JOURNAL OF CHEMICAL EDUCATION
174
THE RELATIVE SOLUBILITIES OF SALTS OF CALCIUM, STRONTIUM, AND BARIUM GEORGE W. WATT University of Texas, Austin, Texas
MOST laboratory manuals used in courses in general ,.hemistry make either inadequate provision or no provision at all for laboratory work that is directly related to the classroom study of the periodic arrangement of the elements and related subject matter. The experimerit described herewith is one of several that, may be used to advantage to illustrate trends in the properties of compounds formed by elements in a given periodic grouu. . Essentially, " . the student determines the minimum volumes of solutions of ammonium sulfate, arnmonium oxalate, and potassium chromate required to form permanent precipitates when added to separate solutions containing equivalent concentrations of calcium, strontium, and barium ions. The resulting data are interpreted in terms of relative solubilities, and from these data the trends in solubility become clear.. directions for the Of the work are supplied to the student in the following form, together with suitable data sheets and pertinent questions to be answered by the student upon completion. Rinse a buret once with tap water, twice with 10-ml. portions of distilled water, and once with 10 ml. of 0.05 M ammonium sulfatesolution. Allow the buret to drain, then fill it with 0.05 M ammonium sulfate solution. Using a clean, dry graduated cylinder in each case, measure out as accurately as possible 10.0-ml. portions of 0.1 M solutions of calcium nitrate, strontium nitrate, and barium nitrate, and place these solutions into separate, clean, dry 100-ml. beakers. Stir the barium nitrate solution and sdd ammonium sulfate solution from the buret one drop at a time. Determine and record the volume of ammonium sulfate solution required for the fimt appearance of a. precipitate of barium sulfate. Treat the strontium nitrate solution similarly, and finally add ammonium sulfate solution to the edcium nitrate solution. If necessary in either case, discontinue addition of ammonium sulfate solution if a precipitate is not ohserved after addition of 40 ml. of the ammonium sulfate solution. Empty the buret, rinse it with tap water, distilled water, and finally with 10 ml. of 0.1 M ammonium oxalate solution. Allow the buret to drain, then place in it about 15 ml. of 0.1 M ammonium oxalate solution. Place 10.0-ml. portions of 0.1 M solutions of the nitrates of calcium, strontium, and barium in separate, clean, dry beakers. To the oalcium nitrate solution add (with stirring) ammonium oxalate solution from the buret one drop a t a time and determine and record the volume of
ammonium oxalate solution required for the first appearance of &precipitateof calcium oxalate. Treat thestrontium and barium nitrate solutions similsrly. and rinse the buret carry out a similar series of experiments using 0.1 M potassium chromate solution and 10.0-ml. portions of 0.05 M solutions of the nitrates of caloium, strontium, and barium. The beakers employed must he entirely clean if the first appearance of the precipitated ehromates is to he observed readily. If, in any case, a precipitate is not observed after addition of 40 ml. of potassium chromate solution, discontinue addition of the solution.
In use with large classes at the University of Texas, this experiment has led to generally satisfactory results. The average time requirement is about one and onehalf hours, which includes performance of each experiment in duplicate and the preparation of a very brief laboratory report. With but very few exceptions, the students reach the correct conclusions, i. e., that the solubilities of the sulfates and chromates decrease while that of the oxalates increases with increase in atomic number of the alkaline-earthmetal, In some cases, students have in difference between the solubilities of strontium an; calcium oxalates as well as the chromates. In the latter case use of a more concentrated solution of potassium chromate improves the distinction between strontium and calcium but makes it equally difficult to distinguish between barium and strontium. Unless instructed to the contrary, the student may reach erroneous conclusions in those cases in which precipitates fail to form because of dilution. Otherwise, the chief difficulties encountered in the use of the experiment are the all too common ones, such as use of d i y glassware, inadvertent dilution of reagents, and similar faulty techniques. There is also a marked unwillingness on the part of the student to accept the first appearance of turbidity as evidence of the separation of a solid phase. The writer wishes to thank Dr. Thomas E. Moore and Dr. Norman Hackerman for their assistance in connedion with the establishment of optimum conditions for use in the experiments described above and with the evaluation of the work of students.
