Time-resolved photoacoustic calorimetry of aqueous peroxodisulfate photolysis in the presence of nitrite anions

The photodissociation of aqueous peroxodisulfate in the presence of nitrite anions was studied between 282 and 292 K by time-resolved photoacoustic calorimetry at 266 nm. Aqueous peroxodisulfate was chosen as the calorimetric reference, thus enabling the measurement of the enthalpy and volume changes associated with the reaction of the photoproduced sulfate radicals with nitrite anions to form SO4-2 and NO2 in the submicrosecond time range. The enthalpy change obtained (-192 +/- 19 kJ mol(-1)) is consistent with available thermochemical data for related species. The interpretation of the measured reaction volume (-13.8 +/- 0.7 mL mol(-1)) points to the ionic charge accumulation on the sulfate anion as the dominating change causing the contraction of the solvent. The comparative analysis of our experimental result and data for partial molar volumes and electrostriction volumes of electrolytes, reinforces the concept that particularly in the case of water, more realistic models for the solvent are needed involving structure rather than continuous features.