The principle of the hexavalent chromium water quality online monitoring instrument is mainly based on spectrophotometry. Under acidic conditions, hexavalent chromium ions react chemically with a specific chromogenic agent (diphenylcarbazide) to form a purplish-red compound. The color depth of this compound is proportional to the concentration of hexavalent chromium ions. The instrument uses a built-in optical system to accurately measure the absorbance of the compound at a specific wavelength. By analyzing the linear relationship between absorbance and hexavalent chromium concentration, the concentration of hexavalent chromium in the water sample can be calculated. The entire process is automatically controlled by instruments, from sample introduction, chemical reactions, absorbance measurement to concentration calculation, achieving automation and high precision for hexavalent chromium monitoring.