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GEORGEL. GILBERT Denison University Granville. Ohio 43023
preparation^.^ Fill the rest of the way with 0.1 M NaOH, except for no. 3 where 0.1 M NH8 is needed. Use the same concentration of indicator as in the lower level. The six bottom layers are:
Density Gradient Columns for Chemical Displays SUBMITTED BY Wllllam B. Guenther University of the South
1) Acetic acid, hromeresol green. (Indicator range pH 3.8-5.4)
Sewanee, TN 37375
2) Acetic acid.. thvmol . blue (not hromthvmal blue). (8.0-9.6)
CHECKED BY
3) 4) 5) 6)
Falcon Ferguson and Erwin Boschmann IUPU at Indianapolis, IN Long-lasting, colorful displays of acid-base, metal ionligand, and redox reactions and relative density are described. A cylinder is filled with successive portions of reactants of decreasing density which is adjusted by use of a n inert solute. Chemical reactions between the reactants exhibit a range 0 1 completton 1n)m 0 to 100% (or equilibrium) in the partially mixed recions between the lasers. The density gradient determines the reagent mixing gradient. (See the explanatory note a t the end of this paper.) Acid-base neutralizations, made visible by suitable indicators, show the full range of a titration curve: acid alone, a range of huffers, conjugate base and excess hase if desired. The rate of change of color (and pH) with distance can be correlated with the slopes of titration curves for the various examples. Redox is shown by the only previously published example we have seen, the beautiful vanadium oxidation state column.' The lifetime of these metastable displays is surprising, some lasting for months in display cabinets. Thus, an important advantage of these demonstrations of complex chemistry is that students can observe them over aperiod of time as they grasp concepts of solution equilibria. T h e phenomenon will he clarified by the descriptions given below. The liquid density demonstration described last makes a simple introduction to the physical principles involved in these displays.
The upper solution can he added through a long-stemmed funnel with the tip drann out and hent to a right nnylc. Crrat car? is nor needed since considrrablr miring isdrsiroble. I'ouring rlwly duun the inclinded cylinder la noss~bleu,ith n steady hand. The cdur pattern forms in a few seconds. Insert a stirring rod and mix very ..eentlv, to disturb the reeion of maim contrast. The same color oattern, somewhat hroadrnrd in butirrrd cosrs, returns every time, shuu,ingt h u l the piwnonwnon is not a n artiinct of mixing nrtion. Of rudrse, the whole cylinder must not be mixed to homogeneity.
Simple Monoprotic Cases Displays involving the simple monoprotic eases illustrate buffer regions and the importance of the choice of a proper indicator. Six 100-ml cylinders aresuitable for closeviewing, while 1- or 2-1vessels may be nerdrd in inrpr rooms. large vessrls orr rarirr to prepar? nnd last longer. Fill the sir nhout two-thirdcwith 0.1 A l solutions o i the acids listed hrlw thnt hare hwn mnde -0.5 Af in NnCI (loq $8 the volume of saturated NaCl is convenient). Add sufficient indicator to give strong color, 0.5 to 1.5%hy volume of the usual indicator
Key to Density Gradient Columns Displays Shown on Cover Len to right, top row first 1) and 2)
Acetic acid-acetates: 1). has brom cresol green. 2) has thymol blue as indicator. 3) and 4) The universal indicator is used far: 3) the H3POI system: 4) the citrate system. 5) Fe(ll1)-CI-.SCN-, EDTA. 6) Co(i1)-NHs. H20, CaCId27inacetone). 7) Ni(i1)-en,en,, enr (buffered ethylenediamine). 8) Density levels for 3 immiscible liquids. Photography by E.P. Kirven.
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Journal of Chemical Education
HCI, bromcresol green. NH&l, thymol blue. HCI, bromcresol green. HCI, thymol hlue.
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0bs~ruarions:'l'he first two cylinders contain layers varying from 0.1 M arrtic, throurh huffers to sodium acetate, and then to excess N ~ O H . he bromcresol green, in no. 1, changes gradually through yellow-green-blue in the buffer range of the acetate system, while the thymol hlue in no. 2 is yellow a t these p H values, changing abruptly to hlue as the titration curve rises almost vertically from p H 7-10. Tubes 3 and 4showjust about the opposite effects since the ammonia svstem is buffered in the thvmol hlue ranee. The titration curves wtth the indialtor ruuyes marked can he posted with the cylinders to drive home the exnlanntions. T h r choice of indicator for finding the equival& paint can then be seen as the one u,hich changes around the pH of the pure conjw gate base (nos. 1,21 or acid (nos;l,4). Cases 5 and 6 show that the DH rise for the strunc arirl and hase case ii wide enough to cover the ranges of both indicators. Comments: A universal indicator can he used (see the following section). NaCl can be put in the NaOH and the layers inverted: there is no reason that base must be on top. A ouzzle case for students to decioher for homework can he constructed by mixing the three k i d s of different strength on the bottom (0.1 M in each of HC1, acetic, and ammonium ion acids). Use NaOH on top and universal indicator in both lavers. Ask students to deduce what is present in each colored layer. (Strongest acid is neutralized first, etc.) Write equations for the reactions and giveK,values to explain why the colors are observed in the order seen. Try acetic acid versus ammonia using bromthymol blue (pH 6-7.6) or universal indicator: a case with no sharp end point, and two buffer regions.
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Alyea. H. N. "Tested Demonstrations"; Journal of Chemical Education: Easton, Pa, 1960: p 45. Haight, G. P. Jr. J. Chem. Educ. 1952, 29, 296. Weast. R. C., Ed. "Handbook of Chemistry and Physics"; Chemical Rubber Co.: Cleveland. OH: Sect D.
Polyprotic Acidgases Use the universal indicator below and construct cylinders for citric and phosphoric acids using NaCl and NaOH as above. Citric acid (pK, values 3, 4.4, and 5.7 at 0.1 M ionic strength) shows gradual changes until the final jump above pH 7. Phosphoric (pK.'s 2,6.8,11.7) has a broad deep green band around pH I , for the second buffer reeion. - . chaneiue " .. sharolv . . hoth helow lvellowl and above tblurr, which corresponds nicely with the jump* in the titrnriun rurvr. Thr acids' rrisudium saltr can be used for the top layer. The more fundamental diagrams ro use for the correlations are alpha (species fraction), and i (proton number) c u ~ v e s . ~ If acid and base are of different concentrations, the colors may move slowly up or down the column over days. Generally these columns last for weeks. Ethylmediamine versus HCI, and glutamic acid versus NaOH are examples of other types of polyprotie systems that can be used.
Prepare \'(Ill from 0.10 M NH4\'O3with amalgamated %n in 1 M H2S0, and fill the cylinder two-thirds wrth this. Placr unreduced \.alladate (V, in01 M H?SO,on tovalc~ne.aithnl1rtlr0.01M KBr& to avoid the permanganate~colorthat produced by the method given in the references in footnote 1. From the hattom, colors develop with gentle stirring: V(I1) lavender; V(II1) complexes, greens; V(IV), light blue; polyvandates (V), yellow. A little 1%H202on top will add reddish peramvanadate complexes if desired. A column of the first four species has been stable for several weeks on repeated occasions. The 1 Mvanadiuml may suit small tubes; however, 0.1 M is quite dark enough for large cylinders. Chromium In the bottom, place a mixture 0.02 M in Cr(III), 0.2 M in NaC1, andO.O1 M in HCI, in the top, one that is 0.01Min EDTA and 0.01M before in NaHCO.. Place a band of water over the bottom laver ,~~ adding rhetop. Very pale (:rtllI~., diffuses (and can br mixed) rhrough the water to meet the EDTA. Birnrbonate cataiy7er the formnrion ofdrrp purpleCrtEDTA)-'.alierr~veralhours, illustrating the slow reaction of the inert complexes ~~~~
Universal Indicator Sodium salts: 10 mg of methyl orange, 20 mg of bromcresol green, 10 mg of bromthymol hlue, and 30 mg of thymol hlue; dissolve all together in 5 ml ethanol and then add 5 ml water. Prepare a set of buffers for display at unit pH values from 1 to The colors are different in each, roughly according to the following. pH re:-ora:ge
1
yelFow-ieen
1
8 9 10 blue-green blue
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Throughout, concentrations can b e varied t o suit larger or smaller containers. Students can make their own in test tubes for ~ r e l i m i n a r vand c o m ~ a r i s o ne x ~ e r i m e n t swith a other &stems should be qualitative analysis scheme. amenable t o this technique. T o prevent metal hydroxide precipitation, buffering is needed in some cases, a s with t h e ammonia and ethvlenediamines above. Workable concentrations must be found. It is a chemical challenge t o devise a set of complexes of successivelv i n c r e a s i n ~formation constants a n d varied colors ro iliustrate the predominating equilibrium principlr. .Metallochrumic indicators might he useful in illu~tratinethe effccts ot nH and comoetinu complexing in EDTA systems.
any
I n explaining the colors of the universal indicator, note t h a t thymol blue has two color-change ranges and contributes t o t h e red below p H 2. Phenolphthalein, recommended in some mixtures t o make a clearer change in t h e p H 9-10 region, is, unfortunately, unstable t o cleavage in base so t h a t t h e color fades, and makes it unsuitable for these column displays.
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Metal Ion Equllibrla
Iron Bottom one-third of column: use a solution 0.1 M in FeC13,0.1 M in HCI, and 1 Min NaCI. In the middle third, place a solution 0.01 M in KSCN and 0.2 Min NaCl, and on the top, 0.01 MEDTA. Observe: yellow Fe(II1)-Cl-, red Fe(II1)-SCN-, yellow FeEDTA-. A diffuse transition in the first change occurs because the formation constants of the chloride and thiocyanate complexes are not very different, about 4 and 100.The sharp transition to the EDTA complex reflects the far greater Kr, about loz5,for this complex. Coban In the bottom half place a mixture 1Min hoth NHICL, and NH1 in the top, a mixture 0.1 M in CoCI2,0.7 M i n HCI, 1 M in NH4C1,and 50% in acetone. Observe hlue at the top from C O C I ~ (add ~ - some acetone to the too if it is not auita hlue). ...oink CoZ+.. "- at the center where chloride isbiluted, and &mine complexes below. Some may be dark Co(II1)species from dissolved 02. HZOZmay he added to the bottom layer if the Co(1II) is desired. Nickel This procedure was adapted from Ramette.'In the bottom place a mixture 0.1 M in NiCL and 0.2 M in HC1 and NaCI: in the too. a mixture 0.6 M in ethvlebediamine (en? and 0.2- M in HCI.~,which will produce a 0.4 Men s k r i o n that js'iJ.2 M i n the dihydrochluride fur buffering. Observe green Ni(ll).,, blue N i b ) " and Ni!en)21*, and red-purple Niten)?'. The lug Kr values are 7.5.6.3, and d.6. ~~~~~
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%Guenther,W. B. "Chemical Equilibrium"; Plenum: New York, 1975; Chaps 4-9 Ramene, R. W. "Chemical Equilibrium and Analysis"; AddlsonWesley: Reading, MA. 1981; p 456. We obtain the green band in columns prepared with H2S04,In agreement with references in footnote 1. When HCIO, was used, a red-brown band appeared between the bottom lavender and the blue V(IV). This soon changed to green. Recent workers have said that V(III),, is blue: see Cotton, F. A,; Wilkinson, G. "Advanced Inorganic Chemistry". 4th ad.; Wiley: New York. 1980; p 717. Brown may be VOV4+.Green may be VO+ or V(OH)2+. Fernelius, W. C.; Blanch, J. E.; Bryant. B. E. Inorg. Synthesis 1957, 5. 130. 188.
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Llquld Density
Water-immiscible liquids placed in a density-gradient column will sink t o t h e point of equal density and collect in spheres to minimize surface area, achieving lowest surface free energy. Aniline has been used, b u t it oxidizes and disintegrates. We have found three liquids t h a t last longer and can, if desired, all be placed in t h e same column, each sinking t o a different level. T h e liquids are: chlorobenzene, density 1.10 glml; ortho-chlorotoluene, d, 1.08 glml; and dibutyl phthalate, d 1.04 glml. T h e column is one of salt water ranging in density from 1.2 t o 1.0 glml. Make three NaCl solutions: A, saturated in water, density 1.2 glml. B, 13%NaCl made by diluting 45 ml of saturated to 100 ml, d , 1.09g/ml. C, 8% NaCl made by diluting 27 ml of thesaturatedto 100 ml, d , 1.06 glml. Fill the glass display cylinder slowly with a fine tip funnel one-fourth the way with each of the three salt solutions and top with water. Add a desired volume (about 3%of the total column volume) of each organic liquid starting with the most dense. Use a dropper with its tip below the top surface to prevent sticking at the air interface. The drops will soon coalesce to large spheres. Prior to use, the organic liquids may he colored with water-insoluble compounds. We have used the tris(acetony1acetonate) complexes of Cr(III), red, and of Ca(III), greea6 These are long-lasting, uncharged complexes. PAN (I-(2-pyridylam-!-naphthol), a metallochromic indicator, gave a useful yellow color stable in o-chlorotoluene. The salt column can he prepared ahead of class. The actual addition of the organic liquids is quite surprising to watch, each one stoooine .. .. at a successivelv. hieher .. level. This is onlv visible at close range. or 11). a verrirnl projrctiun device. For best stability. the salt solutions shmld he made ahead, hrared, and cw,lcd to rid them of r x r w diriulved arr that mmrs our and stick3 firmly to the flcrating balls, raising their level. A single ball column can be prepared using only two NaCl solutions of densities on either side of that of the liquid. Temperature effects can be illustrated by cooling this one for about 20 min in the refrigerator before class. The ball sinks and will slowly rise during class. The oreanic liauids have a temoerature exoansion coefficient abmt rive times r h n i oi water, D.lUi;C verrus ( 0 2 ~ 0 OC for water. Thus, the hnlls rhnnge dmaity inure than the water dws fur agiren temperature change. Volume 63 Number 2
February 1986
149
Further explanations The reproducible stratification of these density gradient reaction columns can be rationalized by considering the two opposing processes that hring about the local balances. When a small volume of solution is displaced below its homogeneous surroundings, the density of the new surroundings will push it back toward its origin and change its density toward that of the new environs by mixing. The new reagent mixing into it also reacts tochange its concentrations toward those of the new surroundings. These two effects bring the materials to balance at density and degree of reaction each depending on vertical distance. Diffusion is a much slower orocess.. deoendinn . - .Wick's law) on the eradient and the nature of the particles involved. he gradi&t soon becomes verv small over anv short distance so that the rate of diffusion decreases as the column stands. These rates could be studied by comparing the moving color bands in columns prepared with reagents a t various concentrations. In tooconcentrated cases, the heat of reaction can upset the me-
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Journal of Chemical Education
chanical balance. If there is no density difference between the layers of reagents, visibly inhomogeneous regions do not disappear after stirring. The only reason I can offer for this phenomenon remaining undiscovered for so long is the chemist's passion for mixing solutions well, especially after seeing the messes students make by pouring reagents into test tubes and trying to decipher the stratified condition resulting. N o t e added in proof: A column having a known gradient of density andlor degree of reaction can be ~ r o d u c e dusing the flow mixing apparatus developed for gradient elution chr~matography.~Columns prepared in this way seem to last even longer than mixed
Rieman, W.; Sargent. R. In "Physical Methods in Chemical Analysis". Berl, W. G., Ed.; Academic Press: New York, 1961; Vol IV, p 185-7
