Advanced Topic 483 of 1102 Kinetics & Equilibrium
Selective Precipitation
Ions are separated by adding reagent so that one salt precipitates first.
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In-depth explanation of Selective Precipitation
Selective precipitation separates ions in a mixture by adding a reagent that forms an insoluble salt with one ion before the others. The reagent is added slowly so that the least soluble salt precipitates first, and the ions that remain in solution can then be separated by a later addition. The separation is judged complete when the first ion has been reduced to a very low concentration before the second begins to precipitate. This method is widely used in qualitative analysis and in the purification of inorganic compounds. The difference in solubility product values between the salts determines how cleanly the separation can be achieved.
Going deeper: Selective Precipitation
Key ideas: Calculate the ion concentration needed to start each precipitation: [Ag+] = Ksp/[Cl-] for each anion. The salt needing the lower reagent concentration forms first. Check separation by computing how much of the first ion remains when the second starts to precipitate; if under 0.1 percent, separation is considered quantitative. Control of pH is often used for hydroxides and sulfides.
Common mistakes and connections: Students think the salt with the smaller Ksp always precipitates first, but this holds only for salts of the same stoichiometry and similar concentrations. Another mistake is forgetting that the concentration of the first anion changes as it precipitates. Coprecipitation and adsorption may spoil the separation in practice. Ties to the solubility product and to qualitative analysis schemes.
Visualization of Selective Precipitation
With 0.01 M Cl- and 0.01 M CrO4(2-), AgCl precipitates at 1.8e-8 M Ag+ long before Ag2CrO4 does, so the ions separate cleanly. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Chloride and chromate ions can be separated by adding silver nitrate, since silver chloride precipitates before silver chromate does.
Frequently asked questions about Selective Precipitation
What is Selective Precipitation?
Ions are separated by adding reagent so that one salt precipitates first.
Is Selective Precipitation a beginner, intermediate or advanced topic?
Selective Precipitation is a advanced topic in Kinetics & Equilibrium. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Selective Precipitation?
Study these earlier topics in Kinetics & Equilibrium first: Solubility Product, Common Ion Effect, Precipitation Prediction.
What comes after Selective Precipitation?
The next topics in Kinetics & Equilibrium are Complex Ion Equilibria and Effect of Complexation on Solubility.
Can I practise Selective Precipitation with quizzes?
Yes. The interactive lesson includes a self-check question on Selective Precipitation, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Selective Precipitation fits in Kinetics & Equilibrium
Selective Precipitation is topic 64 of 91 in Kinetics & Equilibrium.