In the Laboratory
Synthesis and Analysis of Copper Hydroxy Double Salts
W
Laura M. Brigandi, Phyllis A. Leber, and Claude H. Yoder* Department of Chemistry, Franklin and Marshall College, Lancaster, PA 17604; *
[email protected] Many naturally occurring, important materials are double salts; that is, compounds that can be thought of as a combination of two or more simple salts. Several articles have been published in this Journal involving copper double salts, including the preparation and analysis of basic copper carbonate (1), the preparation and analysis of copper and other double salts (2), the purification of basic copper sulfate (3), the reaction of malachite with acid (4), and the determination of percent copper by colorimetry (5). We report here on a project involving the synthesis of several naturally occurring copper double salts using simple aqueous conditions. The ions present in the compound are analyzed using colorimetric, gravimetric, and gas-analysis techniques appropriate for the first-year laboratory. From the percent composition, the empirical formula of each compound can be obtained. The mineral paratacamite, Cu4(OH)6Cl2, can be thought of as a combination of CuCl2 and Cu(OH)2 in a one-to-three ratio—CuCl2[Cu(OH)2]3. It can be prepared by addition of hydroxide to aqueous cupric chloride: 4Cu2+ + 8Cl− + 6OH− → Cu4(OH)6Cl2(s) + 6Cl− (1) The two simple salts can combine in a variety of different ratios, as shown in Table 1, which also shows that the percentage of copper in the different stoichiometries does not vary significantly. However, the chloride percentage varies considerably and can be used to determine the stoichiometry of the compound. Four copper hydroxy double salts, the minerals malachite, Cu2(OH)2CO3; brochantite, Cu4(OH)6SO4; paratacamite, Cu4(OH)6Cl2; and Cu4(OH)6Br2 can be prepared and
Table 1. Percent Composition for Different Stoichiometries of Copper Hydroxy Chloride Salts
analyzed by similar methods, allowing a choice of compounds. The presence of copper, as well as the particular anion in each compound, can be ascertained by qualitative tests. Copper can be identified by formation of the blue–violet ammonia complex, carbonate by reaction with acid (fizzing), halide by precipitation with silver ion, and sulfate by precipitation with barium ion. The presence of the hydroxide ion can be determined by IR spectroscopy if desired. Quantitative analysis of the compounds involves several different procedures. The percent of copper ion can be found by colorimetry, using the dark blue color of the copper ammonia complex. The percent chloride, bromide, and sulfate anions can be determined gravimetrically, and the percent of carbonate ion is found by the collection over water of CO2 gas, evolved on addition of acid. Typical student results for this project are given in Table 2 and reveal that the data for the percent copper and percent anion can be used to establish that, for example, the identity of the compound synthesized in the paratacamite preparation is CuCl2⭈3Cu(OH)2. It is unlikely that time constraints will allow each student to do all parts of this project within two laboratory periods (the total time for preparation and analysis of one compound is three 3–4 hour laboratory periods). The project is structured so that it can be broken down in several different ways. One option is to divide students into groups of three, with one student doing the synthesis of one compound, while a second student is responsible for the copper analysis (including preparation of standard solutions), while the third is responsible for analysis of the anion. Another option is to have each student prepare a compound and then divide the analytical work among students, who then share results. Clearly, many other permutations are possible. Hazards
Double Salt
Simple Salt Ratio
Cu5(OH)2Cl8
4CuCl2⭈1Cu(OH)2
50.01
44.64
Cu4(OH)2Cl6
3CuCl2⭈1Cu(OH)2
50.74
42.57
Cu3(OH)2Cl4
2CuCl2⭈1Cu(OH)2
52.02
38.70
Cu2(OH)2Cl2
1CuCl2⭈1Cu(OH)2
54.77
30.56
Appropriate eyewear should be worn at all times. Although copper compounds are generally not very toxic, they should be handled with care and all solutions and residues disposed of in the appropriate manner. Sodium hydroxide, ammonia, and nitric acid are corrosive and must be handled with great care, preferably in the hood.
Cu3(OH)4Cl2
1CuCl2⭈2Cu(OH)2
57.84
21.51
WSupplemental
Cu4(OH)6Cl2
1CuCl2⭈3Cu(OH)2
59.51
16.60
Cu5(OH)8Cl2
1CuCl2⭈4Cu(OH)2
60.56
13.51
Copper (%) Chloride (%)
Table 2. Theoretical and Experimental Data for Different Stoichiometries of Copper Hydroxy Anion Salts
Compound
Copper (%) Theor.
Exp.
Malachite
57.48 57.58
Brochantite
Anion (%) SD
Theor.
Exp.
SD
4.22
27.14
26.35
0.85
56.20 56.02
1.77
21.24
23.24
0.33
Paratacamite 59.51 59.49
1.11
16.60
17.12
0.43
Cu4(OH)6Br2 49.26 50.58
3.15
30.97
29.34
2.29
1662
Journal of Chemical Education
•
Material Instructions for the students and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Sheeran, D. J. Chem. Educ. 1998, 75, 453–455. 2. Snavely, F. A.; Yoder, C. H. J. Chem. Educ. 1971, 48, 621–622. 3. Rodriguez, E.; Vicente, M. A. J. Chem. Educ. 2002, 79, 486– 488. 4. Stone, C. H. J. Chem. Educ. 1944, 21, 350. 5. Yoder, C. H.; Smith, W. D.; Katolick, V. L.; Hess, K. R.; Thomsen, M. W.; Yoder, C. S.; Bullock, E. R. J. Chem. Educ. 1995, 72, 267–269.
Vol. 82 No. 11 November 2005
•
www.JCE.DivCHED.org
