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Habit changes of lead chloride, PbCl2, caused by growth from pure aqueous solution and the effect of KCl, NH4Cl, CdCl2 and HCl as additives
554
Journal of Crystal Growth 87 (1988) 554—560 North-Holland, Amsterdam
HABIT CHANGES OF LEAD CHLORIDE, Phd2, CAUSED BY GROWTh FROM PURE AQUEOUS SOLUTION AND THE EFFECT OF KCI, NH4CI, CdCI2 AND H~AS ADDITIVES M. VAN PANHUYS-SIGLER
“,
P. HARTMAN and C.F. WOENSDREGT
Institute of Earth Sciences, State University of Utrecht, Postbus 80.021, 3508 TA Utrecht, The Netherlands Received 21 October 1987
Crystallization of lead chloride, PbC12, from pure aqueous solution produces crystals with dominant (211) and smaller (010) and (100) faces at low supersaturation. Increase of supersaturation yields crystals elongated along the c axis. Growth from a previously prepared saturated solution gives crystals elongated along the c axis with (011) as main terminal faces. This habit also occurs when KC1, NH4C1 or CdC12 or mixtures of these are added to a freshly prepared supersaturated solution. Only at low additive concentration and at low supersaturation (211) is dominant. The latter habit is supposed to be caused by adsorption of OH— ions on (211). A deposit of Pb(OH)Cl has been observed. Crystallization from HCI containing solutions enhances the (010) and (121) forms, presumably through preferred adsorption of H~and possibly PbCl~ions. Increasing supersaturation diminishes the effect of the habit modifying process until the stage where dendrites appear.
1. Introduction Lead chloride, PbC12, shows a remarkable habit variation when crystallized under various conditions. As recorded by Groth [1] growth from aqueous solution by cooling produces prismatic crystals, more or less elongated along the C axis with terminal faces {211). The indexing will be based on the space group Pnam with cell dimensions a = 0.7619 nm, b = 0.9044 nrn and C = 0.4533 mn [2]. When crystallized from a solution containing HC1 the terminal faces are {121 } and { 111 }. Crystals with a similar habit were obtained by Abdulkhadar and Ittyachen [3]. The question arises whether this change is due to the chloride concentration, the pH or the supersaturation. Similar results have been described for PbBr2 [1], while Pb(OH)Cl, Pb(OH)Br and Pb(OH)I, cornpounds that also have the PbC12 structure, show terminal faces (011) instead [4]. To get a better insight in the various factors that may cause the habit changes a series of experiments were carried out whereby PbC12 was crystallized from pure *
Present address: c/o Shell Expro, Dept. UE1T/13, Shellmex House, Strand, London WC1R ODX, UK.
aqueous solution and from solutions to which KC1, NH4C1, CdCI2 or HC1 was added.
2. Experimental technique Method A: In an Erlemneyer flask a known amount of PbC12 (UCB, p.a. grade) is dissolved in a known amount of bidistilled water by heating. Eventually cosolutes are added and then the closed flask is put into a water bath that has a temperature slightly below room temperature. Crystallization starts when the temperature in the flask is quite near to room temperature. During the crystallization the supersaturation changes from an initial value to almost zero. This initial value was determined from known solubility curves [5].After crystallization the crystals are filtered, washed and dried. The faces on these tiny crystals of which the length varied from a few tenth mm to about 4 mm, have been determined using a two-circle optical gomometer or a scanning electron microscope. Identification of faces was also corroborated using either a polarisation microscope or single-crystal X-ray methods (Weissenberg and precession cameras).
0022-0248/88/$0150 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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Method B: to obtain a supersaturated solution a known amount of PbC12 is added to a clear, previously prepared saturated solution. The additives KC1, NH4C1 and CdC12 were from BDH, Analar grade, HC1 from Merck, p.a. grade.
3. Results 3.1. Crystallization from pure aqueous solution Using method A and a low initial relative supersaturation a = 0.10, the crystals show large (211) faces with smaller (010) and (100). At a = 0.22 the habit becomes more tabular (010) with additionally (120) faces (fig. 1). At a = 0.32 the habit becomes more elongated along the c axis so that (010) and (100) become dominant (fig. 2), while also (110) appears. At a = 0.43 the crystals are more elongated and sometimes hollow, while at a = 0.53 and 0.63 dendrites appear. With method B at a = 0.10, the habit is quite different from the one obtained with method A. The crystals are elongated along the c axis with (010) and (100) as main forms and (110), (120) and (210) as small faces. At the top the main form is (011) (fig. 3) with occasionally (211). Similar laths, but sometimes hollows, occur at a = 0.18. At higher a the habit becomes needle-like .
Fig. I. PhCi, crystals grown from pure aqueous s~luIion (method A) at a = 0.22.
Fig. 2. PbC12 crystals grown from pure aqueous solution (method A) at a = 0.32.
and at a = 0.79 dendrites occur. In search for an explanation of this unexpected difference in habit it was found that crystals obtained with method A showed extra lines in their X-ray powder diffraction pattern which could be unequivocally be ascribed to orthorhombic Pb(OH)Cl.
3.2. Crystallization in the presence of KC1, NH4CI or CdC12 .
.
Crystallization was carned out according to method A. KC1 decreases the solubility. The initial
Fig. 3. PhCI, c~stalgrown from pure aqueous solution (method B) at a = 0.10.
556
M. van Panhuys-Sigler eta!.
/ Habit changes of lead chloride
0.9 0
0.8 0.7 V
0.6
0
V
0
0.5 0~
0.4
)011~
0 0
0.3
0 0
0.28
~
V 0.1 ~
0
211~ I
0.05
0.10
0.15
0.20
0.25
conc.Cr(mol l~1) Fig. 4. Morphodrome of PbC1
2 grown in the presence of halides: (0) KC1: (D) NH4C1: (v) CdCI2 as cosolute. On the horizontal axis, the amount of the additives is expressed as moles Cl — per liter.
relative supersaturation was calculated using solubility curves [5]. The incorporation of K into PbCl2 was measured using atomic absorption spectrometry and appeared to be very low, between 0.002 and 0.009 at%. The results are presented in the form of a morphodrome (fig. 4) in which a is plotted versus the concentration of the cosolute. Only at low a and at low KC1 concentration is the habit isometric with {21 1) dominant. Higher a leads to needles [001] on which {011 } is the dominant terminal face (fig. 5). Increasing KC1 concentration favour this habit change so that in fig. 4 two areas can be distinguished, one where (211) is the characteristic and often dominant form, the other where (011) is present while (211) fails. Similar results were obtained with NH4C1 or CdCl 2- They appear in fig. 4 with different symbols. Several experiments were carried out with mixtures of the cosolutes: KC1 + NH4C1, KC1 + CdCl2 and NH4C1 + CdC12. As no solubility
curves are known for these systems only a qualitative estimate of a could be made. With this restriction in mind it was found that virtually the same habit changes occur as when only one cosolute was present.
Fig. 5. PbCl2 crystal grown at a = 0.33 from a solution containing 0.0268 mol KC1 per liter.
M. van Panhuys.Sigler et aL
/ Habit changes of lead chloride
3.3. Crystallization in the presence of HC1
557
forms (fig. 7c). Finally, in region E the habit is tabular (010) with large (121) and occasionally small (100), resulting in a rhomb-like appearance (fig. 7d). Thus, increasing HC1 concentration causes (121) to become more and more promi-
Crystallization experiments were carried out at the following concentrations of HC1: 0.013N, 0.096N, 0.8N, 3.2N, 4.37N, 4.8N and 6.17N. The solubility of PbC12 shows a minimum at about iN
nent with accompanying shortening along the c axis. It also causes the crystals to become more flattened according to (010). The effect of the supersaturation is not always clear. At 4.8N HC1 the habit changes with increasing supersaturation from that of region E, via D to C, clearly an increase in length along the c axis. At still higher supersaturations dendrites appear. For lower HC1 concentrations the effect is less obvious and therefore a statistical study was made of the crystals grown from 0.096N HC1 solutions. From 80 crystals obtained in an experiment the length L and breadth B was measured. The ratio R = L/B is considered as a habit characteristic. For each initial a value histograms of the number of crystals versus R were made, whereby the R values were put together in groups n
HC1, while at about 6N HC1 it becomes larger than in pure water. The results are shown in a morphodrome (fig. 6). In region A needles [001] prevail that show the forms (010), (100), (110), (120) with (lii), (011) and (121) as terminal faces. Not all forms are always present. The fleedles have (010) as largest faces, but sometimes (100) is dominant. As to the terminal faces, there is a slight tendency for (011) to be the most prominent one at low supersaturation and very low HC1 concentration. At the higher HC1 concentration (3.2N) (121) is the most prominent terminal form, while in the intermediate ranges (111) is present, together with (011) or (121). In region B the needles are shorter with (010) as dominant form with (121) again the most important terminal form (fig. 7a). In region C the habit is blade-like (fig. 7b) and, on the whole, again shorter, until in region D the crystals are six-sided plates (010) forms (100)(120) and (121), sometimes with with smallside (110) and/or
C. dendrites 1.0
:
0.5
•,~“
?
0
/a
~/~
,,~\\\
:
added HCI(mol 1h) Fig. 6. Morphodrome of PbCI 2 grown from HCI containing solutions. The dashed lines indicate approximately the borders between regions that are characterized by the drawn habits.
558
M. van Panhuys-Sigler et aL
/ Habit changes of lead chloride
4
Fig. 7. PbC1
2 crystals grown from HC1 containing solutions: (a) from 0.8N HC1 at a = 0.08; (b) from 4.37N HC1 at a from 4.8N HCI at a = 0.18; (d) from 6.17N HC1 at a = 0.09.
=
0.14; (c)
30
25
N~
025
10
20
30
B
Fig. 8. Smoothened histograms of the length/breadth ration R of PbC12 crystals grown from 0.096N HCI solutions at five different supersaturations a.
the more so for the smaller R values. Hartley’s maximum F-test [6] proved that the variances of the distributions are not significantly different at the rather low level of 0.05. This provides the basis to carry out an F test to see whether there is a significant difference between the averages (one way analysis of variance [6]). It was found that the different distributions practically have no chance to represent the same population, neither those with low R among themselves, nor those with high R. The conclusion is therefore that for different initial a entirely different distribution are obtamed.
M. van Panhuys.Sigler et aL
4. Discussion The drastic habit change from (211) as dominant form obtained by method A to prismatic crystals with (Oil) in method B can be explained as follows. From pure solution and also from halide containing solutions very often a deposit of Pb(OH)Cl was formed along with the PbC12 crystals. In method B the supernatant solution was used, which is therefore more acidic. This means that the occurrence of (211) is due to preferential adsorption of OH ions on these faces, maybe in the form of a two-dimensional adsorption layer of Pb(OH)Cl. It is to be recalled that addition of a NaCl solution to a Pb-acetate solution does not lead to the crystallization of PbCl2 but of Pb(OH)Cl [4]. Therefore the habit obtained with method B should be considered as the one that least affected bya external factors. The additionis of halides causes competition between the adsorption of OH and Cl-, so that higher concentrations of halides lead to a smaller effect of 0H adsorption and therefore a higher growth rate of (211). As a result the habit becomes more elongated along the c axis, while (011) appears as terminal face. Increase of supersaturation also diminishes the effect of OH— adsorption, so that the morphodrome of fig. 4 becomes understandable. The question as to why (211) is affected most by OHadsorption will be considered in a separate paper [7]. Using the stability constants recorded by Smith and Martell [8] it appears that in pure aqueous solution the major species is PbCl The concentration of free Pb2 + ions is smaller, while about 5% PbCl 2 is not dissociated. The effect of halides is to slightly decrease the PbCl + concentration, while that2 tof At thethe PbCl2 species increases ofat KC1 the highest concentration cost of Pb ~ is formed, just before KPb even PbCl 2C15 crystallizes instead of PbCl2 [5]. The morphodrome of PbCl2 crystallizing from HC1 solutions is abnormal in the sense that the lines separating the various habits, increase with HC1 concentration, instead of decrease as is usual. -
-
~.
/ Habit changes of lead chloride
559
The effect of HC1 is clearly twofold: it decreases the growth rate of (121) and of (010). The morphodrome (fig. 6) and the statistical evaluation show that an increase of supersaturation results in a more elongated habit. The large needles obtamed from 0.013N HC1 and at a = 0.08 could be measured with an optical goniometer: they never showed the form (121). Instead {ii1) and/or (011) were present. They have much the same habit as those obtained from pure solution with method B. Hence the appearance of (121) must be due to preferential adsorption of some ionic species. The prevailing species in solutions with HC1 concentrations of iN or higher is PbCl~ apart from H~ or H3O~. Because at constant supersaturation an increasing HC1 concentration leads ultimately to a dominance of (121) the species responsible for this change 3. must begrowth H~ or The H3O~,probably along with PbC1 rate retardation of (010) seems to have its maximum at SN HC1, beyond which the crystals become thicker again. At the highest pH in our experiments the solution contains a notable amount of PbCl~, so the effect on (010) has probably to be ascribed to adsorption of PbCl The occurrence of these complexes in solution becomes immediately apparent from the solubility data [5]: an increasing amount of HC1 leads first to a sharp decrease of the solubility with a minimum at about iN HC1, which is then followed by a gradual increase. The bimodal distribution found in the experiments at 0.096N HC1 has to be ascribed to nucleation at different supersaturations. The longer (large R) and larger crystals have nucleated at higher a values than the shorter and smaller ones. The fact ~.
that the distribution is not continuous (except for the highest a value) but bimodal, points to the existence of a critical Belowexa~ the adsorption of the supersaturation habit modifyinga~. species erts its largest influence, while beyond a~the growth rate is high enough to overcome the adsorption effect. It should be mentioned here that the habit change is not of ‘the cubic-octahedral type that occurs on NaCl, where K faces appear. A PBC analysis of the PbCl2 structure [7] showed all forms mentioned here to be F forms.
560
M. van Panhuys-Sigler et aL
/
Habit changes of leadchloride
5. Conclusions
growth rates. The effect is probably due to a combined effect of H~and PbCl~ions. Increas-
(1) The habit of PbC12 crystals that is least affected by external factors can be described as elongated along the c axis with (010) and (100) as main forms and (110), (120) and (210) as smaller faces, while at the top (011) is the dominant form. (2) The dominant form (211) occurring at crystallization from a freshly prepared slightly supersaturated aqueous solution is due to adsorption of OH ions. The solvent acts as an impurity. (3) Crystallization in the presence of KC1, NH4C1 or CdCl2 or mixtures of these enhances the growth rate of (211) by the adsorption competition between the OH and Cl ions. Increase of solute concentration and of supersaturation leads to the disappearance of (211), to a more elongated habit and to the occurrence of (011) as dominant terminal form. (4) The effect of increasing HC1 concentration is the slowing down of the (121) and (010)
ing supersaturation gives an elongated habit with dominating (010) and (100).
—
—
—
References [11P.
Groth, Chemische Krystallographie (Engehnann, Leip-
1906) Vol. I. G. 219.Donnay, E.G. Cox, 0. Kennard and [2] zig, J.D.H. Donnay, MV. King, Eds., Crystal Data, 2nd ed. (Am. Crystallographic Association, 1963). [31M. Abdulkhadar and M.A. Ittyachen, J. Crystal Growth 48 (1980) 149. [41zig, P. Groth, Chemische Krystallographie (Engelmann, Leip1906) Vol. I, 297. [5] W.F. Linke and A. Seidell, Solubilities (Amer. Chem. Soc., Washington, DC, 1965). [61R. Till, Statistical Methods for the Earth Scientist (MacMillan Press Ltd., London, 1974). [7] C.F. Woensdregt and P. Hartman, J. Crystal Growth 87 (1988) 561. [8] R.M. Smith and A.E. Martell, Critical Stability Constants, Vol. 4: Inorganic Complexes (Plenum, New York, 1976).
Journal of Crystal Growth 87 (1988) 554—560 North-Holland, Amsterdam
HABIT CHANGES OF LEAD CHLORIDE, Phd2, CAUSED BY GROWTh FROM PURE AQUEOUS SOLUTION AND THE EFFECT OF KCI, NH4CI, CdCI2 AND H~AS ADDITIVES M. VAN PANHUYS-SIGLER
“,
P. HARTMAN and C.F. WOENSDREGT
Institute of Earth Sciences, State University of Utrecht, Postbus 80.021, 3508 TA Utrecht, The Netherlands Received 21 October 1987
Crystallization of lead chloride, PbC12, from pure aqueous solution produces crystals with dominant (211) and smaller (010) and (100) faces at low supersaturation. Increase of supersaturation yields crystals elongated along the c axis. Growth from a previously prepared saturated solution gives crystals elongated along the c axis with (011) as main terminal faces. This habit also occurs when KC1, NH4C1 or CdC12 or mixtures of these are added to a freshly prepared supersaturated solution. Only at low additive concentration and at low supersaturation (211) is dominant. The latter habit is supposed to be caused by adsorption of OH— ions on (211). A deposit of Pb(OH)Cl has been observed. Crystallization from HCI containing solutions enhances the (010) and (121) forms, presumably through preferred adsorption of H~and possibly PbCl~ions. Increasing supersaturation diminishes the effect of the habit modifying process until the stage where dendrites appear.
1. Introduction Lead chloride, PbC12, shows a remarkable habit variation when crystallized under various conditions. As recorded by Groth [1] growth from aqueous solution by cooling produces prismatic crystals, more or less elongated along the C axis with terminal faces {211). The indexing will be based on the space group Pnam with cell dimensions a = 0.7619 nm, b = 0.9044 nrn and C = 0.4533 mn [2]. When crystallized from a solution containing HC1 the terminal faces are {121 } and { 111 }. Crystals with a similar habit were obtained by Abdulkhadar and Ittyachen [3]. The question arises whether this change is due to the chloride concentration, the pH or the supersaturation. Similar results have been described for PbBr2 [1], while Pb(OH)Cl, Pb(OH)Br and Pb(OH)I, cornpounds that also have the PbC12 structure, show terminal faces (011) instead [4]. To get a better insight in the various factors that may cause the habit changes a series of experiments were carried out whereby PbC12 was crystallized from pure *
Present address: c/o Shell Expro, Dept. UE1T/13, Shellmex House, Strand, London WC1R ODX, UK.
aqueous solution and from solutions to which KC1, NH4C1, CdCI2 or HC1 was added.
2. Experimental technique Method A: In an Erlemneyer flask a known amount of PbC12 (UCB, p.a. grade) is dissolved in a known amount of bidistilled water by heating. Eventually cosolutes are added and then the closed flask is put into a water bath that has a temperature slightly below room temperature. Crystallization starts when the temperature in the flask is quite near to room temperature. During the crystallization the supersaturation changes from an initial value to almost zero. This initial value was determined from known solubility curves [5].After crystallization the crystals are filtered, washed and dried. The faces on these tiny crystals of which the length varied from a few tenth mm to about 4 mm, have been determined using a two-circle optical gomometer or a scanning electron microscope. Identification of faces was also corroborated using either a polarisation microscope or single-crystal X-ray methods (Weissenberg and precession cameras).
0022-0248/88/$0150 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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/ Habit changes of lead chloride
555
Method B: to obtain a supersaturated solution a known amount of PbC12 is added to a clear, previously prepared saturated solution. The additives KC1, NH4C1 and CdC12 were from BDH, Analar grade, HC1 from Merck, p.a. grade.
3. Results 3.1. Crystallization from pure aqueous solution Using method A and a low initial relative supersaturation a = 0.10, the crystals show large (211) faces with smaller (010) and (100). At a = 0.22 the habit becomes more tabular (010) with additionally (120) faces (fig. 1). At a = 0.32 the habit becomes more elongated along the c axis so that (010) and (100) become dominant (fig. 2), while also (110) appears. At a = 0.43 the crystals are more elongated and sometimes hollow, while at a = 0.53 and 0.63 dendrites appear. With method B at a = 0.10, the habit is quite different from the one obtained with method A. The crystals are elongated along the c axis with (010) and (100) as main forms and (110), (120) and (210) as small faces. At the top the main form is (011) (fig. 3) with occasionally (211). Similar laths, but sometimes hollows, occur at a = 0.18. At higher a the habit becomes needle-like .
Fig. I. PhCi, crystals grown from pure aqueous s~luIion (method A) at a = 0.22.
Fig. 2. PbC12 crystals grown from pure aqueous solution (method A) at a = 0.32.
and at a = 0.79 dendrites occur. In search for an explanation of this unexpected difference in habit it was found that crystals obtained with method A showed extra lines in their X-ray powder diffraction pattern which could be unequivocally be ascribed to orthorhombic Pb(OH)Cl.
3.2. Crystallization in the presence of KC1, NH4CI or CdC12 .
.
Crystallization was carned out according to method A. KC1 decreases the solubility. The initial
Fig. 3. PhCI, c~stalgrown from pure aqueous solution (method B) at a = 0.10.
556
M. van Panhuys-Sigler eta!.
/ Habit changes of lead chloride
0.9 0
0.8 0.7 V
0.6
0
V
0
0.5 0~
0.4
)011~
0 0
0.3
0 0
0.28
~
V 0.1 ~
0
211~ I
0.05
0.10
0.15
0.20
0.25
conc.Cr(mol l~1) Fig. 4. Morphodrome of PbC1
2 grown in the presence of halides: (0) KC1: (D) NH4C1: (v) CdCI2 as cosolute. On the horizontal axis, the amount of the additives is expressed as moles Cl — per liter.
relative supersaturation was calculated using solubility curves [5]. The incorporation of K into PbCl2 was measured using atomic absorption spectrometry and appeared to be very low, between 0.002 and 0.009 at%. The results are presented in the form of a morphodrome (fig. 4) in which a is plotted versus the concentration of the cosolute. Only at low a and at low KC1 concentration is the habit isometric with {21 1) dominant. Higher a leads to needles [001] on which {011 } is the dominant terminal face (fig. 5). Increasing KC1 concentration favour this habit change so that in fig. 4 two areas can be distinguished, one where (211) is the characteristic and often dominant form, the other where (011) is present while (211) fails. Similar results were obtained with NH4C1 or CdCl 2- They appear in fig. 4 with different symbols. Several experiments were carried out with mixtures of the cosolutes: KC1 + NH4C1, KC1 + CdCl2 and NH4C1 + CdC12. As no solubility
curves are known for these systems only a qualitative estimate of a could be made. With this restriction in mind it was found that virtually the same habit changes occur as when only one cosolute was present.
Fig. 5. PbCl2 crystal grown at a = 0.33 from a solution containing 0.0268 mol KC1 per liter.
M. van Panhuys.Sigler et aL
/ Habit changes of lead chloride
3.3. Crystallization in the presence of HC1
557
forms (fig. 7c). Finally, in region E the habit is tabular (010) with large (121) and occasionally small (100), resulting in a rhomb-like appearance (fig. 7d). Thus, increasing HC1 concentration causes (121) to become more and more promi-
Crystallization experiments were carried out at the following concentrations of HC1: 0.013N, 0.096N, 0.8N, 3.2N, 4.37N, 4.8N and 6.17N. The solubility of PbC12 shows a minimum at about iN
nent with accompanying shortening along the c axis. It also causes the crystals to become more flattened according to (010). The effect of the supersaturation is not always clear. At 4.8N HC1 the habit changes with increasing supersaturation from that of region E, via D to C, clearly an increase in length along the c axis. At still higher supersaturations dendrites appear. For lower HC1 concentrations the effect is less obvious and therefore a statistical study was made of the crystals grown from 0.096N HC1 solutions. From 80 crystals obtained in an experiment the length L and breadth B was measured. The ratio R = L/B is considered as a habit characteristic. For each initial a value histograms of the number of crystals versus R were made, whereby the R values were put together in groups n
HC1, while at about 6N HC1 it becomes larger than in pure water. The results are shown in a morphodrome (fig. 6). In region A needles [001] prevail that show the forms (010), (100), (110), (120) with (lii), (011) and (121) as terminal faces. Not all forms are always present. The fleedles have (010) as largest faces, but sometimes (100) is dominant. As to the terminal faces, there is a slight tendency for (011) to be the most prominent one at low supersaturation and very low HC1 concentration. At the higher HC1 concentration (3.2N) (121) is the most prominent terminal form, while in the intermediate ranges (111) is present, together with (011) or (121). In region B the needles are shorter with (010) as dominant form with (121) again the most important terminal form (fig. 7a). In region C the habit is blade-like (fig. 7b) and, on the whole, again shorter, until in region D the crystals are six-sided plates (010) forms (100)(120) and (121), sometimes with with smallside (110) and/or
C. dendrites 1.0
:
0.5
•,~“
?
0
/a
~/~
,,~\\\
:
added HCI(mol 1h) Fig. 6. Morphodrome of PbCI 2 grown from HCI containing solutions. The dashed lines indicate approximately the borders between regions that are characterized by the drawn habits.
558
M. van Panhuys-Sigler et aL
/ Habit changes of lead chloride
4
Fig. 7. PbC1
2 crystals grown from HC1 containing solutions: (a) from 0.8N HC1 at a = 0.08; (b) from 4.37N HC1 at a from 4.8N HCI at a = 0.18; (d) from 6.17N HC1 at a = 0.09.
=
0.14; (c)
30
25
N~
025
10
20
30
B
Fig. 8. Smoothened histograms of the length/breadth ration R of PbC12 crystals grown from 0.096N HCI solutions at five different supersaturations a.
the more so for the smaller R values. Hartley’s maximum F-test [6] proved that the variances of the distributions are not significantly different at the rather low level of 0.05. This provides the basis to carry out an F test to see whether there is a significant difference between the averages (one way analysis of variance [6]). It was found that the different distributions practically have no chance to represent the same population, neither those with low R among themselves, nor those with high R. The conclusion is therefore that for different initial a entirely different distribution are obtamed.
M. van Panhuys.Sigler et aL
4. Discussion The drastic habit change from (211) as dominant form obtained by method A to prismatic crystals with (Oil) in method B can be explained as follows. From pure solution and also from halide containing solutions very often a deposit of Pb(OH)Cl was formed along with the PbC12 crystals. In method B the supernatant solution was used, which is therefore more acidic. This means that the occurrence of (211) is due to preferential adsorption of OH ions on these faces, maybe in the form of a two-dimensional adsorption layer of Pb(OH)Cl. It is to be recalled that addition of a NaCl solution to a Pb-acetate solution does not lead to the crystallization of PbCl2 but of Pb(OH)Cl [4]. Therefore the habit obtained with method B should be considered as the one that least affected bya external factors. The additionis of halides causes competition between the adsorption of OH and Cl-, so that higher concentrations of halides lead to a smaller effect of 0H adsorption and therefore a higher growth rate of (211). As a result the habit becomes more elongated along the c axis, while (011) appears as terminal face. Increase of supersaturation also diminishes the effect of OH— adsorption, so that the morphodrome of fig. 4 becomes understandable. The question as to why (211) is affected most by OHadsorption will be considered in a separate paper [7]. Using the stability constants recorded by Smith and Martell [8] it appears that in pure aqueous solution the major species is PbCl The concentration of free Pb2 + ions is smaller, while about 5% PbCl 2 is not dissociated. The effect of halides is to slightly decrease the PbCl + concentration, while that2 tof At thethe PbCl2 species increases ofat KC1 the highest concentration cost of Pb ~ is formed, just before KPb even PbCl 2C15 crystallizes instead of PbCl2 [5]. The morphodrome of PbCl2 crystallizing from HC1 solutions is abnormal in the sense that the lines separating the various habits, increase with HC1 concentration, instead of decrease as is usual. -
-
~.
/ Habit changes of lead chloride
559
The effect of HC1 is clearly twofold: it decreases the growth rate of (121) and of (010). The morphodrome (fig. 6) and the statistical evaluation show that an increase of supersaturation results in a more elongated habit. The large needles obtamed from 0.013N HC1 and at a = 0.08 could be measured with an optical goniometer: they never showed the form (121). Instead {ii1) and/or (011) were present. They have much the same habit as those obtained from pure solution with method B. Hence the appearance of (121) must be due to preferential adsorption of some ionic species. The prevailing species in solutions with HC1 concentrations of iN or higher is PbCl~ apart from H~ or H3O~. Because at constant supersaturation an increasing HC1 concentration leads ultimately to a dominance of (121) the species responsible for this change 3. must begrowth H~ or The H3O~,probably along with PbC1 rate retardation of (010) seems to have its maximum at SN HC1, beyond which the crystals become thicker again. At the highest pH in our experiments the solution contains a notable amount of PbCl~, so the effect on (010) has probably to be ascribed to adsorption of PbCl The occurrence of these complexes in solution becomes immediately apparent from the solubility data [5]: an increasing amount of HC1 leads first to a sharp decrease of the solubility with a minimum at about iN HC1, which is then followed by a gradual increase. The bimodal distribution found in the experiments at 0.096N HC1 has to be ascribed to nucleation at different supersaturations. The longer (large R) and larger crystals have nucleated at higher a values than the shorter and smaller ones. The fact ~.
that the distribution is not continuous (except for the highest a value) but bimodal, points to the existence of a critical Belowexa~ the adsorption of the supersaturation habit modifyinga~. species erts its largest influence, while beyond a~the growth rate is high enough to overcome the adsorption effect. It should be mentioned here that the habit change is not of ‘the cubic-octahedral type that occurs on NaCl, where K faces appear. A PBC analysis of the PbCl2 structure [7] showed all forms mentioned here to be F forms.
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M. van Panhuys-Sigler et aL
/
Habit changes of leadchloride
5. Conclusions
growth rates. The effect is probably due to a combined effect of H~and PbCl~ions. Increas-
(1) The habit of PbC12 crystals that is least affected by external factors can be described as elongated along the c axis with (010) and (100) as main forms and (110), (120) and (210) as smaller faces, while at the top (011) is the dominant form. (2) The dominant form (211) occurring at crystallization from a freshly prepared slightly supersaturated aqueous solution is due to adsorption of OH ions. The solvent acts as an impurity. (3) Crystallization in the presence of KC1, NH4C1 or CdCl2 or mixtures of these enhances the growth rate of (211) by the adsorption competition between the OH and Cl ions. Increase of solute concentration and of supersaturation leads to the disappearance of (211), to a more elongated habit and to the occurrence of (011) as dominant terminal form. (4) The effect of increasing HC1 concentration is the slowing down of the (121) and (010)
ing supersaturation gives an elongated habit with dominating (010) and (100).
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References [11P.
Groth, Chemische Krystallographie (Engehnann, Leip-
1906) Vol. I. G. 219.Donnay, E.G. Cox, 0. Kennard and [2] zig, J.D.H. Donnay, MV. King, Eds., Crystal Data, 2nd ed. (Am. Crystallographic Association, 1963). [31M. Abdulkhadar and M.A. Ittyachen, J. Crystal Growth 48 (1980) 149. [41zig, P. Groth, Chemische Krystallographie (Engelmann, Leip1906) Vol. I, 297. [5] W.F. Linke and A. Seidell, Solubilities (Amer. Chem. Soc., Washington, DC, 1965). [61R. Till, Statistical Methods for the Earth Scientist (MacMillan Press Ltd., London, 1974). [7] C.F. Woensdregt and P. Hartman, J. Crystal Growth 87 (1988) 561. [8] R.M. Smith and A.E. Martell, Critical Stability Constants, Vol. 4: Inorganic Complexes (Plenum, New York, 1976).