8
SOLUBILITY OF SULFUR: AN UNDERGRADUATE RESEARCH PROJECT MAXEY BROOKE Sweeny, Texas
THEdirector of undergraduate research often finds himself on the t ~ i horns n of a dilemma. On one hand, he must assign projects whose theory and technique are not too advanced for the undergraduate student. On the other hand, to be truly research, the projects should yield data that are new and add something to the body of total scientific knowledge. A third difficulty, too frequently, is t,he lack of funds assigned for undergraduate research. The determination of the solubility of sulfur in organic solvents meets all these problems. The chemistry student becomes acquainted with the theory of solubility early in his career. The project makes use of only the most elementary laboratory equipment. Not. a great deal of work has been done on the subject: Mellor' lists only 20 odd organic solvents in which the solubility of sulfur has been determined. And last but quite important, thereis little expense involved. Natural brimstone is frequently of sufficient purity for the determinations, but purified sulfur is available at all chemical supply sources and is probably the cheapest reagent listed. Pure solvents are also available at a nominal cost or can be obtained in experimental quantities, without cost, directly from the manufacturers. For low-boiling-point solvents (below 100°C.), the following technique has been developed. A quantity of solvent sufficient for the tests, say 200 ml., along with more than enough lump sulfur to saturate it (25 to 50 g.) is placed in a flask fitted with a reflux condenser. The flask is put in a steam or water bath and allowed to reflux for some time. Usually overnight is sufficient. The flask is then placed in a constant-temperature bath at a temperature somewhat below the boiling point of the solvent for 5 to 6 hours. A precipitation of sulfur crystals indicates that the solntion is saturated at the bath temperature. Since the precipitated sulfur tends to adhere to the sides and bottom of the flask, it is possible to decant small portions, approximately 10 ml., into chilled, tared, weighing bottles. The bottles are quickly stoppered and the solution weighed. The bottles are then opened and placed on the steam or water hath until the solvent is evaporated. The remaining sulfur is then weighed and the solvent determined by difference. Meanwhile, the constant temperature hath is adjusted to a lower temperature, the solution is allowed to come to equilibrium, and the process is repeated until the solubilities of enough temperat.ures to establish a curve are obtained. 1 MELLOH, J. W., ".4 Comprehensive Treatise on Inorganic and Thci~rrtical chemist^^" Longmans, Green & Co., 1935, Val. X.
The results can be expressed in a number of ways: mg. sulfur per g. solvent, g. sulfur per 100 ml. solvent. (Mellor), or mol per cent Ss. Liquids in the medium boiling range (100-200°C.) are unsuited to this type of project since their evaporation involves the use of expensive and specialized vacuum equipment. For high boiling liquids (b. p. above 200°C.) a quite simple technique has been devised. A suitable quantity of solvent (10 g.) and sulfur (10 mg.) is weighed into a L- X 8-in. test tube. A thermometer and stirring device are inserted into the test tube which is placed in a glycerin or wax bath. The bath material should be transparent, or nearly so, with a good light source behind it. The hath is slowly heated and the mixture stirred until the sulfur has gone into solution. .4t this point the two-phase system becomes a clear homogeneous solution. The heat is removed from the hath which is allowed to cool at a rate not exceeding 4' per minute. Stirring is continued and the solution observed as the temperature drops until suddenly it becomes cloudy. The temperature at this point is recorded. The change is quite sharp and can be observed and reproduced within 0.2'. This is known as the critical solution temperature, and the ratio of sulfur to solvent can be regarded as the solubility of sulfur at this temperature. The experiment can he repeated with other sulfur-solvent ratios until enough points are available to plot a curve. The system sulfur-dibutyl phtha1at.e has been investigated by the a u t h ~ r . ~ An example of the utility of this project is a series of experiments to demonstrate the influence of chemical structure on solvent power. Etarda determined the solubility of sulfur in normal hexane. Hildebrand and Jenks4 determined the solubility in benzene. Since the structure of cyclohexane is midway between that of normal hexane and benzene, it would be reasonable to assume that the solubility of sulfur in cyclohexane was midway between its solubility in n-hexane and hensene. The sulfur used in the experiment was a sample of very pure natural brimstone having the analysis: Sulfur.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99.983% Bituminous mattcr.. . . . . . . . . . . . . . . . . . . . . . . . . 0.015 Ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.002
The cyclohexane was Eastman technical grade. BROOKE, J. M., J . Am. C h e n ~Soe., . 72, 5748 (1950). ETARD, A,, Ann. Chzm. I'hgs., 171, 2, 571 (18911. "HILDEBR~ND, J. H., AND C. A. JENKS, J . Am. Chsm. Soc., 43, 2172 (1921). a
AUGUST. 1951
435
The experiments were performed as described in the section on low boiling-liquids. The results: . Temperature, 'C. 11.1 22.2 26.1 44.2 ?7.6 10.6
Solubility, grams slcljur/100 g. solution
.
0.716 1.018 1.090 2.016 2.938 4.385
The results, when compared with the data of Etard and Hildebrand and Jenks. indicated that the solubility of sulfur in C ~ C I O falls ~ ~ approximately X ~ ~ midway between the ~olnbilityin whexane and benzene as was predicted. I t would be of interest to carry this study to its conclusion, investigating the eflect of chain branching, unsaturation, and substituents.
