Spectrophotometric Determination of Mercury in Water

Mercury is one of the most toxic heavy metals found in aquatic environments. Its detection is therefore essential for drinking water safety, wastewater control, and ecological monitoring. Among the various analytical techniques available, the dithizone-based spectrophotometric method remains a classic and widely adopted approach, particularly in laboratories where advanced atomic spectrometers are not accessible. 

This method combines selective complexation, solvent extraction, and photometric measurement to quantify total mercury with satisfactory accuracy.

The analytical principle rests on the reaction between divalent mercury ions (Hg²⁺) and dithizone (diphenylthiocarbazone) under acidic conditions. Dithizone forms a stable, orange‑coloured chelate with Hg²⁺, which is sparingly soluble in water but readily extractable into organic solvents such as chloroform or carbon tetrachloride. The resulting complex exhibits a characteristic absorption maximum in the visible region, typically around 485–490 nm. 

According to the Beer‑Lambert law, the absorbance of this coloured solution is directly proportional to the concentration of mercury, provided that the optical path length and wavelength are fixed. By comparing the measured absorbance against a calibration curve prepared from standard mercury solutions, the mercury content in the sample can be accurately determined.

Before the colour reaction can proceed, the water sample must undergo thorough digestion. Mercury may exist in inorganic forms (Hg²⁺, Hg⁺) or as organic mercurial compounds, and the spectrophotometric method measures total mercury. Therefore, oxidising agents—commonly potassium permanganate and potassium persulfate—are added in an acidic medium, and the mixture is heated to convert all mercury species to Hg²⁺. Excess oxidant is then reduced with hydroxylamine hydrochloride to prevent interference in the subsequent complexation step. 

This pre‑treatment is critical; incomplete digestion will lead to significant underestimation because organic mercury does not react with dithizone.

Once digestion is complete, the sample is adjusted to the optimal acidity (usually around 0.5 mol/L sulfuric acid). Dithizone solution is added, and the orange mercury‑dithizonate complex forms immediately. To eliminate the excess reagent and possible interfering ions, the complex is extracted into an organic phase. This extraction not only concentrates the analyte but also separates it from most co‑extracted impurities. 

After separation, the organic layer is transferred to a cuvette and its absorbance is measured at the selected wavelength. A blank prepared with the same reagents but without the sample is used to zero the instrument, ensuring that only the net absorbance from mercury is recorded.