Using formaldehyde as a novel chemical actinometer for 185 nm vacuum ultraviolet photon flux quantification in water

Abstract

Quantifying ultraviolet (UV) photon flux is important for water photolysis practice. However, it is currently challenging to quantify the photon flux of 185 nm vacuum UV (VUV185) emitted by low-pressure mercury lamp (LPML) because of the interference of coexisting 254 nm UV (UV254). Given that formaldehyde (FA) is insensitive to UV254 within the time scale of interests whereas vulnerable to VUV185, this study for the first time evaluated the possibility of using FA as a chemical actinometer to determine the VUV185 photon flux under varying conditions. Experimental results revealed that the photodegradation of FA followed a pseudo-1st-order reaction kinetic under VUV185 irradiation, and the products of initial concentration (C0) and reaction rate constants (i.e., C0 × k) were proportional to the numbers of LPMLs (under C0 of 50–300 mg/L), suggesting that C0 × k is an excellent indicator for photon flux determination. Increasing dissolved oxygen concentration from 1.0 to 7.8 mg/L, pH from 4.0 to 10.4, and temperature from 22 to 44 °C all imposed negligible interferences (variation ≤ 10%) on the C0 × k values, proving that FA is a good actinometer for VUV185 photon flux detection. Because FA can be measured readily by spectrophotometry method, its use as an actinometer is more convenient than conventional actinometers. Therefore, this study demonstrates that FA is a promising candidate of actinometer for VUV185 quantification in the copresence of UV254.