on the Temperature of Sodium Tripolyphosphate ... - ACS Publications

A large number of starting materials can be employed in sodium tripolyphosphate preparation, providing that the molar ratio of Na/P is 5/3. In an indu...
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Ind. Eng. Chem. Res. 2007, 46, 6401-6407

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Effect of Impurities (Fe3+ and Al3+) on the Temperature of Sodium Tripolyphosphate Formation and Polymorphic Transformation Regina Kijkowska,* Zygmunt Kowalski, Danuta Pawlowska-Kozinska, Zbigniew Wzorek, and Katarzyna Gorazda Institute of Inorganic Chemistry and Technology, Cracow UniVersity of Technology, Warszawska 24, 31-155 Krakow, Poland

The effect of Fe3+ and Al3+, usually present as impurity ions in wet-process phosphoric acid, on formation and polymorphic transformation of sodium tripolyphosphate (STPP ) Na5P3O10) has been investigated using X-ray powder diffraction, IR spectroscopy, and scanning electron microscopy methods. The simultaneous appearance of both high- (Form-I) and low-(Form-II) temperature modifications at the beginning of STPP formation at the temperature range of 250-300 °C in the impurity-free as well as in Al- and Fe-containing samples was recorded. Form-I in the impurity-free STPP, containing both crystallographic forms, underwent low-temperature transformation into Form-II around 350 °C, while Form-II was stable up to 450 °C and transformed into high-temperature Form-I above 450 °C. Ignition of the impurity-containing STPP revealed that Al3+ and Fe3+ stabilized Form-I so that the low-temperature transformation (Form-I f Form-II) did not occur, while Form-II transformed into high-temperature Form-I at a lower temperature (below 450 °C) than in the impurity-free STPP temperature. 1. Introduction STPPssodium tripolyphosphate (Na5P3O10)sis an authorized multipurpose food ingredient under U.S. Federal Legislation (sec. 182.1810) and also EU legislation (Directive 95/2) registered as E451(1). STPP is also of great technical importance.1-6 It is the key constituent of some modern synthetic detergents. The STPP, as a single substance, performs several very useful functions in washing process; it builds Ca and Mg ions into water-soluble complexes, buffers washing media, facilitates dissolving of detergents, and protects the washing machine against corrosion. In contrast, the phosphate-free detergents are based on water-insoluble zeolite. The latter absorbs by ion-exchange Ca but not Mg. To increase the washing efficiency the phosphate-free detergents require several additional chemicals, such as nonbiodegradable polycarboxylates, EDTA, citrate, enzymes, and other ingredients. A large number of starting materials can be employed in sodium tripolyphosphate preparation, providing that the molar ratio of Na/P is 5/3. In an industrial manufacturing procedure thermal or highly purified wet-process phosphoric acid, neutralized with NaOH or Na2CO3 to obtain an orthophosphate solution with Na/P ) 5/3 molar ratio, has been used the most. From the solution obtained water is evaporated, and the resulting dry residue is ignited. Depending on the ignition temperature the STPP appears in one of the anhydrous monoclinic forms; lowtemperature Na5P3O10-II (Form-II) or high-temperature Na5P3O10-I (Form-I).4,7-13 The phase transition, a complex, not entirely described in the literature, process, has been observed in quite a wide (450-500 °C) temperature range. This transformation, according to Van Wazer, is usually not complete, and it is commonly found that STPP heated above 500 °C contains 5-30% of low-temperature Form-II.4 Form-I exhibits “lumping” properties resulting from extremely high solubility in water and simultaneous formation of hexahydrate Na5P3O10‚6H2O crystals from a supersaturated solution. Commercial STPP of diversified * To whom correspondence should be addresed. Fax: (48) (12) 6282036. E-mail: [email protected].

grades contains both crystalline phases and some small varying amounts of tetrasodium diphosphate (Na4P2O7) and crystalline sodium metaphosphates (NaPO3)n. The weight ratio of FormI/Form-II is either controlled by the temperature of ignition or by mixing the final products of either form. When the wet-process phosphoric acid (WPA) is used, the effect of impurities, derived from the phosphate rock, on the quality of STPP should be taken into consideration. For example, preconcentrated up to 77-80 wt % of H3PO4, industrial WPA obtained from Kola apatite in one of the Polish plants contains about 2% of SO42-, 0.1% of F, 0.5% of Al, 0.3% of Fe, 0.15% of Ti, and some other elements at a lower level.14 High purification of the WPA is an expensive process. Considering the detergent (not food ingredient) industry production, the question arises as to what extent the phosphoric acid should be purified and how the impurities may affect the phase composition of the final STPP product. The aim of the investigations carried out in our laboratory was to determine how Al3+ and/or Fe3+, present in phosphoric acid as impurities, affect the phase composition of the STPP. The amount of Al3+ and/or Fe3+ was such that it resulted in 0.05-1.0 wt % of the element in the final dry product. The investigated level of impurities was predicted on the basis of the following reasoning. As the STPP process gives no byproducts, the molar ratio of the impurity element/P in WPA used and in the STPP obtained remains the same. Because 100 wt units of WPA (∼80 wt % H3PO4) is equivalent to 100.1 wt units of STPP, the wt % of the impurities transferred from WPA to STPP is also on the same level. The investigated range (0.05-1 wt %) covers the numbers 0.3 wt % Fe and 0.5 wt % Al in WPA derived from Kola apatite.14 The effect of Al3+ on Na5P3O10 formation and on (Form-II f Form-I) thermal transformation was described in our previous paper.15 The explored temperature range was 350-550 °C. The present paper extends the temperature range to a lower (below 350 °C) region and supplements the previous report with the effect of Fe3+ alone and of the simultaneous presence of Fe3+ + Al3+ as impurities in phosphoric acid. The phase composition of the product

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Figure 1. XRD patterns of Na5P3O10 powders ignited at 300 °C. (A) ) Na5P3O10-II (Form-II) - standard, (B) ) impurity-free Na5P3O10 (reference sample), (C) ) Na5P3O10 (with 0.5 wt % Al), (D) ) Na5P3O10 (with 0.5 wt % Fe), (E) ) Na5P3O10 (with 0.5 wt % Fe + 0.5 wt % Al), (F) ) Na5P3O10 (with 1.0 wt % Al), (G) ) Na5P3O10 (with 1.0 wt % Fe), (H) ) Na5P3O10-I (Form-I) - standard, (I) Na4P2O7 (pyrophosphate) - standard (F1 ) FormI, F2 ) Form-II).

obtained within the temperature range of 250-550 °C was investigated.

Figure 2. XRD patterns of Na5P3O10 powders ignited at 400 °C. (A) ) Na5P3O10-II (Form-II) - standard, (B) ) impurity-free Na5P3O10 (reference sample), (C) ) Na5P3O10 (with 0.5 wt % Al), (D) ) Na5P3O10 (with 0.5 wt % Fe), (E) ) Na5P3O10 (with 0.5 wt % Fe + 0.5 wt % Al), (F) ) Na5P3O10 (with 1.0 wt % Al), (G) ) Na5P3O10 (with 1.0 wt % Fe), (H) ) Na5P3O10-I (Form-I) - standard, (I) Na4P2O7 (pyrophosphate) - standard (F1 ) FormI, F2 ) Form-II).

2. Experimental Section Reagent grade chemicals, phosphoric acid (15 M/L H3PO4) and NaOH (POCH S.A. Poland), AlPO4·H2O (RdH Lab. GmbH & Co, Germany), and FePO4·2H2O (Aldrich Chem. Corp. Inc.), were used without further purification. For the STPP preparation a solution of orthophosphates with a molar ratio of Na/P ) 5/3 was evaporated. The dry product obtained was ignited at temperatures 250 °C, 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, and 550 °C for 2 h. The materials obtained were identified using powder X-ray diffraction (XRD), IR spectroscopy, and scanning electron microscopy (SEM) methods. When impurities were included, an appropriate amount of AlPO4·H2O and/or FePO4·2H2O was dissolved in a concentrated phosphoric acid solution, and then NaOH (20 wt %) was added to obtain an orthophosphate solution with the required Na/P molar ratio. For the XRD, Philips X’pert equipment with graphite monochromator PW 1752/00, radiation Cu KR, Ni filter, 2Θ from 10° to 60° at 30 kV, 30 mA was used. A Fourier Transform IR spectrometer FTIR-FTS 175 (Bio-Rad) was used to record the IR spectra of the samples in a KBr pressed pellet covering the wavenumbers of 400-4000 cm-1. 3. Results A selection of X-ray diffraction patterns, presented in Figures 1-4, illustrates the differences in phase composition of the analyzed samples resulting from the different content of

impurities and temperature of ignition. For comparison, the XRD of Form-I, Form-II, and also of tetrasodium pyrophosphate (Na4P2O7) are included as standards in the figures. For identification of Form-I the unique nonoverlapping diffraction peak at a reasonable intensity at 2Θ ≈ 21.7° and another one at 2θ ≈ 29.0° have been used.16 The position of other overlapping peaks of Form-I/Form-II is located outside the range of 2θ ) 21-31° and is not helpful in phase composition analysis. According to the literature a rapid transformation of Form-II to Form-I should be expected within the temperature range of 450 °C-500 °C.4,7,8 That has been also observed in our work in the impurity-free STPP (Figures 3B and 4B) as well as in the samples with a low concentration (0.05 wt %) of Al or Fe (not included in the paper). In contrast, the presence of Fe and/ or Al (Figure 1C-G) makes a high proportion of Form-I (indicated as F1) already formed in the STPP at a temperature as low as 300 °C. The appearance of small F1 peaks in the impurity-free sample (Figure 1B) will be discussed later in this paper. Within the temperature range of 300-400 °C the presence of impurities results in a mixture of Form-I/Form-II (Figures 1C-G and 2C-G) and that does not depend on whether the sample contains Fe or Al alone or Fe + Al. The intensities of the peaks of the components in the STPP, containing Al/Fe, are showing no regular changes with the amount of impurities as well as with the temperature increase. For example, at 300 °C

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Figure 3. XRD patterns of Na5P3O10 powders ignited at 450 °C. (A) ) Na5P3O10-II (Form-II) - standard, (B) ) impurity-free Na5P3O10 (reference sample), (C) ) Na5P3O10 (with 0.5 wt % Al), (D) ) Na5P3O10 (with 0.5 wt % Fe), (E) ) Na5P3O10 (with 0.5 wt % Fe + 0.5 wt % Al), (F) ) Na5P3O10 (with 1.0 wt % Al), (G) ) Na5P3O10 (with 1.0 wt % Fe), (H) ) Na5P3O10-I (Form-I) - standard, (I) Na4P2O7 (pyrophosphate) - standard (F2 ) FormII).

the intensities of (F1) are higher in the STPP containing 0.5% Fe (Figure 1D) than in the sample, containing 0.5% Fe + 0.5% Al (Figure 1E). However, the intensities of the same samples at 400 °C (Figure 2D,E) are opposite to those at 300 °C. That might be resulting from the different crystallinities of the same sample as recrystallization goes on. The intensities of the peaks become more uniform at 450 °C after the Form-II f Form-I transformation in the impurity-containing STPP is almost completed (Figure 3C-G). In contrast, the impurity-free sample (Figure 3B) at 450 °C is still in the well-crystallized lowtemperature modification (Form-II). A temperature increase up to 500 °C also makes impurity-free STPP transform into hightemperature modification (Figure 4B). A further temperature increase up to 550 °C has no effect on the phase composition of the product, which is in Form-I. Scanning electron micrographs indicate that an impurity-free, reference sample, ignited at 300-450 °C (Figures 5A,E and 6A), consists of large, nonuniformed, platelike crystals. In contrast, SEM of the impurity-containing samples gives the evidence that Fe (Figures 5B,F and 6B,D) similar to Al (Figures 5C,G and 6C,E) inhibits particle growth of the STPP. The inhibitory effect seems to be even stronger when Al + Fe are simultaneously present (Figures 5D,H and 6F). Variations in morphology and a decrease in crystallite size caused by the presence of the impurities do not depend on the temperature of ignition. The smaller particles that appear at the beginning of

Figure 4. XRD patterns of Na5P3O10 powders ignited at 500 °C. (A) ) Na5P3O10-II (Form-II) - standard, (B) ) impurity-free Na5P3O10 (reference sample), (C) ) Na5P3O10 (with 0.5 wt % Al), (D) ) Na5P3O10 (with 0.5 wt % Fe), (E) ) Na5P3O10 (with 0.5 wt % Fe + 0.5 wt % Al), (F) ) Na5P3O10 (with 1.0 wt % Al), (G) ) Na5P3O10 (with 1.0 wt % Fe), (H) ) Na5P3O10-I (Form-I) - standard, (I) Na4P2O7 (pyrophosphate) - standard.

the STPP formation (300 °C) do not grow larger while ignited at higher temperatures (400 °C and 450 °C). 4. Discussion Considering the phase transition in the STPP, a mechanism has no clear description in the literature available so far. The published data have been referred mostly to the dehydration of phosphates mono- and/or disodium without giving much consideration to the polymorphic transformation of the STPP.13,17-22 From the literature cited one can learn that Form-I and FormII of the Na5P3O10 have the same space group (C2/c).7,10-12 The monoclinic unit cell in each structure contains 4 molecules with different parameters. Na5P3O10-I is characterized by a ) 0.961 nm, b ) 0.534 nm, c ) 1.973 nm, and β ) 112°, while in the unit cell of Na5P3O10-II they are a ) 1.60 nm, b ) 0.524 nm, c ) 1.125 nm, and β ) 93°. In both structures the anions P3O105- are geometrically very similar, each having a twofold symmetry axis of the unit cell with a trans configuration of adjacent tetrahedra.10-12 The cation-anion electrostatic bonds form continuous three-dimensional networks with a sheetlike arrangement which is more distinct in Form-II than in Form-I. All sodium cations in Form-II are octahedrally coordinated by 6, while in Form-I some of the Na-ions are coordinated only by 4 oxygen atoms. The triphosphate ions are linked by -O....Na....O- into sheets in the plane parallel to (ıj01) in FormII. In Form-I the sheetlike structure parallel to (100) is less defined because of more ionic contacts involved in intersheet linkage.

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Figure 5. Scanning electron micrographs of powders ignited at 300 °C (A, B, C, D) and 400 °C (E, F, G, H). (A) and (E) f impurity-free Na5P3O10 (reference sample), (B) and (F) f Na5P3O10 (with 1.0 wt % Fe), (C) and (G) f Na5P3O10 (with 1.0 wt % Al), (D) and (H) f Na5P3O10 (with 0.5 wt % Fe + 0.5 wt % Al). The bar ) 20 µm.

According to the literature drying of an orthophosphate solution with Na/P ) 5/3 molar ratio yields a mixture of anhydrous double salt Na3H3(PO4)2, anhydrous Na2HPO4, sometimes Na2HPO4·2H2O, and a small amount of Na4P2O7. Heating the above mixture at successively higher temperature results in further dehydration and subsequent condensation to give crystalline Na4P2O7 and an amorphous phosphate phase, that, on prolonged heating at 200 °C, transforms into crystalline Na2H2P2O7.4,9 The presence of amorphous phase was deduced from the difference between the amount determined by chromatographic and XRD methods. The chromatographic included ortho-, pyro-, tripoly-, and tetrametaphosphates, while the XRD showed crystalline Na4P2O7 and Na5P3O109. Further heating results in the evolution of more water and, at a temperature around 300 °C, the formation of Na5P3O10 according to the following reaction: 2Na4P2O7 + Na2H2P2O7 (amorphous) f 2Na5P3O10 + H2O. Postulated by J. Edwards and A. H. Herzog the mechanism involves dissolution of Na4P2O7 in the amorphous phase, countercurrent migration of Na+ and H+, and, without the need of diffusion of bulky phosphate ions of any

degree of condensation, the formation of possibly amorphous Na5P3O10 at the crystal-amorphous phase interface.9 Most of the phosphate mixture with Na/P ) 5/3 heated to 350-400 °C is converted into low-temperature modification Na5P3O10-II.4,13 However, J. Edwards and A. H. Herzog observed the formation of a high-temperature modification (Form-I) before the lowtemperature Form-II started to form.9 Their XRD data on heating the orthophosphates at 250 °C indicated variation in composition with time in the following sequence: Na4P2O7 (after 15 min) f Na5P3O10-I (after 30 min) f Na5P3O10-II (after 60 min). By Van Wazer, the appearance of high-temperature modification (Form-I), prior to low-temperature modification (Form-II), can be accounted for by the Gay-Lussac Ostwald step rule that says that an unstable form is usually produced before the stable form appears in the case of substances exhibiting several forms.4 In such cases, the amount of an unstable form, if it appears below the transition point, becomes gradually larger, goes through a maximum, and then declines as the stable modification starts to form. According to Corbridge the transition I f II needs a lattice breakup in all directions to form an amorphous phase prior to the growth of Form-II.12 Also, by Van Wazer, the appearance of high-temperature Form-I observed during the lowtemperature condensation of orthophosphates and its subsequent reconversion to Form-II is catalyzed by the presence of amorphous material. 4.1. Possible Forms in which Al and Fe Coexist with STPP. In the present work the impurities were introduced by dissolving AlPO4·H2O and/or FePO4·2H2O in phosphoric acid prior to solution evaporation. Some precipitate was forming while NaOH was added into the solution containing Al and or Fe (but not in an impurity-free solution). Since no crystalline phase containing Fe and/or Al has been recorded in the dry residue as well as in the ignited samples, the impurity elements can be distributed between adsorbed on the surface of any crystalline phase, incorporated in an amorphous phase, and/or incorporated in some host STPP crystal. 4.2. Amorphous Phase. The XRD patterns revealed that the dry residue (before ignition) consisted of the highly crystalline double salt Na3H3(PO4)2 and anhydrous Na2HPO4 (example in Figure 7A). Ignition at 250 °C resulted in crystalline Na4P2O7 and sometimes in some small amounts of Form-I and Form-II of Na5P3O10. When more than one crystalline phase has been recorded by the XRD (example in Figure 7B,C,E) and quantitative assessment of the phase proportions is not possible, the presence of an amorphous phase has no direct evidence. On the contrary, since the XRD of the phosphates with a molar ratio of Na/P ) 5/3 (equivalent to 2Na2HPO4 + NaH2PO4) exhibits only one crystalline Na4P2O7 with the molar ratio of Na/P ) 4/2 (Figure 7D,F) the rest of the sodium phosphates should be in the amorphous phase. The discussed amorphous phase may, according to Edwards and Herzog,9 play a mediatory role between the crystalline substrate, such as Na4P2O7, and newly formed Na5P3O10. It may also, according to Van Wazer, catalyze the polymorphic transformation of STPP.4,8 The impurities in question, being in an amorphous phase, may enhance the discussed effects. 4.3. Al and/or Fe Stabilize the High-Temperature Modification (Form-I) and Promote Transformation of Form-II f Form-I at Low-Temperature Range (below 450 °C). Both, high- and low-temperature modifications appeared at the beginning of the STPP formation at a temperature of 300 °C in the impurity-free as well in Al-, Fe-containing samples. In some cases Form-I appeared even at a temperature as low as 250 °C. While the impurity-free sample was treated by heat at 250 °C (Figure 7B) and at 300 °C (Figure 1B), small peaks of F1 at 2θ

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Figure 6. Scanning electron micrographs of powders ignited at 450 °C: (A) ) impurity-free Na5P3O10 (reference sample), (B) ) Na5P3O10 (with 0.5 wt % Fe), (C) ) Na5P3O10 (with 0.5 wt % Al), (D) ) Na5P3O10 (with 1.0 wt % Fe), (E) ) Na5P3O10 (with 1.0 wt % Al), (F) ) Na5P3O10 (with 0.5 wt % Fe + 0.5 wt % Al). The bar ) 20 µm.

) 21.8° and 29.0° were recorded. Those peaks of Form-I, being at a metastable stage below the transition temperature, disappeared at 350 °C (figure not included) and at 400-450 °C (Figures 2B and 3B) and that is in agreement with the GayLussac-Ostwald step rule. On the contrary, in the samples containing Al and/or Fe ions Form-I that appeared at the temperature range of 250-300 °C was not disappearing while the temperature was rising up to 450 °C (Figures 1-3). The coexistence of the two forms below known from the literature transition temperature region (450-500 °C) may suggest that the impurities stabilize Form-I to such an extent that the transformation of Form-I f Form-II has been hindered. The effect of making Form-I stable in the presence of Fe and/or Al in comparison to the impurity-free STPP has been also observed in IR spectra. An example obtained at 350 °C is shown in Figure 8. An identification of the phases in the mixture of Form-I/ Form-II is based on the appearance of the nonoverlapping IR bands at wavenumbers of 715 cm-1 and 756 cm-1 corresponding to Form-I (marked as *) and of 667 cm-1 corresponding to Form-II (arrow). Stabilizing phenomenon of less stable polymorphs by inorganic ions or organic substances is described in the literature. The well-known example considers vaterite (the least stable polymorph of CaCO3 in water) that was forming by homogeneous precipitation in the presence of some small amounts

of La3+ or Ba2+ ions or organic additives, whereas calcite crystals were produced in the control experiments without the additives.23-27 Some other examples indicate that Mg2+, Sr2+, Ba2+, Fe2+, Ni2+, and Zn2+ favored metastable aragonite formation and inhibited its transformation into stable calcite.24,25 One of the explanations in those reports was based on the adsorption of the additives on the surface of the less stable phase usually forming first. The adsorbed ions inhibited the solvent-mediated transformation into calcite. Additionally, the adsorption of the impurities reduced the growth rate of the aragonite nucleus resulting in size reduction and in morphology changes. If the investigated impurities adsorbed on the surface of the STPP crystals have some stabilizing effect, inhibiting the transformation of the metastable polymorph (Form-I) below the transition point is possible. Also, some morphological changes resulting from the modification of growth rates of different crystal faces, according to the literature,24 are possible. The observed SEM reduction in the crystallite size of STPP (Figures 5 and 6) may also be caused by the adsorbed impurities similar to those reported for CaCO3.24,26,27 The possibility of an incorporation of Al3+ and Fe3+ in the crystal of Na5P3O10 can also be considered. If the ionic radii of the impurity and the host cation are comparable, their electric charges and coordination numbers are the same, then the

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Figure 8. IR spectra of Na5P3O10 powders ignited at 350 °C. (A) ) Na5P3O10-II (Form-II) - standard, (B) ) impurity-free Na5P3O10 (reference sample), (C) ) Na5P3O10 (with 0.5 wt % Al), (D) ) Na5P3O10 (with 0.5 wt % Fe), (E) ) Na5P3O10 (with 0.5 wt % Fe + 0.5 wt % Al), (F) ) Na5P3O10 (with 1.0 wt % Al), (G) ) Na5P3O10 (with 1.0 wt % Fe), (H) ) Na5P3O10-I (Form-I) - standard. Figure 7. XRD patterns of powders with Na/P ) 5/3 ignited at 250 °C (B-F) in comparison to (A) not ignited. (A) ) crystalline dry residue before ignition (o ) Na3H3(PO4)2 and * ) Na2HPO4), (B) ) impurity-free, (C) ) with 0.5 wt % Fe, (D) ) with 0.5 wt % Fe + 0.5 wt % Al, (E) ) (with 1.0 wt % Al), (F) ) (with 1.0 wt % Fe), (G) ) Na5P3O10-I (Form-I) - standard, (H) ) Na4P2O7 (pyrophosphate) - standard (peaks: F1 ) Form-I, F2 ) Form-II, Py-pyrophosphate).

impurity ions can be incorporated without morphological changes.28 If the differences are essentially large, which is the discussed case, a little incorporation of the impurity can cause large effects in the crystal habit.28 The ionic radii with the coordination number (6) of Na+, Fe3+, and Al3+ are 0.116, 0.069, and 0.0675 nm, respectively, while the ionic radii with the coordination number (4) of Na+, Fe3+, Al3+ are 0.113, 0.063, and 0.053 nm, respectively.29 The ionic radii of the impurities are considerably smaller than that of the host Na+ cation. Theoretically, such small cations can be incorporated in some interstitial positions (Frenkel defects). However, the balance of the excess of the electric charge of the interstitially incorporated trivalent ion is rather slightly possible. Another possibility is the incorporation in the host crystal of the Na5P3O10 resulting from the substitution for Na+ cations. If the substitution occurs, then it should cause major changes in the local environment because of the differences in the ionic radii and should induce cationic vacancies (Shottky defects) in order to balance the difference in the electric charge. When temperature is increasing, the cationic vacancies can facilitate mobility of the cations in the lattice. Higher energy (strain energy) of the distorted local elements in the crystal of Al- and/or Fe-containing Na5P3O10 may be a driving force for the polymorphic Form-II f Form-I transformation at lower (below 450 °C) than expected (above 450 °C) ignition temperatures. This goes well with the statement reported by Corbridge that the transition II f I might involve only a partial break up and reorientation of the sheetlike units already present in the structure.

To prove the discussed phenomena, if they are the case, the authors cannot see much of the others, some more experimental evidence has to be found. 5. Conclusion For the STPP preparation a solution of orthophosphates with the molar ratio of Na/P ) 5/3 was evaporated, and the dry residue obtained was ignited at a temperature range of 250550 °C. To prepare the orthophosphate solution with the required molar ratio phosphoric acid was neutralized with NaOH (20 wt %). AlPO4·H2O and FePO4·2H2O, treated as impurities, were dissolved in phosphoric acid. The simultaneous appearance of both, high- (Form-I) and low(Form-II) temperature modifications at the beginning of STPP formation at the temperature range of 250-300 °C in the impurity-free as well as in Al-, Fe-containing samples was recorded. The appearance of high-temperature Form-I as a metastable phase at a low-temperature range can be accounted for the Gay-Lussac step rule. Ignition of the impurity-free STPP, containing both crystallographic forms, caused at 350 °C the polymorphic transformation of Form-I f Form-II. Form-II was stable up to 450 °C, while ignited above 450 °C transformed into high-temperature Form-I. In Al- and/or Fe-containing STPP the impurities stabilized the high-temperature modification (Form-I) and promoted transformation of Form-II f Form-I at lower (below 450 °C) than in the impurity-free temperature range. The stabilizing of the Form-I effect at a low-temperature range may have resulted from Al3+ and Fe3+ adsorption. The impurities adsorbed on the surface of the metastable Form-I may inhibit its polymorphic transformation. Examples supporting that idea have been provided in the text. Possible incorporation of Al3+ and Fe3+ in the crystal of the STPP may induce cationic vacancies and generate strain energy

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in the distorted local elements in the lattice. Those can facilitate mobility of the crystal elements promoting polymorphic FormII f Form-I transformation at lower (below 450 °C) than expected (above 450 °C) ignition temperature. Acknowledgment This paper is a result of research financially supported by the State Committee for Scientific Research, Warsaw, Poland. We would like to acknowledge helpful SEM analysis of Barbara Trybalska (University of Science and Technology (AGH)), Krakow, Poland. Literature Cited (1) Rashchi, F.; Finch, J. A. Polyphosphates: A Review of their Chemistry and Application with Particular Reference to Mineral Processing. Miner. Eng. 2000, 13, 1019. (2) Fuchs, J. Production and Use of Detergent Grade Sodium Triphosphate in the U.S.A. Proceedings of the 1st International Congress on Phosphorus Compounds; IMPHOS: Paris, 1977; p 201. (3) Kowalski, Z.; Kijkowska, R.; Pawłowska-Kozinska, D.; Wzorek, Z. Sodium Tripolyphosphate and other Condensed Sodium Phosphates Production Methods. Pol. J. Chem. Technol. 2002, 4, 3, 27. (4) Van Wazer, J. R. Phosphorus and Its Compounds; Interscience Publishers: New York, 1958; Vol. 1. (5) Grzmil, B. Manufacturing of Pyro- and Tripolyphosphate Complexes of Micronutrients in the Process of Phosphates Condensation. Ind. Eng. Chem. Res. 1997, 36, 5282. (6) Grosse, J.; Nielen, H. Sodium Triphosphate - Raw Materials, Production Process and Quality. Tenside Deterg. 1983, 20, 6, 285. (7) Corbridge, E. E. C. The Structural Chemistry of Phosphorus; Elsevier: Amsterdam, The Netherlands, 1974. (8) Van Wazer, J. R. Structure and Properties of the Condensed Phosphates. II. A Theory of the Molecular Structure of Sodium Phosphates Glasses. J. Am. Chem. Soc. 1950, 72, 644. (9) Edwards, J. W.; Herzog, A. H. The Mechanism of Formation of Sodium Triphosphate from Orthophosphate Mixtures. J. Am. Chem. Soc. 1957, 79, 3647. (10) Dymon, J. J.; King, A. J. Structure Studies of the two Forms of Sodium Tripolyphosphate. Acta Crystallogr. 1951, 4, 378. (11) Davies, D. R.; Corbridge, D. E. C. The Crystal Structure of Sodium Triphosphate, Na5P3O10, Phase II. Acta Crystallogr. 1958, 11, 315. (12) Corbridge, D. E. C. The Crystal Structure of Sodium Triphosphate, Na5P3O10, Phase I. Acta Crystallogr. 1960, 13, 263. (13) Dombrovski, N. M. About Reaction of Triphosphate Formation while Mono- and Disodium Phosphate are Thermally Dedydrated. Zh. Neorg. Khim. 1962, 7, 95. (14) Kijkowska, R.; Pawlowska-Kozinska, D.; Kowalski, Z.; Jodko, M.; Wzorek, Z. Wet-process Phosphoric Acid obtained from Kola Apatite.

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ReceiVed for reView February 27, 2007 ReVised manuscript receiVed July 2, 2007 Accepted July 15, 2007 IE070296I