Aqueous chemistry of chlorine, bromine, and bromine chloride and development of a pulsed-accelerated-flow spectrophotometer with position-resolved observation

Tian Xiang Wang, Purdue University

Abstract

The reversible hydrolysis reactions of Cl$\sb2$(aq) and Br$\sb2$(aq) are general base-assisted in the forward reaction, and general acid-assisted in the reverse reaction. The role of the general acid assistance for proton transfer to oxygen are correlated to the role of the nucleophilicity of X$\sp-$ (Cl$\sp-$, Br$\sp-$, I$\sp-$) as it reacts with X$\sp+$ in HOX. Rate constants of the forward and reverse hydrolysis reactions at $\mu$ = 0.50 M are determined from 0.0$\sp\circ$C to 30.0$\sp\circ$C for Cl$\sb2$(aq), and at 25$\sp\circ$C for Br$\sb2$(aq). The activation parameters ($\Delta$H$\sp{\ddagger}$ and $\Delta$S$\sp{\ddagger}$) for the forward and the reverse reactions of Cl$\sb2$ hydrolysis are measured. The $\Delta$H$\sp{\phi}$ and $\Delta$S$\sp{\phi}$ values for Cl$\sb2$ and Br$\sb2$ at $\mu$ = 0.50 M decrease markedly with an increase in temperature. Bromine chloride in the presence of chloride ion forms the dichlorobromate(I) ion, BrCl$\sb2\sp-$. Equilibrium constants at $\mu$ = 1.00 M, 25.0$\sp\circ$C are determined for (BrCl$\sb2\sp-$) /( (BrCl(aq)) (Cl$\sp-$), (Br$\sb2$Cl$\sp-$) / (BrCl(aq)) (Br$\sp-$), (Br$\sb2$Cl$\sp-$) / (Br$\sb2$(aq)) (Cl$\sp-$), and (Br$\sb3\sp-$) / (Br$\sb2$(aq)) (Br$\sp-$). UV absorption bands are resolved for Br$\sb2$Cl$\sp-$ at 245 nm ($\epsilon$ 24,900 M$\sp{-1}$ cm$\sp{-1}$) and 381 nm ($\epsilon$ 288 M$\sp{-1}$ cm$\sp{-1}$). The UV spectral properties of Cl$\sb2$(aq), Cl$\sb3\sp-$, Br$\sb2$(aq), and Br$\sp-$ are examined and compared. The reaction between Cl$\sb2$(aq) and Br$\sp-$ to form BrCl$\sb2\sp-$ occurs at the diffusion-controlled limit; the rate constant, (7.7 $\pm$ 1.3) $\times$ 10$\sp9$ M$\sp{-1}$ s$\sp{-1}$, is measured by the pulsed-accelerated-flow method. A new PAF-PRO instrument based on a masked CCD and a high-resolution digital-position syringe ram has been constructed. Pulsed-accelerated-flow (PAF) profiles (velocity of 3-16 m/s in a ball-type mixer/reactor) in conjunction with the method of position-resolved observation (PRO) are used to measure rate constants of rapid reactions in aqueous solution (t$\sb{1/2} \succeq$ 14 $\mu$s). A new mixing model is developed to predict chemical reactions with the turbulent mixing in the PAF-PRO cell. The instrument has photometric precision of 0.001 A at 0.1 A with 8 $\mu$s time resolution and 19 $\mu$m positional resolution. The objective of the PAF-PRO instrument is to measure rapid reactions of complex systems such as sequential, parallel or autocatalytic reactions.

Degree

Ph.D.

Advisors

Margerum, Purdue University.

Subject Area

Analytical chemistry

COinS