Environ. Sci. Technol. 1985, 19, 286-287

3.0

4.0 =>o c - I

5.0

CORRESPONDENCE

-

-

-

Comment on “Kinetics of Monobromamine Disproportionation-Dlbromamine Formation In Aqueous Ammonia Solutions”

SIR The paper by Inman and Johnson (1) discusses the kinetics of dibromamine formation in aqueous am- monia solutions which was shown to be greatly catalyzed by the presence of inorganic phosphate. While the authors should be commended for their ingenuity in obtaining difficult to measure kinetic data, we disagree with their conclusion that the proposed mechanism predicts that the rate of dibromamine formation should be invariant with pH at low pH values. The theoretically derived rate ex- pression which they obtained is incorrect as the denomi- nator should be squared. Their conclusions are based on their erroneous equation. The correct expression corre- sponding to their hypothesized mechanism is d[NHBr2] - -

dt k&~1(H+)[Br&l~ + ~&H&~(H+) [PO~I [Br&I2

(1)

where [PO,] = total phosphate concentration and [Br;] = total initial bromine concentration which may be ex- pressed in terms of the authors experimentally determined catalyzed, k,, and uncatalyzed, k,, rate constants as d[NHBr2]

[1 + KH~(H+) + K ~ i K ~ ( H + ) [ p o 4 1 ] ~

k,(H+) [Br$I2 + k,(H+) [PO4] [Br&I2 (2) - -

dt [1 KH~(H+) + K~iKp(H+)[po41]~

where k, = k3KH1 and k, = k&HIKp. These expressions predict that the rate of dibromamine formation should go through a maximum at a pH given by

(3)

pH -PKH1 (4)

pH = log KH1 + log (1 + Kp[P04])

which simplifies to

if the term K [PO,]<< 1. equal to 2.065 x 106

M-l (-PKH1 = 6.31) from their experimental results ob- tained in the absence of phosphate (slope of Figure 8 of ref 1) and their measured uncatalyzed rate constant, k,. This calculated value is in close agreement with the value of -pKH1 = 6.39 obtained by Johannesson (2). Using values of KH1, k,, and k, and the assumption that Kp[PO,] << 1 which makes the third term in the denominator of rela- tionships 1 and 2 negligible in comparison to the second term, we have prepared a plot of “normalized” initial re- action rates vs. pH (Figure 1) following the convention used by Inman and Johnson in Figure 4 of their paper. The normalized rate is expressed in terms of the rate of change of absorbance at 232 nm and is equivalent to the initial rate in a solution having [Br;] = 0.1 mM and [PO,] = 0.02 M. The experimental results of Inman and Johnson are in reasonably good agreement with the calculated values which clearly show a maximum in reaction rate.

The authors have also stated that either HPOt-, H2P- 04-, or PO:- probably catalyzes the reaction so that a negligible dependence of the phosphate effect on pH is

We have cafculated a value of

* ‘ O 3

0

A

0

A

A

0 Experimental ACalculated 6’oA 7.0 20.0 A 18.0 16.0 14.0 12.0 10.0

- In (H + ) I I I I I I I I I I

8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 PH

Flgure 1. Normalized initial rate of NHBr, formation. Comparison of experimental ( 7 ) and calculated values.

expected. Their statement is consistent with the observed rate expression. However, catalysis by only HP02- is also consistent with the observed rate expression. In contrast catalysis by only H2P04- according to their mechanism yields a reaction rate expression with a phosphate-cata- lyzed term which is second order in H+ when pH >> pKa2. Therefore, it appears that within the pH region used in the experiments either that HP04- and H2P042- both act as equivalent catalysts or that only HP04- is effective if the reaction proceeds as they hypothesize.

We would like to propose an alternative model for the formation of NHBr2 in the presence of phosphate which is also consistent with the observations between pH values of 6 and 8.5 and is perhaps somewhat conceptually more simple. The formation of NHBr2 could be acid catalyzed by H2P04- according to the mechanism

(5) NH2Br + H+ - NH3Br+ kr

NH2Br + H2P04- 5 NH3Br+ + HP0:- (6)

( 7 ) NH3Br+ - H+ + NH2Br

NH3Br+ + NH2Br -% NHBr, + NH4+ (8)

(9) H2P04- e HP042- + H+ If k, is large, then the corresponding batch rate expression is

k.

KSZ

k,k,[H+I [Po41 [Br1I2 k3kf[H+][Br$12 + - [H+I + Ka2 - d[NHBr2]

dt 1 1 + t [ H + ] + L

The alternative model invokes only a single simple acid catalysis step to augment NH3Br+ production and does not

286 Environ. Sci. Technoi., Vol. 19, No. 3, 1985 0013-936X/85/0919-0286$01.50/0 0 1985 American Chemical Society

Environ. Sei. Techno/. 1985, 19, 287-287

depend on a rate-limiting step involving a hypothetical bromine-phosphate intermediate. Furthermore, all con- stants are conceptually defined unlike the value of Kp used by Inman and Johnson which is a “composite” parameter that should be a function of pH if different phosphate species catalyze NHBrz formation in a differential manner according to their mechanism.

Registry No. NH2Br, 14519-10-9; NHBr2, 14519-03-0.

Literature Cited (1) Inman, G. W., Jr.; Johnson, J. D. Environ. Sei. Technol.

(2) Johannesson, J. K. J. Chem. SOC. 1959,2998-3001. 1984, 18, 219-224.

Richard L. Valentine,* Chad 1. Jafvert

Department of Civil Engineering and

The University of Iowa Iowa City, Iowa 52242

Environmental Engineering

SIR Valentine and Jafvert (1) have correctly noted an error in our eq 23. They have suggested an alternative mechanism with both a specific acid catalysis step and a general acid catalysis by H2P04- which accounts for the pH variation between pH 8.5 and pH 6.0. Unfortunately the experimental limitation preventing the collection of data a t lower pH makes it impossible to verify their modified mechanism fully. Recently, Gray et al. have taken data using a pH jump procedure at pH values from

0013-936X/85/09 19-0287$01.50/0 0 1985 American Chemical Society

3 to 1 2 (2). On the basis of these results they propose a mechanism similar to Valentine and Jafvert (1).

On the basis of the limited pH range we were able to cover due to the experimental limitations imposed by the chemical equilibria, it is not possible to test Valentine and Jafvert’s proposed mechanism below pH 6. Therefore, the calculated values in their Figure 1 are useless below this value. In any case, their alternative mechanism is an ap- pealing proposal.

It should also be pointed out we neglected to reference the work by Wajon and Morris (3). They measured the rate of monobromamine formation under conditions sim- ilar to ours and found a rate constant of 7.4 X lo7 M-l s-l compared to our value of 4 X lo7 M-l s-l.

Registry No. Monobromamine, 14519-10-9; dibromamine, 14519-03-0.

Literature Cited Valentine, R. L.; Jafvert, C. T. Environ. Sei. Technol., preceding paper in this issue. Pasquini Christina, S.; Azure, M. T.; Workman, H. J.; Gray, E. T., Jr. “Decomposition of Bromamines in Aqueous So- lution: Preliminary Report of the Decomposition Kinetics and Disproportionation of NH,Br”. presented at the 5th Conference on Water Chlorination-Environmental Impact and Health Effect, Williamsburg, VA, June 1984. Wajonl J. E.; Morris, J. C. Inorg. Chem. 1982,21,4258-4261.

Guy W. Inman, Jr., J. Donald Johnson*

Department of Environmental Sciences and Engineering The University of North Carolina at Chapel Hill Chapel Hill, North Carolina 27514

Environ. Sci. Technol., Vol. 19. No. 3, 1985 287