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7.
5.
3 Step-by-step precipitation
When there are several ions in the solution that can form precipitates with the same precipitant, the order of precipitation is different due to the different solubility products of the formed precipitates
.
The sequence of precipitation formation follows the principle of Q Θ > Ksp Θ .
The ions can be separated by step precipitation
.
It is generally believed that when the concentration of the precipitated ion in the solution is lower than 10 -5 mol·dm -3 , the ion precipitation is complete
[Example 7-10] The mixed solution contains 2.
5×10 -2 mol·dm -3 Pb 2+ and 1.
0×10 -2 mol·dm-3 Fe 3+ , if the concentrated NaOH solution is added dropwise (ignoring the volume Change), can Pb 2+ and Fe 3+ be separated? If it can be separated, try to find the pH range of separation
.
It is known that Ksp Θ ,pb(OH) 2 =1.
43×10 -15 , and Ksp Θ ,Fe(OH) 3 =2.
79×10 -39
.
The solution first determines the sequence of precipitation when Pb 2+ and Fe 3+ are gradually added to the NaOH solution
.
When Pb 2+ begins to precipitate
When Fe 3+ begins to precipitate
Therefore, when NaOH solution is added, Fe 3+ precipitates first, and Pb 2+ precipitates later
.
When of Fe 3+ complete precipitation, [of Fe 3+ ] mol · DM is 10 -3 , then a solution of [OH - ] is
At this time, for Pb(OH) 2
.
Q Θ <Ksp Θ , Pb(OH) 2 , no Pb(OH) 2 precipitation will be generated
.
When i.
e.
of Fe 3+ precipitation was complete Pb 2 + yet precipitate, so of Fe 3+ and Pb 2+ can be separated with NaOH solution
When Fe 3+ precipitation is complete
[OH-]=6.
53×10 -12 , pH=2.
81
When Pb 2+ begins to precipitate
[OH-]=2.
The pH range for separating 2.
