Kinetics of hydrated electron reactions with phosphate anions: a laser photolysis study

Abstract

The decay kinetics of hydrated electron (eaq−) formed upon photolysis of aqueous solutions of sodium pyrene-1,3,6,8-tetrasulfonate at λ = 337 nm in the presence of phosphate anions (up to 2 mol L−1) was studied by nanosecond laser-pulse photolysis in a wide range of pH (3.5–10) and ionic strength (I, up to 2 mol L−1) values. At high pH values, where the HPO42− ions dominate, the eaq− decay kinetics depends only slightly on phosphate concentration (rate constant for the reaction is at most 2·105 L mol−1 s−1). The H2PO4− ions react with eaq− at a rate constant of 2.8·106 L mol−1 s−1 (I = 0), which increases linearly with the parameter \(\exp ({{\sqrt I } \mathord{\left/ {\vphantom {{\sqrt I } {1 + \sqrt I }}} \right. \kern-\nulldelimiterspace} {1 + \sqrt I }})\) in accordance with the Debye-Huckel theory. The rate constant for quenching of eaq− by H3PO4 at pH ≤ 4 decreases linearly with the parameter \(\exp ({{\sqrt I } \mathord{\left/ {\vphantom {{\sqrt I } {1 + \sqrt I }}} \right. \kern-\nulldelimiterspace} {1 + \sqrt I }})\) due to the secondary salt effect and equals 1.6·109 L mol−1 s−1 at I = 0. The logarithm of the rate constant for quenching of eaq− by phosphates is linearly related to the number of the O-H bonds in the phosphate molecule.