Chloride-assisted microwave-assisted digestion for reliable ICP-OES determination of tin in industrial wastewater:...

Accurate determination of tin (Sn) in industrial wastewater by inductively coupled plasma-optical emission spectrometry (ICP-OES) is compromised by the susceptibility of Sn⁴⁺ to hydrolysis, precipitation, and volatilization during conventional nitric acid digestion. This study presents the development and full validation of a chloride-assisted microwave digestion protocol that stabilizes tin through in situ formation of stable tin (IV) chloro-complexes (including [SnCl₆]^2- and related species), thereby suppressing analyte loss and enabling reliable quantification in complex industrial effluent matrices. A direct experimental comparison of HNO₃-only and HCl-assisted microwave-assisted digestion showed that, although the difference did not reach statistical significance (two-tailed t-test, t = 1.47, df = 12, p = 0.167), Sn concentrations were consistently lower under HNO₃ digestion across all seven replicates (48.95 vs. 51.51 μg/L), indicating a systematic negative bias attributable to the known instability of Sn⁴⁺ in oxidizing acidic media - where hydrolysis, precipitation as metastannic acid, and partial volatilization collectively compromise analyte recovery. The method was validated in full compliance with ISO/IEC 17025:2017, encompassing linearity, repeatability, intermediate precision, trueness, robustness, and measurement uncertainty. The calibration curve exhibited excellent linearity over the range 10-2500 μg/L (R² ≥ 0.999; slope = 0.8853). Precision was high across all concentration levels (RSD 0.94-6.95%). Spike recoveries in the wastewater matrix ranged from 86 to 100%. Limits of detection and quantification were 12.10 μg/L and 40.33 μg/L, respectively. Robustness assessment using a Plackett-Burman design with seven analytical factors confirmed that minor variations in instrumental and operational parameters did not significantly affect method performance (all E_x < E_crit = 4.58). To the best of our knowledge, this is among the first studies to provide a chloride-assisted microwave-assisted digestion protocol for tin determination specifically in organized industrial zone wastewater, validated in full accordance with ISO/IEC 17025:2017 and including Plackett-Burman robustness testing. The method offers a practically implementable, matrix-appropriate solution for accredited environmental laboratories engaged in regulatory monitoring of tin in industrial effluents.