Stephen Allan Butter Mobil Chemical Company Research and Develo~mentLaboratories P.O. Box 240 Edison, New Jersey 08817
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An Apparatus for Solubility Determinations
It is often necessary to determine the solubility of solids. in various solvents as a function of temperature. The procedure becomes complicated with solvents with low vapor pressures or with solutes sensitive to air. The following is a relatively simple . -procedure that overcomes these complications. The solubility of solids at various temperatures is readily obtained using a modified fritted filter tube. The data are useful for preparing solubility curves of solute conceutration versus temperature and for accurately finding the maximum solute a solvent can contain at a fixed temperature. The procedure involves agitation of excess solute with a known quantity of solvent until equilibrium is reached (see Figure 1). The condenser is then replaced by a stopper and tube and a small positive pressure is applied to filter the solution through the frit (Figure 2). The filtered solution is then evaporated and the dry solid weighed, or, the solution may be analyzed by other means. Altematively, the solid remaining undissolved is dried and weighed, and the solute concentration determined by difference from the amount of solid initially added. Figure 3 shows solubility curves attainable by this procedure for some aqueous salt solutions. It would be instructive to have the student demonstrate the negative solubility coefficients of salts such as lithium and manganese(I1)sulfates.
This procedure admirably lends itself to working with air-sensitive materials. The positive pressure filtration minimizes evaporation of the filtrate which is particularly useful for measuring solubilities in low-boiling solvents. Suitable filter tubes that may be modified are sold by most glass companies. Pressure filters such as that of Kontes Glass Company (K-953000) or Coming Glass Works (34020) are also useful. A medium porosity fritted disc is most often used. Dimensions of the tube in the diagrams are 225 cm X 2.5 cm X 8 mm tubing for the bent portion. Caution Pressure during filtration should not exceed 10 lb/sq in. a n d the apparatus should be behind a protective shield. A simple mercury pressure-relief bubbler may be inserted into the gas line to avoid pressure buildup during filtration (see Figure 2).
Notes Mixing is best achieved using a small flow of nitrogen or air bubbles, which also minimize clogging of the frit. Equilibrium has been reached when successive soluhility measurements are reproducible with increasing agitation time.
Figwe 2. Filtration apparatus
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TEMPERATURE, Figure 1. Equilibration apparatus.
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10 20 30 40 50 60 70 80 90 OC.
Figure 3. Temperature dependence of the water soiubihty of sodium sulfate, ammonium sulfate, and potassium dichromate.
