2HClO2 f HOCl + HClO3 0.302 0.604 0.302 + 2HClO2 f ClO2 + + H2O HOCl HCl 0.5 × 0.189 0.189 0.189 0.5 × 0.189 HOCl + HClO2 f HClO3 + HCl 0.207 0.207 0.207 0.207 predicted concentrations from reactions 3, 11, and 12 experimentally determined concentrations
Figure 3. Effect of chlorous acid concentration on the chlorine dioxide formation from the chlorous acid disproportionation in the presence of chloride (25 °C, 5 mmol/L of Cl-, 4.5 mol/L of H2SO4).
quently, the experimental evidence that a higher initial chlorous acid concentration results in the formation of more chlorine dioxide, as shown in Figure 3, can be explained. In the presence of chloride, the product yields during the chlorous acid disproportionation at strong acidity (Table 5) can be satisfactorily described by reactions 3, 11, and 12:
2HClO2 f HOCl + HClO3
(3)
HOCl + 2HClO2 f 2ClO2 + HCl + H2O
(11)
HOCl + HClO2 f HClO3 + HCl
(12)
as determined by the good agreements between the predicted and experimentally determined concentrations of various chlorine-containing species (Table 6). Reactions 11 and 12 have been identified by other authors during the reaction of chlorous acid with hypochlorous acid (chlorine) at less acidic conditions (Emmenegger and Gordon, 1967; Gordon et al., 1972; Aeita and Robert, 1984). On the basis of table 6, we can draw the conclusion that under the present condition reactions 3, 11, and 12 account for about 60%, 19%, and 21%, respectively, of the total chlorous acid consumption during the course of the reaction. The dramatic effect of chloride on the chlorine dioxide generation from the chlorous acid disproportionation at such a high acidity can be further demonstrated in Figure 4. It shows that in the presence of 50 mmol/L of chloride, the chlorine dioxide generation is very fast even at 25 °C. Also, the chlorine dioxide yield is significantly affected by the chloride concentration; a
HClO2
HOCl
HClO3
-0.604
+0.302
+0.302
-0.189
-0.095
-0.207
-0.207
-1.00 -1.00
0 0
ClO2
Cl-
+0.189
+0.095
+0.207 +0.509 +0.483
+0.20 +0.189 +0.189
+0.302 +0.328
Figure 4. Effect of Cl- on the chlorine dioxide generation from the chlorous acid disproportionation (1 mmol/L of HClO2, 4.5 mol/L of H2SO4, 25 °C).
higher chloride concentration leads to a higher chlorine dioxide yield. These results are consistent with the theory discussed above. The practical implication of this work is that under the conditions of the commercial chlorine dioxide generators, the production of chlorine dioxide from the chlorous acid disproportionation is only possible if chloride is present in the reaction solution. This is in agreement with the earlier hypothesis (Ni and Wang, 1997) that the presence of chloride is essential to create the chlorine dioxide producing condition during the methanol-based chlorine dioxide generation process. Conclusions In the absence of the chloride ion, the chlorous acid disproportionation in a 4.5-mol/L H2SO4 solution can be described as 2HClO2 f HOCl + HClO3 and HOCl + HClO2 f Cl2 + HClO3. It was found that the former is the dominant reaction. No chlorine dioxide is generated. This was hypothesized by the lack of reaction 5, which is essential for the formation of chlorine dioxide, since chlorous acid is not a strong nucleophile. On the other hand, experimental evidence shows that the presence of chloride in the reaction solution under so high acidity results in the generation of chlorine dioxide and that more chlorine dioxide is produced at a higher chloride concentration. These observations were explained by the hypothesis that chloride can convert chlorous acid to a strong nucleophile so that reaction 5 becomes possible.
2372 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998
Literature Cited Adam, L. C.; Fabian, I.; Suzuki, K.; Gordon, G. Hypochlorous Acid Decomposition in the pH 5∼8 Region. Inorg. Chem. 1992, 31 (7), 3534. Aeita, E. M.; Robert, P. V. Kinetics of the Reaction between Molecular Chlorine and Chlorite in Aqueous Solution. Environ. Sci. Technol. 1986, 20(1), 50. Aeita, E. M.; Robert, P. V.; Hernandez, M. Determination of Chlorine Dioxide, Chlorine, Chlorite, and Chlorate in Water. J. Am. Water Works Assoc. 1984, 76, 64. Barnett, B. Ph.D. Dissertation, University of California, 1935. Emmenegger, F.; Gordon, G. The Rapid Interaction between Sodium Chlorite and Dissolved Chlorine. Inorg. Chem. 1967, 6 (3), 633. Gordon, G.; Kieffer, R. G.; Rosenblatt, D. H. The Chemistry of Chlorine Dioxide. Progress in Inorganic Chemistry; John Wiley & Sons Inc.: New York, 1972; Vol. 115, p 201. Hong, C. C.; Rapson, W. H. Kinetics of Disproportionation of Chlorous Acid. Can J. Chem. 1968, 46 (12), 2053. Hong, C. C.; Lenzi, F.; Rapson, W. H. The Kinetics and Mechanism of the Chloride-Chlorate Process. Can. J. Chem. Eng. 1967, 45, 349. Hoq, M. F.; Indu, B.; Ernst, W. R.; Gelbaum, L. T. Kinetics and Mechanism of the Reaction of Chlorous Acid with Chlorate in Aqueous Sulfuric Acid. Ind. Eng. Chem. Res. 1992, 31, 137. Kieffer, R. G.; Gordon, G. Disproportionation of Chlorous Acid. I. Stoichiometry. Inorg. Chem. 1968a, 7 (2), 235. Kieffer, R. G.; Gordon, G. Disproportionation of Chlorous Acid. II. Kinetics. Inorg. Chem. 1968b, 7 (2), 239. Lenzi, F.; Rapson, W. H. Further Studies on the Mechanism of Formation of ClO2. Pulp Pap. Can. 1962, 63 (9), T442.
Ni, Y.; Wang, X. Mechanism of the Methanol Based ClO2 Generation Process. J. Pulp Pap. Sci. 1997, 23 (7), J346. Peintler, G.; Nagypal, I.; Epstein, I. Kinetics and Mechanism of the Reaction between Chlorite and Hypochlorous Acid. J. Phys. Chem. 1990, 94, 2954. Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals, 1st ed.; Pergamon Press: Oxford, New York, 1966; p 12. Pryke, D. C.; Reeve, D. W. A Survey of ClO2 Delignification Practices in Canada. Tappi J. 1997, 80 (5), 153. Schmitz, G.; Rooze, H. Me´canisme des Re´actions du Chlorite et du Bioxyde de chlore. 3 La Dismutation du Chlorite. Can. J. Chem. 1985, 63, 975. Stockburger, P. What You Need To Know before Buying Your Next Chlorine Dioxide Plant. Tappi J. 1993, 76 (3), 99. Taube, H.; Dodgen, H. Applications of the Mechanisms of the Reaction Involving Changes in the Oxidation States of Chlorine. J. Am. Chem. Soc. 1949, 71 (10), 3330. Tenney, J.; Shoaei, M.; Obijeski, T.; Ernst, W. R.; Lindstroem, R.; Sunblad, B.; Wanngard, J. An Experimental Investigation of the Chloride-Chlorate Reaction System. Ind. Eng. Chem. Res. 1990, 29 (5), 916. Yin, G.; Ni, Y. The Effect of Chloride on the HClO2-HOCl Reaction in a 4.5 mol/L Sulfuric Acid Solution. Can J. Chem. Eng. 1998, in press.
Received for review September 2, 1997 Revised manuscript received March 13, 1998 Accepted March 25, 1998 IE970608P
