Structural Analysis of Amorphous Phosphates Using High Performance Liquid Chromatography
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No. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. 13 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society
HPLC AND PHOSPHATES: EXPERIMENTAL TECHNIQUE
The metal-phosphates of interest here consist of chains of corner-linked P0 4 tetrahedra (Fig 1); which are, in turn , bonded to each other by the metal cations. A crystalline phosphate of this type will generally have a phosphate chain of only one specific length in its structure while an amorphous phosphate solid will be characterized by a distribution of "spaghetti-like" chains of different lengths. By applying the HPLC technique, the relative concentration and distribution of phosphate chains in an amorphous solid can be determined quantitatively. Selected crystalline-phosphate standards are used to calibrate the HPLC system so that a particular peak in the HPLC chromatogram can be unambiguously assigned to a known phosphate anion.
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FIG. 1 Schematic drawing of (left) a crystalline metal-phosphate compound with only P 2 chains (two corner-linked P0 4 tetrahedra) and (right) an amorphous metalphosphate with the same overall composition. In the amorphous phosphate, both longer and shorter phosphate chains are present. The metal cations are represented by the shaded spheres. The HPLC technique is applicable to most metal-phosphate solids because of the manner in which these solids dissolve in high-pH aqueous solutions. In the present experiments the phosphate solid is placed in a pH 10 solution of 0.22 M NaCl and 5x10- 3 M Na 4 EDTA. Diffusion of water into the solid leads to hydration, disentanglement, and subsequent transport of entire polyphosphate anion chains into solution. Acid/base reactions between the H+ and OH- ions and the P-O-Metal groups affect the ionic interaction between the chains and assist in the disentanglement and transport of the chains into solution [4-7]. The presence of the EDTA in the solution is apparently critical when dealing with high-field-strength cations such as Mg+ 2 or Fe+3 . Once the "spaghetti-like" chains are in the chromatography solution, they are quite stable and do not exhibit any tendency toward either hydrolysis or
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polymerization for a period of at least 24 h [8]. The solution containing the dissolved phosphate solid is passed through a 0.45 micron filter and then injected into the HPLC system where the chains of linked P0 4 tetrahedra are bound on an ion-exchange column. An HPLC chromatogram is obtained as the sodium chloride molarity at the ion-exchange column is programmed to increase as a function of time, and thereby, sequentially release phosphate chains of various lengths beginning with phosphate anions consisting of only one P0 4 tetrahedron (Pl), then two P0 4 tetrahedra (P2), three P0 4 tetrahedra (P3) and so forth. The HPLC chromatogram, therefore, consists of a series of peaks whose position in time (i.e., the time at which the sodium chloride molarity has reached the value at which specific phosphate anions are released from the column) corresponds to the presence of
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