Rapid Determination of Formaldehyde in Indoor Air by Digital Image Colorimetric Technology

According to the requirements of appendix A of GB/T 18883—2002, indoor air was collected. The collected gas sample was absorbed with 0.05 g·L^-1 phenol reagent solution, and the resulting chromogenic solution was poured into 8 colorimetric dishes. The colorimetric dishes were placed side by side in the wooden box of the self-made digital image colorimetry device. The photography lamp was fixed above the small hole on the wooden box to provide a surface light source for colorimetric testing, with color temperature of 5 500 K and irradiance of 3 000 lx. The smartphone was fixed upside down on the outside of the wooden box, and the camera was 23 cm away from the colorimetric dish. The chromogenic solution inside the wooden box was captured through a small hole. Adobe Photoshop CS6 image processing software was used to process images taken by the smartphone, and formaldehyde was quantitatively detected by extracting the G value of the "G (green)" channel. It was shown that the linear relationship between values of the mass concentration and the logarithmic conversion value of G value of formaldehyde was kept in the range of 0.10-0.60 mg·L^-1, with detection limit (3S/N) of 0.04 mg·L^-1, and RSD (n=8) of the determined values of 2.0%. The correlation coefficients of the standard curves obtained from shooting with different smartphones were not less than 0.997 0, indicating that the proposed method had good universality. Test for recovery was conducted according to the standard addition method, giving recoveries in the range of 101%～103%. The proposed method was used for the analysis of formaldehyde in indoor air of 2 actual test scenes, and absolute values of relative errors between the determined values and those obtained by the phenol reagent spectrophotometry in GB/T 18883—2002 were less than 5.0%.