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Total chromium is one of the most closely monitored heavy metals in water quality management. Industries such as electroplating, metallurgy, tanning, and chemical manufacturing commonly discharge wastewater containing chromium compounds.

 Among them, hexavalent chromium is classified as a Group 1 carcinogen and has been listed as a priority heavy metal pollutant under China‘s national pollution prevention plan. Under the Integrated Wastewater Discharge Standard (GB 8978-1996), the maximum allowable discharge concentration for total chromium is 1.5 mg/L, and for hexavalent chromium, 0.5 mg/L. 

Accurate and timely monitoring of total chromium at discharge points is therefore essential for ensuring compliance and preventing environmental risks. However, the conventional laboratory method for total chromium determination suffers from a persistent efficiency bottleneck that has long constrained the timeliness and effectiveness of water quality monitoring.

The Conventional Method: Slow and Labour‑Intensive

The standard method for total chromium determination is the diphenylcarbazide spectrophotometric method (GB 7466-87). The procedure is complex: trivalent chromium in the sample must first be oxidised to hexavalent chromium using potassium permanganate, excess oxidant is decomposed with sodium nitrite, and the remaining nitrite is further decomposed with urea. The treated sample is then reacted with diphenylcarbazide to form a purple‑red complex, which is measured spectrophotometrically at 540 nm.

The entire process involves multiple steps—oxidation, decomposition, colour development, and photometric measurement—each demanding high reagent purity and precise operational technique. From sample collection to final data output, the analysis typically takes several hours or more. For industries operating around the clock, such as electroplating and tanning, this delay means that by the time the results are available, the corresponding wastewater has long since been discharged, and any exceedance has already occurred. The slow speed, operational complexity, and inability to support continuous monitoring have historically kept total chromium monitoring in a reactive, “post‑event” mode.