Water Calorimetry: A Correction to the Heat Defect Calculations.

Area of Science:

• Radiochemistry
• Chemical Kinetics
• Physical Chemistry

Background:

• Accurate calculation of heat defects in aqueous solutions is crucial for understanding radiation-induced chemical processes.
• Previous reaction models (model III) used literature rate constants that were later found to be incorrect.
• Temperature variations (21 °C and 4 °C) significantly influence reaction pathways and energy balance in irradiated water.

Purpose of the Study:

• To revise a reaction model (model III) by incorporating a corrected rate constant for calculating heat defects in irradiated aqueous solutions.
• To compare the predictive accuracy of the original model (model III) and the revised model (model IIIR) at different temperatures (21 °C and 4 °C).
• To validate model predictions using experimental water calorimetry measurements.

Main Methods:

• Development of a revised reaction model (model IIIR) with an updated rate constant.
• Creation of model versions for both 21 °C and 4 °C.
• Performance of water calorimetry measurements using pure water and hydrogen-saturated water.
• Measurement of hydrogen peroxide yields in oxygen/hydrogen-water mixtures.

Main Results:

• Model IIIR predicts similar heat defects at 21 °C for both models, but a 30% increase at 4 °C compared to model III.
• Model IIIR predicts heat defects of -0.023±0.002 at both 21 °C and 4 °C, consistent with experimental calorimetry data.
• Measured hydrogen peroxide yields in H2/O2-water agree with model IIIR predictions at both temperatures.

Conclusions:

• The revised model (model IIIR) provides accurate predictions for heat defects in irradiated water at different temperatures.
• Experimental water calorimetry measurements are now sufficiently precise to determine heat defects more accurately than simulations alone.
• Consistency across different water systems (pure, H2-saturated, H2/O2-water) serves as a critical indicator of water purity.