36Revista Latinoamericana de Metalurgia y Materiales. Vol. 16, N° 1, 1996.

YELLOW PHASES FORMED FROM HGCL2 + 2KI IN AQUEOUS SOLUTION, INGEL MEDI T AND IN SOLID- STATE REACTION, AND THE FORMATION OFRHYTHMIC BICOLOR LIESEGANG BANDS

Seung-Am Cho+*, Javier Ochoa", Miryan Puerta''", Argenis Hernandez'?", Amado Quintero",+ Department of Materials Science, Venezuelan Institute of Scientific Research (MC), Apartado 21827,Caracas 1020A, Venezuela .•• School of Metallurgical Engineering and Materials Science, Central University of Venezuela, Apartado50361, Caracas 1050A, Venezuela.a) Nonmetallic Materials Section, Institute of Petroleum Technology(INTEVEP),Apartado 76343, Caracas

1070A, Venezuela.b) CVG-Bauxite ofVenezuela (BAUXIVEN), Puerto Ordaz, Venezuela.

ABSTRAeTBy means of protective colIoid technique the transient initial intermediate yelIow precipitate formed in fast

aqueous HgCh + 2KI reaction is identified as orthorhombic J3-HgI2and its finite powder XRD pattem is established.The solid-state yellow products are KHgI3.H20 and K2HgI4.3H20crystals. In an effort to identify yelIow product in gelwe established a condition for vivid and interleaved bicolor, yelIow and red, Liesegang band formation. We proved thatthe tentative powder XRD data of J3-HgI2from yelIow substance formed on Woelm Ah03 is wrong and thenrecornmended the current XRD file be superseded by our new pattem.

l. INTRODUCTIONRecently, a large red Hgl, single crystal

was grown successfully [1] because of itsnumerous interesting optoelectronic properties[2-4] especially its potential X-ray and low-energy v-ray detector capacity.[5,6]. However,the mercuric iodide HgI2 is a material withmany facets of color, polymorphism andproperties. The Hgl- is also a popularsubstance that exhibits temporal and spatialstructural evolution known as the Liesegangbands in a gel medium [7-11]. AIthough red,yellow, orange and white colors as well as a,~, y, W and W' phases have been reported, someof them are still in controversy [2,12]. Atpresent, a stable red a-HgI2 and a metastableyellow ~-HgI2 are generally accepted and theytransform at 400K (127 C) [13,14].

The single crystal structural data ofboth phases are know as:

(15 42 21 2)ce-Hgl-: SGNo.137, D4h -P--- ;and

n m e

~-Hgh (SGN0.36, C~~ -cmc21), [15].

The powder X-ray diffraction pattern ofa-form (file ~ 21-1157) is definite [16].There, however, exists a unique but tentativepowder XRD data of ~-form (file N" 15-34)[17], which, in fact, does not represent ~-HgI2as we disclose in this paper. The absence ofpowder XRD work on ~ modification is due to,its rapid and transient precipitation fromaqueous solution [18,19], in addition to itsphysicochemical vulnerability; the instabilityagainst mechanical [15,20,21] and optical[18,21] disturbances as well as treatment withsolvents [22]. Because of to, all these factorsthe yellow variety ~-Hgh, resulted fromdiverse preparation methods [7, 8, 13-15, 18,19, 23-25] are taken for granted the same

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yellow I3-HgI2phase obtained from sublimationand condensation [15] without structuralconfirmation.

This paper presents the result of ourexperimental identification of those yellowvariety resulted from the reaction HgCh+2KIformed in aqueous, in solid-state and in sodiumsilicate gels, including the controversial yellowsubstance which appeared on active alumina[17,22]. We established the definite powderXRD pattem of I3-HgI2 and recommended thetentative file for 13be replaced by ours. In duecourse of identification of yellow productformed in gels we demonstrated a condition fora bicolor, red and yellow, Liesegang bands.formation. From our experimental observa-tions we also proposed a more substantiveaqueous reaction equation.

ll. EXPER.IM:ENTALWe obtained a precipitation reaction

scheme (Fig. 1) for the fast reaction in aquéoussolution (1).

0.20

0.18

0.16~ 0.1. :

00.12 ::

~ 0.10¡ Red el. - Hgla

0.08 tYellaw 0.06 ')

~ - He Iz 0.0' %.tI 0.02 ¡~~~iiW%~::\~:::,:~:,.:.:a:,:.:f,'.'...'.,.,'.... .... .., ,. .

0123.567 8 '101112131'1~1617J81'20

t (min)FIG. 1. The polymorphic, yellow B-HgI2 and red a-HgI2,

crystalline phase fields in the aqueous reaction fromHgC12 and 2KI in terms ofinitial molarconcentration of reactant [HgC12]as a function oftime at room temperature. The hatched rosy fieldrepresents two phase region in whichtransformation B < a takes place in solution.

HgCh(aq) + 2KI(aq) = HgI2(s) + 2kcl(aq)

(1)

This reaction field diagram is represen-ted by color or polymorphic crystalline phase interms of initial molar concentration of reactant[HgChl as a function of reaction time at roomtemperature. The initial metastable intermediateIernon yellow suspension formed immediatelyand its color changed rapidly into rose and itthen gradually changed to the bright red finalcrystallites.

A series of stock solutions, fromO.OOlM to O.2M HgCl2 and from 0.002M toOAM KI, were prepared from the Merckanalytical grade salts. For each experiment200ml Hg5 solution in a 600ml beaker wasmagnetically stirred. Stoichiometrically corres-ponding 200ml r solution was then rapidlyadded and the color changes during thereaction were carefully tracked with a stopwatch. The maximum retention time of yellowsuspension formed from O.OlM Hg2

+ solutioncould be extended to about 1 minute byreducing the reaction bath temperature to 0° Cby surrounding the beaker with an ice bath.The yellow suspension within the retentiontime was rapidly filtered and dried both in airand in a vacuum drying oven at roomtemperature. The color of both dried powderswas rosy and XRD identified them as red u-HgI2 (Fig. 2). Theresult manifests that theinitial yellow phase transformed into. red phaseduring the filtration process.

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38Revista Latinoamericana de Metalurgia y Materiales. Vol. 16,N° 1, 1996.

80 7S ~O 65 60 55 50 l5 40 35 30 2S lO 15 10

FIG. 2. X-ray powder diffraction spectra ofyellow B-HgI2 and red a-HgI2 precipitates fromed in theaqueous so/ution.

However, the initial yellow precipitateformed from 0.02M Hg2

+ in 1% gelatinesolution could be captured. The gelatine waschosen as protective colloid because it has thestrongest protecting power on rJ-Hgh [26].The yellow sediment settled for 5 days from themilky initial lemon yellow suspension wasevenly spread on a glass plate. The plate wasdried in air for a day and it was then kept in adesiccator until next morning for diffractionwork. The indexed powder XRD pattem ofthe yellow sample is presented in Table 1 andinFig. 2. The indexation was carried out bycalculating the relative diffraction intensities[27]:

(2)

using the rJ-HgI2 single crystal structural data[15]: space group Cmcz¡ (N" 36), unit cellparameters and ionic positions and with the aidofthe Intemational Tables [28], where c, m, Lpand F are: arbitrary constant, multiplicity,Lorentz-polarization factor, and structurefactor respectively.

The solid-state reaction was carried outwith a total 2 g mixture of HgCh and 2KI in amortar at room temperature. The powderswere lightly ground with a pestle for thoroughmixing. . As rosy color had appeared themixture was immediately transferred into adesiccator. Next day we observed many yellowspots in a red powder matrix and they hadgradually grown into round colonies forming aheterogeneous yellow-red color pattem ofabout 10% yellow in area after 22 days (Fig.3), the day of X-ray diffraction. The powderXRD pa Hem (Fig. 4) indicated that thereactants had completely converted into fourproducts; two yellow [29] potassium iodo-mercurates, KHgI3.H20 and K2HgI4.3H20, inaddition to red a-HgI2 and colorless Kcl.

FIG. 3. Color photo of solid-state reaction productpattern formed from powder mixture ofHgCI2 and 2Kl al room temperature.

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:IOLID SAIAIF. REACllON PRODUCISa. (a. -Hg-:~)K (KCL)A (KHgI' .~O)B (10 HgI" .3lJ. O)

a.I

K

a. a.

80 75 70 65 60 55 50 45 40 35 30 25 20 15 10

29FIG. 4. X-ray powder diffractogram of the solid-state reaction products.

Table I. The indices (hk1) of X-ray powder difraction pattern of orthorhombic yellow f3-Hgh and thecomparison of their observed angle 28o) relative intensity lo and lattice spasing do with thecalculated counterparts 28e) le and de.

(hkl)(002)(110)(111)

(004)}(112)

(022)(113)(023)(114)(024)(200)(006)(115)(025)(130)(131)(132)(116)

13.0022.5823.5126.1

27.3829.823l.1834.4035.7938.5839.2339.6540.8541.4642.0243.5645.62

12.7522.3523.27

{25.67}25.82

27.2029.6230.8434.2935.3738.2538.9239.5840.544l.154l.6843.2545.34

24.127.7100.086.5

9.1-22.14l.26.08.1

34.214.112.116.116.126.218.17.0

59.213.779.1

{2.4 }100.0

5.538.375.99.71l.832.410.22l.022.61l.922.716.822.6

do(A)6.8103.9383.7843.426

3.2572_9962.8682.6072_5092.3342.2962.2732_2092.1782_1502.078l.988

6.9363.9743.820

{3.468}3.448

3.2763.0142.8972.6132.5352.3512.3122.2752.2232_1922.1652.090l.998

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40Revista Latinoamericana de Metalurgia y Materiales. Vol. 16, N° 1, 1996.

(hkl) 280 28e lo le do(A) de(A)

(133) 46.04 45.77 16.1 11.1 1.971 1.980(040) 49.32 48.98 10.1 12.3 1.848 1.858(041) 49.62 49.45 16.1 15.6 1.837 1.842(223) 50.24 49.92 26.2 36.1 1.816 1.825

(OO8)} 53.40 r74

}

7.0 r6

}

1.716

C34

(224) 53.08 6.7 . 1.724

(135) 53.21 13.2 1.720

(206) 56.00 55.71 4.4 6.5 1.642 1.648(225) 57.38 56.96 7.0 14.3 1.606 1.615

(136)} r793

}{4.3} f.591

(118)58.20 57.98 4.0 1.5852.5 1.589

(311) 60.90 60.70 4.0 4.7 1.521 1.524(312) 62.20 61.91 7.0 8.4 1.492 1.497

(24O)} C79

} r} 1.457 (58(313) 63.90 63.90 11.1 3.6 1.456

(241) 64.19 11.0 1.450

(151) 66.38 66.23 5.0 6.1 1.408 1.410

(208)} rOO} tZ}

1.389 {1.396(152)

67.42 67.38 8.0 12.4 1.389

(O47)} t26}

{44} 1.355 {1.355(153) 69.35 69.28 6.0 4.4 1.355

(1110) 72.15 72.05 5.0 5.1 1.309 1.309(139) 75.40 75.31 4.0 5.4 1.261 1.261

To identify the yellow product formed in HgCh, were prepared for test tube (Figs. 5asodium silicate gel medium [7,8], three different through 5d) and U tube (Figs. 6a through 6d)

grade of gels were prepared, of which the experiments. In all test tubes, no sooner had

medium grade gel having pH = 6.0 gave the best 5ml supernatant 2KI solutions been added to

resulto The experimental practice size and shape the corresponding tubes consisting of 5ml

of glass tubes are the same as those employed in HgCh and IOml gel solutions than thin yellowour earlier work [30]. Four sets of rectant layers of precipitates appeared on top of gels

solutions, O.OlM, 0.02M, 0.17M and 0.24M in and they gradually changed into red in time

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inverse proportion to concentration approxi-mately as 4h, 3h, lh and 30 mino respectively.In the case of two dilute (O.OlMand 0.02M)tubes, about a day laternew yellow layers ofprecipitates were formed at the bottom of thered layers. The coloration process repeatedandthen formed an interleaved bicolor Liesegangbands. As the depth of precipitates increased allthose bands above the interleaved at below it,eonverted into red (Fig. Sb). In sufficientlylong time all the bands ultimately became red,leaving some scattered residual yellowprecipitates at the bottom of the rings, Le. Fig.5a.However, in the case of two otherconcentrated (O.17M and 0.24M) tubes, thelayers of red precipitates kept growing up untilabout 3days and then followed by theappearance of yellow preeipitates undemeaththe red layers and they stayed yellow on the daythe were photographed (Figs.: Se and Sd). Figs.5a through 5d were taken after reaetiondurations for 16 days, 6 days, 4 days and 4.daysrespectively.

Figs.5a, Fig.5b

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Figs: 5 e 5 d

FIG. 5. Color photos o/ reaction mode in test tubescontaining sodium silicate gels with (/eJt toright): (a) O.OlM, (b) O.02M, (e) O.17M and (d)O.24M HgCI2, taken after reactiondurations 16days, 6 days, 4 days and 4 days respectively.

We attempted to capture the yellowcrystals in all tubes by haversting theprecipitates. We washed the precipitates withdistilled water then filtered and dried in air. Thepowder XRD of all the four samples revealedred phase. The out come shows that themetastable yellow phase had been transformedinto stable red phase during the washing andfiltering stages probably due to the fact that thecolloidal pratection power of sodium silicate gelon yellow phase was the weakest [26]. Foursets, IOml each, of reactantfeed solutions, 2KIon left arm and HgCl2 on rightarm, werepoured to U tubes, each containing 35ml gel. Athin red product layer appeared approximately 5days, 5 days, 3 days and 2 days laterrespectively and they continued to graw. In 6acase, a couple of additional thin red layers wereformed on the right side of the initial layerstarting fram 12 days later and they keptgrawing into spotty bands, then on the 18th daysevera} faint yellow crystals were nucleated. In6c and 6d case, yellow precipitates appearedrespectively 11 days and 5 days later but in thelast (6d) case the yellow band disappeared at8th da{' and the whole gross band deformed atthe same time. The photos (Figs 6a through 6d)were taken respectively 28 days, 9 days, 15 daysand 15 days after the initiation of theexperiment. We did not attempt to capture theyellow crystals in U tubes.

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43Revista Latinoamericana de Metalurgia y Materiales. Vol. 16, N° 1, 1996.

Fig.6a

Fig.6h

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44Revista Latinoamericana de Metalurgia y Materiales. Vol. J 6. N° J. J996.

Fig. 6c

Fig. 6d

nG. 6. Color photos of'reaction mode in Utubes containing sodium silicate gels andfeed solutions (top to bottom): (a) O.OlM, (b)O.02M, (e) O.17M and (d) O.24MHgCl2 on right arms and counter part 2KI solutions on left arms, taken respeetively 28days, 9 days, 15 days and 15 days after the start o[ experimento

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We repeated the experiment of Goya et[17] to confirm the tentative powder XRDta of B-phase (file N 15-34) which was

btained from the controversial yellowbstance formed when red a-HgI2 wasorbed on the active Woelm alumina but not

TI inactive sodium fluoride under anhydrousndition [17,22]. AlI reagent grade powders,

.5- a-HgI2 and NaF both from Merck, 15- a-• 203 from Baker and activity grade 1

omatographic amorphous Al203 from. oelm, were dried for two days at 115 C in a

um drying oven and were transferred to aiccator. Equal (0.2g each) portions of redgI2 were added to three petridishes, eachtaining 2.5g of aA1203, Woelm A1203 and

powders. They were mixed by shakingcovered dishes for IOmin. And were then

. tained moisture free condition in a•..•••eccator for 11 days. Contrary to the earlier

rts [17,22] none of them showed a colorge nor their XRD spectra revealed .anyuct phase as demonstrated in Fig. 7.

F Base

.~~...%%. ,~ >;j

50 t5 iO as ao 25 20

. X-ray powder diffraction spectra of mixturesof red aHg/2 powder with podwers of alpha-alumina, Woelm a/umina and sodium fluoride.

nr, DISCUSSION

We proved that, by means of protectivecolloid technique, the initial yellow product inaqueous reaction was f3-Hgh and thenestablished its correct powder XRD pattem.The nature of transient and rapid precipitationof f3-HgI2 crystal shows that it is the trueintermediate reaction product and is indeed themetastable phase. The gelatinized spread onXRD sample plate remained yellow in adesiccator. Three months and seventeen dayslater we scribbled a chinese character on it witha steel pencil and the character irnmediatelychanged its color to red. Both, the yellow andred colors on the plate also remainedindefinitely in the desiccator as shown in Fig.8, which was taken 11 days after the day of thescribble. The colors stayed as they were evenin air for weeks after the photo was taken .

FIG. 8. Color photo of a scribbled chinese characteron the gelatintzed ye/low spread, which issubstantiated by the phase transformationB(yellow)-Hg12 y a(red)-Hg12.

From our results on precipitationcharacteristics in aqueous solution and inreference to the mode of development ofbicolor bands in gel, we propose the aqueous

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46Revista Latinoamericana de Metalurgia y Materiales. Vol. 16,N° 1, 1996.

reaction equation into more representativefashion as follow:

HgCh(aq) + 2KI(aq) = [I3-Hgh(s) ~

~ a-Hgh(s)] + 2kcl(aq)(3)

orHgCh( aq) + 2KI( aq) = (p ~ Ci) Hgh( s) +2KCI(aq)

(4)

and we prefer (4) for its simplicity andelegancy. The transient characteristics invol-ved with the fast chemical reaction process inthe formation of ~-Hgh in addition to itsphysicochemical vulnerability [15,18,21-23]may have deferred the powder XRD work onthis phasefor more than a century.

The solid-state reaction produced fourproducts rather than two (1) or three (3) as inthe case of aqueous reaction. The reaction insolid might have been catalysed by adsorbedmoisture since the both, KHgI3 and K2HgI4, aredeliquescent [29]. The dissolved range of [T]had favored the formation of two mercurycomplexes, [HgI3]" and [Hg4L in the Hgl--[HgI3r- [HgI4r equilibrium [31]. As a matterof fact the KHghH20 crystal has beenprepared in a HgI2 and KI solution with excessiodide content [32]. Another interesting obser-vation is that the reaction took place heteroge-neously by forming spotty yellow potassiumiodomercurate colonies in the homogeneousred Ci-HgI2matrix. An unsolved problem hereis that whether all the colonies consist of twopotassium iodomercurates simultaneously oreach colony represents one of them separatelyas both are yellow.

In many previous works on crystalgrowth in gel from HgCh and KI [7-11,33]

three observed redbands [7,10,11,33], oneencountered interIeaved bands of red HgI2 andblack HgCh.2HiO [9] and two mentionedyellow phase along with red bands [7,8] ofwhich one mentioned the yelIow crystal wasK2HgI4 without proof [8]. We presume thatour yelIow precipitate in gel, based on themode of bicolor band formation within therange of concentration of solution employed, isP-Hgh rather than K2HgI4. An earIy experi-ment on U tube with excess HgCh showedformation of only extensive red bands [33].We, however, encountered the formation ofboth, band and yelIow phase (Figs. 6a, e andd). The appearance of yelIow phase on theright side of existing red precipitates and theshift of reaction zones toward right arms in 'allU tubes are probably due to differences inmobility of responsible ions (J.!I ~ JlHg2+)through gel. Although we failed to harvest theyellow crystallites in gel for structure work, wedid establish an optimum condition for vividand rhythmic bicolor, yelIow/red, interleavedLiese-gang band formation in the test tube (Fig.5b).

Contrary to earlier report [17], neitherof our three set of powder mixtures, red u-HgI2 with Woelm Ah03, u-Ah03 and NaF,developed yeIlow color nor their XRD spectrashowed any product phase (Fig. 7). The Goyaet al's assignment of the yeIlow product ontheir Woelm alumina to p-HgI2 [17] provesthus to be controversial because it is not onlyunobserved in our repetition but also theirpowder XRD patterndoes not coincide withthat of the yellow P-HgI2 established in thepresent work. The current powder XRD dataof P-Hgh (file N" 15-34) adopted from them[17] must be superseded by our pattern. Theextension of this work wilI be presentedelsewhere under the titIe of switching Chemical

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Reaction Dynamics and Kinetic VS. Thermody-namic Control in Reference to PolymorphicPrecipitation ofHgh Crystals [34].

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