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tested demonstrations Thermochromic Behavior of Cobalt(ll) Halides in Nonaqueous Solvents and on Filter Paper William D. Bare and E. K. Mellon Florida State University Tallahassee, FL 32306-3006
GEORGEL.GILBER~ Denison University Granville, OH 43023
This color change is reversible, and the colors remain constant if evaporation is prevented. The phenomenon adequately demonstrates thermochromic equilibrium, although the "thermometer" with its color-change range of C is of questionable utility. As a class more than 10 O activity, one might challenge students to design systems with sharper wlor-change ranges.
The goal of the Reactivity Network Project is to collect Cobalt Watercolor Paintings That Appear and information about inorganic chemical reactivity in a form Disappear as the Humidity Changes useful for general chemistry teachers, textbook authors, Because of the much larger extinction coefficients (or and curriculum developers. This paper presents two phemolar absorptivities) of the blue complexes as compared to nomena of cobalt chemistry that are esthetically pleasing, the pink (1,2),the aqueous cohalt(I1) chloride system has relatively safe, and suitable for class experimentation and been used for disappearing ink (sympathetic ink) dcmondemonstration. strations (7-10 . Adeslgn is "paintedon paper, and hear is An inorganic system exhibits thermochromic behavior aoolird to disoel water and develoo the characteristic blue when the color changes over a temperature range (I). c~ior.It is pos'sihle to prepare a soiution that has almost no Perhaps the most familiar example of thermochromism is perceptible pink color but is still wncentrated enough to the hlue-violet-pink transformation in whalt(I1) chloride, give a pronounced blue color on filter paper when heated. which has been extensively discussed by Shakhashiri for Cobalt concentrations of0.1-0.01 Mare suitable, and 0.05 chemical demonstrations (2, 3). The pink color has been M wbalt(I1) works well. explained in terms of high-spin octahedral d complexes Multicolored, if transitory, waterwlok can be created on (e.g., C O ( H ~ O 1.) ~The ~ +blue color is usually, hut not invaripaper using varying proportions of halide counterably, associated with tetrahedral geometry (e.g., C O C ~ ~ ~ - ) filter , ions with cobalt(I1).While all the cobalt solutions are light and the violet color resultsfrom mixtures of intermediate pink in aqueous solution and when absorbed by filter paper complexes in solution (1,2, 4, 5). in moist air, striking colors appear with partial dehydration by a heat gun. Cobalt(I1) chloride turns deep blue; the Construction of Cobalt "Thermometers" bromide, contrasting pale blue. The nitrate remains pink. Aqueous solutions of cobalt(I1) chloride hexahydrate are The iodide under moist conditions yields first a light pink pink at room temperature. Solutions in methanol are redtint that yellows over time, especially under light. The violet; in ethanol and isopropanol, blue. "Liquid initial blue-green tint in the dehydrated state becomes thermometers" (61,wbalt(I1) halide systems that change more green over time due to the autoxidation (air-oxidacolor at specified temperatures, can be tuned by varying the tion) of iodide to ttiiodide, probably catalyzed by cobalt(I1) proportions of water and various alcohols to customize and visible light. The oxidation product, cobalt(I1)hydroxsolvent mixtures. This illustrates one of the more importide, is also unstable to autoxidation, yielding black ant commercial applications ofthermochmmic phenomena: Co(OH)O(aq)(11)over time. A more stable green tint can color indicators for temperature change. be produced by adding iron(II1) chloride to the stock coA simple example of the thermochromic behavior of alcobalt(I1) choride (8). hol-water mixtures is seen by adding 4 drops of 1.0 M A wide variety of vivid watercolor pigments is produced aqueous CoC12 to 2.0 mL of pure ethanol. Pink at very low by mixing the cobalt(I1)halides, or, more conveniently, by temperatures, the solution turns violet as the temperature adding varying amounts of NaBr or NaI or both to stock approaches 15 'C, and becomes blue above 25 'C. cohalt(I1) chloride or cobalt(I1) nitrate. The various solutions are then painted on filter or watercolor paper with a Class Activity hobby brush. Depending on the concentrations used, the Many such systems may be tuned for color or temperadesign is invisible or difficult to discern. The picture is ture by adding various quantities of methanol, ethanol, developed with heated air from a heat gun (Hejet (Fiseher) pmpanol, n-butanol or their mixtures to stock aqueous works better than a hair dryer), and the process is reversed coc12. by moist air or steam. The hydration-dehydration is reCaution:The alcohols aboue are highly flammable, and versible over many cycles, but the gradual degradation of methanol uapor is toxic. Avoid open flames and sparks in iodide occurs as mentioned above. the uicinity, and conduct operations in a fume hood to avoid Prepare 1.0 M stock solutions of cobalt(I1) nitrate, breathing the uapors. imn(II1) chloride, and various sodium halides. Using a The following equation describes the proeess simply. pipet (Beral) and a 96-well spot plate, prepare various mixtures and apply one-drop quantities to labeled pieces of filter paper. Dilute the mixtures with distilled water as 12+= [CD(E~OH)~C~~I + 6H20 2EtOH + 2C1-+ [CO(H~O)~ necessary. Prepare a color key by dehydration of the samViolet f Blue Pink %=s ples on paper with a heat gun or on a hot plate (low heat). Using the color key as a guide, prepare pigments and create art. Finished work using iodide can be protected Volume 68 Number 9 September 1991
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