Anal. Chem. 1909, 6 1 , 1253-1259
1253
Composition of Aluminum Phosphate Solutions. Evidence from Aluminum-27 and Phosphorus-31 Nuclear Magnetic Resonance Spectra Michael A. Wilson* and Philip J. Collin
CSIRO Division of Coal Technology, P.0. Box 136, North Ryde, New South Wales 2113, Australia
J. W. Akitt Chemistry Department, University of Leeds, Leeds, United Kingdom L S 2 9 J T
Low- (2.11 T) and hlgh-field (9.4 1 ) 27AI and 31P nuclear magnetic resonance (NMR) spectrometry has been used to study the structure and reactlvity of aluminum chloride solutlons In the presence and absence of OH-, H3PO4, H,P04-, HPOt-, and PO-: specles. Evidence Is presented for the existence of higher molecular welght complexes than the [AI04AIl,( OH),,( H20)12]7+catlon In the presence of hydroxide Ion. At high field, at least flve types of complexed aluminum are observed by "AI NMR In the presence of phosphate species. Three specles at -3.6, -6.2, and -7.6 ppm are observed In solutlons containing predominantly H3PO4, and two specles are observed In the presence of H,PO4- at -3.6 and -7.6 ppm. I t Is proposed that the specles at -3.6 ppm has a AI:P ratio of 1. At least 11 resonances from dlfferent complexes In aluminum phosphate solutions can be observed by 31PNMR. I t Is shown that prlnclpal dlfferences In chemlcal shlft are due to the nature of the ligand (L = HsPO4, H2P04-, HPOt-) rather than the number of llgands (Ll, L,, L,, L,, etc.) In the complexes. High-temperature (up to 95 "C) studles show that the complexes can exchange ligand wlthout equlllbratlng with uncomplexed alumlnum. I t Is also shown that the acldltles of the complex solutions are greater than those of the unmixed solutlons alone. This acidity mainly results from dlssoclation of water molecules In the coordlnatlon sphere of alumlnum.
INTRODUCTION Aqueous solutions of aluminum and phosphate are important in such diverse fields as catalysis, soil science, and medicine ( I ) . Studies of the structure of aluminophosphates are important in understanding Alzheimers disease (2, 3 ) , dental cements (4), the formation of a new range of molecular sieve catalysts (5), and the availability of phosphorus to plants in soils (6-8). There is now reasonable agreement (9-22) that solutions of aluminum salts contain the species A1(Hzo)63+ and A1(Hz0)50H2+,and in the presence of base, an oligomer but the and the polymeric cation [A104A11z(OH)~4(Hz0)lzl'+, type and concentration of species formed depend on the way the aluminum salt is hydrolyzed. In addition, complexes can form from any other ligands in solution. Thus the nature of the species present when phosphate ion is added to aluminum chloride solutions has not been established, although Akitt et al. (23)and O'Neil et al. (4) showed that up to six resonances are observed by 31Pspectrometry and Karlik et al. (2) showed that three resonances are observed by 27Alspectrometry. In this paper we study the structure of solutions of aluminum chloride in the presence and absence of trisodium phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate, and phosphoric acid by using both low-
(2.11 T) and high-field (9.4 T) 27Aland 31PNMR. These results extend the experiments of Akitt et al. (23)and Karlik et al. (2)to a wide range of different concentrations and show that at least five types of phosphorus-containing complexes can be observed by 27AlNMR and at least 11 by 31PNMR.
EXPERIMENTAL SECTION Solutions. Partly neutralized aluminum salt solutions were prepared by adding dropwise, with stirring, 0.2 M sodium hydroxide to 5 mL of 0.2 M aqueous aluminum chloride at room temperature. Various concentrations of solutions of aluminum chloride and sodium aluminate from 0.2 to 0.029 M were also prepared. Aluminophosphate solutions were prepared by addition of 0.1-2.0 mL of 0.2 M aluminum chloride solution to 2.0 mL of &Pod, (0.190 M), NaH2P04(0.200 M), Na2HPO4(0.180 M), or Na3P04(0.220 M) at room temperature. The solutions were made up to a total volume of 4.5 mL by addition of water and 0.5 mL of deuterium oxide (as an NMR lock signal). Some experiments were performed in the presence of dilute acid and at elevated temperatures up to 95 "C. Spectrometry. Most 27AlNMR spectra were obtained on a JEOL FX9OQ spectrometer operating at 23.3 MHz for 27Al. A 90" (65 FS) pulse with 5000-Hz sweep width was used. A total of 800 scans were collected in 8K data points with an acquisition time of 0.819 s and pulse delay of 1s. The latter was sufficient to ensure complete relaxation. All samples were studied at low field. It was clear, however, that much spectral detail was obscured in the low-field spectra. Accordingly, spectra were obtained on a Bruker HX400 instrument on seven samples selected to cover as wide a range of solution compositions as possible, where hydrolysis was minimal and where all the species present were detectable in the spectra. nAl spectra were obtained at 104.2 MHz at 25 "C with a total acquisition time of 0.786 s. Chemical shifts were measured with respect to Al(HzO),3+monomer in 0.2 M aluminum chloride solution. Likewise, low-field 31Pspectra were obtained on a JEOL FXSOQ spectrometer at 36.2 MHz using a 5000-Hz sweep width and a 45" (35ks) pulse. Data were collected in 8K points using 640 scans. Pulse delay was 4 s with an acquisition time of 0.819 s. Relaxation experiments demonstrated that these conditions gave quantitatively accurate data. High-field 31Pwork was also carried out on a Bruker HX400 spectrometer. High-field 31Pspectra were obtained at 162.0 MHz with a total acquisition time of 0.254 s. Spectra were obtained initially at 25 "C, but it was found that much better resolution was obtained at -5 "C, at which temperature the spectra exhibited up to 11, possibly 12, lines. Integrations and Spin Counting. The concentrations of 31P species present or A1 monomer were determined directly from the 27Alor 31Pspectra. By collection of equal numbers of scans on each solution, the amounts of aluminum or phosphorus in solution could be monitored. This technique, coined "spin counting", is useful for detecting species that are unobserved, either because of precipitation or, in the case of the quadrupole aluminum nucleus, because of large electric field gradients due to molecular asymmetry.
0 1989 American Chemical Society 0003-2700/89/0361-1253$01.50/0
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
RESULTS AND DISCUSSION Solutions of Aluminum Chloride. Since 27Al NMR spectra of similar solutions have been reported previously (10, 22), only the new findings will be discussed briefly here. Seven resonances were observed in high-field 27Alspectra. Apart from the monomer ion [Al(H20)63fplus A1(H20)50H2+]at 0 ppm, resonances were also observed at 79.9 ppm from aluminate [Al(OH),-], 62.9 ppm from tridecamer [A10,Al12(OH)u(H20)12]7+, herewith called the A113 ion, and phosphorus containing-species at -1.4, -3.6, -6.2, -7.6, and -9.0 ppm. Only two resonances were observed at low field. It should be noted that it is only the central tetrahedral aluminum that is obions at ambient served in spectra of [A104A112(OH)u(H20)12]7+ temperature. The surrounding 12 aluminum octahedra are sufficiently distorted to make their aluminum resonance so broad as to be invisible. Thus in measuring the amount of nAl a.s this species we found it necessary to multiply the signal observed from tridecamer by 13. The chemical shifts of monomer, tridecamer, and aluminate ions were independent of aluminum and hydroxide ion concentration; however, the ~ )the monomer ion was conline width at half-height ( w ~ , of centration dependent as previously reported (10). On the other hand, the line width of the [A104A112(OH)24H20)12]7+ species is concentration independent with wljz = 6 Hz. These results suggest that the interaction of the A13+ hydration sphere with bulk solvent is concentration dependent, whereas that of the tridecamer species is not. The tridecamer results may reflect the fact that the water protons are too far removed from the central tetrahedral aluminum to affect the observed line widths, or that proton exchange is very fast. The line widths of the A1(H20)s3+,[A1(H20)50H]2+resonance in our experiments were considerably greater than those recorded by Akitt and Elders (10) due to the higher concentration and viscosity. The concentrations of A1 monomer [All, and [Al13]ion present were studied as a function of time after mixing at room temperature. It is clear that the equilibrium concentration of monomer is reached within minutes of mixing. This is not true of the Al13ion whose Concentration increases slowly and needs more than 10 days to reach equilibrium. Heating the solution up a t 80 " C after equilibrium is established (about 32 days) had little or no effect on the apparent concentration of monomer ion but greatly reduced the concentration of All, ion present. When all the tridecamer aluminum is accounted for, the sum of the concentration of aluminum as monomer and tridecamer still does not equal the total concentration of aluminum in solution. For example at equilibrium for a solution with [OH]/[Al] = 2 and [All, = 0.066 M, the [Al13]+ [All, = 0.056 M. Thus another species is present. At high [All, this species is an oligomer (22), which can be detected as a very broad resonance at or around 4 ppm from monomer. The concentrations of the aluminum species in solution at equilibrium are plotted in Figure 1 as a function of the total aluminum concentration in solution and [OH]/[Al], ratio. At high A1 concentrations and low [OH]/[Al] the oligomer and monomer species appear to be present, but the oligomer concentration appears to peak a t or about [AI], = 0.14 M, [OH]/Al]T = 0.4. The concentration of All, species increases up to [OH]/[Al], = 2.5. A t higher concentrations, [OH]/[Al], = 3.0, Al(OH), precipitates, and a t [OH]/[A~]T= 4.0, aluminate ion is formed. These results are in broad agreement with those reported by Bottero et al. (20), except the concentration of oligomer is estimated to be greater and extend over a greater range of [OH]/[& ratios. Of more significance is the fact that we also detected an increase in unobservable aluminum above an [OH]/[AI], ratio of about 2.3 but below that of precipitation. This increase is accompanied by a
Total AI ( M )
Flgure 1. Effect of [OH]/[AI] ratio at equilibrium on concentration of AI species in solution: W, aluminate ion; 0 , monomer; 0,AI,, species; 0, oligomer; A, polymer. The unbroken line represents the total observable aluminum.
decrease in A113 concentration and may be due to the formation of polymers other than the oligomer, probably those involved in precipitation. It is probable that it is this material that is also formed at the expense of Al13ion on heating to 80 "C. Reaction of Sodium Phosphate with Aluminum Chloride. The experiments with aluminum chloride and hydroxide ion are relevant to the experiments with added phosphate since these salts are basic and similar species may be present in solution. Nevertheless, the Al13 ion was not observed in any of the solutions containing phosphate as additive. This suggests that the All, ion does not have time to form before complex formation and is not stable in the presence of phosphate. That is, the complex is formed from the monomer and possibly the oligomer. 27AlNMR detected the aluminate ion in solution up to the addition of 0.5 mL of AlCl,; however, the solutions were virtually aluminum-free as further aluminum chloride was added (Figure 2a). There appeared to be a small change in the chemical shift of the aluminate ion from about 78.6 to 77.3 ppm as aluminum chloride was added. Spin counting showed that up to 0.5-mL addition, all aluminum could be accounted for as aluminate ion, but thereafter all aluminum had precipitated from solution. The change in the 31Pchemical shift with neutralization of 2 mL of sodium phosphate with aqueous aluminum chloride was also studied. As expected, on neutralization the 31P chemical shift falls as HP042-ions are generated and reaches a value expected for a solution of HPOZ- salt at about 1.0-mL addition. Further neutralization reduces the chemical shift slightly, so that there is some additional association to form H2P04-. However, reaction is accompanied by formation of a precipitate of aluminum phosphate. This precipitation was monitored by spin-counting 31PNMR (Figure 3a). It can be seen that almost all the phosphorus has precipitated out of solution on the addition of 2 mL of aluminum chloride. Reaction of Sodium Monohydrogen Phosphate with Aluminum Chloride. Studies with HP042-allow the titration curves with PO:- to be followed further. No aluminum was observed in solution until after the addition of greater than 1 mL of aluminum chloride solution (Figure 2b), and then both complex and free aluminum species are observed. Low-field
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
Uncombined P
A\
0.2
Total P i n solution
,*
r
O2I
0.0 /
//'
/r-x
Monomer scale)
-
.-.
I
Total P in solution
-0
I
I
05
1.0 1.5 20 O . 2 M AI Clg (ml)
--I----------hi
_ i
10
0.5
+
Flgure 2. The effect of AICI, on the proportion of Ai in solution as observed by low-field (2.11 T) "AI NMR. Symbols: 0 , complex; X, monomer; 0,aluminate ion. Key: (a) PO4%titration, 0.22 M (2.0 mL); (b) HPOt- titration, 0.18 M (2.0 mL); (c) H2P04-titration, 0.20 M (2.0 mL); (d) H,PO, titration, 0.194 M (2.0 mL). The line obtained for the sum of all forms of AI if all AI is observed is shown as TA,.
spectra show only one broad line from the complex as previously reported (2). However, high-field spectra clearly show the presence of a number of species at -1.4, -3.6, and -7.6 and a shoulder at -9.0 ppm (Figure 4a). Addition of aluminum chloride causes 31Presonance to shift to higher field with reduction in pH as H2P04is formed. Only 0.5 mol equiv is necessary to reduce the chemical shift to that observed for H2P04. Figure 3b shows that phosphorus is lost from solution as precipitate during the addition of up to 1.0 mL of AlCl,. A t this point approximately 60% of the phosphorus is removed. However, above 1.0 mL of AlCl, the concentration of soluble phosphorus progressively increases and reaches a maximum at 2.0 mL. Figure 5 shows the high-field spectrum of the product with [HP04-] = 0.2 M and [Al]T = 0.4 M. The effect of cooling to 268 K is clearly shown by a comparison of Figure 5a with parts b and c. Resonances sharpen so that at least 10 resonances from complexed phosphorus can be observed. The most important at -8.5 and -12 ppm are probably those previously reported by Akitt et al. (23), but there are other important resonances at -17.1, -13.8, and -5.9 ppm. For the purposes of Figure 3 all resonances at -12 and greater are grouped together. It is clear that there is relatively little change in concentration of complexes after addition of 2.0 mL of 0.2 M AlC13. Reaction of Sodium Dihydrogen Phosphate with Aluminum Chloride. The high-field 27A1spectra (Figure 4b-d) consist of two resonances at -3.6 and -7.6 ppm. There is little or no sign of the resonance observed at -1.4 ppm derived from HPOf solution. The major resonance is at -7.6 ppm (Figure 4b), but on addition of more aluminum (Figure 4c,d) the -3.6 resonance grows in intensity relative to the -7.6
1.0 1.5 2.0 0.2M AIC13 (mi)
-
10
,
Flgure 3. Effect of AICI, addition on proportion of phosphorus in solution as observed by 31PNMR. Symbols: total; A,uncomplexed;
0, at -8 ppm; X, at a-12 ppm. Key: (a) PO," titration, 0.22 M (2.0 tiration, 0.27 M mL); (b) HP0:titration, 0.18 M (2.0 mL); (c) H,PO,(2.0 mL); (d) H,PO, titration, 0.194 M (2.0 mL).
Table I. Estimates of Proportions of Aluminum in Aluminophosphate Solutions from High-Field Spectra initial solution concentration, M
[All
[HPO>-] [H,pO,-l
A
B
C
D
E
F
G
0.4 0.18
0.1
0.2
0.4
0.1
0.2
0.4
0.2
0.2
0.2 0.2
0.2
0.2
[Sh,PO,I
product solution concentrations, lo3 M A
B
C
D
E
F
G
-1.4 -3.6 -6.2 -7.6 -9.0
135 58
19
58
112
29 10
77
73
60
30 21 26
54 24 34
70 49 21
tot complexed uncomplexed
232 168
96 4
131 69
172 234
77 23
112 88
140 260
chem shift
ppm resonance and peak asymmetry indicates some intensity at -1.4 ppm. The concentration of complex ions increases with A1 concentration (Figure 212); in particular, that complex resonating at -3.6 ppm increases by a factor of more than 5. This is also shown by the data in Table I, which lists estimates of concentrations from high-field spectra. Solution D has a concentration of -3.6 ppm complex of 112 X M whereas that of solution B is 19 X M. The resonance at -7.6 ppm decreases with an increase in aluminum concentration. These results show that the -7.6 ppm complex is dependent on a high concentration of phosphorus species whereas the formation
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989 Y
Chemical shift. 6 !ppmI
Figure 4. High-field (9.4 T) "AI spectra of aluminophosphate complexes. Initial concentrations of solutions: (a) [AICI,] = 0.4 M, [HPO?-] = 0.18 M; (b) [AICI,] = 0.1 M, [HpO,-] = 0.2 M; (C) [ A U J = 0.2 M = [H,PO,-1; (d) [AICI,] = 0.4 M, [H,PO,-] = 0.2 M.
1b 1
d
i -5
-10
:hemu2
ChemiCGi
L -15
-20
-25
s h i f t iilppml
Snlft
b!ppPl
Figure 5. High-field (9.4 T) ,'P NMR spectra of aluminum phosphate complexes. Concentrations: (a) [HP02-] = 0.18 M, [Ai] = 0.4 M at 298 K; (b) [HPO,*-] = 0.18 M, [AI] = 0.4 M at 268 K; (c) part b expanded.
of complex resonating at -3.6 ppm is dependent on a high aluminum concentration, and they suggest that the Al/P ratio for the complex resonating at -7.6 is lower than that resonating at -3.6. Possibly the -3.6 resonance arises from a 1:l complex
C h e w icl
5hi'I
o (ppv.)
Figure 8. High-field (9.4 T) ,'P NMR spectra of aluminum phosphate complexes. Expanded spectra are shown; full spectra are included as insert. Concentrations: (a) [H,PO,-] = 0.2 M, [AI] = 0.1 M; (b) [H,PO,-] = [AI] = 0.2 M; (c) [H,PO,-] = 0.2 M, [AI] = 0.4 M.
and that a t -7.6 from complexes in which some aluminum atoms have two or more ligands attached. The concentrations of the various forms of phosphorus present in solutions of NaH2P04neutralized with A1C13 are shown in Figure 3c. Uncombined phosphorus is still present after addition of 2.0 mL of A1C13, but this reduces considerably after the addition of excess (10 mL) AlCl,. The chemical shift of the uncombined phosphate changes from 0.2 to -0.1 ppm. This is consistent with protonation of H2P04-groups. The remaining phosphorus is complexed. Typical high-field spectra obtained a t 268 K are shown in Figure 6. Clearly, even more resonances are present in the spectra than in the HP0:- experiment. The resonance at -8.5 ppm is now further resolved to reveal a new species a t -8 ppm. There is a new species a t -15 ppm, and the -12 ppm resonance appears to be partly further resolved. Adding more aluminum (compare parts a, b, and c of Figure 6) increases the concentration of the -8.5, -15.0, and -17.1 (now a t -17.0) ppm resonances relative to that of the other resonances and introduces new resonances at -16.0 and -13.3 ppm. Furthermore, producing the equimolar solutions by adding sodium carbonate to hydrolyze the aluminum and then adding phosphoric acid produced identical spectra. Reaction of Phosphoric Acid with Aluminum Chloride. In these solutions increasing the A1 concentration again increases the concentration of complexes (Figure 2d), but now three rather than two complexes are visible at -3.6, -6.2, and -7.6 ppm (Figure 7). The -6.2 ppm resonance appears to increase appreciably in concentration above equimolar quantities of AI and P (Figure 7c and Table I, solution G ) . Likewise the -3.6 resonance appears to increase at the expense of the -7.6 ppm resonance. The concentrations of the various forms of phosphorus in the experiments with phosphoric acid are shown in Figure 3d.
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
0,
-4
Chornlcal shift
1257
-? 6 (pprn)
Figure 7. High-field (9.4 T) 27AINMR spectra of complexes. Initial concentrations of solutions: (a) [AI] = 0.1 M, [H3P04] = 0.2 M; (b) [AI] = 0.2 M, [H,PO,] = 0.2 M; (c) [AI] = 0.406 M, [H3P04] = 0.198 M.
The concentration of free phosphoric acid is reduced to 36% after addition of 2 mL of 0.2 M AlC1, and to 13% on addition of 10 mL. That is, the concentrations of complex progressively increase with an increase in A1 concentration. The 31Pspectra of the solutions are less complicated than those of the other solutions (Figure 8) but again consist of resonances at -8.0, -8.5, and -12 ppm. The resonances at -15, -17.0, -18.0, and -20.0 ppm are missing or present in only trace quantities, but a new resonance at -16.5 ppm is also present. The latter is very intense at high aluminum concentration (Figure 8c). Variable Temperature Studies. These experiments were performed at low field (2.11 T), but they are sufficient to show that the complexes exchange at 80 "C. The 27Al spectra showed that the amount of monomeric aluminum ion decreases in intensity with increasing temperature, and there is an increase in complex resonating at -3.6 ppm. This result is as expected if the consumed free aluminum forms a 1:l complex with phosphorus ligand as proposed. The chemical shift of the coalesced peaks at 80 "C is at -5.5 ppm, which is intermediate between the major resonances at -3.6 and -7.6 ppm. The free aluminum does not become indistinguishable from complex. That is, there is rapid ligand exchange of complexed aluminum environments and so of ligand but not rapid exchange of A1 as shown in eq 1, where charges have been dropped for clarity. 31PNMR showed that the conslow % 2L + Al(H2O)e A1L + L 7 AlLz (1) centration of uncomplexed phosphorus decreased with heating up to 65 "C and eventually becomes negligible. This decrease coincided with an increase in concentration of the 3-12 ppm resonances. No real change in the relative proportion of -8 ppm resonance occurred, and the ratio of 3-12 resonances to -8 ppm resonances increased from 0.6 to 1.0 over this
Chemical shilt,6(ppm)
Figure 8. High-field (9.4 T) 31PNMR spectra of aluminum phosphate complexes. Concentrations: (a) [H3P04] = 0.2 M, [AI] = 0.1 M; (b) [H3P0,] = 0.2 M, [AI] = 0.2 M; (c) [H3P04] = 0.198 M, [AI] = 0.4 M.
temperature range. A t 95 "C only one peak was observed and the chemical shift was about -9 ppm, which is that expected from coalescence of the resonances obtained at lower temperatures. It appears therefore that, unlike aluminum, fast chemical exchange is occurring with uncomplexed phosphorus. This was confirmed by cooling the sample to room (298 K) temperature. At room temperature the original spectrum was reproduced. 31Pspectra obtained by cooling further to 268 K are better resolved than those obtained at 298 K, and there are some other distinct differences. The resonance at -8.5 ppm appears to be slightly smaller and broadened relative to the -8.0 ppm resonance in the 298 K spectra. Table I1 illustrates the concentrations of complexed and uncomplexed phosphorus in the various solutions prepared for high-field NMR. Cooling the solution from 298 K to 268 K also increases the amount of uncomplexed phosphorus, particularly for H3P04solutions. Mechanism and Structure of Complexes. A myriad of possible complexes may be formed from the variety of A1 species in solution as phosphorus ligands are exchanged into the coordination sphere of aluminum. It is possible however to exclude a large number of these. We have shown that there is no All, ion present and there is no evidence for any tetrahedral aluminum in phosphorus-containing complexes. Thus we may concern ourselves only with reactions of octahedral aluminum. We have also shown that complex formation is not observed until the pH of solutions containing predominately H2P04-species is reached. Thus it is probable that this ion and H3P04are potentially the most important hgands. Five types of complexes can be observed by 27AlNMR. Apart from a species at --9.0 ppm, only one at -1.4 ppm is observed in the presence of HPOd2-solution. Two more species are observed at -3.6 and -7.6 ppm, of which a small amount of the former forms from the latter at a high con-
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
Table 11. Concentrations of Complex Ions in Aluminum Phosphate Solutions Used in High-Field Experiments" concn soln
temp, K
[PI
[All
A A
268 298
0.2 0.2
0.4 0.4
B B C C D D
268 298 268 298 268 298
0.2 0.2 0.2 0.2 0.2 0.2
0.1 0.1 0.2 0.2 0.4 0.4
E E F F G G
268 298 268 298 268 298
0.2 0.2 0.2 0.2 0.2 0.2
0.1 0.1 0.2 0.2 0.4 0.4
P-uncomplexed
P-complexed
Rb
0.005 nc
0.195 0.200
0.84 0.83
0.071 0.066 0.033 0.030 0.023 0.014
0.129 0.134 0.167 0.170 0.173 0.186
1.34 1.40 1.28 1.30 1.01 1.08
0.1130 0.0975 0.0754 0.0629 0.0476 0.0354
0.087 0.103 0.125 0.137 0.152 0.165
1.13 1.34 1.12 1.22 1.08 1.18
HP042-
H2POc
H3P04
nAll concentrations in mol dm-3;see Table I for composition of solutions. b R is the ratio of complexes P to complexed A1 calculated from A1 data. Negligible.
Table 111. Concentrations of Complexes in Aluminum Phosphate Solutions
initial solution concentration, M
[All
IHP012-1 [H@OCI [H804I
A
B
C
D
E
F
G
0.4 0.18
0.1
0.2
0.4
0.1
0.2
0.4
0.2
0.2
0.2 0.2
0.2
0.2
product solution concentrations, lo3 M chem shift
B -
C 24 39 69 5
-5.9 -7.0 -8.0 -8.5 -12.0 -13.8 -15.0 -16.0 -16.5 -17.0 to -19.0 -20.0
13 19 126 16 -
5 21 27 53 -
-
-
11 8
23
23
3
-
total
196 0.84
129 1.34
R a
A
-
D -
F
G
-
14 77 31
-
4
13 68 30 13"
15
-
-
29" -
7
8
-
1
-
167 1.28
172 1.01
124 1.12
151 1.08
-
23 51 51Q 11 13
E 16 45 21 -
-
86 1.13
-
Two peaks.
centration of Al, and a further complex at -6.2 ppm, which only forms in the presence of H3P04. There is strong evidence for the -3.6 ppm species originating from a 1:l complex, which can exchange directly with a multiligand complex (-7.6 ppm) on heating. The structure of the other complexes is unknown, but the fact that the -1.4 ppm complex only forms in high concentrations of aluminum and has a chemical shift deshielded from the other complexes and closer to A1(H20)63+ suggests that this may originate from an aluminum oligomer, possibly the dimer AlzP where the shielding effect of the phosphate is shared between two aluminum atoms. Little additional aluminum is incorporated when the P/A1 ratio of solutions is increased above 1. The amount of aluminophosphate complered 31Pand 27Alcan be determined from the NMR spectra and solution concentrations. The elemental data for the complex can be estimated from the 27Al and 31Pspectra and are shown in Table I1 as R for the high-
field experiments. It is clear that R is 3 1 but not >2 for all but the solutions of H P O l - with excess aluminum chloride, including all the experiments performed at low field. The ratio R is close to 1 for the H2P04- and experiments with highest concentrations of aluminum. Thus it is clear that only one or two phosphorus-containing ligands are complexed. Thus the major species present could be A1(H3P04)3+,Al(H3P04)$+, A1(HzP04)2+,Al(HzP04)2+,Al(HP04)+, Al(HP04)2-, but we must also consider the possibility of species containing two or more aluminums and one phosphorus ligand. Table I11 lists the relative concentrations of complexes established from the high-field 31Pexperiments. Theoretically the elemental ratio for the complex mixture should be calculable from the intensity of the resonances from the various complexes, since P/Al = CPi/C(Pi/ni? (2) i
1
ANALYTICAL CHEMISTRY, VOL. 61,
r
Moles H' generated
0.3
x
AI added moles x lo3
Figure 9, Plot of moles of complexed phosphorus and acid generated from H3P0, solution.
where P is the 31Psignal intensity for each resonance and ni is the Al/P ratio for i complex. Solutions G, E, and F in Table 111are useful for these comparisons since they are relatively simple in terms of numbers and amounts of complex. By assuming n,,n2,n3,n4 equals 0.33,0.5,1, 2, or 3 it is possible to calculate assignments that agree with the observed P/A1 ratios. It is clear that no set of ni values fits all solutions, so that each resonance arises from species with variable stoichiometry. That is, the resonances must reflect the nature of the ligand rather than stoichiometry. Akitt et al. (2) assigned a resonance at -7.6 ppm to (H3P04),n Z 2, a resonance at -12.3 ppm to Al(HzP04)2+and A1(H2P04)2+,and a resonance at -15.7 ppm to A1(H,P04)3+. These probably equate to the resonances at -8.0 to -8.5, -12.0, and -16.0 ppm observed in these experiments. They were able to show that in the presence of excess H3P04all A1 is complexed, as the -7.6 resonance and the RIA1 ratio in the complex was then 3. The fact that the -8.5 ppm resonance builds up in the presence of excess Al, i.e. at lower pH and in solution, strongly suggests it is associated with H3P04species. If this resonance is from complex containing 2 or 3 mol of H3P04 and that at -12 ppm originates from (HzP04-),, where n = 1, 2, then the resonance at -16.5 ppm must be from an aluminum complex containing 2 or 3 aluminum atoms to account for a P/A1 ratio of only 1.08 for solution G (Table 111). We note that the PIA1 ratio R increases in the solution series G, F, E as the -16.5 ppm resonance decreases, a result not unexpected if the -16.5 ppm resonance arose from di- or trimeric aluminum species. The 31Pspectra of solutions E and F put some severe limitations on the nature of the species present in solution. With R = 1.13 and 1.12, respectively, there can be few species present with ni = 2 (eq 2). The resonance a t -16.5 ppm assigned to species with ni = 0.5 or 0.33 is relatively low in intensity, particularly for solution E. Combinations of ni for eq 2 only yield acceptable solutions if ni = 1for the -8.5 ppm resonance. Thus it is necessary to modify Akitt's original assignments and ascribe this resonance to a species of Al(H3P04),3+ where n also is equal to l. These assignments for the -8.5 and -12 ppm resonances are also supported by the high-temperature studies, which show that &PO4 is lost from solution with formation of resonances at -12 ppm or greater chemical shift and no change in intensity of the -8.5 ppm resonance. With increasing temperature the polyligands are formed preferentially, particularly those from dissociated species such as HzP04-. Likewise the complexed
NO. 11, JUNE
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species observed by nAl NMR are readily interchangeable on heating the solution, which suggests the two complexes are structurally similar. Significantly, exchange occurs without making monomeric A1(HzO)p species indistinguishable from complex, but exchange of ligand involves exchange with noncomplexed phosphate. From the pH of the solutions it is possible to calculate the number of moles of H+ generated if no complex formation occurred. These calculations assume that the mixing of solutions causes no additional dissociation, and that the acidity of the phosphate aluminum chloride solution is due to the weighted sum of moles of H+ introduced from the two solutions. It is clear that the final solutions are more acidic than predicted and more acidic than both phosphate or aluminum chloride solution, and additional acidity must occur due to complex formation. The acidity due to complex formation is plotted in Figure 9 as the observed acidity minus that predicted from mixing. It can be seen that there is good agreement between the total number of moles of H+ released and number of moles of phosphorus in complexed. That is, for every mole of incorporated, one He is released. The majority of this acid does not arise from H3P04, since H3P04is the most important ligand. Thus it is proposed that for every phosphoric acid ligand incorporated, approximately one H+ is lost from the aluminum coordination sphere. ACKNOWLEDGMENT We thank Professor N. N. Greenwood (Leeds) for access to high-field facilities and Dr. Fontain for obtaining high-field spectra. Registry No. Al(H20)63+,15453-67-5; A1(H20)6(OH)2+, 18499-02-0; AI(OH)i, 14485-39-3. LITERATURE CITED (1) Kniep, R. Angew. Chem., Ind. Ed. Engl. 1986, 25, 525-534. (2) Karlik, S. J.; Elgavish, G. A.; Piliai, R. P.; Eichhorn, G. L. J. Mag. Reson. 1982, 49, 164-167. (3) Crapper, D. R.; Krishnan, S. S.;Dalton, A. J. Science 1973, 780, 511-512. (4) O'Neill, I . K.; Prosser, H. J.; Richards, C. P.; Wilson, A. D. J. Biomed. Mat. Res. 1982, 76,39-49. (5) Wilson. S. T.; Lok, B. M.; Messina, C. A,; Cannan, T. R. Flanigen, E. M. J . A m . Chem. SOC. 1982, 104, 1146-1147. (6) Shayan, A.; Davey, G. C. Soil Sci. SOC.Am. J . 1978, 42, 878-882. (7) Robarge, W.; Corey, R. B. SoilSci. SOC.Am. J. 1979, 43, 481-487. (8) Sims, J. T.; Ellis, B. G. Soil Sci. SOC. Am. J. 1983, 47, 912-916. (9) Akitt, J. W.; Farthing, A. J. Mag. Reson. 1978, 32. 345-352. (IO) Akitt, J. W.; Eiders, J. M. J. Chem. SOC.,Faraday Trans 1 1985, 87, 1923- 1930. (11) Akitt, J. W.; Milic, N. B. J. Chem. SOC.. Dalton Trans. 1984, 98 1-984. (12) Akitt, J. W.; Farthing, A. J. Chem. Soc., Danon Trans. 1981, 1624-1 628. (13) Akitt, J. W.; Farthing, A. J. Chem. SOC., Dalton Trans. 1981, 1617-1621, (14) Akitt, J. W.; Farthing, A.; Howarth. 0. W. J. Chem. Soc., Daton Trans. 1981, 1609-1614. (15) Akitt, J. W.; Farthing, A. J. Chem. SOC., Danon Trans 1981, 1606- 1608. (16) Akitt, J. W.; Greenwood, N. N.; Khandelwal, B. L.; Lester, G. D. J. Chem. Soc., Dalton Trans. 1972, 604-610. (17) Akitt, J. W.; Greenwood, N. N.; Lester, G. D. J. Chem. SOC.A 1989, 803-807. (18) Akitt, J. W.; Mann, B. E. J. Mag. Reson. 1981, 44, 584-489. (19) Akitt. J. W.; Elders, J. M. J . Mag. Reson. 1985, 63. 587-589. (20) Bottero, J. Y.; Cases, J. M.; Flessinger, F.; Poirier, J. E. J. Phys. Chem. 1980, 84, 2933-2939. (21) Bottero. J. Y.; Tchoubar, D.; Cases, J. M.; Flessinger, F. J. Phys. Chem. 1982, 86, 3667-3673. (22) Brown, P. L.; Sylva, R. N. J. Chem. SOC., Dalton Trans. 1985, 1967- 1970. (23) Akin, J. W.; Greenwood, N. N.; Lester, G. D. J. Chem. SOC.A 1971, 2450-2457.
RECEIVED for review October 5 , 1988. Accepted February 28, 1989.
