An Experimental Study of the Liesegang Phenomenon and Crystal Growth in Silica Gels Amandus H. Sharbaugh Ill Franklin County Medical Center, Greenfield. MA 01301 Amandus H. Sharbaugh Jr. 28 Hemlock Dr.. Clifton Park. NY 12065 Thirty years ago, the authors made an extensive experimental study of reactions in gels-the subject of a two-year science ~roiect.The priman, thrust of the proiect was to stimulatk the interesiof students through hands-on execution of some simple experiments, including the observation and interpretat& of the results therefrom. About 100 diffusion experiments were carried out in separate test tubes. One compound (the inner electrolyte) was dissolved in a gel mixture before it solidified and was quickly poured into a test tuhe. After the gel had set, a solution of a second compound (the outer electrolyte) chosen to react with the first was ooured on the solid eel surface. The resulting reaction was observed as the outer electrolyte diffused downward throueh the "brush heap" structure of the ael. The growth of t i e resulting crystais and precipitates was extremely interesting to watch and often very colorful and spectacular. The specimens were about equally divided between the growth of periodically handed precipitates (Liesegang rings), and those experiments specifically designed to study the fundamentals of Liesegang ring formation. Diffusion to the bottom of the test tube (or cessation of chemical reaction) required on the order of a month, and the volumes of outer electrolyte and gel were held constant to permit comparisons. Initially, the most impressive thing about the project was the breadth of coveraae of different crvstals and Lieseaana , years latkr, an additionaiim; ring phenomena. ~ u t n o w 30 portant result has surfaced in that we have discovered that silica gels exhibit an uncanny ability to preserve perfectly the original crystal and ring formations. Since the gels had heen individually photographed in color in 1957, we also have a unique opportunity to compare their original appearance with that 30 years later. Reactions in Gels Crystalsand handed precipitates in gels are often found in nature in the most unexpected settings. It is thought that they may be the precursors to handed agates, gold veins in quartz rock, crystalline material in animal tissue, and even gallstones (1).In addition, the growth of crystals in gels has been used recentlv to produce large single crvstals important in modern technology. For additionaidetails, the reader is referred to the excellent article by Suib in a recent issue of this Journal (2) and several reviews and books (3-7). Remarks on Gel Preparation for Dlffuslon Experiments Unless otherwise stated, all gels reported in this paper were made by mixing equal volumes of water glass or sodium silicate (density 1.06gImL) and NI2 acetic acid. The desired amount of inner electrolyte was dissolved in the acid prior to mixing, and the sodium silicate was always added to the acid, stirring quickly and vigorously. Fifty milliliters of the mixture was poured into a test tube (2.5 X 19 em) and allowed to set (about ane-half hour). Such a mixture is practieallv neutral. The volume of the eel in the test tuhe was held constant.Thedepthof thegel inthe~etulmwashetween 14and 15rrn, while thedepth of the added electrolyte was held toabout 2'2 rm.
The gel in the preceding recipe can be made either acidic or basic as well as neutral. If an acidic gel is desired, 1N acetic acid is used. Alternativelv. a basic eel results if we sim~lvincrease the density of the sodium iiiicate (say from 1.06 to 1.16iAd use an equal am&t of N12 acetic acid. Distilled water was always used in order to avoid cloudinesswith certain insoluble chlorides.Some care must he exercised to keep the test tuhe in averticalposition and avoid movement during gelation, for the rings faithfully reflect the starting contour of the gel surface. Sealing the natural cork stoppers with shellac reduced evaporation to a minimum. Infusion of water twice at 10"ear intervals was accom~lishedhv the use of a hwodermic needle and syringe pushed thro&h the cork sropper. A f& gels made wlth agar and gelatin drd not survive cven the mildest temperature cyrles encountered in the first year or two after preparation.
Comparison of One-Year-Old Gels with 30-Year-Old Gels The linear dependence of the diffusion distance upon square root of time (to he expected from Fick's law) permits an extrapolation to determine the time for diffusion to the bottom of the test tube. In silica gel a t room temperature, this extrapolated time is ahout 18 weeks. In most of these exoeriments. the reaction stopped when the ~recipitation f r k t had advanced t o about one-half the length of the tube so the bulk of the initial active reaction was completed in a month or two. Slides prepared from photographs of the specirhens made after preparkion were projected to allow close 1-2 examination and comparison with the gels as they appear today. As expected, there were very few changes. The most remarkable thing is the propensity for precise preservation of the original crystals and precipitates as long as the gel. does not dry out. Even the luster and color of metallic colored crystals has deteriorated little, if any, from their original appearance. In only one or two cases in the copper chromate studies (see Table 1) did we find an additional fuzzy band after 30 years.
Table 1. Measurements ol Copper Chromate Band Spaclng' Inner: 0.05 N d" dJd,,
d"
0.05 N dJd,,
d"
0.1 N dJd,,
'outer elearom-C~SO,. inner elecmlyte-~gcro,, d.
d"
0.1 N dJd,,
= distance (cm) from gel
surface m top edge d band n. a l n ~ ~ r n p l eband. te
Volume 66
Number 7 July 1989
589
nMorted reactlm in slllca gel: (leftto ilght)(a) Pbla (b) ttglr. (c)Cu tetrahedra. (d, e) besic HgCI*. (0dwble bardlng. (g) lead tree.
Figure 2. As6ated reactions In silica gel; (left to right, outer electrolyte followed by Inner electrolyte)(a)0.25 N CuSO,, 0.0s NaSrO,: (b) 0.5 N CuSO,, 0.05 NarCr04;(c) t N CuSO,: 0.05 Na~crn.: (6) 2 N CUSO,,0.05 Na,cm4; (e-g) MnC1.HIS.
Reactions and Crystal Growth In Silica Gelst
Displacement Reactlofls Silica gel was made 0.02 N with respect to lead acetate. After the gel had set, a small piece of iron or aluminum metal was inserted into the top surface of thegel'Sincethese higher in the electrochemical series, they displace the lead ions, causing the lead to qn~tallizeout. Beautiful branching lead "trees" grew in aboutthree weeks to the bottom of the test tube. The tree, composed of conducting lead, allowed the flow of electrons from the iron and the aluminum to give rise to the reduction of the lead ions (see g in Fig. 1).
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Journal of Chemical Education
Silver crystals were obtained by the same principle but with an interestine variant. A dece of aluminum metal was daced at the bottom ofa test tube and a silica gel, without any addid electrolyte, was poured and solidified around and over it. Then a solution of N/2 silver nitrate was to diffusethrough the gel down to the top edge of the of The aluminum then displaced the silver ions (as expected from their relative positions in the electrochemical series, and a silver gcbush,,started to grow upward. The bUBhbecame more dense as it into regions of
'
The reader is referredto refs 2 and 5 for additional experimental details for some of the expriments desuibed in this section.
Figure 3. Assorted reactions in silica gel; (len to right) (a) Pbl,, (b-h)doubiy banded systems. Refer to text.
gradually increasing silver concentration. In one experiment of this type, when tap water was used instead of the usual distilled water, a set of silver chloride hands (Lieseeme rims) formed as the silver nitrate diffused down to the piece i f &tal: Copper Tetrahedron Crystals A test tuhe was prepared with the silica gel made 0.05 N with respect to copper sulfate. After the gel had set, a reducing solution of 1%hydroxylamine hydrochloride was placed on its surface. In a few weeks perfectly shaped tetrahedra of copper were visible to the unaided eye. The crystals were larger a t the hottom of the test tuhe due to the dilution of the reducing solution (fewer nucleation sites) and the slower rate of diffusion. As will he seen, this change in size is almost always observed in crystal formation in gels when diffusion techniques are used. Lead Iodide Crystals A gel composed of 25 mL of N acetic acid and 25 mL of 1.06 density water glass containing 4 mL of 1N lead acetate was allowed to set. Then a solution of 2 N potassium iodide was allowed to diffuse into the gel. Spectacularly beautiful yellow lead iodide crystals form. When 10 mL of the lead acetate solution was added to the gel instead of 4 mL, the crystals formed in 20 bands about 1mm in thicknessand 1mm apart. The crystals were in the form of gleaming hexagonal platelets and became larger as the solution diffused down the tube. Finally, the handing ceased, and the remainder of the gel was filled with randomlv dis~ersedolatelets. This was the onlv time when the band s~acing'hidnot faliaw a geometric progressidn (see tube a in Fig. 3). Mercuric Iodide Crystals Mercuric iodide crystals are distinguished hy their striking bright red color (see Fig. lh). They were made by allowing N/2 mercuric chloride to diffuse intoa gel composed of equal parts of 1.06 density water glass and N acetic acid and made NI10 with respect to KI. Stringy threadlike red needles formed with a handing visible only by transmitted light. Twenty such very closely spaced hands about 1-2 mm in thickness formed over 6 cm. After thehanding ceased, some individual crystals as long as 7 mm formed in the region of high dilution and slow diffusion. The addition of 1e of elucose comoletely eliminated any tendency to handing, althgugc the charac'ter of the red crystals was unchanged. The addition of 5 g of glucose changed the threadlike mercuric iodide crystals to fine red particles barely visible to the eye and introduced a fine band structure. Fifty hands were counted over a 6-cm distance. The glucose acts as a protective colloid and inhibits the growth of the lead iodide crystals.
Flgwe 4. Effect of concentration of outer electrolyte upon diffusion rate; outer eiectrolyt-CuSO,, inner electrolyte-Na2Cr04.
Basic Mercuric Chloride A hnarc silica gel was prepared hy using equal volume^ uf 1 0 8 density sodium silicate (note the h i ~ h e than r usual density) solution and N/2 acetic acid. No other inner electrolvtc i* added. T h m the solidified eel was covered with saturated mercuric chloride (about NI2). ~ r o & i s h black crystals formed i n a striking regular gradation in size from barely visible, but present in great profusion, to accasionalcrystals about 5mm in size at the hottomof the tube (see tuhe d in Fig. 1). According to Holmes (5)these crystals are probably HgCIr2H20. When a very slightly basic gel was used (1.06 sodium silicate and N/2 acetic acid), the crystals grew into long fernlike fronds up to 3 cm in length with evidence i f bands spaced several millimeters apart (see e in Fig. 1). The character of the handing was drastically changed with the addition of glucose to the geL2 One Not a surprising result when we learn that glucose is often used in ice cream and sugar preserves to discourage the formation of crystals of ice and sugar. Volume 66
Number 7
July 1969
591
Figure 5. Assorted reactions of copper chromate In silica gel; (lefi to right) (
a 4 CuCrO,, (e) CuCrO,
gram of glucose reversed the usual order of gradually increasing band separation. In this ease we observed an initial formation of gray hands several millimeters in thickness and spaced by % mm. These gradually decreased in thickness until, at the bottom of the tube, very fine handing occurred where 25-30 hands in a centimeter were counted. When 6 g of glucose was added to the preceding recipe, the banding was completely suppressed; a continuous precipitate of darker gray basic chloride was formed until the concentration of the diffusant was so low that the precipitation ceased. Manganese Sulfide Rings These hands were made by the method of Tower (8,9)where the sulfide ion was introduced into the silica gel by bubbling hydrogen sulfide gas for %1 min through the unset gel. This operation must always be done in a hood or outdoors due to the toxicity of the gas. Firstly, with this procedure, it is difficult to control the precise concentration of S2-. Secondly, the addition of the acid by solution of the gas accelerates the setting of the gel; this can lead to damage of the gel structure when the bubbling occurs in a partially set gel. Introduction of the sulfide ion by dissolving definite concentrations of sodium sulfide were unsuccessful because the gels, so prepared, failed to set due to the alkalinity. Most of the experiments were done by mixing equal volumes of 1.07 density water glass and NI2 acetic acid. Concentrations of N, 1'1%N, and 2 N manganous chloride were used with no observable difference in the appearance of the hands. In general, a total of about 20 tan geometrically spaced bands formed in each test tube. As is often observed, the top centimeter or so was solid with little evidence of bandine. while the remainine hands were composed of discrete partirlrs of manganoua sulfide (see g i n Fig. 2).Tl1eappraranre wasdracricollgrhnnged whm rhearerlc acid =,as omitred irom thr abow r r e i p ~( s a p r, f m Fig. 21. One centimeter of compact gray precipitate was farmed and then followed by 30 fine bands of tan precipitate over 5 em and finally changing hack to the gray color over the last % em. This gel was slightly basic (not enough to prevent setting), sndsa the white color may have been due to the formation of a complex basic manganous sulfide or manganous hydroxide. DiffusionStudies of the Llesegang Phenomenon
,
Copper Chromate in Silica Gel An experiment was mn to determine the effect of concentration of the outer electrolyte on diffusion rate. Equal volumes of copper sulfate solutions having the concentration of 2 N, N, and N12, were
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Journal of Chemical Education
grown at 4 'C.
(f) CuCrO,
grown at 4 'C, then 25 'C
allowed to diffuse into equal volumes of silica gels that were made 0.05 N with respect to sodium chromate. Observations over a 141-h period yielded plots that were linear with the square root of time (see Fig. 4). Furthermore, the higher the concentration of the copper sulfate, the faster was the diffusion. These results were to be anticipated from Fick's law with respect to both time and concentration gradient. If the concentration of copper sulfate had heen less than 0.05 N, the sodium chromate would have diffused from the gel into the copper sulfate and, of course, no banding or precipitation would have occurred in the gel. The effect of temperature on the process was studied by allowing a l N copper sulfate solution to diffuse into silica gel in three different test tubes. In each tube the gel was made N110 with respect to sodium chromate. The three test tubes were kept at reasonably constant temperature of4"C (refrigerator), 21 "C (room),and 52 'C (water bath). The precipitates grown at 4 'C and 24 'C obeyed the linear relation with square root of time, but the precipitate growth at 52 OC increased linearly with time rather than with square root. No rings formed in the refrigerator up to the end of the observation time (140 h) (see e in Fig. 5). However, in another experiment the test tube was removed from the refrigerator and then usual formation of rings commenced (see f i n Fig. 5). At 52 'C, some other phenomenon overrides the usual diffusion-controlled mechanism, the nature of which is unclear. Effect of Superimposed Electric Field Upon Rate of Diffusion Since the time necessary t o form a completed set of rings can be a s long a s several months, it was decided t o see whether this time could b e reduced by superimposing a n electric field t o speed u p the movement of the diffusing ions. After some false starts, t h e arrangement shown in Figure 6 was evolved. When t h e electrodes were located in the eel itself, gases produced by electrolysis ruptured the solid ;el. It will be noted that the arraneement in Fiaure ti uses liauid "electrodes" and avoids this Gouble. T h e results are shown graphically in Figure 7. T h e expected square root dependence was found only with zero applied volts. As increasing , diffusion disamounts of voltage were s u ~ e r i m p o s e d the tancr gradually aPPrwwhrd iinearity with lime a s would he expected for ionic charre3 moving in a n electric field sufficiently large t o overridethe diffusion dominated movement. A t 2Z1I2and 45 V, t h e growth of the precipitation was strictly
STAINLESS
srrrrwmr
1111
BATTERY
STAINLESS STLLL~IRE
F l p e 7. Effen ot superimposed voltage on time diffusion In silica gels; outer elecwolyie--CuSO,, inner elecbolyte--Na&Q.
Table 2. Character ofCopper Chromate Bandlng wlth Systematic Changes In the Concentrations of Outer and Inner Electrolytes N m l i t y of ReclplEiectrolr tatlon Outer Inm Zone CUSOI NaGm, (cm) APPARATUS FOR GROWING RINGS WITH SUPERIMPOSED ELECTRIC FIELDS Figure 6. Apparatus for growing Llesegang rings wlth auparinmosad electric flelds.
linear with time. Also, a t amlied voltages of 5 V and lamer, the Liesegang ring foimatibh ceased. This points up anjm: portant fact that the processes operative in ring formation are sensitive to the veiocity of charge movement. Examination of the curves in Figure 7 shows that, indeed, one can accelerate the ring formation some three- or fourfold before loss of the periodic character of the precipitation. Spacing of the Rings
The distanca of the initial (ton) . - . edee of the comer chromate bands from the gel surface was measured fo; i0 different samples. The remarkable claritv of the intervenine regions and sharpness of the copper chromate bands allowed these distances to be measured to within about f'I4mm (see a, b, c, d in Fig. 2). Representative values of the ratios of adjacent bands (d,ld,.l) are shown in Table 1.Considering the accuracy of the measurement, the ratio is remarkably constant,and trendsare well beyond themeasurement error. The nearlv constant value of the ratio indicates a eeometric series, a fact that bas been reported in earlier literature (10, 1 I ) . One c~hservationof maneanous sulfide rines in silica eel (9) yields the average valueUof1.21, which i s i n the rGge found here. Often the last band was incompletely formed, and only a hazy beginning of precipitation occurs. However, this alwavs occurs a t the location where the hand is to be expectedirom the geometric progression relation. Wo. Ostwald developed a theon, for the formation of Liesegang rings over 60 years ago (12); This theory is based upon t
