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The Precipitation of Ferrous Hydroxide: A Lecture Demonstration Submitted by: 0. W. L a u Chinese University of Hong Kong Shatin, N.T., Hong Koug Checked by: Erwin Boschmann IUPUat Indianapolis Indianapolis, I N Dissolve ahout 0.2 g uf ft!rroui nmmonium sulfate in H test tuhe in ahout 5 ml of distilled water and then add four drum of 2 M sodium hydroxide solution. An instantaneous prec&- itation of the white to verv pale green ferrous hydroxide oc- curs. After one minute or; a riddish-brown precipitate of ferric hydroxide (best described as hydrous ferric oxide) ap- pears on the sides of the test tube and near the surface of the solution. However, the bulk of the precipitate sinks to the bottom of the test tuhe and the color gets darker green and some turns blue. All the precipitate turns greenish-blue on shaking the test tuhe. The precipitate settles to the bottom again in about 15 min and it now appears dark hlue. This persists for a few weeks. Ferrous ammonium sulfate should he wrd instead of ferrous sulhte because the former is more stal~le and lrss suswptible tu nir oxirlarion. Remarks Ferrous hydroxide, like other transition metal hydroxides, is insoluble in water. The insolubility of these hydroxides arise primarily from the very unfavorable entropy terms since the entropiesof solution of polyvalent ions are quite negative and the entropy of solution of hydroxide itself is also negative (1). That ferrous hydroxide will be oxidized by air can be pre- dicted from the electrode potential of the Fe(III)iFe(II) couple in basic medium (2). The experiment also shows that the rate of uxidation of the ft.rn,ui ion in l~asic medium is (:onsirlerahle. In fart. irun (111 hvdroxide isu,hite, hut in theoresenccofnir . . it quickly darkens owing to oxidation (3). The awearance of the hlue precipitate is a hit unexpected to the students and this compbund-is, in fact, an example of a class of inorganic compounds which contain ions of the same element in two different formal oxidation states (4). The most obvious and striking feature of many of these so-called "mixed valence" compounds is the presence of intense absorption in the visible region of the spectrum, not present in compounds containing either valence state alone. Thus, whereas the colors of almost all Fe(II1) and Fe(I1) compounds are colorless, pale orange, or pale green, the color of almost all mixed valence . . systems simultaneouily containing FetIIl) and FetIl) ia deep hlue tu Mark, of which I'russian hlue is anuther well-known example. The experiment described is indeed a verv simple one: however, it can serve as a lecture demonstration for suih topi& as solubility of ionic compounds, electrode potentials of transition elements and their modification by formation of GEORGE L. GILBERT Denison Unlversly Granville, Ohlo 43023 either an insoluble compound or a complex ion, and mixed valence compounds. Literature Cited (1) Phillips, C. S. G.,and WiUlams,R. J. P., "lnarganic Chemistry,"OIford University Plea, Oxford. 1965. Yol. 1, p. 259. (21 Day, Jr. M.C.,andSelbin, J.,"TheanticalhorganieChemistry,'.2nd &.,Van Nostrand, Reinhold Co. New York, 1969.p. 358. (31 Catton,F.A.,a"d Wi1kin~on.C.. "Advanced lnorganieChemiJtry?3rd ed.,Interreience . Publishers, New York, 1972. p. 860. 141 EmelLur. H. J., and Shave. A. G., (Editoral, "Advances in Inorganic Chemistry and Kadiochemistry." Academic Pros Inc, New York, 1967, Vol. 10, p. 253. The Variation of Vapor Pressure with Temperature Submitted by: Derek A. Davenport and Viswanathan Srinivasan Purdue University West Lafayette, I N 47907 Checked by: Madeline P. Goodstem Central Connect~cut State College New Britain, CT Demonstration Provide three cans of butane lighter fluid whose nozzles have been adapted to fit inside each of three identical bicycle tire gauges.' Place one can in ice, another in a beaker of water maintained at about 50°C, the third being left at room tem- perature. When temperature equilibrium has been achieved (-5 min with occasional shaking) take the pressure of each can. Lay the three tire gauges on a temperature-calibrated base-line drawn on an overhead projector. A shadowgraph (in a double sense) of the non-linear increase of vapor pressure with temperature will appear on the screen. A single butane can and a single tire gauge can also be used with the readings heing marked one by one in the overhead projection, hut this procedure is more time consuming and less dramatic. Remarks Since warehouses frequently reach close to 50°C there would seem to he no danger in heating butane cylinders to this temperature. Only at much higher temperatures do the lapped seams peel apart.' As a matter of principle a safety shield should be used. Davenport, Derek A., J. CHEM. EDUC., 53,306 (1976). Tested Demonstrations is a monthly feature designed to present lecture demonstrations and experiments in a format convenient for elassroom use. Readers interested in either submitting or checking demonstrations should contact the column editor. An outline of format requirements was given on page 166 of the March 1976 issue of th~s Journal. 474 1 Journal of Chemical Education
