Environ. Sci. Technol. 1985, 19, 286-287
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 ammonia 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 expression which they obtained is incorrect as the denominator should be squared. Their conclusions are based on their erroneous equation. The correct expression corresponding to their hypothesized mechanism is d[NHBr2] dt k&~1(H+)[Br&l~ + ~ & H & ~ ( H + ) [ P O[Br&I2 ~I (1) [1+ K H ~ ( H ++) K ~ i K ~ ( H + ) [ p o 4 1 ] ~
-
where [PO,] = total phosphate concentration and [Br;] = total initial bromine concentration which may be expressed in terms of the authors experimentally determined catalyzed, k,, and uncatalyzed, k,, rate constants as d[NHBr2] dt
-
k,(H+) [Br$I2 + k,(H+)[PO4][Br&I2 [1
K H ~ ( H ++) K~iKp(H+)[po41]~
(2)
where k, = k3KH1and k, = k&HIKp. These expressions predict that the rate of dibromamine formation should go through a maximum at a pH given by pH = log KH1 + log (1 + Kp[P04]) (3)
*
‘
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4.0
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5.0
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0
=>o
c I
A A 0
A
6’o 0 Experimental ACalculated
7.0 20.0A
I
I
Environ. Sci. Technoi., Vol. 19, No. 3, 1985
12.0
14.0
- In I
I
10.0
(H + )
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-catalyzed 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
+ H+
(4)
if the term K [PO,]
