Fractional Precipitation
Definition and meaning of Fractional Precipitation in chemistry.
Fractional precipitation is a separation technique in which a reagent is added slowly to a solution containing two or more ions that can each form a precipitate with it, so that the compound whose solubility product is exceeded first precipitates essentially completely before the more soluble compounds begin to form.
In more detail
The method works because different salts of a common precipitating ion have different solubility products (Ksp); the compound whose Ksp is exceeded first, at the lowest reagent concentration, precipitates first. By adding the precipitant gradually and monitoring the concentration, chemists can precipitate and remove one ion nearly completely before the next one begins to precipitate, achieving separation. This underlies classical qualitative analysis schemes and industrial ore purification. Successful separation requires a sufficiently large difference between the effective solubilities of the competing precipitates; because Ksp expressions have different forms for compounds with different ion ratios (e.g., 1:1 versus 2:1), Ksp values cannot be compared directly across such compounds, molar solubility, not the raw Ksp number, determines which precipitates first.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Governing principle | Solubility product (Ksp) differences |
| Common reagent example | AgNO3 (silver nitrate) |
| Key requirement | Large difference in molar solubility between competing precipitates |
In a solution containing both Cl⁻ and CrO4²⁻ ions, slow addition of AgNO3 precipitates AgCl (Ksp ≈ 1.8×10⁻¹⁰) almost completely before Ag2CrO4 (Ksp ≈ 1.1×10⁻¹²) begins to form. Although Ag2CrO4 has the smaller Ksp, its 2:1 stoichiometry gives it a higher molar solubility (about 6.5×10⁻⁵ M) than AgCl (about 1.3×10⁻⁵ M), so chloride is removed first. This Ksp-based sequencing is the basis of the Mohr method for chloride titration, where the appearance of red-brown Ag2CrO4 signals the titration endpoint.
Frequently asked questions
Why must the precipitant be added slowly?
Slow, controlled addition keeps the reagent concentration low so only the ion whose solubility product is exceeded first precipitates, preventing co-precipitation of the more soluble compound.
Is fractional precipitation the same as fractional crystallization?
No. Fractional precipitation separates different ions using a common precipitating reagent based on differing solubility, while fractional crystallization separates compounds of similar composition based on differing solubility as temperature changes.