Spectrophotometric Analysis of Divalent Mercury (Hg(II)) Using Dithizone: The Effect of Humic Acids and Ligands

Spectrophotometric analysis of divalent Hg(II) using dithizone has been widely used. Yet, a number of analytical issues and concerns associated with this method remain to be addressed. We studied the effect of humic acids (Aldrich and Acros humic acids) and pH on Hg(II) analysis and clarified several analytical and operational issues. Our study shows that the humic acids lower the slopes of the Hg(II) calibrations and thus the sensitivity of the method. Nevertheless, the calibrations retain good linearity and thus still remain valid and useful in the presence of the humic acids at the tested levels of up to 100 ppm. The effect of the humic acids appears to be similar under both acidic and basic conditions. Our tests using cysteine (model agent for thiol group) and oxalate (carboxylic group) reveal the cause for the effect of the humic acids. The study shows that cysteine has the strongest effect on the Hg(II) analysis (largest calibration slope decreases), followed by humic acids and then oxalate. As for the pH effect, in the absence of the humic acids, basic conditions lead to lower sensitivity but still with good linearity at pH up to 9. Yet, the method fails to perform satisfactorily at pH ≥ 10. Our further extended study on the effect of ligands (chloride, hydroxyl, citrate, oxalate, and cysteine) confirms the effect and role of the thiol and carboxylic groups of humic acids in affecting the Hg(II) analysis. These ligands widely present in environmental samples can interfere with the Hg(II) analysis by lowering its sensitivity while still leaving its calibration linearity unaltered. Our operational study shows that the concentration of dithizone solution (dithizone in chloroform) should always be kept excessive and adjusted based on the level of Hg(II) analyzed to ensure complete complexation of Hg(II) with dithizone. Adoption of the dithizone solution used for the Hg(II) extraction, instead of chloroform, to zero the spectrophotometer proves to be useful and effective in minimizing analytical errors. The improved, refined method of spectrophotometric analysis of Hg(II) using dithizone can still serve as a useful analytical tool. Yet, a lack of due attention to and appropriate measures for handling the effect of humic acids and other ligands can result in analytical errors and research artifacts. This can consequently compromise the analytical validity of this method. Appropriate analytical calibrations should be conducted with the effect of humic acids or ligands in consideration, and only the specific calibration in the presence of the humic acid or ligand of concern at the relevant level(s) should be employed appropriately to calculate the results of the analytical unknowns.