THE SOLUBILITY PRODUCT CONSTANT FOR COPPER IODATE: AN EXPERIMENT B. H. PETERSON Coe College, Cedar Rapids, Iowa
E A R L Y in a course in qualitative analysis the student becomes acquainted with the theoretical principles of chemical equilibrium. In the laboratory work for this course one works with the applications of chemical equilibrium via the solubility product, instability, ionization, and hydrolysis constants. Of these constants perhaps the solubility product is the most important, and an experimental determination of this constant is desirable. The solubility of an ionic compound is usually so large as to make the solubility product constant meaningless, or, so small as to require special techniques for its measurement. Copper iodate is a salt of moderate solubility, and a titrimetric analysis of an aliquot portion of a saturated solution of this salt provides the data necessary for the calculation of the solubility product. Advantage is taken of the reaction between copper iodate and iodide ions in acid solution with an accompanying titration of the liberated iodine with a thiosulfate solution. The ionic equations for these reactions are.
C,,(IO& C u + + + 210a21'
- -+ - + cu++
210,-
12Hf
CuI
+ 121--
+ 2S203--
21-
+ 131' + 6H2O
SO--
The copper iodate is prepared from reagent grade chemicals. Twenty-five grams of CuSO4.5H20 and 42 g. of KIO, are dissolved in 200 ml. of water, and these solutions are added slowly with constant stirring to 1 I. of water. The precipitate is filtered, washed with water, and air dried. The yield will he about 40 g. Since an excess of the K I 0 3 is difficult to remove by washing, it is recommended that the CuSO.,5Hz0be in slight excess. An approximately 0.1 N Na&08 solution is provided. The student standardizes this solution against a portion of a 0.1000 N KI03solution, also provided, using direc-
tions obtained from a quantitative analysis textbook. A starch indicator solution, reagent grade potassium iodide, and a 2 M acetic acid solution are also provided. Distilled water is used for all solutions. Approximately 1 g. of Cu(IO& is placed in 100 ml. of water, warmed for a short time, then allowed to cool slowly to room temperature. This solution is filtered, and 25.0-ml. portions of the filtrate are removed for analysis. To each portion is added 1 g. of KI. After several minutes 10 ml. of the acetic acid solution are added, and the solution is titrated with the thiosulfate solution to the point where most of the iodine color has disappeared. The starch indicator solution is then added, and the titration is continued until the blue color has disappeared.
If Z = concentration of C u + + in number of gram-ions per liter, then K., of CU(IO.)* = v m 2 ~ =) ma ~ Z
=
(normality of NazSpOl)(ml.of N~S.Os!(milliequivalent wt. of Cu! (4O)/atomic wt. of Cu
For a sample calculation:
Therefore,
K., of C U ( I O ~ !=~ 4[(0.1041)(13.2)(0.001)(40)/13Ia =
3.0 X
lo-'
This value agrees quite well with the value, 2.0 X 10-7, calculated from solubility data given by Seidell.' I SEIDELL, A,, "Solubilities of Inorganic and Metal Organic Compounds," 3rd ed., I). Van Nostrand Ca., Inc., New York, 1940, Val. 1, p. 491.
JOURNAL OF CHEMICAL EDUCATION
