Development and validation of mercury-free homemade reagents for spectrophotometric COD analysis: A green, cost-effective approach for resource-limited laboratories analyzing low-chloride petrochemical effluents

• Mercury-free reagent formulation reduces hazardous waste by ∼70% and eliminates mercury exposure risk. • Method validated only for low-chloride matrices (<10 mg/L) performance declines above 500 mg/L Cl⁻ without adaptation. • Not validated for saline, domestic, or pharmaceutical wastewaters; scope limited to surfactant-free petrochemical effluents. • Requires strict control of vial cleanliness, reagent aging, and digestion conditions for reproducibility. • Formal cost-benefit analysis confirms 85% cost reduction while maintaining analytical performance within the defined scope. This study validates a mercury-free, locally producible reagent system for spectrophotometric COD analysis using the HACH DR2400, maintaining full factory calibration compatibility without recalibration. The optimized formulation (2.2 mL Ag 2 SO 4 catalyst S4 + 0.8 mL K 2 Cr 2 O 7 digestion solution D2) achieved absorbance values of 2.95–2.98 at 420 nm (low-range) and 0.34–0.36 at 620 nm (high-range), matching HACH factory curves across five independently prepared batches. Validation with certified KHP standards and real industrial effluents from ethylbenzene/styrene monomer units demonstrated recovery of 99.9–101.0% with precision RSD < 3% (n=15). The method is validated exclusively for low-chloride (<10 mg/L), surfactant-free petrochemical effluents; mercury elimination is justified by inherently low chloride content and partial AgCl precipitation from elevated silver sulfate concentration. Green metrics confirmed environmental benefits (Eco-Scale 82/100 "excellent"; AGREE 0.78/1.0), while economic analysis revealed 85% operational cost reduction versus commercial kits. Critically, this work does not claim conceptual novelty in mercury-free COD chemistry, rather, it delivers a practically validated adaptation addressing three operational gaps unmet by existing literature: (1) factory calibration compatibility without recalibration, (2) sanctions-resilient local reproducibility using Merck-grade reagents, and (3) quantified cost-benefit analysis enabling adoption in resource-constrained settings. This work offers a scientifically rigorous solution for laboratories analyzing low-chloride (<10 mg/L), surfactant-free petrochemical effluents under resource constraints. Application to saline, domestic, or surfactant-containing matrices requires matrix-specific validation.