Anal. Chem. 1984, 56, 193-196
Enforcement Standards Laboratory; DNA Task B990MXPF, Work Unit 00064, William J. Witter, DNA Task Manager. Presented in part at the 9th Annual FACSS meeting in Philadelphia, PA, September 1982. Certain commercial equipment, instruments, or materials are identified in this
193
paper to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
X-ray Diffraction Spectrometry for the Analysis of Crystalline Solid Phases J. R. Garcia, Marta SuPrez, C. G. Guarido, and Julio Rodriguez* Departamento de Quimica Inorgbnica, Facultad de Qufmica, Universidad de Oviedo, C/ Calvo Sotelo sln, Oviedo, Spain
X-ray dlffractlon In the quantitatlve analysis of crystalline solid phases was used. A nonconventionai method of callbrallon for mlxtures of materials with the same chemical composttlon and isomorphic structure Is reported. Thls method, whlch can be used when some of the components cannot be obtained In pure form, was applied to the evolution study of a-titanlum phosphate In the H+/LI+ and H+/Na+ Ion exchange. Dlstrlbutlon curves of solld phases were obtained.
The first synthetic ion exchangers for commercial applications were inorganic materials. After the initial development of fusion permutites and gel permutites (1, 2) for use in water-softening processes, a rapid expansion of organic ionexchange materials developed (3). In contrast, the study of inorganic solids was undertaken almost solely in relation to problems of structural chemistry and diffusion in solid systems. Due to new technical problems, such as the separation of ionic compounds in radioactive water, there has been a resurgence of interest in inorganic exchangers. In these operations, highly selective materials which have stability against high temperatures in acid medium and against strong ionizing radiation are required. Organic resins are not adequate for these applications because their selectivity and exchange capacity are altered by the presence of ionizing radiation. The fact that many insoluble oxides absorb anions or cations from an aqueous solution leads to problems in analytical separations. This has been attributed to several causes but it was not studied in the ion-exchange field until the use of an insoluble zirconium phosphate in the separation of uranium and plutonium from fission products was reported (4,5). As a result of the renewed interest in inorganic materials, the study of the insoluble acid salts of tetravalent metals has developed considerably. Compounds with a great variety of metals (Zr, Ti, Sn, Ce) and anionic groups (phosphate, arsenate, vanadate) were used, and the behavior of their amorphous ( 1 , 6) and crystalline (7-10) forms was studied. The most widely studied compound, among these materials, is a-zirconium bis(monohydrogen orthophosphate) monohydrate (a-ZrP), a lamellar solid (11). As a consequence of its special arrangement, the reflection in the X-ray diffraction powder pattern, at lower angular values, corresponds with the value of the spacing between layers (12, 13). Control of the saturation degree of ion-exchanger material is usually carried out by conventional methods of chemical analysis. They are slow procedures but their accuracy is great. However, in industrial processes, where there is a need to know
the actual degree of ionic conversion a t a fixed moment, the speed of the determination may be more decisive. Ti(HP04)2.H20(a-Tip) is isomorphic with a-ZrP (14)and both show cation exchange properties. a-TiP is an adequate material for retaining Li+ and Na+ ions (15-20). In this process, it behaves as a bifunctional exchanger, the presence of distinct crystalline phases being observed: TiHM(P04)2-xH20 and Ti(MP0&.yH20, where M = Li, Na (17-20). When the hydrogen ion is substituted by another cation in a-Tip, the spatial arrangement of each layer remains unaltered (21). Nevertheless, the layers will be situated a t different distances, the value of which will be a function of the size of the substituted ion and the degree of hydratation of the crystalline phase formed (10, 17, 18). The present paper reports the quantitative determination of the concentration of solid phases on samples of a-TiP unsaturated in Li+ and Na+ by measuring the intensities of the X-ray diffraction lines characteristic of the interlayer spacing and the most adequate conditions for their realization. In the following discussion, for the sake of brevity, the various ionic forms are simply indicated by their counterions (under a bar) and water content, while their interlayer distances are reported in parentheses (8). Thus, for example, Ti(HP04)2.H20and Ti(NaP04)2-H20will be written as ".H20 (7.6 A) and " a . H 2 0 (8.4 A). EXPERIMENTAL SECTION Reagents. All chemicals used were of reagent grade. The NaOH solutions were standardized with HC1, which had previously been standardized against Na2C03. The a-TiP was obtained following the method of Alberti et al. ( I @ , using 10 M H3P04and reflux times of 50 h. The titanium phosphate gel was prepared by precipitation from a hydrochloric solution of TiC14with diluted H3POI. The crystalline solid (a-Tip) was washed with deionized water until free of chloride (test with AgNO,), dried at 60 "C, and ground to a particle size of less than 30 fim. This solid was then characterized by chemical analysis, thermal analysis (DTA and TGA), IR spectrometry, and X-ray diffraction. Analytical Procedures. The determination of the concentration of phosphorus and titanium in the solids was carried out gravimetrically (22). The released phosphate groups were measured spectrophotometrically (23),using a Perkin-Elmer Model 200. The lithium and sodium ions in solution were determined by atomic absorption spectrometry, using a Perkin-Elmer Model 372. The diffractometer used was a Philips Model P V 1050/23 (A = 1.5418 A; (28) scan rate, 0.25'/min; chart speed, 2 cm/min). Ion Exchange Studies. The exchanger was equilibrated with the 0.1 M (MC1 + MOH) solution at 5.0 and 25.0 "C (f0.1), following a variation of the procedure described by Clearfield et al. (24). The hydroxide was added with additions of 1 mL (5 "C) or 2 mL (25 "C) (every second addition was followed by a 60 (5
0003-2700/84/0356-0193$01.50/00 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
ri" 2t r by[/-
lo
M-L,~ M=N~'
1
1
LfOH (rneq/ga-TPI T=250°C __-1
I
1
1
1
1
,
1 2 3 L 5 6 7 8 9 1011
i 1 1 .ii 6 'I
I
Fi ure 3. Variation of the relative intensity of the ion-exchange phases H /Lif treated at 310 OC.
B
ib 1'1 15 1;
MOH (rneq/ga-TP)
90
T z 5.0 O C __-
Figure 1. H+/M+ exchange isotherms on a-Tip: (a) M = Li, (b) M = Na. b -
a -
roa2l,K
10021q4 t$i3
F ure 4. Variation of the relative intensity of the ion-exchange phases H /Na+ treated at 95 OC.
5J
O C ) or 20 (25 O C ) min wait), The equilibration time was 48 h. The solid was present in the solution in an approximate ratio of 500 mL:l g.
phases were obtained (20): HH (7.4 A), (6.8 A), and (7.6 A). In the H+/Na+system at 95 "C, the following phases were obtained (17, 18): HH.H20 (7.6 A), "a (6.9 A), and NaNa.H20 (8.4 A). By determining (by cutting out the peak areas and weighing them) the relative intensities of the reflections associated with each of these phases and their relation to the amount of hydroxide initially added, the behavior of the ion-exchange material can be observed, as the substitution progresses (Figures 3 and 4).
RESULTS The conversion degree of a-Tip, as a function of the amount of metallic hydroxide initially added, was obtained by chemical analysis (Figure 1). Before X-ray diffraction, the ion-exchange samples were heated, so that the interlayer distances of each crystalline phase were sufficiently separated for them to be completely differentiated. The H+/Li+ exchanged solids were treated at 310 "C, while those of the H+/Na+ system were heated at 95 "C (Figure 2). In the H+/Li+ system, at 310 O C , the following crystalline
DISCUSSION The quantitative analysis of the elemental composition of a material is generally easy. Nevertheless, difficulties increase when we need to know the concentration of a fixed crystalline phase in a mixture consisting of several compounds. X-ray diffraction is an appropriate technique for analysis of crystalline substance mixtures, when each component originates characteristic diffraction lines, which are independent of those of the other components. The application of X-ray diffraction to quantitative analysis is based on the fact that the reflection intensities of a crys-
Angle, 26
Flgure 2. X-ray patterns of ion-exchange samples: (a) H+/Li+ treated at 310 OC, (b) H+/Na+ treated at 95 OC.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
195
talline phase in a sample depend upon the relative concentration of this phase. The ratio between intensity and concentration is seldom linear due, mainly, to the interelemental absorption of each component of the sample. In general, the intensity of a determined reflection is inversely proportional to the mass absorption coefficient of the material (25). Assuming that this material is made up of an homogeneous mixture of i crystalline species, with particle size small enough such that extinction and microabsorption effects are negligible and the particles are not preferentially oriented, the total intensity of the reflection originated by the j t h phase, is given by W:
where pm* is the mass absorption coefficient of the sample excluding the j phase, j is the subscript referring to the studied phase, W j is the weight fraction, pj is the density, pj* is the mass absorption coefficient, and K j is a constant depending upon the nature of j and other physical and instrumental factors
Kj = CIOVTGpmFhkJj (2) where C is an experimental constant, Io is the intensity of the incident beam, VT is the total volume of the sample, L is the Lorentz factor, p is the polarization factor, and Fhki is the structure factor adjusted to take into account the temperature effect. The layered compounds are notorious for exhibiting preferred orientation, which affects the intensity measurements in X-ray diffraction. The only strictly correct procedure to minimize errors in intensity measurements is to reduce considerably the average particle size. Thus, for a standard deviation in intensity values smaller than 0'51 ,the particle size must be smaller than 5 pm. However, it is very difficult to obtain such a small particle size by standard laboratory procedures. As stated before, the a-TiP we have used was ground to a particle size smaller than 30 pm. Errors in intensity measurements were minimized by dividing each sample into five portions and taking the average value of the corresponding five independent determinations. From Figures 3 and 4 it can be seen that, over a wide area of the composition range studied, three proportionally large crystalline phases coexist. This corresponds to the most commonly found situation where there is a mixture of more than two components in a sample. Nevertheless, as might be expected, their mass absorption coefficients will be similar, as a consequence of their isomorphism and similarity in chemical composition. If pj* = p m * , eq 1 is reduced to
(3) This equation involves a linear relationship between intensity and concentration. When we attempt to transform the experimental measurements to concentrations, a fundamental difficulty arises: the impossibility of attaining the crystalline phases in their pure state, except for the initial phase. This is evident for the half-exchanged phase (Figures 3 and 4)) because its characteristic reflections cannot be isolated in both series. Moreover, the sum of the mass fractions corresponding to the three crystalline phases fails to make up the whole unit, because the ionic substitution involves decomposition of the exchanger, thereby forming soluble phosphates and amorphous titanium oxides which remain in the solid fraction (15-20). These facts make the realization of standard curves from synthetic samples impossible.
LiOH (meq@a-TP)
Flgure 5. Distribution of solid phases in the H+/Li+ ion exchange.
In the systems studied, the attainment of the distribution of the solid phases, by X-ray diffraction alone, is not possible. The problem can be solved if the substitution degree, as an additional datum, is used (Figure 1). Since the relative intensity of the (002) reflections of each crystalline phase is not affected by the extent of the hydrolysis, eq 3 can be expressed in terms of the molar concentrations = fjIj (4)
cj
where C j is the molar concentrations of the j phase in the mixture, Ij its relative intensity, and f j a constant depending upon each crystalline phase. The molar concentration, in each sample, corresponding to the crystalline phases taken as a whole, will be
c = fiEIGi + f f i I i E i + f G G h
(5)
When C = 1, C j is the molar fraction of the j phase in the crystalline mixture. Moreover , only the half-exchanged and full-exchanged phases contribute to the exchange molar fraction (8) 2 x c = ffiI= 2fGIS (6) The combination of eq 5 and 6 leads to good results when, instead of obtaining each of the factors f , the value of their quotient is looked for, making it necessary to choose a reference factor. The above alternatives will lead to different analytical expressions. We considered only those experimental points in which the pH of the equilibrium solutions is equal to or lower than 6 because, at higher values, the hydrolysis product can act as a cationic exchanger distorting the exchange molar fraction of the a-titanium phosphate (19). In the H+/Li+ system, taking fEias the reference factor, we get the expression
+
Y
X
m
?1
which has the form y = mx -t n. Plotting y against I , the values of m = 2.27 and n = 0.70 are obtained. In the H+/Na+system, taking fK as the reference factor, we get the expression 2x lm -1-ax I
m
=
2-2x I " a f m+ ~-
1-2x I " a
fm
-Y
Y
x
m
fm
fnrr n
(8)
196
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
,on
T = 5.0 O C
/
comparable (26),the conclusions of this paper may be used to explain the behavior of more complicated natural systems, such as clays and clay minerals. Registry No. Ti(HP04)z.Hz0,67789-38-2;Ti(NaP04)z.Hz0, 71656-34-3; TiNaH(PO,),, 62086-62-8; Ti(HP04)z,13772-30-0; TiLiH(P04),, 87509-56-6; Ti(LiPO&, 84188-17-0.
80 70 €0
50
LO 30
LITERATURE CITED
8
20 E 10
+
s5
(1) Amphlett, C. 6. "Inorganic Ion Exchangers"; Elsevier: Amsterdam,
90
* 80 70
60 50
LO 30 20 10
1 2 3 L 5 6 7 8 9 1011 1213 NaOH (meq/ga-TP)
Flgure 6. Distribution of solid phases in the H'/Na+
ion exchange.
which has the form y = mx + n. Plotting y against x , the values of m = 2.67 and n = 1.39 are obtained. Conventionally, it is considered that f~ = 1.0. Thus, the following intensity factors are obtained: System H+/Li+: fz = 1.0, = 1.4, f ~=x 3.2 System H+/Na+: f H =~ 1.0, &qa = 1.4, fNx =a 2.7 With these factors, the relative intensities (Figures 3 and 4) can be transformed into distribution curves of solid phases (Figures 5 and 6). The exchange molar fractions obtained by chemical analysis and X-ray diffraction showed good agreement over the entire composition range. Thus, X-ray diffraction is an adequate technique for obtaining the saturation degree of a-titanium phosphate in Li' and Na' ion retention. The method of analysis described in this work can be used to study the behavior of lamellar materials, similar to a-Tip, such as certain arsenates and phosphates of tetravalent metals. Moreover. since the a-TiP and montmorillonite structures are
fzi
1964. (2) Heifferich, F. "Ion Exchange"; McGraw-Hill: New York, 1962. (3) Nachod, F. C.; Schubert, J. "Ion-Exchange Technology"; Academic Press: New York, 1956. (4) Russel, E. R.; Adamson, A. W.; Schubert, J.; Boyd, G. E. U . S . A . E. C.1943, Report CN-508. (5) Beaton, R. H.; Cooper, V. R.; Frles, B. A.; Chapelle, T. J.; Scheft, I.; Stoughton, R. A. U . S. A . E. C. 1943, Report CN-633. (6) Veseiy, V.; Pekarek, V. Talanta 1972, 19, 219. (7) Clearfieid, A.; Nancollas, G. H.; Blessing, R. H. "Ion Exchange and Solvent Extraction"; Marinsky, J. A., Marcus, Y., Eds.; Marcel Dekker: New York, 1973; Vol. 5. (8) Alberti, G.; Costantino, U. J. Chromatogr. 1974, 102, 5. (9) Alberti, G. Acc. Chem. Res. 1978, 11, 163. (10) Clearfield, A.; Alberti, G.; Costantino, U.; Howe, A. T.; Ruvarac, A,; Abe, M. "Inorganic Ion Exchange Materlals"; Clearfieid, A., Ed.; CRC Press: Boca Raton, FL, 1982. (11) Clearfield, A.; Stynes, J. A. J. Inorg. Nucl. Chem. 1964, 26, 117. (12) Clearfieid, A.; Smith, G. D. Inorg. Chem. 1989, 8. 431. (13) Troup, J. M.; Clearfield, A. Inorg. Chem. 1977, 16, 3311. (14) Volkov. A. I.; Novlkov. G. I.; Ivkovich, N. A. Khlm. Khlm. Tekhnol. (Minsk) 1976. 10. 46. (15) Alberti,' G.; Cardini-Galli, P.; Costantlno, U.; Torracca, E. J. Inorg. Nucl. Chem. 1987. 29. 571. (16) Takaguchi, K.; Tomita, I. J. Chromatogr. 1978, 718, 263. (17) Clearfield, A.; Frianeza, T. N. J. Inorg. Nucl. Chem. 1978, 40, 1925. (18) Aiberti, G.; Costantino, U.; Luciani, M. L. Gazz.Chim. Ita/. 1980, 110, 61. (19) Suirez, M.; Garcia, J. R.; Rodriguez, J. J. Phys. Chem., in press. (20) Garcia, J. R. Thesis, Universidad de Ovledo, Spain, 1983. (21) Alberti, G.; Costantino, U.; Allulli, S.; Tomasslnl, N. J. Inorg. Nucl. Chem. 1978, 40, 113. (22) Kolthoff, I. M.; Sandell, E. B.; Meehan, E. J.; Bruckenstein, S. "Quantitative Chemical Analysis"; Nigai. Buenos Aires, 1972. (23) Michelsen, 0. B. Anal. Chem. 1957, 29, 80. (24) Clearfield, A.; Oskarsson, A.; Oskarsson, C. I o n Exch. Membr. 1972, 1, 91. (25) Klug, H. P.; Alexander, L. E. "X-ray Diffractlon Procedures for Poiycrystalline and Amorphous Materials"; Wiley: New York, 1974. (26) Lelgh, D.; Dyer, A. J. Inorg. Nucl. Chem. 1972, 34, 369.
RECEIVEDfor review July 14, 1983. Accepted September 19, 1983.
