The aim of the current study is to investigate the dissolution process of alkaline-earth metal sulfides SrS and CaS in ultrapure MilliQ water and define the size dependence of the formed particles on the amount of added salt due to the similarity of their chemical properties (e.g. a cubic crystal structure, ion radius). The pH values of SrS and CaS aqueous solutions increased when an additional amount of salt was added into these closed equilibrium systems, as the average quantity and size of the formed particles rose respectively in the measured range of 10−1500 nm. The nanoparticles (detected by Nanosight® LM10) appeared in the prepared aqueous solutions containing 0.092 ± 0.01 mM of SrS(s) (pH = 9.97 ± 0.02) and 0.097 ± 0.01 mM of CaS(s) (pH = 9.94 ± 0.02) or above the aforesaid salt amount, which was about 18 times lower concentration than our previously determined values for [SrS] = 1.671 mM and [CaS] = 1.733 mM (pH = 11.22 ± 0.04). Up to these amounts of added salt in the closed equilibrium systems of H2O–SrS and H2O–CaS, all particles had dissolved due to the better solubility of smaller ones, which is related to their larger specific surface area, and thus, to the increase in solubility. Therefore, this principle allows to calculate the value of the solubility product (KSP) for nanoscale particles in different equilibrium systems by using the nanoparticle tracking analysis (NTA) method.
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