General Chemistry • Solubility and Complex Ion Equilibria
In acidic, H2S-containing solutions, metal sulfides can dissolve or precipitate according to the overall equilibrium MS(s) + 2 H3O+ ⇌ M2+ + H2S + 2 H2O. This calculator can derive the overall constant Kspa from Ksp, Ka1, and Kw, and it can also test whether a chosen set of conditions will cause MS(s) to precipitate using Qspa.
Recognized keys: mode, MS, M, Ksp, Ka1, Kw, Mref, H2Sref, Mconc, H2Sconc, pKind, pVal, precision.
Kspa is an effective equilibrium constant for MS(s) in acidic, H2S-containing solution using the overall reaction MS(s) + 2 H3O+ ⇌ M2+ + H2S + 2 H2O. It is defined so that Kspa = ([M2+][H2S]) / ([H3O+]^2).
The calculator uses Kspa = Ksp / (Ka1 x Kw). This connects sulfide solubility to the first ionization of H2S and water autoionization in acidic solution.
Compute Qspa = ([M2+][H2S]) / ([H3O+]^2) for the current conditions and compare it with Kspa. If Qspa is greater than Kspa, MS(s) is favored to precipitate; if Qspa is less than Kspa, precipitation is not expected.
Yes. If pH is provided, the calculator converts it to [H3O+] using [H3O+] = 10^(-pH) before evaluating Qspa.
The method treats activities as concentrations and uses Ka1 for H2S with fixed Kw at 25 C by default. If ionic strength, temperature changes, or additional equilibria are important, real precipitation behavior can differ.