Operation of Dlsplay After preparation, the display message is erased in our case by eluting with 0.25 MHCI. This requires -200 mL and takes about 5 min. The display message is then developed by elution with -300 mL 0.1 M NaOH taking about the same time. By following the erasing solution with a water flush (-50 m i ) the subsequent de\.elopmrnr can be made more qu~ckly,and this is often convenient in demonstrations. ~lthou~ weh have not elaborated our system any further than this, it would he possible by the use of more than one indicator to generate more than one message a t different pH's. We have, for example, packed a large conventional glass chromatography column with hands of resin with different indicator impregnates to produce a "rainbow column" 2. This version of the demonstration requires no special apparatus and a wide variety of indicators might he used3. While our "pH activated displays"are unlikely to become competitive with modern electronic devices, they can he good fun!
Why Cupric Chloride Crystals Are Green SU~M~TTEO s~
Felix Sabba Vlrglnla Polytechnic Blacksburg, VA 24061 Cralg Zebrowski Denison University Granville. OH 43023 CHECKEDBY
Erwln Boschmann, Heidl Boschmann, and Falcon Ferguson IUPU-lndlanap~ll~ Indianapolis. IN 46202 Although copper(I1) sulfate hydrate, CuSOqSH20, crystals are hlue, the hydrated copper(I1) chloride, CuC12.2H20, crystals are green. This demonstration shows that the copper(I1) chloride is really made u p of two ions, blue Cu(OH2);+, which is also responsible for the color of copper sulfate, and yellow CuCIj-, the combination of colors producing green. This is shown by using the technique of ion flotation to float the CuC1:- ion. The only equipment required is some means of producing small bubbles in a eentle stream in a saturated solution of cupric chloride. A f&pnn,sity glass disc could he used as a sparger, o r a fine-porosity Hirsch funnel with thestem ot'the ~~
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funnel connected to a compressed air source. Care should he taken to have good control of the air stream as too high a pressure could hreak the porous disc. The Hirsch funnel should he filled about halfway with the saturated cupric chloride solution. The collector to be used is a long-chain aminium ion such as dodecyl aminium, CI2Hz5NH:. This is cationic and will combine with the anionic CuC$-. The simplest way to prepare the collector is t o dissolve the amine, which is commercially available, in ethanol and add a few drops of concentrated hydrochloric acid to the solution. The aminium chloride may precipitate out, but on warming i t will redissolve in the alcohol. Immediately, the bubbles will lift the yellow aminium salt of ~ u ~ 1 : to - the surface producing a golden yellow froth. Though easily visible from close by, this is not too easy to see from the hackof a lecture theater. However, if a few milliliters of isoctanol is added to the froth, the huhbles will hreak and the yellow product will dissolve in the octanol, giving an easily visible yellow layer on the surface with the blue cupric solution beneath. In contrast, if the c u ~ r i cchloride solution is diluted before the flotation. all that is produced is a white foam, illustrating the point that below a certain chloride ion concentration, the chloride ligands are replaced by water ligands. Similarly, a saturated solution of copper sulfate will produce only a white foam. It must he emphasized that the collector must he an aminium salt. The cluaternarv ammonium soaps, although cationic, will not effect the separation. Directions 1) Prepare a solution of CuCLzby adding 13 g of cupric chloride to 50 mL of H20and stirring until dissolved. 2) Place this solution in a 250-mL graduated cylinder in which a bubbler has been inserted or into a Hirsch funnel. 3) Prepare a solution containing:
5.0 mL ethanol 1.0 g dodecylammine 3 drops conc. HC1 4) Bubble air through the cupric chloride solution. 5) Add about 10 drops of the aminium chloride in ethanol to the bubbling cupric chloride and mix well. 6 ) Turn off the air and add 10 mL of isoetanol: mix well. 7) The yellow precipitate will dissolve in the isoetanol above the lower blue layer. A similar demonstration can show that the mauve color of cobalt chloride is compounded of the pink Co(OH&+ and the hlue CoC1:-. However, this does not show uo as well in a large classroom because the color contrast is not as good as with the cupric chloride, but it is excellent for close viewing.
Volume 63 Number 12 December 1986
1091
