Densities of covalent solids can often be determined by the method of flotation. In this method, a series of liquids covering a range of densities is assembled. It is important that the solid to be investigated be insoluble in all of the liquids chosen. For amino acids, the following selection of solvents is reasonable:
| Solvent | Name | Density, g/mL |
|---|---|---|
| C2H4Cl2 | dichloroethane | 1.2351 |
| CH2Cl2 | dichloromethane | 1.3266 |
| CHCl3 | chloroform | 1.4832 |
| CCl4 | carbon tetrachloride | 1.5940 |
| CH2BrCl | bromochloromethane | 1.9344 |
| CH2Br2 | dibromomethane | 2.4970 |
| CHBr3 | bromoform | 2.8899 |
| CBr4 | carbon tetrabromide | 2.9609 |
General Flotation Procedure A small amount of each solvent (1 mL or so) is placed in a 1-dram vial, and a few crystals of the solid of interest are added to each vial. The solid will float on those solvents that exceed it in density, and will sink in those solvents that have densities lower than that of the solid. For the sake of discussion, suppose that the solid of interest sinks in dichloromethane, but floats on chloroform. The conclusion is that the density of the solid lies somewhere in the range between 1.3266 and 1.4832 g/mL. Several solvent mixtures of intermediate densities can then be prepared by mixing varying volumes of the bracketing solvents. Again the solid will be found to float on one mixture and sink in the adjacent mixture. Its density will then be known to lie within an even smaller range. This process can be continued to measure the density with any desired degree of accuracy.
Solvent Mixtures Because the solvents in the table have similar molecular structures, their volumes will be additive on mixing. Thus if 0.5 mL of dichloromethane is mixed with 1 mL of chloroform, the density of the mixture will be given by total mass/total volume = (0.5*1.3266 + 1.0*1.4832)/(1.0 + 0.5) = 1.4310 g/mL.
Detailed Flotation Procedure. Put a very small amount of your amino acid in each of 3 1-dram vials. To the vials, add exactly 0.5 mL of, respectively, CCl4, CH2Cl2, and C2H4Cl2, and quickly cap the vials to prevent evaporation. Observe which pair of solvents brackets the density of your amino acid. For the sake of discussion, assume that your amino acid floats in carbon tet and sinks in dichloromethane. Add a small amount of amino acid to a fourth vial, and add exactly 0.5 mL CHCl3. Again observe which solvent pair brackets your amino acid. Suppose that your amino acid floats in carbon tet and sinks in chloroform. Then at this point you know that your amino acid has a density in the range between 1.483 and 1.594 g/mL.
To the vial containing your amino acid and 0.5 mL of chloroform, add a 0.1-mL increment of carbon tetrachloride, cap the vial, and carefully swirl the vial to mix the solvents. BE SURE THE SOLVENTS ARE UNIFORMLY MIXED (if they are not, you will see density whorls and currents in the liquid). Observe whether your amino acid remains on the bottom or tends to float up. Suppose the solid remains on the bottom. Add a second 0.1-mL increment of carbon tetrachloride, cap the vial, and swirl to mix. Again observe. Continue adding 0.1 mL increments of carbon tetrachloride until the solid begins to float up. Suppose it begins to float up after addition of 0.7 mL of carbon tetrachloride. Then the density of your amino acid lies between the densities of the following solvent mixtures:
Calculate the densities of these two mixtures to obtain the density range for your amino acid to good precision.