Researcher

Sozit Kurtu

Objective

To qualify near-melting point behavior with regards to solubility, specifically in the case of piperonal (m.p. 35-37C) in n-hexane from 40C to -8.5C. The additivity of volumes is an assumption and the densities of the solvent and solute are obtained from the ChemSpider prediction.

Procedure

  1. Make 0.16M, 0.076M, 0.032M, 0.016M, and 0.007M solutions of piperonal in n-hexane at room temperatue.
  2. Set water bath to 40C to equilibrate.
  3. Insert tubes in a way that facilitates periodic removal to examine samples. Make sure that all samples are completely submerged in the bath.
  4. Decrease temperature in increments of 5C, checking each sample after equilibration for precipitation.
  5. Sonicate periodically to prevent supersaturation.
  6. Continue until sample precipitates, or until the lower limit of water bath.

Results

Calculations

exp185.jpg

A scatter plot comparing temperature of crystallization with concentration of solution.

Discussion

  1. Pure n-hexane was used instead of mixtures of hexane isomers in a similar previous experiment.
  2. The solutions were made directly in one dram vials, rather than in a volumetric flask, as in Experiment 184. The masses in the calculation sheet are the actual measured concentration.
  3. A one-point solubility for piperonal in hexane was taken and found to be 0.15 at room temperature. This experiment appears to have allowed a higher concentration until ~15C.
  4. The samples appear to display a linear relationship between temperature and solubility. Extrapolating to 25C suggests a solubility of 0.341M, significantly higher than the previously recorded value.
  5. The upper limit concentration in this experiment, 0.16M, was difficult to dissolve at room temperature, so I would not anticipate a much greater concentration for observation closer to the mp. Despite the above extrapolation, I would anticipate an optimum concentration of ~0.2M.
  6. While the melting points of cyclohexanecarboxylic acid and piperonal are similar, their behavior in hexane is highly disparate. While cyclohexanecarboxylic acid was found to be soluble in all proportions in hexanes, piperonal shows a much more limited solubility. This may be due to the extra stability offered by the dioxane group.

Conclusion

The properties of hexane as a solvent near the solute's melting point can not be universally extrapolated from Experiment 184, where cyclohexanecarboxylic acid remained essentially miscible at ~45 degrees below its melting point. More investigation is required as to the nature of its miscibility with cyclohexanecarboxylic acid as opposed to piperonal.

Log

2010-10-13

12:25 - Made sample A by weighing solute and then solvent in 1-dram vial.
12:27 - Made samples B-E.
13:10 - Set bath to 40C. Actual = 20C.
13:20 - Bath temp 35C, inserted tubes.
13:25 - Set bath to 35C. Actual = 40C.
13:35 - Set bath to 25C. Actual = 31C.
13:50 - Set bath to 15C. Actual = 20C. C- samples cloudy @ 20C.
13:52 - A has crystalline residue at bottom @ 18C.
13:55 - Set bath to 10C. Actual = 15.5C. B & C beginning to form crystals @ 15C and 13C, respectively.
14:02 - Set bath to 5C. Actual = 10C.
14:03 - Sonicate for 20s; slight redissolution due to heat.
14:09 - Set bath to 0C. Actual = 6C.
14:16 - Set bath to -5C. Actual = 0.5C.