I need help with finding minimum pH and and sulfite dose for reducing a metal to

Question

I need help with finding minimum pH and and sulfite dose for reducing a metal to precipitate. Formation constants given.

Probl kaline ea ctive is i, (25 pts) A wastewater contains 0.15 chromate (CrO). The chromate is to be removed by reducing it to Cr" with sulfite (SO.2) (which is oxidized to sulfate) and then precipitating it as Cr(OHxo. Permitted effluent concentrations are 0.1 mg/L of Cr and 0.2 mg/L total dissolved chromium. Determine the minimum pH and sulfite dose necessary to meet these requirements. You may find the following information helpful. CrO42-+ 4H2O + 3e- Cr(OH)3(s) +50H Ea_-0.12 Stepwise formation constants for Cr hydroxide complexes: Ki- 1091, K2 -1079. pKso, for Cr(OH)se) is 30.2.

Explanation / Answer

1) Given pKso = 30.2 for Cr(OH)3(s)

pKso is struvite solubility product. pKso = -log10Kso = antilog(-pKso)

Kso = 10-30.2 = 6.31*10-31

For chromium hydroxide: Cr(OH)3 <-------> Cr3+ + 3OH¯

Kso = [Cr3+] [OH-]3 ====> 6.31 x 10-31 = (x) (3x)3

x= 1.3*10-8M = [Cr3+]

[OH-]3 = 3X =3.9*10-8M

pOH = -log 3.9 x 10¯8 = 7.41

pH = 14 - pOH = 14 - 7.41 = 6.6 (acidic)

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