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3.4D: The Unavoidable Loss of Recovery

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  • A loss of recovery should be expected when performing a crystallization. Although there are ways to maximize the return of crystals, a portion of the desired compound will always be lost. The reasons for this are both inherent to the design of the process and mechanical.

    As previously discussed, a portion of the compound of interest will remain dissolved in the mother liquor and be filtered away. Figure 3.24 shows suction filtration in order to recover benzil (a yellow solid) that had been crystallized from ethanol. In Figure 3.24c it is apparent that the filtrate is also yellow (the liquid that has passed through the filter paper), making it obvious that some benzil remained dissolved in the solvent. Further evidence that some compound is always lost to the mother liquor can be seen when solvent evaporates from drips on glassware or the benchtop, revealing residual solid (Figure 3.25a+b).

    Figure 3.24: Loss of yield due to the solubility of compound in the cold solvent: a) Suction filtration, b) Recovered yellow solid, c) Yellow filtrate, indicating some yellow compound remained dissolved in the mother liquor.

    The loss of solid material can also be witnessed with every manipulation of the solid. Only the majority of the crystals can be delicately scraped off the glassware, Buchner funnel, and filter paper, and there will always be a residue that is left behind (Figure 3.25c).

    Figure 3.25: a) Streak of solid (benzoic acid) from an evaporated drip on the outside of a flask, b) A magnificent crystal pattern from an evaporated drip on the benchtop, c) Residual solid (benzil) on a flask after collection by suction filtration.

    When there is such an obvious loss of yield from solid clinging to glassware, it may seem wise to use solvent to rinse additional solid out of the flasks. A few rinses with cold solvent are indeed recommended, but it is not recommended to use solvent excessively in an attempt to recover every granule of solid. The more solvent that is used, the more compound will dissolve in the cold mother liquor, decreasing the yield. The loss of material due to residue on the glassware is an unfortunate, but accepted aspect of this technique.

    To demonstrate the loss of yield with crystallization, several pure samples of acetanilide and benzil were crystallized. Using pure samples allowed for any loss of material to be only due to the solubility in the mother liquor and adhesion to the glassware, not by the exclusion of impurities, which also have mass. From many trials of crystallizing acetanilide from hot water using different scales (between \(0.5 \: \text{g}\)-\(1.5 \: \text{g}\) each time), the recoveries were surprisingly consistent, between \(60\)-\(65\%\) (Figure 3-26). From many trials of crystallizing benzil from hot ethanol using different scales (between \(0.5 \: \text{g}\)-\(4.5 \: \text{g}\) each time), the recoveries were also quite consistent, between \(87\)-\(92\%\) (benzil is the yellow solid in Figure 3.24). In all situations, the recovery of solid was never \(100\%\). Also, the typical recovery for the two systems was different, with the most logical explanation being that there was a larger percentage of compound lost to the mother liquor with acetanilide than with benzil.

    Figure 3.26: Crystallized acetanilide.

    It is common for new organic chemistry students to be disappointed by a low yield (anything less than \(95\%\)!), and to worry that it is somehow their fault. Students are often inclined to cite "user error" as a main cause for a loss of yield in any process. Although spilling solid on the benchtop or addition of too much solvent will of course compromise the yield, the modest recovery of acetanilide in this section should demonstrate that sometimes low yields are inherent to the process and the chosen solvent.

    Contributor

    Lisa Nichols (Butte Community College). Organic Chemistry Laboratory Techniques is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. Complete text is available online.

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