edited by GEORGE L. GILBERT Denison University Granville, Ohio 43023
pH-Activated Dlsplays SUBMITTEDBY
D. C. Sherrington and J. Carruthers Unlverslty 01 Stralhclyde 295 Cathedral Street
Glasgow G1 l X L , Scotland CHECKED BY
Edward Koubek Department 01 t h e Navy
United States Naval
Academr
In the course of our work on polymer-supported reagents and catalvstsl we have had occasion to olav " around with the idea of attaching dyes to anion exchange resins via ionic forces and in particular have looked at simple acid-base indicators. Many of these can be tightly and permanently hound such that the host resin changes color reversibly with pH with, as far as we can tell, an infinite lifetime. This arises because in the charged form (anionic) the indicator is water soluble hut becomes hound to a resin cation site. In the neutral form many of the well-known indicators have only low soluhilitv in water and are traditionallv. disoensed from. . tor exanlplc.,prupanol solutions. A.;a result the neutral form also becomes tightly hound to resitis in contact u,ith aqueous iolutions~presumably ria hydruphohic interactions. This permanent binding in both forms therefore allows continuous reversible color changes within the resin to take place as the pH of the surrounding solution is cycled, without anv leachine of the indicator. Ha\.ing disroverrd this phenomenon we sct ahout trying t c g exploit it in some form ot'tr~hnolugical application hut so f d r have manayrd to come up with only one. Furrhermore, this has moved ooerationallv inconvenient and is alreadv redundant in the fight of moiern LCD and other electron& developments. Nevertheless our "pH-activated displays" are chemically and visually interesting and make an attractive contribution to appropriate lecture demonstrations. A
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Preparation of Display Our demonstration display is in effect a flat column constructed from two Perspex sheets (75 X 12 X 0.3 cm) (see figure). A flat, oblong sandwich cavity is created inside (for resin) by bonding strips of Perspex (2 cm wide) around the nerioherv . . , of one sheet and the second sheet is then holted. tlangrlike, to [he base with a thincork gasket to furm n liquid seal. This armnrement ennhles the "column" to be exuosed along its entire iength. The cavity is packed with untreated resin, and then carefully the required display message (MERRY XMAS in our case!) is sculptured out from the resin. This negative is then carefully filled with treated resin, and finally the cover is replaced and the whole assembly is bolted together. The device is also provided with a liquid inlet and outlet tuhe suitahly bonded to the top and bottom of the base assembly. The display is mounted vertically and connected by a short rubber tuhe a t the top t o a chromatngraphy column (75 cm). This provides a conveniently large hydrostatic head to drive solution through the display. The outlet of the display also carries a small rubber tuhe with a pinch-clip tap.
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Preparatlon of Resln Any conventional strong anion exchange resin is suitable, but for good flow rates, and hence fastest display changes, the larger particle sizes (14-52 mesh) are best. Depending on the size of display to he created, an appropriate weight of resin must first be imoremated with the indicator of choice. . Taking phenolphthalein as an example, this is done simply by stirring the resin in excess sodium hydroxide solution (-1 M ) to which the indicator has been added. The intensity of color can be adjusted by varying the amount of indicator; in addition, particularly intense coloration can be achieved by flow in^ the indicator solution through - a chromatoara~hv column packed with resin.
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Hodge. P.; Sherrington, D. C.. Eds., "Polymer-supported Reactions in Organic Synthesis"; Wiley: Chichester, 1980. We are arateful to our checker E. Koubek for this name. Hutton, W. J. Chem. Educ. 1984,61,172. 1090
Journal of Chemical Education
The demonstration display.
