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

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 (VUV 185 ) emitted by low-pressure mercury lamp (LPML) because of the interference of coexisting 254 nm UV (UV 254 ). Given that formaldehyde (FA) is insensitive to UV 254 within the time scale of interests whereas vulnerable to VUV 185 , this study for the first time evaluated the possibility of using FA as a chemical actinometer to determine the VUV 185 photon flux under varying conditions. Experimental results revealed that the photodegradation of FA followed a pseudo -1 st -order reaction kinetic under VUV 185 irradiation, and the products of initial concentration ( C 0 ) and reaction rate constants (i.e., C 0  ×  k ) were proportional to the numbers of LPMLs (under C 0 of 50–300 mg/L), suggesting that C 0  ×  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 C 0  ×  k values, proving that FA is a good actinometer for VUV 185 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 VUV 185 quantification in the copresence of UV 254 .