Kinetics of oxidation of nitrite by aqueous bromine

Abstract

The kinetics of oxidation of nitrite to nitrate by aqueous bromine have been investigated using a spectrophotometric stopped flow technique. In the pH range 4.2-5.8 (acetate buffer) the rate law is: - d[Br21,/dt = [Br21[N02 -I2 (a + b/[Br-1) (where [Br,], = [Br2]+[Br,-1) with a = (4.61-0-1) x lo4 l2 m01-~ s-l and b = (3.3 1-0.1) x lo4 1. mol-l s-l at 298.2 K and with the temperature dependences, - R d(lna)/d(l/T) = (46k 4) kJ mol-l and - R d(ln b)/d(l/T) = (45 k 2) kJ mol-'. In the pH range 0.8-2.5 (perchloric acid) the rate law is : - d[Br2],/dt = [HN0212[Br21 (w + v/[Br-l)/(l+ z[H+ItBr,l,) with w = (5.9+0.2)x lo4 l2 m01-~ s-l, v = (3.41-0.1)~ lo4 1, mol-l s-I, and z = (1.90i 0.06) x lo7 l2 mol-2 at 298.2 K. In addition: - R d ln(w/z)/d(l/T) = (31 1 4 ) kJ mol-I and - R d ln(v/z)/d(l/T) = (46 f 4) kJ mol-l In the pH range 2.8-3.3 (chloroacetate buffer) a combination of these two rate laws adequately describes the kinetic results. These rate laws have been interpreted in terms of two reversible initial reactions: 6) NO2- +Br2 + N02Br +Br- (followed by attack on N02Br by NO2-) (ii) NO2-+NO2- (or HNOJ + N204'- (or HN204-) (followed by attack by Br2 upon N204'- or HNZO4- or upon N203 formed from HN204-).