J. Phys. Chem. 1980, 84, 2933-2939
stoichiometry assumed in this and previous studies.16 Crystals I and IV are also supportive of this stoichiometry. The framework geometries of the five crystals, and of dehydrated CQ-A for comparison, are given in Table 111. The averaged structure of the framework changes little as a function of cationic content. In crystal I, whose Ca2+ content was the highest, the (Si,Al)-.0(3) distance is closest to the one in CQ-A; perhaps this is because only Ca2+ions are bonded to O(3) in these two structures. The nonframework geometry is given in Table IV. The Cs-0 distances for threefold-axis Cs+ ions range from 2.93 to 3.20 A; this range includes not only structural differences, but also the effect of disorder implicit in refinements in the space group Pm3m.2*9(Since only an average conformation can be learned from structural refinements in Pm3m, geometry affected by the presence of partially occupied cation positions is inaccurately described.) Similar distances are observed in N ~ & S ~ - AK5Cs7-A,l ,~ and T1&sg-A.2 In all structures, Cs+ ions at &ring sites have D4,,symmetry. These are lloosely associated with four O(1) oxide ions of the zeolite framework at distances ranging from 3.31 to 3.36 A, in all stiructures except that of crystal I. The somewhat longer distance, 3.407(6) A, observed there is likely to be a disorder effect (vide supra); the conformations of those two &rings which are associated with Cs+ ions can be expected to be different from that of the empty %ring. Acknowledgment. This work was supported by the National Science Foundation (Grant CHE77-12495). We are indebted to the University of Hawaii Computer Center.
2933
Supplementary Material Available: Listings of the observed and calculated structure factors for all five structures (Supplementary Tables I-V, 10 pages). Ordering information is given on any current masthead page. References and Notes (1) For a discussion of zero coordination, see Firor, R. L.; Seff, K. J. Am. Chem. Soc. 1977. 99, 6249. and references therein. See also ref 2 and 3. Subramanian, V.; Seff, K. J. Phys. Chem. 1979, 83, 2166. Pluth, J. J.; Smith, J. V. J. Phys. Chem. 1979, 83, 741. The nomenclature refers to a zeolite of composition Na12Si,,Ail,048, exclusive of water molecules if a hydrated crystal is considered. Vance, Jr., T. B.; Seff, K. J. Phys. Chem. 1975, 79, 2163. Fraenkei, D.; Shabtai, J. J. Am. Chem. SOC. 1977, 99, 7074. Charneii, J. F. J. Cryst. Growth 1971, 8 , 291. (a) Firor, R. L.; Seff, K. J. Am. Chem. Soc. 1978, 100, 3091. (b) Sherry, H. S.; Walton, H. F. J. Phys. Chem. 1967, 71, 1457. Cruz, W. V.; Leung, P. C. W.; Seff, K. J. Am. Chem. SOC. 1978, 100, 6997. Principal computer programs used in this study: T. Ottersen, CO~RARE data reduction program, University of Hawaii, 1973; full-matrix least-squares, P. K. Gantzel, R. A. Sparks, and K. N. Truebiod, u s 4 , American Crystallographic Association Program Library (old) No. 317 (revised 1976); Fourier program, C. R. Hubbard, C. 0. Quicksall, and R. A. Jacobson, Ames Laboratory Fast Fourier, Iowa State University, 1971; C. K. Johnson, OATEP, Report No. ORNL-3794, Oak Ridge National Laboratory, Oak Ridge, Tenn., 1965. “International Tables for X-ray Crystallography”; Kynoch Press: Birmingham, England, 1974; Voi. IV, pp 55-60. Reference 11, pp 73-76, 86. Reference 11, pp 149-150. Ogawa, K.; Nitta, M. Aomura, K. J, Phys. Chem. 1978, 82, 1655. I t is assumed that the two Cs+ ions inside the sodalite unit would locate as far apart as possible, that is on opposite sides of the origin, at the fractional coordinates x = y = L = 0.08 and -0.08, as determined in this and other crystal structures (see ref 1 and 5). The ionic radius of Cs’, 1.69 A, is taken from “Handbook of Chemistry and Physics”; CRC Press: Cleveland, Ohio, 1974; 55th ed.,p F-198. Seff, K. Acc. Chem. Res. 1976, 9 , 121.
Studies of Hytlrolyzed Aluminum Chloride Solutions. 1. Nature of Aluminum Species and Compositiion of Aqueous Solutions J. Y. Bottero,”+J.
MI. Cases,+F. Flesslnger,t and J. E. Polrlert
Centre de Recherches sur la Valorlsotion des Mlnerais, E.N.S.G. et L.A. 235 du C.N.R.S.,B.P. 452, 54001 Nancy Cidex, France; and Societe Lyonnalse des Eaux et de I’Eclairage, 75016 Paris, France (Receive& February 12, 1979; In Final Form: May 12, 1980)
The hydrolysis-precipitation process of AI(OH)3from aluminum chloride was studied by 27AlNMR spectroscopy and by pH titration. The A1 concentration was lo-’ M, and the temperature 25 O C . The (OH)/(Al) ratio was varied from 0 to 2.5. The results are discussed in terms of a set of presumed species, and the potentiometric titr,ation curve is interpreted quantitatively by using a computer technique.
Introduction Numerous studies have been made of the hydrolysis of aqueous aluminum solutions. The nature of the complex cations is of particular concern to many researchers because of their presuimed role in soil formation,l the interest they represent in mineral and analytic chemistry, and their importance in the elimination of colloids and organic matter through coagulation and flocculation2 in water treatment. We know3 that under the conditions in which it is used to clarify natural the aluminum ion (Al(H20)e)3+ Centre de Recherclhes sur la Valorisation des Minerais. Societe Lyonnaise des Eaux et de 1’Eclairage. 0022-3654/80/2084-2933$01 .OO/O
can exist only in low concentrations. At pH values above 3, the ion is hydrolyzed to produce more or less soluble polynuclear forms5called hydroxo complexes,8 polycations: or hydroxo polymers.1° The interpretation of the results varies with the methods of preparation and identification used as follows: (1)Using potentiometric titration methods, Brosset et al.ll proposed the following varieties: A1(A12(OH)5)n(3+n)+ or simpler complexes and (A18(OH)m)4+.The ratio r = (OH)/(Al) was limited to 2.5. The composition of these complexes is in agreement with Sillen’s “core links” theory.12J3 Elsewhere, Van Cauwelaert et al.8J4 discusses the series (A14(0H)8)4+ to (A113(OH)32)7+.Recently Vermeulen et all5and Stol et al.16 have emphasized 0 1980 American Chemical Society
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The Journal of Physical Chemistry, Vol. 84, No. 22, 1980
TABLE I: pH and Turbidity of the r = (OH)/(AIT) 0.5 PH 3.59 turbidity, FTU 0.6
Bottero et al.
Various r = (OH)/(AlT) Treating Solutions 1 1.5 1.8 2 2.2 3.69 3.87 3.99 4.06 4.28 0.6 0.58 0.65 0.60 0.80
the importance of kinetic phenomena during the hydrolysis. (2) Using the coagulation of silver halide sols, Matijevic et al.17J8conclude that the main form present is the complex (A18(OH)zo)4+. (3) Using chemical methods of identification, Hsu and Bates? following a series of dialysis experiments, proposed a continuous series of polymers whose basic unit is a ring of the type (A16(OH)12(H20)12)6+. The rings unite at the edges as r increases (two rings: (A~)~o(OH)ZZ(HZO)~~)~+; ...), thus prefiguring seven rings: (A1~4(OH)60(Hz0)24)12+; the bidimensional structure of the crystallized hydroxide. Hem et al.19~20326 draw similar conclusions from their complexation experiments with ferron. (4) Using small-angle X-rays, Rausch and Bale,21after heating the hydrolyzed solution for 1 h at 70 "C, with r = (OH)/(Al) values between 1.5 and 2.25, found a polymer with a radius of gyration of 4.3 A which they assume is (Al1304(OH)24(H20)1z)7+. This had already been proposed by Johanson,22following his studies of the crystals of basic aluminum sulfate. ( 5 ) Using nuclear magnetic resonance, Akitt et al.23-25 conclude that the following forms which vary with the pH values are present: (Al(HzO),)3+,(A1z(OH)z(HzO)8)4+, (A~~~~~(OH)Z~(HZO)~Z)~+, and probably ( A ~ ~ O W Z O -
OM"d4+.
There are still therefore a number of unknown factors concerning the exact nature and composition of aluminum aqueous solutions as a function of the concentration of aluminum, the neutralization ratio r, the temperature, and the aging time. Because of the important role of these solutions in water treatment by coagulation of colloids and organic matter, we have undertaken the present research.
Experimental Methods Starting with a 0.5 M mother solution of aluminum chloride (obtained from A1Cl3*6HZ0, Merck, reference no. 1084) and with a NaOH solution (Titrisol, Merck), we prepared solutions whose final concentration in Al was 0.1 M. In this preparation, we put into a 300-mL chamber, thermostatically regulated at 20 "C under nitrogen sweeping, 50 mL of the 0.5 M AlC13 solution. Using a peristaltic pump, we added under violent agitation 200 mL of a solution containing the amount of sodium hydroxide necessary to obtain the molar ratio r = (NaOH)/(AlCl,) = (OH)/(AIT) desired. The time required for this addition was fixed at 1 h. NMR spectra have been obtained at 23.45 MHz in the Fourier transform mode with a Brucker HX 90 interfaced to a Nicolet 1080 computer. The broad band unit allowing the observation of any nucleus was built at the Laboratoire de Chimie Thgorique (UniversitB de Nancy I); 10-mm 0.d. tubes were employed, the substance used as intensity reference being dissolved in deuterium oxide (whose deuterium signal placed the field frequency lock), in a Wilmad coaxial cell. A 3-kHz spectral width was selected. The free induction decays were accumulated using 2K words, while the Fourier transform was performed on a 16K array, by using the zero filling technique; 90" pulses were used; the time elapsing between two pulses was sufficient to allow a complete return of the magnetization to equilibrium. In order to find the amount of aluminum bound in the different species, experiments were made by using reference
2.3 4.37 3
2.4 4.50 9
2.5 4.75-5.20 30-150
-17ppm4
80 ppm
Figure 1. NMR spectra of "AI from treating solutions with varlous r values and an age t = 24 h.
samples of the ion Al(OH), at 5X and 2 X M, placed in the capillary tube of the coaxial cell. The peak of the reference monomer Al(OH)4-is located at 79.9 ppm from the signal of Al(HzO)63+.Thus by simple integration of the peaks we can calculate the amount of aluminum bound in each detectable species. The potentiometric titration experiments were made under the conditions described above for the preparation of solutions at a given r = (OH)/(AlT). The pH of the solution was measured with a Tacussel TS 80 pH meter, equipped with an EPL 1 register. We used a type C10 reference electrode and a type T B 10 HS glass electrode with an immersion depth of 6 cm. The turbidity of solutions was measured with a Hach 2100 A turbidimeter. The results are expressed as formazine turbidity units (FTU) (cf. Table I). The turbidities for all four standards are based on formazine dilutions. The standards are rated at 0.61, 10, 100, and 1000 FTU (furnished with the instrument) and contained in sealed glass tubes. The first is a liquid chlorobenzene solution, and the last three are liquid latex solutions.
Results and Discussion Nuclear Magnetic Resonance Spectra. Apart from the ion signal taken as reference A1(OH)4-, we observed a maximum of three signals whose importance varied with the value of r = (OH)/(A~T). From r = 0 to r = 2.5, a signal appears at 17 ppm from the reference, corresponding to aluminum ions in tetrahedral c~ordination.~~ As we shall see later, the height and the area of this peak increase up to a value of r = 2.2 and then decrease (Figure 1). From r = 0.5 to r = 2.4 a signal appears at 79.9 ppm from the reference, corresponding to aluminum ions in octahe-
Hydrolyzed Aluminum Chlaide Solutiorm
npUn 2. Representation of hpolymer Ai,a04(OH)~H10),1'+ horn Jchsnsson. Thetebghedmnofoaygmatanshlheanterofhegap contains he fow-coordinsted AI atom.
dral This signal corresponds to aluminum ions in monomeric species of the type Al(HzO)63+,Al(OH)(HZO),1+,and A1(OH)z(HzO),+. This peak does not shift. Its width at half-maximum increases with r; when r is between 2.2 and 2.3, it is ca. 3 times as wide as when r = 0.5. The height and the area of this peak show a corresponding decrease from r = 0.5 tor = 2, followed by an increase up to r = 2.2 and then a decrease. The peak located a t 17 ppm from the reference si al was attributed to the polymer AIIzVI(OH)zIAIP 04(HzO)12'. It has been described by Johansonn and by Akitt et al.23 (Figure 2). It was actually possible to detect only the tetrahedrally coordinated aluminum ions in the AI1304(OH),(Hz0)1~+ complex because the signals from the octahedral ions were too broad. The four-coordinated aluminum ion is presumed to be located a t the center of a structure with a symmetrical environment so that the electric field gradient a t its position is very weak or nonexistent. The 12 other octahedral aluminum8 are presumed not to have a symmetrical environment. The electric field gradient at their level is therefore relatively high, and the octahedron may be distorted. All this causes so great a
he m
l of U~yskalt2hmnkby. Vd. 84, No. 22, 1080 2 9 s
broadening of the peak,due to the quadrupolar relaxation, that the signal cannot be detected at high resolution. For these reasons, it seems highly unlikely that a condensed form containing more than two aluminum ions in oetah&al coordination would be detectable at high resolution. The peak at 79.9 ppm and r = 2.2, characteristic of aluminum in octahedral coordination, was thus attributed to the dimer whose formula is Alz(OH),(HzO)l~ck)f.In fact, for this value of r the pH of the solution is 4.28, and the activity of the predominant monomeric variety, Al(OH)z(HzO),+,calculated from the thermodynamic constants, is less than lo-" M. Under these conditions the monomeric species cannot be detected in high-resolution NMR because of their low concentration. From the areasof the peaks, it was possible to calculate the amount of aluminum bound in the monomeric, the dimeric, and the condensed A11304(OH),(HzO)lJ+ forms. Figure 3 shows the variation in the percentage of aluminum bound in the various species as a function of r and of the pH. If one allows for experimental errors, the sum of the concentrations expressed as a percentage of the total aluminum is between 98 and 105% when r is between 0.5 and 2.3. This result is consistant with the number of aluminum ions bound in the polymer. Beyond this value part of the aluminum seems to be in a form which we shall try to determine. Nevertheless, we should note that the measurements of turbidity made on these solutions reveal a rapid increase in turbidity when r is greater than or equal to 2.3 as shown in Table I, which is characteristic of a nonsettling gel. To sum up, we may say that the forms believed to be present through direct investigations in the literature or by our trials are the following: Al(HzO)63+,Al(OH)(Hz0),2+,Al(OH)z(HzO),+, A1z(OH).(HzO)i+LB1)+, and A1z(OH)z(HzO),'+. To this series we add a polymer containing a four-coordmated aluminum to which we shall give the provisional formula Al13(OH),04(HzO)l~+. Potentiometric Studies. For values of r above 2.2, high-resolution NMR does not enable us to determine the total number of species present in the solution. We have tried to counteract this difficulty by the use of potentiometric titration, taking into account the results obtained in this study or elsewhere on the nature of the aluminum species present. Figure 4 (curve I) shows the evolution of the experimental potentiometric titration curve as a function of the neutralization ratio r for an aging time of 24 h. I t is then possible to construct a model from the species we have assumed to be present and using automatic calculation to obtain a potentiometric titration curve which may be superimposed on the experimental m e . Such an approach requires that the solution, after 24 h, be in a state of pseudoequilibrium so that we may use the thermodynamic characteristic constants of equilibrium states. The validity of the model cannot be best judged by the agreement of the experimental potentiometric data with those obtained by calculation but rather by comparison with the quantitative results obtained by high-resolution NMR (Figure 3). To do this, we must know the values of the activity coefficients of the different species. We have used Glueckauf s formula" applicable for concentrated el&lyte solutions and we have compared the results with those obtained with the Debye-Hiickel law.=
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Bottero et al.
The Journal of Physical Chemistty, Vol. 84, No. 22, 1980
A All0/-AIT -0-
1OfJc
0-- ---0
Pi M R
A-A
Debye- Huckel
-0-
-
I
dlueckeuf h - 4 0
90-
80
-
70
-
eoc 50-
-.. *.
*.
-0
30
20
PH 3
4
35
45
I
1
1
1
I
,
I
I
011
05
1
15
I
21
12
2 15
r = IOH/ALtl
Figure 3. Percentage of aluminum bound and species identified through NMR and calculatlon vs. rand pH values: (0)NMR; (0)Glueckauf; (0)100 NMR quantitative results.
The calculation is based upon the following equilibria which exist in the solution: AP+ + HzO P A1(OH)2++ H+ (1,U
K2,1 = 10-5.02(ref 29, 30) (1,2)
A13+ + 2Hz0 P A1(OH)2++ 2H+ Kl,z = 10-8s71(ref 31)
(2,2)
2AP+ + 2Hz0 P Alz(OH)z4++ 2H+
K2,2=
(ref 31)
(2,X) Al(OH)Z+
+ Al(OH)Z+ +r! A12(OH),('-")+ + ( X - 4)H+
(13,24)
Kz,, = x 13AP' + 28Hz0 P A11304(OH)247+ 32H"
K13,24 =
(ref 32)
The sudden increase in turbidity for values of r > 2.3 might be due to the formation in the solution of the nonsettling gel previously described by Rubin and Hayden.,, We shall write this form of aluminum hydroxyde as Al(OH),* in order to better differentiate these species from amorphous Al(OH)3.29 (1,3) A13+ + 3Hz0 P A1(OH)3*+ 3H+ K = 10-10.4(ref 33)
(A) Debye-Huckel;
The principle of the automatic calculation is the following: (a) For each value of the pH between 3 and 5, with an increment of 0.05 pH unit, we determine the amount of OH- bound in the different aluminum species. The solution containing 0.1 M A1 is subject to the constraint 0.1 = Ci[Ali(OH)p(3i-P)+] (1) 2
with i = 1, 2, 13 and p = 0, 1, 2, 24. The law of mass action applied to each of the equilibria described above enables us to obtain the concentration of each form as a function of the A13+ concentration
where j = 1,2, 32; K j , is the equilibrium constant related to the reaction i,p;(d(OH),"+) and (A3+) are respectively the concentrations of the ions Ali(OH),*+ and AP+; a = 3i - p ; and fi,p and fl,o are the respective activity coefficients for the ions Alj(OH)p"+calculated according to the following: 1. Glueckaufs Formula. = -~Z~,p2pl/~/(l + 13ri,pp1/2)+ log 0.018rnri(ri + h - v) 2.3v(l + 0.018mr) h-v h -log (1+ 0.018rnr) - - log (1- 0.018mh) (3)
+
V
Y
The Journal of Physical Chemistty, Vol. 84, No. 22, 1980 2937
Hydrolyzed Aluminum Chloride Solutions
A'
i
i
+3
3,s
4,o
',5
Flgure 4. Comparison of the experimental and calculated rvs. pH titration curves of the AICI3 solution: curve I Is the experlmental curve; curve I1 is the calculated curve wlth AI(H,0)B3+, Ai(OH)(H,O);+, AI(OH)2(H20)4+;curve I11 is the above-mentioned plus Ai,(OH),(H,O);+; curve I V is the same plus Aii,04(0H)24(H20)i~+; curve V is the same as 111 plus A12(OH),(H,0),_,(6-x)+ and Aii304(OH),,3+; curve V I Is ail these forms plus AI(OH)3+.
with -~Zi,;pl/~(l + Bri,pp1/2)= Debye-Huckel's law; a = 0.5; Zi,p = charge of the ion Ali(OH[),*+; B = 0.33; ri,p= radius of the ion Ali(OH),*+; rn = molality = 0.1; ri = 4,/Vwo = (apparent molar volume of electrolyte (unhydrated))/(mole volume of pure H,O);and v = vl+ + vl= number of ions (positive and negative) formed by one molecule. We have taken for the values of ri those relative to the upper concentration of each ionic species: rN3+ = 5.9; r''~lZ)'4+= 3.8; r"Alls" = 19.7. h = hydration number of electrolyte (assumed constant). We took for the "&"polymer h values equal to 40 and with hAp+ = 12,34h ~ =+10, and hA12(OH)*= 22.
Not having knowledge of the charge of the species Al(OH),*, we take the activity coefficient of this species to be f1.3* = 1. This assumption implies a zero or very low charge. 2. Debye-Huckel Law. log f i , p = -aZi,p2p1/2(1+ Bri,pp1/2) (4) Combining eq 1 and 2 we may write
where i = 1, 2, 13 and j = 1, 2, 32.
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The Journal of Physical Chemistry, Vol. 84, No. 22, 1980
1
TABLE 11: Schematic Calculation Process for Potentiometric Studies
' P h z Pho
Pi,n - Pc,n-l pc,n-l
I
5 10-3
When this condition is satisfied, the concentrations of the various aluminum species are known, and it is then possible to determine the ratio of neutralization r from eq 7.
+ 0 05
CP[A~,(OH)~~+I
r = OH/AlT =
the activity coeffi c i e n t s ( e q u a t i o n s 3,4)
solution of equation5
C a l c u l a t i o n 01 the concen t r a t i o n o f t h e different f o r m s by e q u a t t o n 2
e.
k a l c u l a t i o n o f t h e ionic
Bottero et al.
a
stop
To solve eq 3 and 4 we must calculate the concentrations of the ions present or, in other words, determine by using the laws the ionic strength pc = 1/22cCnz,2 (6) n
where the summation includes all of the ions contained in the solutions: Na+, C1-, OH, H', and Al,(OH),"+. (b) To determine the ionic strength of the solution, we take an arbitrary value p i j = 0.5 and make the calculation to verify eq 5. Then, from eq 2 and 5, we calculate pc,l. If pi,l differs from pc,l, we repeat the calculation, taking pi,l = pc,l and so on, by successive repetition, until
P
(7) (AlT) The procedure is summarized in Table 11. The calculation was made on an Iris 80 (CII) computer at the University Institute of Automatic Calculation at Nancy. (c) In Figure 4 we have shown the theoretical potentiometric titration curves obtained by calculation for the following species. curve I 1 A1(HzO)2+,A1(OH)(Hz0)62+, A1(OH)z(Hz0)4+.curve III: the above-mentioned species plus A1z(OH)z(HzO)~+. curve IV: the same species as above plus A11304(OH)z4(Hz0)1z7+ and Alz(OH),(HZO)~~,(")+.curve V A1(Hz0)2+,A1(OH)(Hz0)62+, M(OH)z(HzO)4+,Mz(OH)z(HzO)~+, Mz(0H),(Hz0)1~(6z)+, A11304(OH)z2+.curve VI: all of these species and Al(OH),*. In order to improve the superimposition of the theoretical curve on the experimental one, we were obliged (1) to modify the charge of the polymer A11304(OH)2~+ and the constant K133, which became A11304(OH)B3+ and K13? = 10-lo5,(2) to obtain the constant Kz,, equal to 10-5.6in such a way that the concentration of the dimer was 15% at r = 2.2 (cf. Figure 3) and the s u m of the concentrations of the various aluminum species expressed as A1 was equal to 0.1, (3) to modify very slightly the constant K1,3 given by Rubin and H a ~ d e n (we , ~ kept the value Kl,? = 10-lo.l, and (4) to consider the dimer A12(OH)24+ previously described by Sillen and Akitt, since it would make looping possible at 0.1 M AIT for the values of r below 0.5. We can see (Figure 3) that the use of the Debye-Huckel law gives a better agreement with NMR results than Glueckauf's formula for r values lower than 2. For r values greater than 2 both formulas give similar results. This fact can be explained by the variation of the activity coefficient of the species as showed in Table 111.
Conclusions In solutions of aluminum chloride at a concentration of lo-' M and partially neutralized by sodium hydroxide with a ratio r = (OH)/(AlT) varying from 0.5 to 2.5 and after
TABLE I11 : Activity Coefficients Calculated with Glueckauf anId Debye-Huckel Formulas De bye- Huckel Glueckauf ( h = 40) ~ 1 3 + Al+ Al3+ Al+ NJOHL~+ AI,, Al*(OH)24+ PH 0.0181 0.1558 0.0879 0.6876 0.1413 0.7394 0.0791 3.1 0.0181 0.6876 0.0791 0.1558 0.0879 3.2 0.1413 0.7394 0.0181 0.0880 0.6876 0.0791 0.1558 3.3 0.1413 0.7394 0.6877 0.0181 0.0791 0.1558 0.0880 3.4 0.1413 0.7394 0.0181 0.1591 0.0881 0.6878 3.5 0.1335 0.7447 0.0801 0.0203 0.1712 0.0948 0.6986 3.6 0.1565 0.7626 0.0841 0.0242 0.7148 0.177 0.1060 3.7 0.1623 0.7705 0.0862 0.0257 3.8 0.1802 0.1105 0.7207 0.165 0.7747 0.0874 0.0267 0.724 3.9 0.1675 0.0881 0.1822 0.1131 0.7772 0.0273 4 0.1832 0.1147 0.7261 0.1686 0.7785 0.0885 0.0076 0.7271 4.1 0.1838 0.1155 0.1692 0.7793 0.0887 0.0274 4.2 0.1150 0.7265 0.1692 0.7793 0.0887 0.1838 0.0287 4.3 0.1721 0.1185 0.7307 0.7828 0.0898 0.1867 0.0277 4.4 0.7275 0.1693 0.7794 0.0888 0.1839 0.1158 0.0277 4.5 0.1693 0.7794 0.1839 0.158 0.7275 0.0888 0.0277 4.6 0.7275 0.1693 0.7794 0.0888 0.1839 0.1159 0.0277 4.7 0.1693 0.1160 0.7275 0.7794 0.0888 0.1839 0.0277 4.8 0.7275 0.1693 0.7794 0.0888 0.1839 0.1160 0.0277 4.9 0.1693 0.1839 0.1160 0.7275 0.7794 0.0888
A43 0.1563 0.1563 0.1563 0.1563 0.1563 0.1633 0.1746 0.179 0.1816 0.1832 0.1840 0.1835 0.1869 0.1843 0.1843 0.1843 0.1843 0.1843 0.1843
Hydrolyzed Aluminum Chloride Solutions
a maturing time of 24 h, we have shown through NMR the presence of the following species: Al(HzO)63t, Al(OH)(H20)62+,A1(OH)2Wz0)4t, A12(0H),(H20)10-,(6-“)+, A113(OH)2804(H20)~t. The arbitrary hypotheses made to explain the potentiometric titration curves, including the existence of the calculated values of the activity coefficients, are in agreement with these species and Alz(OH)2(H20)84+anld a nonsettling gel A1(OH)3*,33 This study supports some of the results obtained by Akitt23-26 and Rausch and Bale21but does not enable us to find all of the species previously postulated in the literature on potentiometric or chemical studies whose conclusions are based largely on Elillen’s core links theory. If we consider thle previous studies made in this area on the effects of neutralization rates, temperature, and methods of preparing the solutions, the cause of the discrepancies may be attributed to two factors: the concentration of the solutions studied and the methods used. Concentration of the Solutions Studied. Many studies have been made with aluminum chloride solutions whose concentrations are less than M-values much lower than those used in this study. H s u , ~Bersillon,26and S t o P use solutions whose concentrations are lower and higher than 10-1M. They are nevertheless able to characterize aluminum species which verify Brosset’s mlodel,lOJ1i.e., the formation of hydroxo polymers with bidiimensional growth through the adjunction of aluminum ions in octahedral coordination. (Stol concedes that the polymers cannot have more than 15 aluminum ions.) hi these concentration ranges, the in situ use of direct methods such as NMR or small-angle X-ray scattering does not enable us to obtain the same results. A polymer has beein suggested which has an aluminum ion in tetrahedral coordination which necessarily implies a tridimensional structure not predicted by the core links theory. For conceiitrations below BO-3 M, it is difficult to make comparisons, as the direct methods are ineffectual. Methods Used. (Consideringthe results described above, we feel that potentiometric studies are not the ideal means of investigation. What they do basically is to establish a certain number of equilibria and to adjust the results between them. Ni3turally, when the correlations are obtained, they verify the hypotheses. All one can say is that the model has internal consistency. In this case, the results are particularly consistent with the ion A113(OH)2804(H20)83+,whose structure is very different from that of the bidimensional polymers. It is interesting to note (and this supports our hypothesis) that the results of the calculation are in agreement with the direct determination by NMR. In conclusion, we may say that the results of the potentiometric titration methods, unsupported by direct and
The Journal of Physical Chemlstty, Vol. 84, No. 22, 7980 2939
sophisticated methods of investigation, must be considered suspect.
Acknowledgment. We sincerely thank Mssrs. D. Canet, J. P. Marchal, and J. Brondeau of the theoretical chemistry laboratory of the University of Nancy I for their assistance during the NMR trials and helpful discussions. This work was supported in part by DGRST no. 77.7.1056 contract. References and Notes (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16)
(17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35)
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