In the Classroom edited by
Tested Demonstrations
Ed Vitz
Fast Ionic Migration of Copper Chromate
Kutztown University Kutztown, PA 19530
submitted by:
Adolf Cortel IES El Cairat, Gorgonçana 1, 08292 Esparreguera, Barcelona, Spain;
[email protected] checked by:
Gordon F. Hambly Department of Chemistry, John Abbott College, Ste. Anne de Bellevue, QC H9X 3L9 Canada
According to the ionic model of bonding, a solid is an assembly of oppositely charged ions packed by electrostatic forces. A polar solvent can free the ions, which then migrate in an external electric field. Because of the importance of these concepts, many demonstrations of the existence of ions in a solution and ionic migration due to an electric field have been described. Among these, demonstrations of the migration of colored Cu2+ and CrO42᎑ ions are popular (1–4 ). In this demonstration some modifications have been introduced to allow a fast displacement of these ions. Copper(II) chromate is dissolved in concentrated ammonia, where Cu2+ forms the complex ion [Cu(NH3)4]2+. The deep blue color of this ion is an advantage because it is more visible than the pale blue Cu2+ ion. The medium is a strip of filter paper soaked in an electrolyte with a relatively high concentration of ammonia in order to avoid the dissociation of the copper complex; as the electrical conductivity of ammonia solutions is low, ammonium chloride is added to increase the conductivity. Owing to the low viscosity of this medium compared to the gels used in previously described demonstrations, the ions move very fast and they can complete a forward–backward trip within minutes. In addition, the demonstration is set up very quickly, since there is no need to wait for the cooling of the gel.
Figure 1. The yellow (left) and blue (right) bands of CrO42- and [Cu(NH3)4]2+ ions migrate over a strip of filter paper soaked in an electrolyte with a high content of ammonia. A voltage of 30 V is applied at the Petri dishes using two paper clips as electrodes.
Figure 2. Sequence of photographs of ionic migration taken at 1-minute intervals. When the bands of CrO 42᎑ and [Cu(NH3 )4] 2+ ions were about 1 cm apart the voltage was reversed and the ions moved backwards.
Materials Electrolyte: dissolution of 2 g of NH4Cl in a mixture of 50 mL of water and 5 mL of concentrated ammonia. Copper(II) chromate: can be prepared by mixing equal volumes of 1 M solutions of CuSO4 and K2CrO4. The rustycolored precipitate of CuCrO4 is filtered or centrifuged and rinsed with distilled water. Prior to the electrophoresis a small quantity of CuCrO4 is dissolved in a vial with several drops of concentrated ammonia, giving a dark green solution.1 Two Petri dishes Paper clips Glass capillary Strips of filter paper (about 2 cm wide and 15 cm long) Direct current supply and banana connectors with crocodile clips in one end. It is possible to do the demonstration using several voltages. A supply of 30 V ensures a compromise between safety and speed.2 Three 9-V dry cells in series (27 V) may be used instead of a dc supply. Procedure Two Petri dishes about 3 cm apart are filled with the electrolyte and the end of an opened paper clip is immersed in each dish.3 A line is drawn with a pencil across the middle
of a strip of filter paper; next, the strip is soaked with the electrolyte and placed on a piece of filter paper to remove the excess liquid. Using a glass capillary filled with the solution of copper chromate in concentrated ammonia, a line is drawn over the pencil line on the strip. The strip is placed as a bridge with its ends immersed in the Petri dishes. The power supply is connected to the clips using connectors fitted with crocodile clips and is then switched on.
JChemEd.chem.wisc.edu • Vol. 78 No. 2 February 2001 • Journal of Chemical Education
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In the Classroom
At the voltage suggested (30 V) the separation of the green line into two narrow bands, yellow and blue, moving toward the opposite electrodes, can be seen in one minute. Three minutes is enough to complete the separation of the colored bands. Meanwhile, the sense of movement of both ions can be explained according to their electric charge: the negative yellow CrO42᎑ ions move toward the positive electrode and the positive blue [Cu(NH3)4]2+ ions move toward the negative electrode (Fig. 1). It is very interesting to invert the connections to the supply.4 The ions move backwards and their movement is again explained by their electric charge. In a couple of minutes the green band corresponding to the mixture of both ions forms at the origin, but it decomposes again as the ions follow their way toward the electrodes (Fig. 2). Hazards Chromium(VI) compounds are suspected carcinogens. They are also irritants for the skin and eyes and, particularly, for the respiratory tract. Their disposal has been accurately described in previous demonstrations (1). Notes 1. Perhaps the most rewarding observation made with this demonstration is that there are solids that release oppositely charged particles when they dissolve, as explained by the nature of ionic bonding. So I find it very interesting, and an important part of the demonstration, to use solid copper(II) chromate and to show how it is dissolved in ammonia. However, if you prefer to carry out a quick demonstration, you can put small (and about equal) amounts of copper sulfate and potassium chromate in a vial and dissolve the mixture in a few drops of concentrated ammonia. 2. If dc supplies at about 30 V are not available or they are considered not to be safe enough, three 9-V dry cells may be connected
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in series to supply the low current needed for the electrophoresis. A true turbo-electrophoresis can be done using a higher voltage. With a 200-V dc supply the separation can be done in seconds! (the bands are about 5 mm apart in only 20 s). At this voltage there are some drawbacks. First, this is a dangerous voltage, which must be handled with a lot of care. Second, the electrical current heats the strip (the electrical power is V 2/ R) and some of the solvent evaporates. Evaporation can be avoided by using an electrolyte with higher resistance (e.g., 0.5 g of ammonium chloride dissolved in a mixture of 50 mL of water and 10 mL of concentrated ammonia). The lower concentration of NH4Cl increases the resistance and the higher amount of ammonia compensates for the losses due to evaporation. The loss of ammonia is responsible for the slow disappearance of the deep blue color of the copper complex if the migration is done at high voltage. If the same electrolyte used to do the migration at 30 V is used at 200 V, evaporation is very rapid, there are fumes of ammonia, and migration eventually stops because the strip dries out. 3. The clip connected to the positive electrode is oxidized, giving a greenish precipitate of iron(II) hydroxide. Since clips work well, there is no need to use more expensive electrodes. 4. Reversing the voltage is an essential part of the demonstration. Some students may have done experiments on paper or thin layer chromatography, and the reversal of movement will emphasize that in ionic migration the force that drags the bands arises from the electric field and not from the movement of the liquid.
Literature Cited 1. Shakhashiri, B. Z. Chemical Demonstrations; The University of Wisconsin Press: Madison, 1992; Vol. 4, pp 150–155. 2. Little, J. G. J. Chem. Educ. 1990, 67, 1063–1064. 3. Llorens-Molina, J. A. J. Chem. Educ. 1988, 64, 1090. 4. Stock, J. T.; DeThomas, A. V. J. Chem. Educ. 1966, 43, 436–437.
Journal of Chemical Education • Vol. 78 No. 2 February 2001 • JChemEd.chem.wisc.edu
