What minimum pH must be maintained in a saturated H 2 S solution (0.1 M) to cause precipitation of both Mn 2+ & Fe 2+ from a solution, in which each ion is present at a concentration of 0.01 M ? (K a of H 2 S = 9.6 × 10 –21 ; K sp of MnS = 2.5 × 10 –13 ; K sp of FeS = 6.4 × 10 –18 )
Text Solution
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For FeS and MnS to be precipitated from a solution of Fe 2+ and Mn 2+ , I P MnS > K sp(MnS)
[Mn 2+ ][S 2– ] > K sp of MnS
[10 –2 ][S 2– ] > 2.5 × 10 –13 ∴ [S 2– ] > 2.5 × 10 –11 M
So, at [S 2– ] = 2.5 × 10 –11 M or more, precipitation of FeS and MnS will occur.
H 2 S 
2H + + S 2–
∴ [H + ] 2 [S 2– ] = 
× [H 2 S] = 9.6 × 10 –22
∴ 
[2.5 × 10 –11 ] = 9.6 × 10 –22
= 3.84 × 10 –11 ∴ [H + ] max = 8 
× 10 –6 M
Thus, if [H + ] = 8 
× 10 –6 M or less, the precipitation of FeS and MnS will take place.
Therefore, pH (min.) = 6 – ½ log 
= 5.21
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