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The electrokinetic potential of sparingly soluble sulfates. I
THE ELECTROKINETIC POTENTIAL OF SOLUBLE SULFATES. I
SPARINGLY
A. S. Buchanan and E. Heymann Department of Chemistry, University of Melbourne, Melbourne, Australia Received November 19, 1948
I. INTRODUCTION We have suggested recently (l) that the ~-potential of BUS04 in its saturated solution depends on the structure (geometrical and otherwise) of the surface. "Ignited crystals of natural barite have a much lower positive potential than samples of precipitated BUS04 which were recrystallized h'om conc. H2S04 and ignited afterwards. The former have very even surfaces while the latter have irregular surfaces. Our investigations have shown, further, that the ~-potentia] of BaSO4 in its own saturated solution cannot be explained solely by adsorption of Ba ++ and S04 := from solution, but is a characteristic of the crystal itself. It is suggested that the positive potential arises mainly from a preferential release of S04 = from the crystal lattice; however, preferential ion adsorption becomes more important in electrolyte solution. Meanwhile, Grimley and Mort (2) have published a theoretical discussion on the surface potential of AgBr in electrolyte solutions. In two important respects their conclusions appear to coincide with our deductions from the experimental material on BaSO~. In the first instance, the ~harge on the surface of the crystal in its own saturated Solution is due to an inherent property of the crystal itself. Grimley and Mott consider the ~-potential to be due to an excess of vacant lattice sites or interstitial ions in a region of the order of 10-6 cm. thick near the interface, while we have concluded that the degree "of disorder (geometrical or otherwise) determines the magnitude of the potential assumed by the solid, the more disordered surface having a higher potential. Further developments of the work of Grimley and Mort may afford an explanation, in terms of lattice defects, of these results with BaSO4. Secondly, ions from solution, although undoubtedly changing the potential, are, according to Grimley and Mort, "not appreciably adsorbed actually on the surface of silver bromide." Our experimental results lead us to the conclusion that ions reversibly adsorbed from solution, 137
138
A.s.
BUCHANAN
AND
E.
HEYMANN
and changing the potential of BaSO~, are not permanently built into the lattice, but are held more or less loosely near the crystal surface and probably remain hydrated. In our opinion, the contribution by Grimley and Mott is significant in that, for the first time, a theoretical treatment emphasizes the importance of properties of the solid, rather than of the solution, in determining the surface potential of an ionic solid. We have extended our investigation to other sparingly .soluble sulfates, viz., SrSQ, PbS04, CaSQ.2H20, and investigated solutions in water and in alc0hol-water mixtures. II. EXPERIMENTAL
The experimental technique of determining the streaming potential and resistance of the electrolyte in the porous plug has been described previously (1). Rigorous precautions were taken to exclude surface-active impurities. Perforated platinum electrodes coated with silver and silver chloride were used; with these electrodes there was no evidence of polarization in solutions of chlorides, nitrates and sulfates at concentrations below 10-8 N. Recrystallized BaS04, SrSO4, and'PbSO~ were obtained by dissolving precipitated samples of these compounds in hot concentrated sulfuric acid, from which they crystallize after concentration and cooling (cf. 1). The BaSO, and SrS04 were ignited at 700°C.; PbS04 was ignited at 520°C. in order to avoid decomposition. Crystals of up to 1 ram. in diameter may be obtained in this way. Fairly large crystals (up to 0.5 ram. diameter) of BaSO~ were also obtained by very slow precipitation at high dilution by adding very dilute H2S04 from a dropping funnel to twice reerystallized BaC12 dissolved in N HC1 at 90°C. BaSO4 obtained by ordinary precipitation methods is unsuitable for investigation because the electrokinetic equations break down for plugs containing very fine particles (1), probably because of anomalous viscosity effects in narrow channels (3, 4). . The treatment of the natural samples of BaS04 and SrS04 was the same as that previously described for barite (1). Natural gypsum, CaSO4.2H20, was investigated without any treatment except washing with warm conductivity water which was free from surface-active impurities. The crystals were analyzed using gravimetric and spectrographic methods. The sample contained 0.46% MgO, 0.11% CO2 as carbonate, 0.1-0.2% SiO2, and traces of Fe and Na. Precipitated gypsum was made by slowly adding dilute solutions of twice recrystallized calcium nitrate and sulfuric acid from dropping funnels to 3 1. of well stirred boiling water. In this way, thin needle-shaped crystals were obtained. Larger crystals required for retaining the finer crystals in
SPARINGLY SOLUBLE SULFATES. I
139
the cell between the perforated electrodes were obtained by keeping a saturated solution of CaSO~ in N/IO H2S04 in a vacuum desiccator over cone. H2SO4. Crystals of 3 mm. length were obtained in this way. All electrolyte solutions were made from chemicals of A.R. purity, recrystallized if necessary and analyzed spectroscopically. Double distilled water was used, KMn04 and a few drops of dilute H2S04 being added during the second distillation to eliminate any organic surfaceactive impurities that may be present. Neglect of these precautions leads to variable results. The cell constants of our plugs (cf. Section III) were determined from resistance measurements of plugs filled with N/IO or N/50 KC1 solution. In the case of gypsum, 0.1 N solution of KBr in a 44.1% (wt.) alcoholwater mixture was used. We estimate the overall accuracy of our ~-potential measurements as 1.5% for BaS04, 3-4% for SrS04 and PbS04, and ~ 5 % for gypsum. The accuracy decreases with increasing solubility of the solid, that is, increasing conductivity of the system. III. CALCV~ATIO~ OF THE ~-POTENTIAL For the calculation of ~ from measurements of the streaming potential and plug resistance the Helmholtz-Smoluchowski equation, as modified by Briggs, was used, viz.-4~r~ E # ~" =
n
p
,
(1)
where ~ is the viscosity and D the dielectric constant of the solvent, E the streaming potential and P the pressure under which the liquid is forced through the porous plug, # = C/R is the specific conductivity of the liquid contained in the porous plug and thus includes the bulk conductivity (~) and the surface conductivity. C is the cell constant of the plug and R its resistance between the two perforated electrodes. Thus, we obtain
4r~
EC
= D "R--P"
(2)
A comparison of # with Kwas made in all experiments. With our systems, consisting of coarsely crystalline materials, the surface conductivity in no case amounted to more than 1-2% of the bulk conductivity and was usually less. In all calculations we used the value of the dielectric constant of the respective solvent. However, the dielectric constant in the diffuse double layer may be different from that of the bulk of the solution. There is no possibility at present of correcting for this. If one is concerned with very
140
A. S. BUCHANAN AND E. HEYMANN
dilute aqueous solutions only, the correction may have the same value in all cases, and ~ calculated in the manner outlined above, though its absolute value maY be in error, may still be a useful means of characterization for comparative purposes. This appears to be a tacit assumption made by most investigators. Its justification may be doubted, however, if one wishes to compare results obtained in aqueous solutions with those for solutions in alcohol-water mixtures. In such cases it may be preferable, in accordance with a suggestion by Guggenheim (5), to .use the electric moment of the double layer p as a basis of comparison, viz., ¢D #=
4~r
We have quoted our results in terms of f throughout this paper. In order to make a calculation of u from our experimental values of ~ possible, we also give the values of D used in our calculations (Table I). TABLE
I
Water 6°C. Water 18°C. Water 30°C 8.06% (wt.) alcohol in water 20°C. 20.88% (wt.) alcohol in water 20~C. I V . DEPENDENCE
OF T H E
D 85.5 80.8 76.5 75.6 68,2
~'-POTENTIAL ON THE ORIGIN OF THE SOLID
Table II shows the ~'-potentials of a number of samples in their own saturated aqueous solution. All samples were ignited. The solid is positive in all cases. All f-potential values are reproducible. Spectroscopic analysis revealed impurities, particularly in the ease of the natural samples. However, the differences in impurity content between the natural and synthetic samples are not sufficiently marked t o account for the large differenvies in f-potential. In the case of BaS0a, it was shown previously (1) that incorporation of some of the common impurities of natural barite in the precipitated and reerystallized samples did not produce a ~-potential similar to that of natural barite. We must, therefore, conclude that the differences between natural and synthetic samples are not due to chemical impurities. With both BaS04 and SrS04, the natural samples with regular surfaces have a much lower ~-potential than the precipitated and reerystallized samples possessing irregular surfaces. The irregularity of the recrystalliz~ed samples is presumably produced on ignition of the crystals which, before ignition, may have included some acid sulfate and sulfuric acid from the reerystallization process. That the high potential is due to surface irregularity and not to chemical impurities is further suggested
SPARINGLY SOLUBLE SULFATES.
I
TABLE II
]41
f in m y . (20oc.)
Barium sulfate (1) Natural Barite (sample B), clear well-built crystals 5.3; 5.4 (2) BaSOt, crystals (0.2-0.5 ram. diameter) obtained by slow precipitation at high dilution in N IIC1 15.0 (3) BaSO~, sample IV, precipitated and recrystMlized from hot cone. H2S04; crystals with very irregular surfaces (electron microscope) 26.6; 26.8 (4) Natural Barite (sample B), crystals wetted with cone. H2S04 and ignited afterwards 27.0; 27.7 Strontium sulfate (1) Natural Celestite (Gloucestershire, England), slightly translucent well-formed crystals 1.4; 1.8 (2) SrSO4, precipitated and reerystallized from hot cone. H~SO4, crystals with irregnlar surfaces 13.8; 14.4 (3) Natural Celestite, crystals wet%ed with cone. H~SO¢ and ignited afterwards 15.0; 15.3 Lead sulfate" (1) PbSO4, reerystallized from hot cone. H~SO4, crystals (0.2-0.5 mm. diameter) with irregular surfaces 16.0; 16.0 Unfortunately, no samples of natural Anglesite of sufficient purity were obtainable, and therefore a comparison of the natural and synthetic materials could not be made. b y the fact t h a t natural barite and celestitc, having a small positive potential, on wetting with cone. H2S04 and subsequent ignition, acquire a large positive potential, similar in magnitude to t h a t of the recrystallized samples. T h e geometrical surface irregularity of the samples which have a large ~'-potential has been established b y optical and electron microscopy. We are indebted to Dr. A. L. G. Rees and to A. J. Hodge, of the Division of Industrial Chemistry of the Council for Scientific and Industrial Research, Melbourne, for this investigation. In the light of the ideas of Grimley and M o t t (2), it would appear t h a t the lattice defects in the layer immediately adjacent to the surface .are operative in producing a higher charge density in a geometrically irregular surface than in an even one; or else the number of lattice defects m a y be greater in the recrystallized samples than in the naturM ones. 1 There m a y be some doubt as to whether theoretical conclusions from a comparison of the ~-potential values of the 3 sulfates are justified in view of the limited experimental material available (Table III). Moreover, it is doubtful whether the ~-values are suitable for comparative purposes, since the potential at the crystal surface is a function of the surface disorder, or surface lattice defects, and there is no certainty that. i This explanation differs somewhat from the one given previously (1).
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A.
S. B U C H A N A N
AND E.
HEYMANN
TABLE III ~"my. (all positive)
Origin
Natural Recryst. Pptd. dil. I:[C1
BaSO4
PbSO~
SrSO~
5.3 26.7 15.0
16.0
1.5 14.0
for instance, all the recrystallized samples have even approximately the same degree of Surface disorder. Bearing these reservations in mind, it does appear, however, that, for samples of similar origin, the ~--values are in the order of BaS04 >> PbS04 > SrS04. The order of solubility is SrSO4 > PbSO~ > BaSO~, and, therefore, the more soluble compound seems to have the smaller ~-potentiM. A similar result is obtained for the t-potentials and solubilities of the three sulfates in alcohol-water mixtures (Table IV). TABLE IV ~-Potentials (in my.) in Alcohol-Water Mixtures (20°C.) (All Positive)
Water 8.06% (wt.) alcohol 20.88% (wt.) alcohol
BaSO~ (reeryst.)
PbSO~ (recryst.)
SrSO~ (recryst.)
26.6; 26.8 28.9; 28.9 28.9; 28.7
16.0; 16.0 -11.9; 11.6
13.8; 14.4 -10.6; 10.8
One cause of this influence of the solubility of the solid on the t-potential may be a reduction of the thickness of the diffuse double layer as the concentration of ions in solution increases. However, it appears from our experiments in concentrated solutions~ that the influence of increasing electrolyte concentration on the F-potential is smaller than has often been assumed. Solubility of an ionic solid depends on the lattice energy and on the energy of hydration of the ions concerned. One would expect the ~-potentials of the 3 sparingly soluble sulfates to be related to these quantities also. A larger free energy of hydration should counteract preferential retention of the cation by the lattice, and thus the t-potential should decrease with increasing free energy of hydration of the cation. Actually, computations by Bernal and Fowler (6) suggest that the heat of hydration of Sr ++ is about 10% greater than that of Ba ++, which would be 2 cf. the third paper of this series.
SPARINGLY SOLUBLE SULFATES. I
143
compatible with the above reasoning; on the other hand, that of Pb ++ is slightly greater than that of Sr ++. It must be borne in mind, however, that the difference between the heats of hydration of Ba ++ and Sr ++ is rather Small to account for the large difference in f-potential. Furthermore, it is uncertain whether the order of heats of hydration is the same as that of the free energies of hydration; nevertheless, it appears that the entropies of hydration of the 3 cations in question are not very different (cf. 7). V. THE TEMPERATURE COEFFICIENT OF THE ~'-POTENTIAL OF B A S 0 4
Several authors determined temperature coefficients of ~-potentials, but the systems chosen were rather complex (e.g., cellulose, clay). Both decreases and increases of ~ as well as changes of sign were observed (8). It appeared desirable to determine the temperature coefficient for a simple system, such as BaSO4 against its saturated solution. Fig. 1 shows that the ~'-potential increases with decreasing temperature, d~/dT being - 0.20 mr./degree.
¢ my
30 25
5
I0
15
oC
20
2,5
30
FIG. 1. ~'-potential (in mv.) of recrystallized BaSO4 plotted against temperature in oC. A variation of ~" with temperature may be related to the increasing thickness of the diffuse double layer with increasing temperature; this may, however, be counteracted by the fact that the saturation concentration increases somewhat with increasing temperature (2.10 X 10-3 g./1. at 5°C. and 3.0 X 10-3 g./1. at 30°C.). Another possibility is that preferential adsorption of Ba ++ over SO4= increases with decreasing temperature. Fig. 2 shows the variation of the ~-potential (A~) with the concentration of BaC12 and Na2SO4 at two temperatures. Although both the increase of ~ due to BaCl= and the decrease of ~ due to Na2S04 are greater at 30°C. than at 10°C,, there is no marked change with temperature in the difference between these two quantities which may be regarded as a measure for the preferential adsorption of Ba ++. For instance, in N/50,000 solutions (which correspond roughly to the concentration of a saturated BaS04 solution) we have at I0°C. A~ (BaC12): +8.5 mv. and
144
A: s. BUCHANAN AND E. HEYMANN
A~ (Na~S04): - 6 . 0 my.; and at 30°C. A~ (BaC12): +4.5 my. and A~" (Na2SO4) : - 2 . 5 my. The difference between the effects of BaCl2 and Na2SOt is ~-~-2my. for both temperatures. There is thus no evidence that the variation of the ~-potential with temperature is due to a variation in the preferential adsorption of Ba ++ over SOt=.
~v 60
~---"
--'-'-
.-----
~.BaCI 2 iOoC
30°C 50 4o
30
/ /
?/
20 '~
....
I0
N~SO
~
I
2
3 '~, 5 6 IO 4EQUIVALENTS PER LITRE
FIG. 2. Variation of ~ (in my.) of recrystallized BaSO4 with the concentration of electrolytes (in 10-4 equivalents at 2 temperatures. V I . INFLUENCE OF THE SOLVENT AND OF ELECTROLYTES ON THE ~'-POTENTIAL OF B A S 0 4 , S R S 0 4 AND P B S 0 4
To compare the influence of electrolytes on the ~-potential of the 3 sulfates, it is preferable to use, instead of water, an alcohol-water mixture as solvent, in order to reduce the solubility of PbS04 and SrSO4, and thus the conductivity of the systems, sufficiently to make an accurate determination of ~ by the streaming potential m e t h o d possible. The influence of alcohol on the initial ~-potential is noticeable but not great (Table IV). Fig. 3 shows the influence of electrolytes on the ~-potential of BaSO4 both in water and in 20..88% (~'vt.) alcohol-water mixtures. All electrolyte effects are reversible except some obtained with lanthanum salts, which exhibited slight irreversibility. In all cases in which electrolytes have a marked effect, such effects are greater in alcohol-water mixtures than in water. 3 This is probably due to a decrease in the attraction between the 3 Similar results were obtained by Ruyssen (9) and by Reyerson, Kolthoff and Coad (10). However, these investigators employed fine precipitates of BaSO4, which, for reasons mentioned previously (Section III), appear to us to be less suitable for electroldnetic investigation than recrystallized BaSO4.
SPARINGLY SOLUBLE SULFATES.
lOC
145
I
Ba CI 2 al~
Cs o
--.'- ~
/
----'-
La(.NO~,s alc.
mv
--'--'-"
60
--
. .....
---j
....
~'"
--
--7-
B_a C_I~ wat_e._r . . . .
.<
...........
S(NO~.*;t;r
. . . . .
40 20 0
rll~_
.........
-% a'~-.~
NapS(
water
t alc, --"-'----- I----._._
I
2
3 -4 10
4 5 ' 6 Equivalents perLitre
7
8
9
IO
FIG. 3. Plot of the ~'-potential (in inv.) of recrystallized BaS04 against the concentration of various electrolytes (in 10 -4 equivalents/1.)" dissolved in water and in alcohol (20.88 w t . - % ) - - w a t e r mixtures (20°C.).
ions concerned and the solvent, on partly replacing water by alcohol, as a consequence of which adsorption on the crystal surface increases. That these effects are real and not due to the uncertainty about the value of the dielectric constant in the double layer (cf. Section III) is evident on comparison of the curves for sodium sulfate in water and in an alcoholwater mixture. While, in aqueous solution, Na~SO4 merely lowers the positive potential to small values, a change of sign occurs in the alcoholwater mixture (f = - 19 my. at 6 X 10-* N). This can only be explained by increased adsorption of S04= from the alcohol-water mixture. Fig. 4 shows the change of the f-potential (At) produced by addition of electrolytes for BaS04, PbS04, and SrS04 in 20.88% alcohol-water mixtures. The magnitude of the electrolyte effects, both positive and negative, is in the order of BaSO4 > PbS04 > SrS04. Thus the influence of electrolytes on the f-potential of these 3 sparingly soluble sulfates decreases with increasing solubility of the sulfate. 4 On the basis of these experiments it seems probable that preferential adsorption is less strong on the surface of a moderatly soluble solid than on that of a sparingly soluble one. 4 The order of solubilities in 20.88% alcohol-water mixtures is shown qualitatively by the specific conductivities at 20°C.: BaSO4, 1.7 X 10-6; PbS04; 3.3 X 10-6; SrS04, 6.40 X 10 -6 ohm. -1 cm. -1. The ionic mobilities of Ba ++, P b ++, and Sr ++ in the alcoholwater mixture are not known b u t arc not likely to differ appreciably from one another. The corresponding specific conductivities in water are: BaSO4, 3.8 X 10-6; PbSO4, 34.0 X 10-6; SrSO,, 117 X 10-8 ohm. -I cm. -1, suggesting t h a t the addition of alcohol reduces the solubility of SrSO4 a n d PbS04 much more strongly t h a n t h a t of BaSO4.
L
146
A.s.
my
~.~
BUCHANAN AND E. HEYMANN
.._..__-
/ 20
PbCI2 PbSO~ '
"~~4 ~'~"" ~
/
SrCI2 :SrS04
0 - 20 ~x~
"~
- 40
"~-~
NaSO~, (PbSO4)
'N,2SO cB,so2 I
2
3
4
5
6
7
8
g
I0
I0 -4 Equivalents per Lifre Fro. 4. Plot of the change of the ~'-potential (6~') in my. produced by various electrolytes against the concentration (in 10-4 equivalents/L) in alcohol (20.88 w t . - % ) - -
water mixture (20°C.) for recrystallized BaSO4, SrSO4, and PbS04.
Solubility may be regarded as a measure of the stability of a crystal lattice in contact with the solvent (cf. Section IV). It seems possible that the solid possessing a higher solubility in which the ions are not held so firmly by the lattice should exhibit reduced preferential ion adsorption from solution. Unfortunately, no values for the lattice energy of the solids concerned ~re available. The influence of lanthanum nitrate on the ~-potential of BaS04, SrSQ, and PbS04 is shown in Fig. 5. Lanthanum nitrate, in spite of the tervalency of La +++, increases the potential of BaS04 less than BaC12
r¢.~O-
!
2
3
4
5
6
I0 -4 EQUIVALENTS PER LITRE
FIG; 5. Plot of' the ~-potential (in inv.) of recrystallized BaSO4, SrSO4, and PbSO4 against the concentrt~tion of various electrolytes, including lanthanum nitrate, in alcohol (20.88 wt.-%)--water mixture (20°C.).
SPARINGLY SOLUBLE SULFATES. I
147
does, probably for reasons of geometry. In the case of PbSQ, the influence of lanthanum nitrate is approximately equal to that of PbC12. With SrSQ the influence of lanthanum nitrate is very much greater than that of SrC12, probably due to a fairly close similarity between the ionic radii of Sr ++ and La +++ (as compared with those of La +++ and Ba ++) which makes for strong adsorption of La +++ by SrSQ. Unfortunately no sufficiently concordant values for the ionic radii concerned are given by various authors. Some doubt may arise as to whether there is a large proportion of tervalent La+++ in solutions of lanthanum nitrate at concentrations smaller than 10-8 N. The experimental material, as well as general considerations, suggests that hydrolysis is less than with the corresponding aluminium salts, but the possibility of the existence of more complex ion species of lower valency cannot be ruled out. Nevertheless, the influence of lanthanum nitrate in very dilute solution on the f-potential of solids which are not heteropolar in nature, such as cellulose, is greater than that of alkaline earth chlorides (11), probably because geometrical considerations mentioned above are important in adsorption on the latrice of an ionic crystal but do not greatly affect adsorption on cellulose. It is probable, therefore, that even very dilute solutions of lanthanum nitrate contain a fair proportion of La +++. Fig. 4 shows that the potential-increasing effects of BaCI2 on BaS04, PbC12 on PbSO4, and SrC12 on SrSO~, particularly in dilute solution (1-2 × 10-~ eq./1.), are only little greater than the potential-decreasing effects of alkali sulfates on BaSQ, PbS04, and SrSQ, respectively. Thus, considering BaSQ, PbSQ, and SrSO4 in their saturated solutions, it is unlikely that preferential adsorption of the metal ion over that of S04= can account fully for the large positive potentials of these 3 sulfates. We therefore conclude that the f-potential of these sulfates in their saturated solution in alcohol-water mixtures can be accounted for fully only on consideration of properties of the solids themselves (cf. Section IV). VII. TH~ f-POTENTIAL OF GYPSUM The f-potential of gypsum, CaSQ.2H20, cannot be determined in water because of its high solubility, but only in alcohol-water mixtures. In contradistinction to BaSO4, SrS04, and PbS04, which have positive potentials, the potential of natural gypsum is negative, - 2 6 :t: 1 my. It is well reproducible, although less accurately than that of the other sulfates, and shows hardly any change on prolonged streaming. Change of the composition of the solvent mixtures between 24 and 52% (wt.) alcohol produces very little variation of f. Exclusion of surface-active impurities cannot be as rigorous in this system 5 as in the previous ones; 5 G y p s m n cannot be ignited.
148
,~. S. BUCHANAN AND E. HEYMANN
however, the disturbing effect of such impurities is less noticeable with gypsum than with the other sulfates, probably because surface-active substances, which usually render the potential more negative, are adsorbed less on the negative surface of gypsum than on the positive surfaces of BaS04, SrS04, and PbS04.
mv 0
La(NQ ,)3 .......
CaCI2
-20
I
2
3
164 EQUIVALENTS
H~SO, - ' ~ - = - . . . . Na Cite 4 5 8 PER LITRE
FI(~. 6. Plot of the ~--potential (in inv.) of natural gypsum (CaSO4.2H~O) against the concentration of various electrolytes in alcohol (44.1 wt.-%)--water mixture (20°C.).
Fig. 6 shows the influence of electrolytes on the ~-potential of gypsum. CaCl~ makes the potential less negative, while alkali sulfates and H2S04 render it more negative, the influence of H2S04 being smaller than that of other sulfates. Nevertheless, the influence of all these electrolytes is comparatively small Furthermore, there is little difference between the magnitudes of the potential increase due to sulfates and the potential decrease due to calcium salts. Thus t h e strongly negative potential of gypsum i n its saturated solution is not likely to be accounted for by preferential adsorption of S04 =, but must be explained in terms of properties of the solid similarly to the explanation given previously for BaS04. Sodium citrate makes the potential more negative but not quite as much as Na2S04. Thus, the citrate ion, in spite of its tervalency, is less effective than S04 =, probably because its size and field of force are very different from that of S Q =. On the other hand, La +++ has a stronger influence on the potential than Ca ++ and makes the potential positive. This, however, may partly be due to adsorption of hydrolysis products, the effect of lanthanum nitrate, as distinct from all other salts, not being quite reversible. 8 There is a marked difference in electrokinetic properties between naturM and precipitated gypsum. Not only is the ~-potential of precipitated gypsum very sm~ll, but it is difficult to reproduce, and the ratio of E / P varies with the pressure, contrary to the Helmholtz-Smoluchowski equation and to our experience with all systesm thus far investigated. Moreover, no reproducible influence of electrolytes on the ~-potential of precipitated gypsum can be detected. 6 With thorium nitrate variable positive potentials and formation of gelatinous hydrolysis products were observed.
SPARINGLY
SOLUBLE
SULFATES.
I
149
Photomicrographic and crystallographic examinations have shown that our samples of precipitated gypsum consist chiefly of twinned crystals, the growth of which has been relatively much greater in the direction of the C-axis than of the others, thus forming thin monoclinic needles. The crystals are very much thinner than those normally obtained with natural gypsum. According to Wooster (12), the characteristics of gypsum crystals are those of layers of Ca ++ and S04= joined by water molecules, and the coherence of the structure in one direction is due only to water molecules. The linkages between the layers of Ca ++ and S04= are relatively weak, thus accounting for pronounced cleavage planes. Only a small fraction of the surface area of the long and thin precipitated crystals--namely the ends of the needles--has, therefore, strong residual fields. The greater part of the other surfaces consists mainly of structural water molecules, and no Ca ++ or S04= in the actual surface, and is not likely to adsorb S04= appreciably. In these circumstances, a double layer of considerable charge density may be formed only at the end of the needles but not at the other planes. This would account for the smallness of the F-potential, its lack of reproducibility--since the measured potential is the result of contributions of double layers of high and low charge density, and will depend on the degree of randomness of packing of the crystals--and its insensitivity to addition of electrolytes, since little adsorption will occur on the major fraction of the total surface area. The well reproducible large potential of the natural gypsum, which is influenced by electrolytes, owing to ion adsorption, in a reproducible fashion, may be explained, according to this hypothesis, by the greater ratio of surfaces with high charge density, and appreciable adsorptive power, to those of low charge density and small adsorptive power. It is interesting to note that, while natural crystals of BaSO4, SrS04, and PbSO4 have small (positive) potentials, natural crystals of CaS04. 2H20 have a high (negative) potential. The small potential of natural BaSO4 (as compared with precipitated and recrystallized BaS04) has been suggested as being due to the comparative difficulty of releasing S04- from the well ordered surface (1). In the case of gypsum, the preferential release of Ca ++ may be less impeded either by the smallness of Ca ++ or by the weak forces in a layer lattice crystal in which part of the cohesion is d u e t o water molecules. The weakness of these forces may also be responsible for the smallness of the electrolyte effects. X. SVM~ARY
1. The electrokinetic potentials of natural, precipitated and recrystallized samples of BaS04, SrS04, and P b S Q have been investigated in water and alcohol-water mixtures using the streaming potential method.
150.
A. S. BUCHANAN AND E. HEYMANN
The potentials are positive. The natural samples which have very even surfaces always exhibit smaller potentials than precipitated and recrystallized samples having uneven surfaces. The potentials of the three sparingly soluble sulfates in water and alcohol-water mixtures decrease in the order of increasing solubility. 2. The temperature coefficient of the f-potential of BaS04 in its saturated solution is negative. This is not likely to be due to an increase in preferential ion adsorption with the lowering of temperature. 3. The effects of electrolytes have been investigated in water and in alcohol-water mixtures. For the same sparingly soluble sulfate the effects of electrolytes are greater in alcohol-water mixtures than in water. On comparison of various solid sulfates, it is found that the magnitude of the electrolyte effects decreases with increasing solubility of the solid. An explanation of the effect of various electrolytes on the f-potential in water and alcohol-water mixtures is attempted. 4. The f-potential of natural gypsum investigated in alcohol-water mixtures is negative. Electrolyte influence is smaller than with the other solid sulfates. Precipitated gypsum does not give reproducible values for the f-potential. 5. The results of this investigation suggest that the f-potential of a heteropolar solid is not solely determined by preferential ion adsorption, but is largely a characteristic of the solid itself. Samples of different origin, or preparation of the same compound, have widely different f-potentials. It is shown that this is not due to impurities but to different degrees of surface irregularity. Reference is made to a recent theory by Grimley and Mort relating the f-potential to lattice defects in the surface. REFERENCES 1. BUCHANAN, A. S., AND I-IEYMANN, E., Nature 161, 649 (1948); Proc. Roy. Soc. London 195A, 150 (1948). 2. GlCIMLEY,W. B., ANn iV[OTT,IN. F., Faraday Soc. Discussion 1, 3 (1947). 3. BOOTH, F., Nature 161, 83 (1948). 4. ELTON, G. A. H., Proc. Roy. Soc. London 194A, 258 (1948). 5. GUGGENHEIM,E. A., Trans. Faraday Soe. 36, 139 (1940). 6. BERNAL, J. D., AND FOWLER, R. H., J. Chem. Phys. 1, 515 (1933). 7. RICE, O. ~., Electronic Structure and Chemical Binding. New York, 1940, p. 408. 8. Literature vide ABRAMSON,]7I. A., Electrokinetic Phenomena, New York, 1940. 9. RUVSSEN, R. G., J. Phys. Chem. 44, 271 (1940). 10. REYERSON, L. H.j KOLTHOFF, I. ~-~/[.,AND COAD, K., J. Phys: Colloid Chem. 51, 321 (1947). 11. I~AEINOV,G., AND HEYMANN, E., J. Phys. Chem. 47, 655 (1943). 12. WOOSTER, W. A., Z. Krist. 94, 375 (1936).
SPARINGLY
A. S. Buchanan and E. Heymann Department of Chemistry, University of Melbourne, Melbourne, Australia Received November 19, 1948
I. INTRODUCTION We have suggested recently (l) that the ~-potential of BUS04 in its saturated solution depends on the structure (geometrical and otherwise) of the surface. "Ignited crystals of natural barite have a much lower positive potential than samples of precipitated BUS04 which were recrystallized h'om conc. H2S04 and ignited afterwards. The former have very even surfaces while the latter have irregular surfaces. Our investigations have shown, further, that the ~-potentia] of BaSO4 in its own saturated solution cannot be explained solely by adsorption of Ba ++ and S04 := from solution, but is a characteristic of the crystal itself. It is suggested that the positive potential arises mainly from a preferential release of S04 = from the crystal lattice; however, preferential ion adsorption becomes more important in electrolyte solution. Meanwhile, Grimley and Mort (2) have published a theoretical discussion on the surface potential of AgBr in electrolyte solutions. In two important respects their conclusions appear to coincide with our deductions from the experimental material on BaSO~. In the first instance, the ~harge on the surface of the crystal in its own saturated Solution is due to an inherent property of the crystal itself. Grimley and Mott consider the ~-potential to be due to an excess of vacant lattice sites or interstitial ions in a region of the order of 10-6 cm. thick near the interface, while we have concluded that the degree "of disorder (geometrical or otherwise) determines the magnitude of the potential assumed by the solid, the more disordered surface having a higher potential. Further developments of the work of Grimley and Mort may afford an explanation, in terms of lattice defects, of these results with BaSO4. Secondly, ions from solution, although undoubtedly changing the potential, are, according to Grimley and Mort, "not appreciably adsorbed actually on the surface of silver bromide." Our experimental results lead us to the conclusion that ions reversibly adsorbed from solution, 137
138
A.s.
BUCHANAN
AND
E.
HEYMANN
and changing the potential of BaSO~, are not permanently built into the lattice, but are held more or less loosely near the crystal surface and probably remain hydrated. In our opinion, the contribution by Grimley and Mott is significant in that, for the first time, a theoretical treatment emphasizes the importance of properties of the solid, rather than of the solution, in determining the surface potential of an ionic solid. We have extended our investigation to other sparingly .soluble sulfates, viz., SrSQ, PbS04, CaSQ.2H20, and investigated solutions in water and in alc0hol-water mixtures. II. EXPERIMENTAL
The experimental technique of determining the streaming potential and resistance of the electrolyte in the porous plug has been described previously (1). Rigorous precautions were taken to exclude surface-active impurities. Perforated platinum electrodes coated with silver and silver chloride were used; with these electrodes there was no evidence of polarization in solutions of chlorides, nitrates and sulfates at concentrations below 10-8 N. Recrystallized BaS04, SrSO4, and'PbSO~ were obtained by dissolving precipitated samples of these compounds in hot concentrated sulfuric acid, from which they crystallize after concentration and cooling (cf. 1). The BaSO, and SrS04 were ignited at 700°C.; PbS04 was ignited at 520°C. in order to avoid decomposition. Crystals of up to 1 ram. in diameter may be obtained in this way. Fairly large crystals (up to 0.5 ram. diameter) of BaSO~ were also obtained by very slow precipitation at high dilution by adding very dilute H2S04 from a dropping funnel to twice reerystallized BaC12 dissolved in N HC1 at 90°C. BaSO4 obtained by ordinary precipitation methods is unsuitable for investigation because the electrokinetic equations break down for plugs containing very fine particles (1), probably because of anomalous viscosity effects in narrow channels (3, 4). . The treatment of the natural samples of BaS04 and SrS04 was the same as that previously described for barite (1). Natural gypsum, CaSO4.2H20, was investigated without any treatment except washing with warm conductivity water which was free from surface-active impurities. The crystals were analyzed using gravimetric and spectrographic methods. The sample contained 0.46% MgO, 0.11% CO2 as carbonate, 0.1-0.2% SiO2, and traces of Fe and Na. Precipitated gypsum was made by slowly adding dilute solutions of twice recrystallized calcium nitrate and sulfuric acid from dropping funnels to 3 1. of well stirred boiling water. In this way, thin needle-shaped crystals were obtained. Larger crystals required for retaining the finer crystals in
SPARINGLY SOLUBLE SULFATES. I
139
the cell between the perforated electrodes were obtained by keeping a saturated solution of CaSO~ in N/IO H2S04 in a vacuum desiccator over cone. H2SO4. Crystals of 3 mm. length were obtained in this way. All electrolyte solutions were made from chemicals of A.R. purity, recrystallized if necessary and analyzed spectroscopically. Double distilled water was used, KMn04 and a few drops of dilute H2S04 being added during the second distillation to eliminate any organic surfaceactive impurities that may be present. Neglect of these precautions leads to variable results. The cell constants of our plugs (cf. Section III) were determined from resistance measurements of plugs filled with N/IO or N/50 KC1 solution. In the case of gypsum, 0.1 N solution of KBr in a 44.1% (wt.) alcoholwater mixture was used. We estimate the overall accuracy of our ~-potential measurements as 1.5% for BaS04, 3-4% for SrS04 and PbS04, and ~ 5 % for gypsum. The accuracy decreases with increasing solubility of the solid, that is, increasing conductivity of the system. III. CALCV~ATIO~ OF THE ~-POTENTIAL For the calculation of ~ from measurements of the streaming potential and plug resistance the Helmholtz-Smoluchowski equation, as modified by Briggs, was used, viz.-4~r~ E # ~" =
n
p
,
(1)
where ~ is the viscosity and D the dielectric constant of the solvent, E the streaming potential and P the pressure under which the liquid is forced through the porous plug, # = C/R is the specific conductivity of the liquid contained in the porous plug and thus includes the bulk conductivity (~) and the surface conductivity. C is the cell constant of the plug and R its resistance between the two perforated electrodes. Thus, we obtain
4r~
EC
= D "R--P"
(2)
A comparison of # with Kwas made in all experiments. With our systems, consisting of coarsely crystalline materials, the surface conductivity in no case amounted to more than 1-2% of the bulk conductivity and was usually less. In all calculations we used the value of the dielectric constant of the respective solvent. However, the dielectric constant in the diffuse double layer may be different from that of the bulk of the solution. There is no possibility at present of correcting for this. If one is concerned with very
140
A. S. BUCHANAN AND E. HEYMANN
dilute aqueous solutions only, the correction may have the same value in all cases, and ~ calculated in the manner outlined above, though its absolute value maY be in error, may still be a useful means of characterization for comparative purposes. This appears to be a tacit assumption made by most investigators. Its justification may be doubted, however, if one wishes to compare results obtained in aqueous solutions with those for solutions in alcohol-water mixtures. In such cases it may be preferable, in accordance with a suggestion by Guggenheim (5), to .use the electric moment of the double layer p as a basis of comparison, viz., ¢D #=
4~r
We have quoted our results in terms of f throughout this paper. In order to make a calculation of u from our experimental values of ~ possible, we also give the values of D used in our calculations (Table I). TABLE
I
Water 6°C. Water 18°C. Water 30°C 8.06% (wt.) alcohol in water 20°C. 20.88% (wt.) alcohol in water 20~C. I V . DEPENDENCE
OF T H E
D 85.5 80.8 76.5 75.6 68,2
~'-POTENTIAL ON THE ORIGIN OF THE SOLID
Table II shows the ~'-potentials of a number of samples in their own saturated aqueous solution. All samples were ignited. The solid is positive in all cases. All f-potential values are reproducible. Spectroscopic analysis revealed impurities, particularly in the ease of the natural samples. However, the differences in impurity content between the natural and synthetic samples are not sufficiently marked t o account for the large differenvies in f-potential. In the case of BaS0a, it was shown previously (1) that incorporation of some of the common impurities of natural barite in the precipitated and reerystallized samples did not produce a ~-potential similar to that of natural barite. We must, therefore, conclude that the differences between natural and synthetic samples are not due to chemical impurities. With both BaS04 and SrS04, the natural samples with regular surfaces have a much lower ~-potential than the precipitated and reerystallized samples possessing irregular surfaces. The irregularity of the recrystalliz~ed samples is presumably produced on ignition of the crystals which, before ignition, may have included some acid sulfate and sulfuric acid from the reerystallization process. That the high potential is due to surface irregularity and not to chemical impurities is further suggested
SPARINGLY SOLUBLE SULFATES.
I
TABLE II
]41
f in m y . (20oc.)
Barium sulfate (1) Natural Barite (sample B), clear well-built crystals 5.3; 5.4 (2) BaSOt, crystals (0.2-0.5 ram. diameter) obtained by slow precipitation at high dilution in N IIC1 15.0 (3) BaSO~, sample IV, precipitated and recrystMlized from hot cone. H2S04; crystals with very irregular surfaces (electron microscope) 26.6; 26.8 (4) Natural Barite (sample B), crystals wetted with cone. H2S04 and ignited afterwards 27.0; 27.7 Strontium sulfate (1) Natural Celestite (Gloucestershire, England), slightly translucent well-formed crystals 1.4; 1.8 (2) SrSO4, precipitated and reerystallized from hot cone. H~SO4, crystals with irregnlar surfaces 13.8; 14.4 (3) Natural Celestite, crystals wet%ed with cone. H~SO¢ and ignited afterwards 15.0; 15.3 Lead sulfate" (1) PbSO4, reerystallized from hot cone. H~SO4, crystals (0.2-0.5 mm. diameter) with irregular surfaces 16.0; 16.0 Unfortunately, no samples of natural Anglesite of sufficient purity were obtainable, and therefore a comparison of the natural and synthetic materials could not be made. b y the fact t h a t natural barite and celestitc, having a small positive potential, on wetting with cone. H2S04 and subsequent ignition, acquire a large positive potential, similar in magnitude to t h a t of the recrystallized samples. T h e geometrical surface irregularity of the samples which have a large ~'-potential has been established b y optical and electron microscopy. We are indebted to Dr. A. L. G. Rees and to A. J. Hodge, of the Division of Industrial Chemistry of the Council for Scientific and Industrial Research, Melbourne, for this investigation. In the light of the ideas of Grimley and M o t t (2), it would appear t h a t the lattice defects in the layer immediately adjacent to the surface .are operative in producing a higher charge density in a geometrically irregular surface than in an even one; or else the number of lattice defects m a y be greater in the recrystallized samples than in the naturM ones. 1 There m a y be some doubt as to whether theoretical conclusions from a comparison of the ~-potential values of the 3 sulfates are justified in view of the limited experimental material available (Table III). Moreover, it is doubtful whether the ~-values are suitable for comparative purposes, since the potential at the crystal surface is a function of the surface disorder, or surface lattice defects, and there is no certainty that. i This explanation differs somewhat from the one given previously (1).
142
A.
S. B U C H A N A N
AND E.
HEYMANN
TABLE III ~"my. (all positive)
Origin
Natural Recryst. Pptd. dil. I:[C1
BaSO4
PbSO~
SrSO~
5.3 26.7 15.0
16.0
1.5 14.0
for instance, all the recrystallized samples have even approximately the same degree of Surface disorder. Bearing these reservations in mind, it does appear, however, that, for samples of similar origin, the ~--values are in the order of BaS04 >> PbS04 > SrS04. The order of solubility is SrSO4 > PbSO~ > BaSO~, and, therefore, the more soluble compound seems to have the smaller ~-potentiM. A similar result is obtained for the t-potentials and solubilities of the three sulfates in alcohol-water mixtures (Table IV). TABLE IV ~-Potentials (in my.) in Alcohol-Water Mixtures (20°C.) (All Positive)
Water 8.06% (wt.) alcohol 20.88% (wt.) alcohol
BaSO~ (reeryst.)
PbSO~ (recryst.)
SrSO~ (recryst.)
26.6; 26.8 28.9; 28.9 28.9; 28.7
16.0; 16.0 -11.9; 11.6
13.8; 14.4 -10.6; 10.8
One cause of this influence of the solubility of the solid on the t-potential may be a reduction of the thickness of the diffuse double layer as the concentration of ions in solution increases. However, it appears from our experiments in concentrated solutions~ that the influence of increasing electrolyte concentration on the F-potential is smaller than has often been assumed. Solubility of an ionic solid depends on the lattice energy and on the energy of hydration of the ions concerned. One would expect the ~-potentials of the 3 sparingly soluble sulfates to be related to these quantities also. A larger free energy of hydration should counteract preferential retention of the cation by the lattice, and thus the t-potential should decrease with increasing free energy of hydration of the cation. Actually, computations by Bernal and Fowler (6) suggest that the heat of hydration of Sr ++ is about 10% greater than that of Ba ++, which would be 2 cf. the third paper of this series.
SPARINGLY SOLUBLE SULFATES. I
143
compatible with the above reasoning; on the other hand, that of Pb ++ is slightly greater than that of Sr ++. It must be borne in mind, however, that the difference between the heats of hydration of Ba ++ and Sr ++ is rather Small to account for the large difference in f-potential. Furthermore, it is uncertain whether the order of heats of hydration is the same as that of the free energies of hydration; nevertheless, it appears that the entropies of hydration of the 3 cations in question are not very different (cf. 7). V. THE TEMPERATURE COEFFICIENT OF THE ~'-POTENTIAL OF B A S 0 4
Several authors determined temperature coefficients of ~-potentials, but the systems chosen were rather complex (e.g., cellulose, clay). Both decreases and increases of ~ as well as changes of sign were observed (8). It appeared desirable to determine the temperature coefficient for a simple system, such as BaSO4 against its saturated solution. Fig. 1 shows that the ~'-potential increases with decreasing temperature, d~/dT being - 0.20 mr./degree.
¢ my
30 25
5
I0
15
oC
20
2,5
30
FIG. 1. ~'-potential (in mv.) of recrystallized BaSO4 plotted against temperature in oC. A variation of ~" with temperature may be related to the increasing thickness of the diffuse double layer with increasing temperature; this may, however, be counteracted by the fact that the saturation concentration increases somewhat with increasing temperature (2.10 X 10-3 g./1. at 5°C. and 3.0 X 10-3 g./1. at 30°C.). Another possibility is that preferential adsorption of Ba ++ over SO4= increases with decreasing temperature. Fig. 2 shows the variation of the ~-potential (A~) with the concentration of BaC12 and Na2SO4 at two temperatures. Although both the increase of ~ due to BaCl= and the decrease of ~ due to Na2S04 are greater at 30°C. than at 10°C,, there is no marked change with temperature in the difference between these two quantities which may be regarded as a measure for the preferential adsorption of Ba ++. For instance, in N/50,000 solutions (which correspond roughly to the concentration of a saturated BaS04 solution) we have at I0°C. A~ (BaC12): +8.5 mv. and
144
A: s. BUCHANAN AND E. HEYMANN
A~ (Na~S04): - 6 . 0 my.; and at 30°C. A~ (BaC12): +4.5 my. and A~" (Na2SO4) : - 2 . 5 my. The difference between the effects of BaCl2 and Na2SOt is ~-~-2my. for both temperatures. There is thus no evidence that the variation of the ~-potential with temperature is due to a variation in the preferential adsorption of Ba ++ over SOt=.
~v 60
~---"
--'-'-
.-----
~.BaCI 2 iOoC
30°C 50 4o
30
/ /
?/
20 '~
....
I0
N~SO
~
I
2
3 '~, 5 6 IO 4EQUIVALENTS PER LITRE
FIG. 2. Variation of ~ (in my.) of recrystallized BaSO4 with the concentration of electrolytes (in 10-4 equivalents at 2 temperatures. V I . INFLUENCE OF THE SOLVENT AND OF ELECTROLYTES ON THE ~'-POTENTIAL OF B A S 0 4 , S R S 0 4 AND P B S 0 4
To compare the influence of electrolytes on the ~-potential of the 3 sulfates, it is preferable to use, instead of water, an alcohol-water mixture as solvent, in order to reduce the solubility of PbS04 and SrSO4, and thus the conductivity of the systems, sufficiently to make an accurate determination of ~ by the streaming potential m e t h o d possible. The influence of alcohol on the initial ~-potential is noticeable but not great (Table IV). Fig. 3 shows the influence of electrolytes on the ~-potential of BaSO4 both in water and in 20..88% (~'vt.) alcohol-water mixtures. All electrolyte effects are reversible except some obtained with lanthanum salts, which exhibited slight irreversibility. In all cases in which electrolytes have a marked effect, such effects are greater in alcohol-water mixtures than in water. 3 This is probably due to a decrease in the attraction between the 3 Similar results were obtained by Ruyssen (9) and by Reyerson, Kolthoff and Coad (10). However, these investigators employed fine precipitates of BaSO4, which, for reasons mentioned previously (Section III), appear to us to be less suitable for electroldnetic investigation than recrystallized BaSO4.
SPARINGLY SOLUBLE SULFATES.
lOC
145
I
Ba CI 2 al~
Cs o
--.'- ~
/
----'-
La(.NO~,s alc.
mv
--'--'-"
60
--
. .....
---j
....
~'"
--
--7-
B_a C_I~ wat_e._r . . . .
.<
...........
S(NO~.*;t;r
. . . . .
40 20 0
rll~_
.........
-% a'~-.~
NapS(
water
t alc, --"-'----- I----._._
I
2
3 -4 10
4 5 ' 6 Equivalents perLitre
7
8
9
IO
FIG. 3. Plot of the ~'-potential (in inv.) of recrystallized BaS04 against the concentration of various electrolytes (in 10 -4 equivalents/1.)" dissolved in water and in alcohol (20.88 w t . - % ) - - w a t e r mixtures (20°C.).
ions concerned and the solvent, on partly replacing water by alcohol, as a consequence of which adsorption on the crystal surface increases. That these effects are real and not due to the uncertainty about the value of the dielectric constant in the double layer (cf. Section III) is evident on comparison of the curves for sodium sulfate in water and in an alcoholwater mixture. While, in aqueous solution, Na~SO4 merely lowers the positive potential to small values, a change of sign occurs in the alcoholwater mixture (f = - 19 my. at 6 X 10-* N). This can only be explained by increased adsorption of S04= from the alcohol-water mixture. Fig. 4 shows the change of the f-potential (At) produced by addition of electrolytes for BaS04, PbS04, and SrS04 in 20.88% alcohol-water mixtures. The magnitude of the electrolyte effects, both positive and negative, is in the order of BaSO4 > PbS04 > SrS04. Thus the influence of electrolytes on the f-potential of these 3 sparingly soluble sulfates decreases with increasing solubility of the sulfate. 4 On the basis of these experiments it seems probable that preferential adsorption is less strong on the surface of a moderatly soluble solid than on that of a sparingly soluble one. 4 The order of solubilities in 20.88% alcohol-water mixtures is shown qualitatively by the specific conductivities at 20°C.: BaSO4, 1.7 X 10-6; PbS04; 3.3 X 10-6; SrS04, 6.40 X 10 -6 ohm. -1 cm. -1. The ionic mobilities of Ba ++, P b ++, and Sr ++ in the alcoholwater mixture are not known b u t arc not likely to differ appreciably from one another. The corresponding specific conductivities in water are: BaSO4, 3.8 X 10-6; PbSO4, 34.0 X 10-6; SrSO,, 117 X 10-8 ohm. -I cm. -1, suggesting t h a t the addition of alcohol reduces the solubility of SrSO4 a n d PbS04 much more strongly t h a n t h a t of BaSO4.
L
146
A.s.
my
~.~
BUCHANAN AND E. HEYMANN
.._..__-
/ 20
PbCI2 PbSO~ '
"~~4 ~'~"" ~
/
SrCI2 :SrS04
0 - 20 ~x~
"~
- 40
"~-~
NaSO~, (PbSO4)
'N,2SO cB,so2 I
2
3
4
5
6
7
8
g
I0
I0 -4 Equivalents per Lifre Fro. 4. Plot of the change of the ~'-potential (6~') in my. produced by various electrolytes against the concentration (in 10-4 equivalents/L) in alcohol (20.88 w t . - % ) - -
water mixture (20°C.) for recrystallized BaSO4, SrSO4, and PbS04.
Solubility may be regarded as a measure of the stability of a crystal lattice in contact with the solvent (cf. Section IV). It seems possible that the solid possessing a higher solubility in which the ions are not held so firmly by the lattice should exhibit reduced preferential ion adsorption from solution. Unfortunately, no values for the lattice energy of the solids concerned ~re available. The influence of lanthanum nitrate on the ~-potential of BaS04, SrSQ, and PbS04 is shown in Fig. 5. Lanthanum nitrate, in spite of the tervalency of La +++, increases the potential of BaS04 less than BaC12
r¢.~O-
!
2
3
4
5
6
I0 -4 EQUIVALENTS PER LITRE
FIG; 5. Plot of' the ~-potential (in inv.) of recrystallized BaSO4, SrSO4, and PbSO4 against the concentrt~tion of various electrolytes, including lanthanum nitrate, in alcohol (20.88 wt.-%)--water mixture (20°C.).
SPARINGLY SOLUBLE SULFATES. I
147
does, probably for reasons of geometry. In the case of PbSQ, the influence of lanthanum nitrate is approximately equal to that of PbC12. With SrSQ the influence of lanthanum nitrate is very much greater than that of SrC12, probably due to a fairly close similarity between the ionic radii of Sr ++ and La +++ (as compared with those of La +++ and Ba ++) which makes for strong adsorption of La +++ by SrSQ. Unfortunately no sufficiently concordant values for the ionic radii concerned are given by various authors. Some doubt may arise as to whether there is a large proportion of tervalent La+++ in solutions of lanthanum nitrate at concentrations smaller than 10-8 N. The experimental material, as well as general considerations, suggests that hydrolysis is less than with the corresponding aluminium salts, but the possibility of the existence of more complex ion species of lower valency cannot be ruled out. Nevertheless, the influence of lanthanum nitrate in very dilute solution on the f-potential of solids which are not heteropolar in nature, such as cellulose, is greater than that of alkaline earth chlorides (11), probably because geometrical considerations mentioned above are important in adsorption on the latrice of an ionic crystal but do not greatly affect adsorption on cellulose. It is probable, therefore, that even very dilute solutions of lanthanum nitrate contain a fair proportion of La +++. Fig. 4 shows that the potential-increasing effects of BaCI2 on BaS04, PbC12 on PbSO4, and SrC12 on SrSO~, particularly in dilute solution (1-2 × 10-~ eq./1.), are only little greater than the potential-decreasing effects of alkali sulfates on BaSQ, PbS04, and SrSQ, respectively. Thus, considering BaSQ, PbSQ, and SrSO4 in their saturated solutions, it is unlikely that preferential adsorption of the metal ion over that of S04= can account fully for the large positive potentials of these 3 sulfates. We therefore conclude that the f-potential of these sulfates in their saturated solution in alcohol-water mixtures can be accounted for fully only on consideration of properties of the solids themselves (cf. Section IV). VII. TH~ f-POTENTIAL OF GYPSUM The f-potential of gypsum, CaSQ.2H20, cannot be determined in water because of its high solubility, but only in alcohol-water mixtures. In contradistinction to BaSO4, SrS04, and PbS04, which have positive potentials, the potential of natural gypsum is negative, - 2 6 :t: 1 my. It is well reproducible, although less accurately than that of the other sulfates, and shows hardly any change on prolonged streaming. Change of the composition of the solvent mixtures between 24 and 52% (wt.) alcohol produces very little variation of f. Exclusion of surface-active impurities cannot be as rigorous in this system 5 as in the previous ones; 5 G y p s m n cannot be ignited.
148
,~. S. BUCHANAN AND E. HEYMANN
however, the disturbing effect of such impurities is less noticeable with gypsum than with the other sulfates, probably because surface-active substances, which usually render the potential more negative, are adsorbed less on the negative surface of gypsum than on the positive surfaces of BaS04, SrS04, and PbS04.
mv 0
La(NQ ,)3 .......
CaCI2
-20
I
2
3
164 EQUIVALENTS
H~SO, - ' ~ - = - . . . . Na Cite 4 5 8 PER LITRE
FI(~. 6. Plot of the ~--potential (in inv.) of natural gypsum (CaSO4.2H~O) against the concentration of various electrolytes in alcohol (44.1 wt.-%)--water mixture (20°C.).
Fig. 6 shows the influence of electrolytes on the ~-potential of gypsum. CaCl~ makes the potential less negative, while alkali sulfates and H2S04 render it more negative, the influence of H2S04 being smaller than that of other sulfates. Nevertheless, the influence of all these electrolytes is comparatively small Furthermore, there is little difference between the magnitudes of the potential increase due to sulfates and the potential decrease due to calcium salts. Thus t h e strongly negative potential of gypsum i n its saturated solution is not likely to be accounted for by preferential adsorption of S04 =, but must be explained in terms of properties of the solid similarly to the explanation given previously for BaS04. Sodium citrate makes the potential more negative but not quite as much as Na2S04. Thus, the citrate ion, in spite of its tervalency, is less effective than S04 =, probably because its size and field of force are very different from that of S Q =. On the other hand, La +++ has a stronger influence on the potential than Ca ++ and makes the potential positive. This, however, may partly be due to adsorption of hydrolysis products, the effect of lanthanum nitrate, as distinct from all other salts, not being quite reversible. 8 There is a marked difference in electrokinetic properties between naturM and precipitated gypsum. Not only is the ~-potential of precipitated gypsum very sm~ll, but it is difficult to reproduce, and the ratio of E / P varies with the pressure, contrary to the Helmholtz-Smoluchowski equation and to our experience with all systesm thus far investigated. Moreover, no reproducible influence of electrolytes on the ~-potential of precipitated gypsum can be detected. 6 With thorium nitrate variable positive potentials and formation of gelatinous hydrolysis products were observed.
SPARINGLY
SOLUBLE
SULFATES.
I
149
Photomicrographic and crystallographic examinations have shown that our samples of precipitated gypsum consist chiefly of twinned crystals, the growth of which has been relatively much greater in the direction of the C-axis than of the others, thus forming thin monoclinic needles. The crystals are very much thinner than those normally obtained with natural gypsum. According to Wooster (12), the characteristics of gypsum crystals are those of layers of Ca ++ and S04= joined by water molecules, and the coherence of the structure in one direction is due only to water molecules. The linkages between the layers of Ca ++ and S04= are relatively weak, thus accounting for pronounced cleavage planes. Only a small fraction of the surface area of the long and thin precipitated crystals--namely the ends of the needles--has, therefore, strong residual fields. The greater part of the other surfaces consists mainly of structural water molecules, and no Ca ++ or S04= in the actual surface, and is not likely to adsorb S04= appreciably. In these circumstances, a double layer of considerable charge density may be formed only at the end of the needles but not at the other planes. This would account for the smallness of the F-potential, its lack of reproducibility--since the measured potential is the result of contributions of double layers of high and low charge density, and will depend on the degree of randomness of packing of the crystals--and its insensitivity to addition of electrolytes, since little adsorption will occur on the major fraction of the total surface area. The well reproducible large potential of the natural gypsum, which is influenced by electrolytes, owing to ion adsorption, in a reproducible fashion, may be explained, according to this hypothesis, by the greater ratio of surfaces with high charge density, and appreciable adsorptive power, to those of low charge density and small adsorptive power. It is interesting to note that, while natural crystals of BaSO4, SrS04, and PbSO4 have small (positive) potentials, natural crystals of CaS04. 2H20 have a high (negative) potential. The small potential of natural BaSO4 (as compared with precipitated and recrystallized BaS04) has been suggested as being due to the comparative difficulty of releasing S04- from the well ordered surface (1). In the case of gypsum, the preferential release of Ca ++ may be less impeded either by the smallness of Ca ++ or by the weak forces in a layer lattice crystal in which part of the cohesion is d u e t o water molecules. The weakness of these forces may also be responsible for the smallness of the electrolyte effects. X. SVM~ARY
1. The electrokinetic potentials of natural, precipitated and recrystallized samples of BaS04, SrS04, and P b S Q have been investigated in water and alcohol-water mixtures using the streaming potential method.
150.
A. S. BUCHANAN AND E. HEYMANN
The potentials are positive. The natural samples which have very even surfaces always exhibit smaller potentials than precipitated and recrystallized samples having uneven surfaces. The potentials of the three sparingly soluble sulfates in water and alcohol-water mixtures decrease in the order of increasing solubility. 2. The temperature coefficient of the f-potential of BaS04 in its saturated solution is negative. This is not likely to be due to an increase in preferential ion adsorption with the lowering of temperature. 3. The effects of electrolytes have been investigated in water and in alcohol-water mixtures. For the same sparingly soluble sulfate the effects of electrolytes are greater in alcohol-water mixtures than in water. On comparison of various solid sulfates, it is found that the magnitude of the electrolyte effects decreases with increasing solubility of the solid. An explanation of the effect of various electrolytes on the f-potential in water and alcohol-water mixtures is attempted. 4. The f-potential of natural gypsum investigated in alcohol-water mixtures is negative. Electrolyte influence is smaller than with the other solid sulfates. Precipitated gypsum does not give reproducible values for the f-potential. 5. The results of this investigation suggest that the f-potential of a heteropolar solid is not solely determined by preferential ion adsorption, but is largely a characteristic of the solid itself. Samples of different origin, or preparation of the same compound, have widely different f-potentials. It is shown that this is not due to impurities but to different degrees of surface irregularity. Reference is made to a recent theory by Grimley and Mort relating the f-potential to lattice defects in the surface. REFERENCES 1. BUCHANAN, A. S., AND I-IEYMANN, E., Nature 161, 649 (1948); Proc. Roy. Soc. London 195A, 150 (1948). 2. GlCIMLEY,W. B., ANn iV[OTT,IN. F., Faraday Soc. Discussion 1, 3 (1947). 3. BOOTH, F., Nature 161, 83 (1948). 4. ELTON, G. A. H., Proc. Roy. Soc. London 194A, 258 (1948). 5. GUGGENHEIM,E. A., Trans. Faraday Soe. 36, 139 (1940). 6. BERNAL, J. D., AND FOWLER, R. H., J. Chem. Phys. 1, 515 (1933). 7. RICE, O. ~., Electronic Structure and Chemical Binding. New York, 1940, p. 408. 8. Literature vide ABRAMSON,]7I. A., Electrokinetic Phenomena, New York, 1940. 9. RUVSSEN, R. G., J. Phys. Chem. 44, 271 (1940). 10. REYERSON, L. H.j KOLTHOFF, I. ~-~/[.,AND COAD, K., J. Phys: Colloid Chem. 51, 321 (1947). 11. I~AEINOV,G., AND HEYMANN, E., J. Phys. Chem. 47, 655 (1943). 12. WOOSTER, W. A., Z. Krist. 94, 375 (1936).