Precise equilibrium constants from spectrophotometric data:: BrCl in Br2/Cl2 gas mixtures

The equilibrium constant for the reaction, Br-2(g) + Cl-2(g) reversible arrow BrCl(g), is estimated using the classic spectrophotometric method with precise data and a multispectrum fitting approach. Analysis of spectra for 18 Cl-2:Br-2 mixing ratios yields Kdegrees = 9.1 at 22degreesC, with a nominal statistical error (1 sigma) of 0.04. This error translates into a remarkable precision of 0.4 cm(-1) in the dissociation energy of BrCl, easily beating that for the current best spectroscopic estimate ( 5 cm(-1)). However, a sensitivity analysis shows that Kdegrees is susceptible to small systematic errors and minor changes in the least-squares fit model, leading to a more conservative estimate of 2% error. The derived Kdegrees is consistent with statistical mechanical estimates that employ the current value for the BrCl dissociation energy, but it differs considerably from some recently measured and used values, which range from 6.4 to 10.4. The revised estimate of the BrCl dissociation energy is D-e = 18 248 +/- 2 cm(-1).