Ind. Eng. Chem. Res. 1994,33, 239-245
239
Kinetics of the Uncatalyzed and Cu(I1)-Catalyzed Decomposition of Sodium Hypochlorite John A. Church Colgate-Palmolive Company, 909 River Road, P.O.Box 1343, Piscataway, New Jersey 08855-1343
Rate equations for the decomposition of aqueous alkaline NaClO were integrated with inclusion and exclusion of the effects of first-order Cu(I1) catalysis, yielding new time-dependent expressions for [ClO-I, evolved oxygen, and [C103-1. Using a gravimetric technique, uncatalyzed and Cu(I1)-catalyzed rate constants for oxygen evolution and C10- loss were measured and consolidated with previous results. Activation parameters of the uncatalyzed rate-limiting reactions to CIOz- and oxygen could be approximated as linear functions of ionic strength k , allowing estimation of the respective rate constants kl and k, a t 40-70 "C in the p range 2-5. For Cu(I1) catalysis a t 40-70 OC in the p range 4.2-4.7, E,, AH*, and AS* values of 83 300 J g-mol-', 80 550 J g-mol-', and -14.6 J K-l g-mol-l, respectively, were derived from previous and current results, allowing kcu also to be estimated and used in the system of integrated equations.
Introduction Alkaline NaClO solutions, even if carefully purified, spontaneously decompose to yield NaC1, NaC103,02, and minor amounts of NaC102 (Wilson, 1972). Certain metals and/or their oxides/hydroxides-notably Cu, Ni, Co, and Ir-catalyze additional decomposition and gas release. Fe, a relatively weak catalyst alone, has been reported to promote Cu catalysis when both metals are present as solid oxides (Lewis, 1928). The most complete treatments of the uncatalyzed and metal-catalyzed reactions include the work of Lister (1953, 1956a,b), Zenker (1954), Lister and Petterson (1962), Prokopcikas and Butkevicius (19641, Prokopcikas (19641, and Gray et al. (1977). Recently, Adam et al. (1992) have discussed the decomposition of HClO in the pH range 5-8, while Rosen and Zhu (1992) have studied the effects of surfactants on NaClO decomposition. An important goal is the ability to conveniently predict the extent of NaClO loss and evolved 02 for arbitrary conditions of ionic strength, temperature, initial NaC10/ metal concentrations, and aging times; work reported herein addresses this long-standing need. In the homogeneous case involving alkaline NaClO solutions with or without low levels of dissolved Cu, existing data would allow such predictions to be made in certain instances if the required integrated rate equations were available. Where insoluble catalytic metals are present, heterogeneous catalysis occurs with complications of poorlyreproducible 02 rates (Lister, 1956b). The present work encompasses the development of the necessary integrated rate equations, as well as the determination of uncatalyzed and Cu(I1)-catalyzed rate constants over significantly wider ranges of ionic strength p and temperature T than reported previously, using a new gravimetric method to follow 02evolution. Rate constant dependencies upon p and T have been systematized for the first time to facilitate computation of necessary quantities at arbitrary aging intervals. The influence of variations in [OH-] on the uncatalyzed degradation kinetics of NaClO was considered by Lister (1956a), who found that over an [OH-] range of 0.02-0.32 M there were no direct kinetic effects except those due to changes in ionic strength. Likewise,no influence was seen on Cu catalysis in the [OH-] range 0.17-0.62 M (Lister, 1953). Gray et al. (1977) found an inverse dependence of Cu-catalyzed decomposition rates on [OH-] at very high
Cuand OH-levels, i.e., 27 ppm and 1.3-7.0 M, respectively. All work reported herein was carried out at [OH-] = 0.030.05 M (typical of commercial hypochlorite solutions), which also avoided complications from the rapid decomposition of appreciable amounts of free HClO as would be found near neutrality (Lister, 1952; Adam et al., 1992). In the uncatalyzed case, C10- is consumed by the secondorder reactions (Lister, 1956a;Lister and Petterson, 1962)
-c1-+ ki
2c10-
-+
c10,- + c10-
fast
c10,-
(1)
c10,- + c1-
(2)
ko
2c10- 0, 2c1(3) Equations 1and 2 account for approximately 95% of the consumption of C10- when catalysts are absent. When dissolvedCu is present, additional reactions involving both monomeric and dimeric Cu(I1)complexes can occur (Gray et al., 1977),with the monomeric pathway of eq 4 projected
kcu
Cu(OH),"
fast
intramolecular peroxo species
c10-
Clo-
Cu(0H);
+ 0, + 2Cl-
(4)
to dominate at lower [CUI,e.g., 0 is given by modifying eq 8 to dColdt = k,CA2+ gc, (14) Equation 14 may be integrated by substituting the right side of eq 13 for CA,yielding for the amount of oxygen released at time t: Co = (g/c){lnf + (kJc)[ln(l- a ) + 1/(1- a ) In f - l/fl - ln(1- a ) ] (15) and f = 1- [ a exp(-gt)l. where a = p / ( p + g), p = cCA(O), For small g, eq 15 can be shown to reduce to eq 9. Considering C103- formation in the presence of Cu(II), substitution of the right side of eq 13for CAin eq 10yields upon integration
C, = CD(0)+ m[(l/g) - Wn(l/g) - ll/pJ[(g + p)/(np)l - [(In n)lpIl (16) where m = kgC~(o)/(2c) and n = g + p - [p/exp(gt)l. Again disregarding the small amount of ClOz-, material balance from the stoichiometry requires that eq 17 be satisfied at all times: CD = CD(0)+ [cA(O) - CA - cO1/3 (17) Using the appropriate substitutions from eqs 13 and 15 for CAand Co, the identity of eqs 16 and 17 can be shown (Lister, 1992). Correspondingly, values of CDcalculated by these equations (using results for CAand Co from eqs 13 and 15 in eq 17) agreed to high precision for several arbitrary test cases. For the simpler case where CE = 0, the identity of eqs 11 and 17 is readily demonstrated. A computer program was written for numerical evaluation of eqs 13, 15, and 16, using k,, k1, and kcu values as derived below. Where CE = 0, instead of using eqs 7, 9, and 11 it is simpler to use the former eqs with an artificially low CE value, e.g., 1 X lo4 ppm (1.57 X M), to avoid the generation of undefined functions. At such &values, results from the program convergeto those generated by using eqs 7,9, and 11,further confirming the analysis. Experimental Section The NaClO used was a single batch of fresh commercially-prepared material containing NaCl equivalent to NaC10; p, invariant upon decomposition, was calculated to be 4.75. Trace metal determination by inductivelycoupled plasma showed undetectable levels of Cu, Ni, and Co (c0.02 ppm) and an Fe level of 0.07 ppm. Stock Cu(I1) solution, 1% in 10% HN03, was prepared from reagent CuC12. Milli-Q reagent water was used for all dilutions. The NaClO solution, originally 2.35 M, was stored at 4 "C to minimize decomposition before use; initial [ClO-I was determined for each run and was never below 2.06 M in the stock solution. Hypochlorite concentrations were determined iodometrically, allowing for slight interference from C102-, present at concentrations on the order of 1%of the [ClO-I (Lister, 1956a),and oxidizing 4 equivalents of weakly acidic I- as opposed to 2 equivalents by C10-. A correction formula was derived from data in the latter work, where C10- and Cl02- were separately determined over a wide [ClO-I range. To adequate precision, [ClO-I = (av Cl2/ 71.92) - 0.0027, where av C12 ("available chlorine") is in units of grams of C12 per liter (Wojtowicz, 1993). [The empirical factor 71.92 derived from the plot of the literature data is not the same as the molecular weight of Cl2 (70.91) used in iodometry.] C103- does not interfere under these test conditions. A gravimetric method was devised to facilitate measurements of oxygen evolution from multiple samples without the need for thermostated gas burets. As no previous reference to this type of method has been found, details are given here. Narrow-mouth, screw-capped clear round polytetrafluoroethylene (PTFE) 125-mL bottles weighing 42-43 g were used to contain 75 mL of NaClO solutions. A small-diameter (1.2 mm) hole was drilled in each cap to allow evolved oxygen to escape. Corrections for the simultaneous background loss of water vapor were made by weighing a concurrent bottle containing an NaCl solution of the same p as the NaC10. Experiments showed that, at the same total p , a 50-50 NaC1-NaC103 solution lost water vapor at a rate indistinguishably different from NaCl alone; therefore the background water vapor rate in
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 241 decomposing NaClO is insensitive to the changing composition of the solutions. In this method, water vapor rates increase slightly with increasing oxygen rates. The relationship was determined by introducing oxygen at varying flow rates via a hypodermic needle through the cap and below the surface of 55 "C water in one of the bottles, water vapor loss being followed by weighing the bottle and attached needle at intervals. The necessary deductions from the apparent oxygen rates in this work were determined to be 1-2 % in most cases and never more than 3%; hence they were considered negligible in view of ordinary experimental error. Cap hole diameter was selected by trial and error, larger holes giving background rates too high to permit differentiation between 02 and water vapor and smaller holes being subject to occasional clogging. (Proper hole size may be produced by a standard 1.2-mm drill; a US No. 56 drill is an acceptable alternative.) Background water vapor losses were strictly linear over extended time intervals. The background rate for NaCl was 0.0046 g/h at 60 "C and p = 4.75, increasing with decreasing p in essentially direct proportion to the increasing vapor pressure of more dilute NaC1. By comparison, 75 mL of 2.35 M NaClO at p = 4.75 and 60 OC (no added Cu) initially loses 0.0068 g/h of 02 above this background; the initial total 02 rate a t 0.5 ppm Cu(I1) increases to about 0.0291 g/h above background. Selected bottles with matched NaCl rates were used. With progressively more dilute pure NaClO at lower p and T , data scatter increases owing to decreased 02 evolution. In such cases, volumetric gas measurements from relatively large amounts of NaClO (Lister and Petterson, 1962) would be the preferred technique. Increased gas evolution in the presence of catalysts substantially extends the applicable range of the gravimetric method. Owing to the small gradual loss of water, the solutions become slightly more concentrated during a run. Even with very extended runs, however, such increases were never more than0.7 % (typically0.3-0.5% ) and hence were neglected. Also to be considered would be additional weight losses from volatilization of HC10, present in very small amounts in alkaline hypochlorite. However, in 2 M NaClO at 50 "C and [OH-] = 0.05 M, the free [HClOI would be only about 1 X 10-6 M as calculated from an approximate pK, of 7.40 at that temperature (data extrapolated from Morris, 1966). The data of Imagawa (1951) relate the partial pressure of HClO vapor over a 2.4 M NaC103 solution at 50 "C to its concentration in such a solution; from this relationship, the partial pressure of HClO at concentrations obtaining in the present work can be estimated as
