Solubility and complexation reactions.

Question

Recent research on the solubility of greenockite, a thermodynamically stable form of cadmium sulfide (CdS), reveals that mildly acidic sulfide can effectively minimize the solubility of CdS. The pertinent complexation reactions are (neglect any other complexation reactions):

CdS + H⁺ --> Cd²⁺ + HS^- log Ksp = -14.36

CdS + H^{+ -->} CdHS⁺ log K₁ = -4.73

CdS + H⁺ + HS^- --> Cd(HS)₂⁰ log K₂ = -1.0

CdS + H⁺ + 2HS^- -->Cd(HS)₃^- log K₃ = 2.08

CdS + H⁺ + 3HS^- --> Cd(HS)₄^2- log K₄ = 3.53

CdS + H₂O + CdOHS^- + H^+. log K₅= -16.83

a) Derive the plotting equations and draw the solubility diagram of CdS using the solubility and complexation ractions given above.

b) Determine the final pH and the concentration of all species after 4×10-2 moles of H2S are added to 1 liter of solution which is at equilibrium with CdS(s). Assume a closed system

Answer 1

Hi Busayo O.,

I think you mean CdS + H2O -> CdOHS- + H+ as your 6th equation?

In any event, you must solve iteratively, I think (too many overlapping equilibria to solve exactly). So let the H2S dissociate as if in water, and plug those H+ and HS- values into the 6 equilibria. (separately). See which equilibria "want" to soak up H+ and HS- the most; solve those first and then allow the equilibria which don't "want" to soak up these ions as much to tweak the H+ and HS- levels. Run back through iteratively, including the initial H2S dissociation equilibrium, until you achieve stable results.

I don't think a plotting diagram will help you much, since you have 2 independent ions (H+ and HS-) that govern dissociations of CdS. Even if you could view a layered 2-D plot, it won't help you since you can't arbitrarily choose an H+ and HS- level, because they are linked differently for each soluble component.

-- Cheers, --Mr. d.