Polymer-stabilized zinc sulfide colloids as a candidate calibration material for sulfide determination in water

Accurate quantification of sulfide in water depends critically on reliable calibration materials. However, existing calibration materials pose significant challenges for routine sulfide analysis, whether they are in the form of Na₂S solutions susceptible to atmospheric oxidation or ZnS suspensions lacking physical homogeneity. To address these deficiencies, we developed a series of highly stable ZnS colloids by using zinc acetate, sodium sulfide, and a polymeric stabilizer. Four polymers were demonstrated to be effective, namely, chitosan, hydroxyethyl starch, hydroxyethyl cellulose, and hydroxypropylmethyl cellulose. Under optimized experimental conditions, each ZnS colloid formulation maintained satisfactory homogeneity and stability for over 6 months. Moreover, the colloids exhibit no spectral interference with the widely adopted methylene blue spectrophotometry at around 660 nm and do not cause clogging of online analyzer tubing, suggesting their suitability for instrument self-calibration. Particle size and zeta potential analyses revealed that adding a stabilizer effectively prevented particle aggregation, consistent with predominantly steric hindrance imparted by the adsorbed polymer layers. Collectively, the proposed ZnS colloids represent a promising alternative to conventional sulfide calibration materials for method development, instrument calibration, and data quality control.