Potassium iodide as a chemical actinometer for 254 nm radiation: Use of iodate as an electron scavenger

A solution of 0.6 M iodide and 0.1 M iodate in 0.01 M berate buffer (pH 9.25) can be used as a chemical actinometer to measure the incident fluence from a low-pressure mercury lamp that puts out more than 85% of its energy at 254 nm. The actinometric solution is optically opaque to light below 290 nm and is optically transparent to wavelengths greater than 330 nm, Hence, the solution absorbs all of the germicidal wavelengths but little if any of the ambient light normally present in the laboratory. Iodate acts as an electron scavenger and prevents the back reaction of the free electron with the iodine atom following UV excitation of KI. Irradiation results in the linear formation of triiodide, which is quantitated by measuring its absorbance at 352 nm. The quantum yield for this system is approximately 0.75 +/- 0.03 at 20.7 degrees C or approximately three times greater than that obtained previously using nitrous oxide as an electron scavenger. A model is proposed to account for this difference. A precise expression to account for the concentration and temperature dependence of the quantum yield is given by Phi = 0.75(1 + 0.23[C - 0.577])(1 + 0.02[T - 20.7]) where C is the concentration of iodide and T is the temperature. The concentration of iodide can be obtained from the absorbance at 300 nm prior to irradiation using 1.061 M-1 cm(-1) as the molar extinction coefficient. This actinometric system meets the quality criteria established by the International Union of Pure and Applied Chemistry with the caveat that it is designed to measure only germicidal radiation (i.e. wavelengths less than 290 mm).