Source: Laboratory of Dr. Ana J. García-Sáez — University of Tübingen

Most samples of interest are mixtures of many different components. Sample preparation, a key step in the analytical process, removes interferences that may affect the analysis. As such, developing separation techniques is an important endeavor not just in academia, but also in industry. 

One way to separate mixtures is to use their solubility properties. In this short paper, we will deal with aqueous solutions. The solubility of a compound of interest depends on (1) ionic strength of solution, (2) pH, and (3) temperature. By manipulating with these three factors, a condition in which the compound is insoluble can be used to remove the compound of interest from the rest of the sample.¹

1. Precipitation of CaCO₃

1. Prepare 5 mL of 1 M CaCl₂.
2. Prepare 5 mL 1 M Na₂CO₃.
3. In a small centrifuge tube (1.5 mL), add 750 µL of CaCl₂ and 750 µL of Na₂CO₃.
4. Wait 2 min for the reaction to occur. The solution should turn cloudy.
5. Centrifuge the mixture at 10,000 × g for 5 min.
6. Decant the supernatant.
7. Add 1 mL of cold water to the pellet.
8. Resuspend the

Solubility equilibria is employed in many purification processes.  Calcium can be removed from water using sodium carbonate. The solubility product (K_sp) of CaCO₃ is 4.8 × 10^-9. Mixing 1 M of CaCl₂ and 1 M of Na₂CO₃ produced CaCO₃ precipitate. The precipitate was separated from the rest of the solution using centrifugation.

Casein (a key protein in milk) has an isoelectric point at pH 4.6 and formed

Precipitation reactions are applied to many sample preparation processes. As mentioned before, they can be used to remove salts or specific ions depending on their solubility equilibria. They can also be used to remove proteins and other biomolecules from mixtures.

Recrystallization is often employed to further purify solids. This process removes trapped impurities within the solid. Among others, recrystallization can be used to purify salts and organic molecules.

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1. Kotz, J., Treichel, P., Townsend, J. Chemistry and Chemical Reactivity. 8^th ed. Brooks/Cole, Belmont, CA (2012).
2. Arakawa, T., Timasheff, S.N.  Mechanism of Protein Salting In and Salting Out by Divalent Cation Salts: Balance between Hydration and Salt Binding. Biochemistry. 23, 5912-5923 (1984).