L. G.CONTI, R. d’ALEBSANDRO,
350
AND
v. DI NAPOLI
On the Exchange of Chromate Groups in Fresh Lead Chromate1 by Luigi G. Conti,* Rodolfo d’Alessandro, and Vito di Napoli Laboratorio di Metodologie Avanzate Inorganiche del C.N.R., Istituto di Chimka Generale e Inorganka, Universitb di R o w , 00186 Rome, Italy (Received March 81, 1970)
Publication costs assisted by the National Research Council, Italy
The exchange of chromate groups between fresh precipitates of lead chromate and solutions containing radioactive chromate (61Cr042-) has been determined as a function of time, together with the kinetics of monoclinic-orthorhombic phase transition. The results lead to a reexamination of current views on aging of precipitates.
Introduction
to preserve this phase. Adsorbed Violetto Brillante Follone S4B (ACNA, S.p.A., Milano, Italy) is particularly effective in stabilization. The present investigation was undertaken with the aim of examining a chromate exchange curve of fresh orthorhombic lead chromate or determining the effect of the phase transition on the exchange curve itself. To this end, low concentrations and acidities of the starting solutions were used.
Twentg-nine years ago the isotopic exchange at room temperature between solutions containing radioactive lead (212Pb2+) and fresh precipitates of lead chromate was studied by Kolthoff and Eggertsen2 and the results were considered to support strongly a theory of aging,a according to which the exchange in fresh precipitates well below their Tammann temperature should be explained as an effect of repeated and rapid recrystallizations (by motion of crystal surfaces in contact with the Experimental Section solution) of the individual microcrystals. In this way A radioactive NazCr04 solution12 furnished the the microcrystals would eventually become more per61Cr042groups used in the exchange experiments. Its fect. original chromium concentration, activity, and pH If these recrystallizations really take place, three 130 pglml, 30 mCi/ml, and 9.7, respectively. It were main conrwcjuences will follow: (1) the exchange and was diluted 2.2:100 and used as such. The chemicals aging rates inmt depend on the solvation energy of the used for precipitating lead chromate were analytical ions involved, (2) exchange values greater than 100% grade Merck products. should be possible, and (3) equal exchange rates for the The exchange experiments were done at room temcation and the snion should be found. The first perature of 18”, as follows: 10.00 ml of a 9.79 X point has been gerierally confirmed by the findings of M Pb(NOa)2 solution was added in a time of 22 f 1 Kolthoff and his coworkers; the second prediction sec under good and constant mechanical stirring (using seemed also to be verified by the exchange behavior of 10.00 ml of a 1.185 X M K2Cr04 a Pyrex stirrer) to lead chromateZwhich showed Pb exchange curves with characteristic peaks much above the asymptotically (1) Based on part of the Dr. Chem. Thesis presented by Rodolfo approached equilibrium value (100% exchange). The third point has been confirmed by Kolthoff and van% d’Alessandro at the University of Rome, March 1969. (2) I. M. Kolthoff and F. T. Eggertsen, J . Amer. Chem. Soc., 63, Riet4 i n their study of lead sulfate with radioactive 1412 (1941). sulfate arid lead. It should be noted that fresh lead (3) For the main ideas of the theory and experimental facts see I. M. Kolthoff, Analyst, 77, I000 (1952); H. A. Laitinen, “Chemical sulfate gives exchiange curve^^,^ which asymptotically Analysis,” McGraw-Hill, New York, N. Y., 1960, p 156. approach the equilibrium value without showing the (4) I. M. Kolthoff and B. van’t Riet, J . Phys. Chem., 63, 817 (1959). characteristic maxima of lead chromate. (5) I. M . Kolthoff and C. Rosenblum, J . A m e r . Chem. Soc., 56, 1658 Some years ago it was found in this l a b o r a t ~ r y ~ - ~ (1934). (6) G. Collotti, L. G. Conti, and M. Zocchi, Acta CrPystallogr., 12, that an orthorhombic-monoclinic phase transition 416 (1959). occurs in the fresh precipitates of lead chromate. It (7) M . Zocohi, Dr. Chem. Thesis, University of Rome, 1956. appears quite certain that lead chromate always crys(8) L. G. Conti, Report, International School OS Nuclear Science tallizes in the orthorhombic6 structure and then, at a and Engineering, Seventh Session, Argonne National Laboratory, 1958. rate which depends on the characteristics of the solu(9) G. Collotti, L. G . Conti, and N. Zocchi, unpublished results. tion, undergoes a phase transition to the stable mono(10) S. B. Brody, J. Chem. Phys., 10, 650 (1942). l 1 structure. Low acidities and concentrations cliniclo> (11) S. Quareni and R. De Pied, Acta Crgstallogr., 19, 287 (1965). of the reactant solutions prolong the life of the ortho(12) Purchmed from the Radiochemical Centre, Inorganic Departrhombic phi~se. ’Washing with alcohol and ether helps ment, Ameraham, Buckinghamshire, England. The Journal of Physkzk Chemistry, Vol. 76, No. 8,1971
EXCHANGE OF GHRGIMATIE GROUPS IN FRESH LEADCHROMATE solution, which was '2.5 X M in KNQa, in a 100-ml Jena glass centrifuge tube. [The Pb(NO& solution was slightly acidified with HCQ, the concentration of the acid being that necessary to make the pH of the solution in contact with the precipitate of lead chromate equal to 6.80.'3] The precipitate was aged for 5 min, including the time spent for the addition of the Pb(NO& solution, and then, at time to, 1.00 ml of the dilute NaPCrO, solution was added to the suspension in a time of 20 sec. After the desired time (the mechanical stirring was never interrupted during the various operations) the suspension was centrifuged for 8 min at 3000 rpm. BL 1.00-ml portion of the solution was pipetted in a time of about 20 sec at time t, evaporated under infrared, radiation in a thin-walled aluminum dish, and its radioactivity A , was measured. The radioa,ctivity of the mother solution at time to, AD, was determined repeating the sequence of operations just described, except thstt the tracer was added t'o the solution separated from the precipitate by centrifugation after the aging period. The 0.32-MeV 61' rays from W r were counted by a 3 in. X 3 in. NaX(T1) scintillation counter coupled with a 400-channel pulse-height analyzer (Laben, Spectroscope model 408). Such an apparatus permitted integration of the contents of all channels between the two minima, around the 0.32-MeV photopeak. After a suitable counting time, the integrated total was printed out. The error was appreciably less than 1%in all measuremenits. A1 activity measurements were carried out at fixed geometry (7 mm from the sample to the counter). Owing to the small thickness of the samples, absorption of "y rays was negligible. The X-ray quantiitative analyses of the mixtures of the two polymorphic! forms of lead chromate were done as follows. Precipitates were obtained from the same stock solutions used in the exchange experiments. Portions (1.500 1.) of each solution were mixed in a time of about 20 sec in a 4-1.. Pyrex beaker under vigorous mechanical stirring at room temperature of 18". After the desired time stirring was interrupted, the precipiby filtration in one case (3 hr old partial decantation and centrifugao eases, The precipitate was im*stwith alcohol and then with ethyl I 60 min was necessary for the separation procedures. X-Ray diffraction patterns were recorded after a few minutes by means of a Philips djffractometer tinit equipped with a proportional detector, at a scanning speed of 0.5'(2B)/min, and using Cu K a radiation. Percentages were calculated following the method described by Zocchi7 which is based on the intensities of the 210 and 102 reflexions of the orthorhombk imodjfilcation and on the intensity of the 120 refiexion of the monoclinic modification. Use was made of the following expression
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where the intensity I ( m ) ~ , ( r ) 1 0 2 ,at 26 -w- 27.1", is due to the contributions of the two nearly coincident 120 and 102 reflexions of the monoclinic and orthorhombic modifications, respectively. The intensity Io)zlo,at 20 = 26.00", is that of the 210 reflexion of the orthoand its rhombic form. Factor f is defined asI(r)loz/.l~r)zlo value was determined' to be 0.70. Factor P is defined as IO(m)120/Io(r)210, where the 10's refer t o the pure modifications. Factor F was determined' following the phase transition in wet PbCrQl samples which wera placed in the sample holder of the diffractometer. Its value, for typical precipitates, is 2.35, The percentages found by this method are subjected to an error of about *3.' Considering the various factors of uncertainty in the determination of the "true" kinetics of phase transition (solution volumes and stirring conditions necessarily different from those in the exchange experiments; longer separation procedures), the percentages are probably affected by an error of 5.
Results and Discussion Following Kolthoff) the apparent fraction of lead chromate which has participated in the exchange process is given by apparent exchange =
(A0
-AW, A ,M,
I _ _ _ _
where A. is the initial radioactivity (at time to) of the solution in contact with the precipitate, A , the radioactivity of the solution at time t, M s the number of moles of chromate group in solution, and M , the number of moles of lead chromate. Exchnges calculxhed by this formula from experimental data, and corresponding times (t - to) are given in Figure 1. Figure 1 also shows the kinetics of the phase transit',ion occurring in the precipitate (in this case, times correspond to the addition of alcohol to the wet precipitates). The quantities M , and M , were obtained from the concentrations of the reactant solutions. The experimental values of A , could be affected by systematic positive errors, and therefore the apparent exchange values by systematic negative errors, if colloidal lead chromate were present in suspension in. the mother solution after centrifugation and counted as such together with the solution. A check for this possibility was made examining both the mother solution and the precipitate. Both were obtained following the procedure described in the Experimental Section; the only difference was that 1.00 ml of water was added to the sus(13) Small variations of pH with time were presuniably due to absorption of COz by the stock solutions, which were kept in polyethylene bottles, and were corrected by addition of few drops of a dilute KOH solution to the KaCrOa stock solution.
The Journal of Physical Chemistry, Vol. 76,No. 5, 1971
solubilization of the orthorhombic microcrystah and crystallization of the monoclinic ones. (Apparent exchanges due only t o this effect depend on the number of moles of monoclinic PbCr04 forrned after time to, M,,, as
time Ct-tO)
in hours
Figure 1. Penetration of 61Cr0,*- groups into lead chromate and orthorhombic-mo noclinic phase transition.
pension instead of 1.00 ml of the radioactive chromate solution. The. solutions obtained from suspensions of various ages (in the case of the 5 min old suspension the separation wag carried out by rapid filtration) remained perfectly clear for a period of several days, even though they were made 0.05 and 0.1 M in potassium nitrate and Vrere acidified with perchloric acid. Gravimetric determinations of precipitates of various ages, including the 5 miri old precipitate (separated by rapid filtration), in ail caiies gave for the colloidal solid a value lower than 1% of the total amount of lead chromate. (It should be noted that our search for colloidal lead chromate likewise shows that precipitation is completed within the fin3t 5 min.) Whereas this small value could be ascribed t3 experimental errors, we have calculated the effect of this 1’%of colloidal solid in the apparent exchange cuwe. ]Exchange values equal to 0.1, 0.2, and 0.3 should be raised by 1.4, 2, and 2.5010, respectively. The correction to make for the maximum exchange, approxima,tely 1.8, is not easy to calculate. Assuming that the colloidal solid is orthorhombic lead chromate (previous works in this laboratory showed a rather uniform particle size of about 0.1 p for orthorhombic lead ehrornate and of 1-10 p for the needleshaped monoclinic microcrystals prepared from M Pb(NO& and KzCr04solutions) we find that the maximum exchange should be raised by appreciably less than 4%. It can be seen immediately in Figure 1that exchanges higher than unity are clearly due to the phase transition. This observation and unpublished work in this laboratory by electron nzicroscopy9 indicate that the phase transition proceeds through the solution, that is, by The Journal of Physical Chemistry, Vol. 76,No. 8, 1971
which expression can be much higher than unity.) It can also be seen in Figure 1, perhaps less clearly, that an inflection exists in the exchange curve in correspondence with the inception of the phase transition (for precipitates about 3 hr old). This means that the initial part of the exchange curve of orthorhombic lead chromate has the same shape as the curves of lead ~ u l f a t e ,i.e., ~ , ~ is concave downward. We do not know if it approaches unity asymptotically as in the case of lead sulfate but, as it will be explained below, there are good reasons for believing so. The shapes of the exchange curves of lead sulfate strongly suggest a mechanism of diffusion in the microcrystals for the lead and sulfate exchange processes. Such a mechanism can easily explain in a qualitative (but convincing) way also the behavior of lead chromate in the experiments of Kolthoff and Eggertsen2 (lead exchange) and in ours. The decreasing portions of the exchange curves should be due mainly ijo the tracers going back to the solution from the enriched monoclinic microcrystals by diffusion. In this way the curves would approach unity asymptotically. This can be seen very clearly from Kolthoff and Eggertsen’s work.2 Curve 1 (15 sec “old” precipitates), Figure 1, in their paper differs little in shape from a diffusion curve and the little maximum should indicate a phase transition starting somewhat before 100% exchange is reached in the orthorhombic microcrystals. Curve 6 in the same figure (1 hr “old” precipitates) should correspond to the end of the transition and indicate diffusional exchange with the monoclinic microcrystals. The time required for the phase trailsition to be completed, about 1 hr, if our interpretation of MoltJ~offand Eggertsen’s results is correct, is by no means too short. Transition times as short as few minutes have been observed in this laboratory7v9 under various experimental conditions. This interpretation accounts in a very natural way for the otherwise strange fact that, plotting the peak values of Kolthoff and Eggertsen’s exchange curves against the ‘(age” of the corresponding precipitates, a new curve increasing first from the value 1 and then decreasing to the value 1 I s obtained. I n another paper14 Kolthoff and Eggertsen gave (in Table 111) apparent Pb-exchange values (up to 100%) for lead chromate aged at room temperature and at 105’, which suggest presence of one modification and diffusional (14) I. AM.Kolthoff and F. T. Eggertsen, J. Phys. Chem., 46, 458 (1942).
EXCHANGE OF CHROMATE GROUPS IN FRESHLEADCHROMATE exchange. It i s not (clearwhich modification they were concerned with Having seen that the exchange in fresh precipitates can be explained by a diffusion process (complicated by eventual phase transition), it remains t o present a model which can justify this diffusion in crystals well below their Tammann temperature. E t was suggested8 some time ago that fresh microcrystals are Likely to possess a high density of dislocations, in the core of which the lattice material is more or less hydrated. Thurr the activation energy necessary for diffusion to take place along dislocations could be furnished by water present as energy of hydration. (Some experimental evidence for the role of water in dislocations can be found in the work of Tucker and Gjbbs,I5 who introdkced impurity ions into single crystals of cu-Al%Ooby exposure to moisture or by boiling such crystals with soliutions.) Besides, the presence of water in disloc:%tioxlcores should facilitate the motion of the dislocations themselves leading to their annihilation16 or to some stable configuration. I n this way a t o t d rearrangement of the lattice of fresh precipitates and the exchange of iffusional type can be accounted fore1' Another line of evidence is in favor of our dislocation model. Fresh precipitates obtained from highly supersaturated solutions might present many kinds of imperfections and exchange could be perhaps justified on a different basis. However, fresh monoclinic lead chromate is formed under conditions of very low supersaturation, i e . , that due to the difference in solubility
353
between the two polymorphic forms, and cannot possess other than imperfections introduced in the process of slow crystal growth, namely, mainly dislocations. As we have discussed above also fresh monoclinic lead chromate exchanges at a rate not very dissimilar from that of the orthorhombic salt. (Notice also that the monoclinic microcrystals are larger than the orthorhombic ones.) The conclusion drawn from all this is that high exchange rates (high diffusion coefficients) should be generally observed in experiments with fresh wet crystals, not necessarily only with fresh wet precipitates. Our dislocation model for exchange allows one to understand the experiments performed by Kolthoff and van't Riet on lead sulfate that show similar exchange rates for the cation and the anion. Evidently dislocations move slowly relatively t o the ionic diffusion rates along single dislocations. Due to the role of water in crystals, our model for exchange (in the absence of phase transitions) could be described also in terms of two-dimensional recrystallizations. With this distinction Kolthoff's theory would retain much of its validity.
Acknowledgment. This work was supported in part by the National Research Council, Italy, (15) R. N. Tucker and P. Gibbs, J. A p p l . Phys., 29, 1375 (1968). (16) A decrease of lattice defect densities with time has been detected in fresh AgBr by 81Br magnetic resonance absorption, L. G. Conti and R. d'Alessandro, J . Phys. Chem. Solids, in press. (17) A paper by D. Turnbull on alloys, Defects CVy8E. Solids, Rep. Conf., 203 (1955), suggested this idea.
The Journal of Physical Chemistry, Vol. 76, No. S, 1071
