An Experiment With Manifold Purposes The Chemical Reactivity of Crystal Defects upon Crystal Dissolution Annaluisa Fantola Lazzarini and Ennio Lazzarini lstituto di lngegneria Nucleare, Politecnico, Via Ponzio 3413, 20133 Milano, Italy A few years ago we carried out some experiments concerning the fate of F centers in alkali halides upon crystal dissolution. It has been ascertained that these centers give rise to hydrated electrons, e,, which were counted by using the method of competition kinetics ( I ) . At the same time, it was implicitly proven that the e& are generated in amounts suitahle for investigating their chemical reactivity (2). In addition, the method devised for counting e, and therefore the F centers, has been applied to determination of the oscillator strength of the F centers in several alkali halides ( 3 , 4 ) . Since the techniques used in these experiments are not too complicated and the instrumentation required is available in all radiochemical laboratories, we have tried to adapt these measurements for use as a classroom experiment in either a radiochemistry course or a solid state physical chemistry course. This experiment will introduce: 1) crystal defects and their reactivity upon crystal dissolution; the hydrated electron and its reactivity; 3) an application of the radiochemical method of analysis; 4) the technique of competitive kinetics (even though it is applied to a rather particular case). 2)
The experiment is introduced as reported below. The basic principles are sketched out in the first section, while the second section is devoted to the practical procedures; the third section suggests further readings and experiments. In order to save space the reader will he frequently referred to papers of ours for particular aspects and some details of the experiment. Basic Principles The F center is a crystal defect which consists of an electron trapped in an anion vacancy. F centers can he produced in several manners-the simplest one is by X- or gamma-ray irradiation of crystals. Alkali halide crystals are chiefly used in these experiments. In the first approximation, an F center can he described in terms of the Bohr theory for the hydrogen atom by substituting the dielectric constant of the vacuum with that of the crystal. In the case of alkali halides the ahsorption transition leading to the first excited state of F center (1s 2p transition) occurs in the visible region of the spectrum so that while normal alkali halide crystals are transparent to visihle light, those containing an adequate amount of F centers are colored. (F stands for the German word "farhen" which means "color".) The energy of the absorption transition 1s 2p depends on the kind of crystal considered. For instance, under gamma-ray irradiation the NaCl and KC1 crystals become yellow-brown and violet, respectively. By absorption of light of suitahle frequency or by heating the crystal, the electron of F center can he promoted to the conduction hand and can recombine with its counterpart: the V centers. The V centers in alkali halide crystals are the radicals .X or .X,, X being the halogen atom. Of course, the promotion of the F center electron to the conduction band and its recombination with a V center induces "bleaching" of the crystal. Basic information about point defects and their association can he found in reference ( 5 ) . When crystals containing F centers are dissolved in water, the cation cage, which traps the electron of the F center, is destroyed, and the electron is freed into the solution giving
-
-
rise to a hydrated electron, e& The hydrated electron consists of an electron surrounded by four oriented water molecules placed on the corners of a tetrahedron (6).The fundamental chemical characteristic of e, is an unusually high electron donor capacity which makes its reactivity very high. Consequently, thee; mean life is very short in all media. Among the typical reactions of e& there is the reduction of Co(II1) to Co(I1) complexes. In certain cases, e.g., for [Co(EDTA)](ethylenediamine tetraacetatocohaltate), the reduced complex is stable, hut in other ones, e.g., for CoAe3+(A = NHQ),the reduction of the complex ion is followed by its disruption (7). Thus, the amount of (CoAe)" which reacted withe; can he deduced from the amount of Co2+ion formed. The separation of Co2+from the reaction mixture can be easily carried out by solvent extraction (8).If alkali halides, e.g. NaC1, containing F centers are dissolved in (CoA&13 aqueous solutions, Co2+ ions are formed in an amount proportional to that of e; generated. The Co2+ amount is proportional, hut not equal, to that of e, generated because other species capable of reacting with e; are or can he present in solution. For instance, the following reactions occur with H+ ions and the molecular .02 dissolved in water (8): KH+= 2.2 X 1010 malec' scl e&+Ht-.H e-BQ
-
+ .02
EI+
.0; 4 H0.2 K.02= 1.9 X l O l o mole-l ssl
I t is interesting to recall that the reaction rate for (CoAd3+ is 8.2 X 101° mole-' s-' (9). concentration of Co2+ ions-approkhes more-and more that of e, formed upon crystal dissolution. This is, in principle, the h a m idea of competition kinetics. The equation relating the amount of r(Co2+) of Co2+ ions formed by reduction of (CoA#+ complex to the amount r(ep,) of e; generated can he deduced as follows (1). The chemlcal reactions occurring in the solution are
1iq
~.,+SI-PI e +S;-*Pi
(21
where S; is the ith compound (excluding the Co(II1) complex) present in solution and P; the product formed by its reaction withe& Among the compounds Si there are the radicals .C1 and 432, i.e., the V centers. The reaction rates are - d l e & m r ~ = Kcocrrri 1(CoAs)3+l[e&ldt = d[Co2+1
(31
where [XI is the concentration of the compound X and [eJx thee, amount reacting with X. By dividing eqn. (3) plus eqn. (4) by eqn. (31, one obtains:
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Percent, P, of 60C02r Activity Extracted in Organic Phase as a Function of the Initial (CoAd3- Concentration M e (g ion/L) in
'Before the dissolution of 2.5 g of NaCl clystals inadiated in darkness wlth gamma rays up to an absorbed dose of about 90 KGy ( 9 Mrad). lnadiafion time: 40 hr; irradiation temperature: 0%
I I I I I I I ( I I I I I I _ 5 10 1 / i~n l d ~ i t e r /ion ~
*
Plot of the data in the table. (See eqn. ( 6 ) ) Slope b = 3.56 0.06: Intercept a = We;) = (1.34 f 0.04) X 10' Lig ion; Correlation coefficient r = 0.9994.
c a n also b e written a s follows:
1 dCo2+)
-
1
+-X-
1
r(eaq) de;J
21 K ~ [ S ~ I X
Kc,cnr,
[(CoAd3+1
(6)
T~~~a straight line must he obtained plotting the reciprocal of r(Co2+) against t h e reciprocal of t h e concentration of co(nl) in the in which a constant of NaCl containing F centers is dissolved. T h e intercept of this straight line is t h e reciprocal of r(e&) and thus of t h e F centers present in t h e dissolved NaCl crystals. T h e determination of r(Co2+) c a n b e carried o u t easily b y using ~~~~~labelled (CoA6)3+ complex. T h e initial concentration of ( C O A ~ ) ~ i n+t h e probe solution, multiplied b y t h e fraction P of t h e t o t a l activity extracted i n t h e organic phase gives r(Co2+). T h i s is d o n e according t o t h e m e t h o d cited previously (8). Techniques Preparation of Crystals Containing F Centers In our experiments, polycrystalline NaCl of analytical purity (Carlo Erha R.P.) is used. The F centers are farmed hy 60Cogamma-ray irradiation carried out in darkness at 0°C and a t a dose rate of 2.6 K G y h (0.26 Mradihr). The total ahsorhed dose is usually -180 KGy (18 Mrad). Under these irradiation conditions the formation of "aggregate centers" (e.g., F'center which consists of two electrons trapped in the same anionic vacancy) is negligible (1,3). The dose rate is determined by the Fricke dosimeter. I t is not ~ossihleto calculate the exact concentration of F centers formed in a certain polycrystalline sample by using only the irradiation data. Indeed the energyrequired for the F center formation depends not only on the dose rate and ahsorhed dose, but also on the defects and impurities already present in the crystals (10) and on their "history", e.g., the energy required for the formation of an F center is larger for fresh crystals than fur irradiated and bleached ones. Nevertheless, as this energy ranges between 2 and 10 KeV for NaCl crystals, the order of magnitude of the F center concentration attained can be deduced by assuming a mean value of 5 KeVD center (actually, values raneine hetween 3.3 and 4.0 KeV are usuallv found in our enperimenta~eo~ditions). Therefore, the mean F center concentration in our NaCl crystal is about 2.2 X 1017F centerslg NaC1. If 2.5 g of these crystals are dissolved in 10 mi of water (the NaCl solubility in water is about 330 glliter at room temperature), the concentration of e& delivered into solution should he g eqlliter, which is quite a suitable amount for carrymg out several experiments.
Generation of e& and C o ( 4 Separation The irradiated crystals were accurately weighed (2.5 g-samples) in darkness in an Al foil and dissolved in 10 ml of "probe" solutions,
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Journal of Chemical Education
which consist of (Co&)Cl3 aqueous solutions of suitable concentrations. The chosen concentrations were 1,1.3,2,3,5,10X 10W4M.As said above, the e& generated promote the reduction of (&Ad3+ to (CoA#+ complex which dissociates into (Co(Hz0)s)2t and ammonia. Last, but not least, it should he underlined (and experimentally verified) that (Co(Hz0)8)2+is not formed by dissolution of ""irradiated NaCl crvstals. The (CO(H,O)~)~+ ion is separated from the aqueous solution containing NaCl and the surviving (CoAs)Zt ions as follows: the aqueous solution is transferred to a separatory funnel and then about 1.5 g of NH4CNS and 15 ml of 2% pyridine in CHCL is added. Co(I1) compound is extracted through the organic phase as a Co(I1) complex with pyridine and thiocyanate. The aqueous layer was extracted twice with 15 ml of theorganicsolvent, and the threeorganic fractions were added together in a 50-ml standard container which was brought to volume. The aqueous layer was also transferred in a standard con. tainer together with the aqueous washings of the funnel (two fractions ,,f-15 ml) and the container was brought to volume. ~h~ amount of Co(I1) extracted can he determined by spectrophotometrie methods, hut we found the radiochemical determination to be faster and more precise.
Preparation of 60CoLabelled (CoAe)C13and 60Co Countings The ("CoA6)CI3 complex was prepared as described in reference (I). A suitable value for the specific activity of CaC12 is ahout 1.5 MBqIm mol (-40 pCi/m mol). By using 0.8 g of CoClr6Hz0, about 0.5 g of ("Co&)C18 can he obtained. I t must he remembered that (CaA#+ salts with anions very reactive toward the e, (e.g., N o r , NO;, CrOf-, etc.) must he avoided. The W o countings were carried out by means of a conventional gamma-ray NaI(T1) detector equipped with a discriminator. Some particular aspects of the labelled material preparation and 60Ca countings can be found in references (I), (2), and (3). The results of a typical experiment are reported in the tahle and plotted in the figure according to eqn. (6). From the intercept of this straight line (1.36 X lo4 Llg ion) and by remembering that each determination was carried out by dissolving 2.5 g of irradiated NaCl crystals in 10 ml of probe solution, one deduces that the F center amount per gram of these NaCl crystals was 10W x 6.02 x 1OZ3/(1.36X 104X 2.5) = 1.8 X 1017
F centerslg NaCl
Final Observations M a n y interesting a n d f u n d a m e n t a l aspects of t h e experim e n t a r e neelected in t h i s resenta at ion for soace savine - reasons, e.g., t h e salt effect a n d t h e influence of t h e progressive formation of Co2+ ions and scavangers of e& are n o t discussed. T h e s e a n d other topics a r e considered i n full details i n references (11, (21, a n d (3) t o which t h e reader is referred. Moreover, some suggestions for further chemical measurements concerning t h e reactivity of hydrated electrons (e.g., t h e determination of t h e r a t e constant ratio for two Co(II1) complexes) can he obtained from reference (2). Finally, experiments in solid state chemistry can b e devised. F o r instance, t h e effect of crystal crushing a n d heating, t h e role of substitutional impurities, etc., o n t h e energy required for t h e F center formation b y gamma-ray irradiation can h e investigated.
Literature Cited A. L., Lettrre olNuouo Cimento, Sde3.9.147 (1974). (2) Lazzarini, E..and Fantois La%zarini.A. L.. J.I n o r Nucl. Chem., 38,789 (1976). (3) Bori, L., Fantola Lazrsrini, A. L., and Lazlarini, E., P h y s Stat. Sol. ( b ) , 66, 285 (19741. (4) Bosi, L., Fantola Lazzarini. A. L., Lazzarini, E.,Marigliano Rarnaglia, V., and Taglieeozm A.,Phys. Slot.Soi. ( b ) , 69.519 (1976). (1) Lazzarini,E.,andFantolaLazzarini,
(5) Hannay. N.B.,'"Solid-state Chemistry: Prentice-MI, Endewood Cliffs, 1967, pp. 42-60. (6) Hart, E.J., and Anbar, M., "The hydmted electron: J.Wiley, London, 1970,p. 20. (7) Buendale, J. H., Fielden, E. M., and Keene, J. F., Proc Roy. Sac., A 286, 320 (1965). (8) Jonescu, S.,and Grlgorescu-Sabau, C., Prm. Geneve Conference, 28,148 (1968). (9) Reference (6).AppendixX. p. 233. (10) E b 1 . H . W. cited in Schulmsn. J. H.,snd Compton, W.D., "Color Defects in Solids: Pergamon Press. London, 1962,p. 220. (11) Fernelius,W.C. (Editar),"InorganicSyntheseso(M~G~~~~Hill,NiiY~~k,1946,vol. 2,p 217.
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