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Some physico chemical studies on synthetic calcium arsenate hydroxyapatites
131
Notes
J. inorg, nucL Chem.. 1978, Vol. 40, pp. 131-132. Pergamon Press. Printed in Great Brilain
Some physico chemical studies on synthetic calcium arsenate hydroxyapatites (Received 2 September 1976; received for publication 22 June 1977) Due to isostructural and isoelectronic nature, arsenate can be incorporated in calcium hydroxyapatite, Cato(PO4)6(OH)2, (HA) which is an important inorganic crystalline component of bone[l] by PO~---, ASPS- exchange reaction resulting in the formation of solid solutions of calcium arsenate hydroxyapatites according to the equation: Cal0(PQ)6(OH) 2 + n AsO~-Ca,o(PO4)s_,,(AsQ),(OH)2 + nPO 3-. When this exchange reaction is made to progress till completion, pure arsenate hydroxyapatite, Calo(AsO4)6(OH)2, (AsHA) is resulted. Though the presence of isomorphously incorporated arsenate in naturally occurring phosphatic minerals is well known[2] and the possibility of similar substitution at high temperatures has been shown[3], solid solutions of calcium arsenate hydroxyapatite have not been prepared in aqueous media. The present paper deals with determination of lattice constants and assignment of the major frequencies observed in the IR spectra of the samples obtained by Coprecipitation. The solubility trend of the prepared samples on (i) the pH of the medium in biologically important range 5.0 to 8.0 and (ii) on the arsenate content at a given pH were investigated. EXPERIMENTAL The conditions used for the preparation of samples were reported earlier[4]. The samples used for analyses were acetone washed and dried at 100°C for 6 hr. Calcium and phosphorus in the samples were determined complexometrically[5]. The combined arsenate and phosphate in solid solutions was precipitated as magnesium ammonium phosphate and arsenate which was dissolved in the minimum volume of 0.1 M hydrochloric acid and subsequently determined complexometrically[5]. In another experiment containing phosphate and arsenate, the latter was reduced to arsenite by sulphur dioxide and arsenic sulphide, As2Ss was precipitated by hydrogen sulphide in 9 M hydrochloric acid after removal of excess of sulphur-dioxide[6]. The precipitate was separated by filtration through a IG 4 crucible. The phosphate content alone in the filtrate was then determined. The arsenate content was then known by difference. The average weight percent errors were found to be: -+0.05, -+0.08 and -+0.1 for calcium, phosphate and arsenate respectively. The X-ray powder patterns were obtained on a Debye Scherrer Unicam Camera using CuK~ radiation. The IR spectra was recorded as KBr pellets with Perkin-Elmer model 137 E spectrophotometer. The solubility studies of the samples were made at 31 -+0.5°C by equilibrating 0.2g of the powdered sample to 200
mesh with 100ml of suitable buffer combinations[7]. Calcium content in the medium after filtration was determined after the attainment of saturation determined by preliminary experiments. RKSULTSANDDISCUSSION The results of chemical analyses of the samples were given in Table 1. Based on the analytical data formulae for these compounds were proposed. The calculated g atom ratio of Ca/P + As approached the theoretical value of 1.6718]; the observed divergence could be attributed to the experimental errors involved in the determinations. The X-ray diffraction patterns of all the samples contained broad and diffused lines consequent upon the colloidal dimensions of the crystals. The values increased with increase in arsenate content of the samples. These results confirmed that the samples prepared were homogeneous solid solutions. Assignments of the major frequencies [9] observed in the IR spectra of the samples have been made in Table 2. The effect of substitution of PO43- by ASPS- resulted in shift of v~ (980cm-~) and vs (1080cm-') vibrations of PO~ and that of OH- (3600cm -~) to lower frequencies. This is due to the reinforcement of increased mass by the diminshed force constants of bonds. Graphical representations of the solubilities of the samples as a function of arsenate content in a mole of the sample was made in Fig. 1. Each curve represents the trend in the solubility behaviour of the samples at a chosen pH. In general (i) reduction of solubility of a given sample with increase in pH of the medium and (ii) increase in solubility with increase in arsenate content of the samples at a given pH were observed. The solubility of HA is due to the hydrolysis and the following equilibria are established: Calo(PO4)6(OH)2 ~ 10Ca2+ + 6HPO2 + 8OH
Table 2. Assignment of observed frequencies (cm ') in IR spectra
PO~Compound A B D F
980 sh 970 s 965 br 955br
G
--
vI
v3
1080 s 1063 s 1060 s 1057-1060 s --
AsO~ OH-
-855 m 850 m 836m
3600 m 3600m 3500m 350(Im
836 m
3500m
sh, shoulder; s, strong; br, broad; m, medium.
Table 1. Results of chemical analyses and lattice constants of calcium arsenate hydroxyapatites
Compound
Ca
Wt% P As
A B C D E F G
39.19 38.20 40.00 39.60 39.60 39.60 38.20
17.34 -14.30 4.00 13.50 12.09 10.50 1 8 . 0 0 5.89 2 9 . 1 8 3.72 3 5 . 2 5 -43.00
Proposed formula Calo(PO4)6(OH)2 Cag.s(PO4)4.7(AsO4)l.3(OH)2 Calo(PO4)4.1(AsO4)t.9(OH)2 Cag.9(PO413.4(AsO4)z6(OH): Cag.9(PO4)I.9(AsOa)4.1(OH)2 Ca9.9(POa)l.z(AsO4)4.s(OH)2 Cag.s(AsO4)5.s(OH)2
g atom ratio, Ca/P + As 1.63 1.63
1.66 1.65 1.65 1.65 1.68
Lattice constants (,~) a c 9.42 9.44 9.48 9.68 9.69 10.01 10.16
6.95 7.17 7.26 7.40 7.40 7.45 7.50
(1)
132
Notes more to the right resulting in a decrease in calcium content in solution and the observed pH dependence is understandable. At a given pH, the increase in solubilities of the samples with increase in arsenate content may be due to reduction of lattice energy of HA consequent upon introduction of bigger arsenate ion in the lattice.
900 -
700--
Acknowledgements--We thank Dr. B. R. Sant, Deputy Director, Regional Research Laboratory, Bhaubaneswar for the powder patterns and Mr. B. S. Bisht, Scientist, CDRI, Lucknow for the IR spectra of the samples.
~'soo -
Department o.f Chemistry Khallikote College Berhampur-760 001 India
°9 _E 3oo
8
7.
~"
I 0 0 HA
20
40
1
t
I 60
I 80
Mol percenL x
IOO AsHA.
Fig. 1. Dependence of solubility of samples of calcium arsenate hydroxyapatites on arsenate content at the following pH values of the medium of equilibration: (1) 5.0, (2) 5.5, (3) 6.0, (4) 6.5, (5) 7.0, (6) 7.5, (7) 8.0. 6HPO~- + 6Ca 2+ ~ 6 CaHPO4
(2)
4Ca 2+ + 8OH- ~ 4Ca(OH) 2.
(3)
Analogoues equations can be written for AsHA. With increase in pH of the medium the equilibrium (3) shifts
N. S. CHICKERUR P. P. MAHAPATRA
REI~_,RENCES 1. W. F. Neuman and M. W. Neuman, Chem. Rev. 1, 55 (1953). 2. J. W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. 9, p. 171. Longmans, London (1957). 3. M. Masseys, J. C. Troube, G. Boal and G. Montel, Bull. Soc. Chim. (France) 3, 2308 (1969). 4. N. S. Chickerur and P~ P. Mahapatra, J. Inst. Chemists 47, 13 (1975). 5. R. P. Singh, N. S. Chickerur and T. S. B. Narasaraju, Z. Anal. Chem. 117, 237 (1968). 6. A. I. Vogel, A Text Book of Micro and Semimicro Qualitative Inorganic Analysis. Longmans, London (1962). 7. M. Donbrow, Instrumental Methods in Analytical Chemistry-Their Principles and Practice, Vol. 1. Pitman, London (1966). 8. J. R. Van Wazer, Phosphorus and its Compounds, Vol. 1. Interscience, New York (1958). 9. K. Nakamoto, Infrared Spectra o/Inorganic and Coordination Compounds. Wiley, New York (1971).
Z inorg, nucl. Chem., 1978, Vol. 40, pp. 132-134. Pergamon Press. Printed in Great Britain
Kinetics of catalysed aquation of malonato- and (o)phthalatopentaamminecobalt(lll) complexes by metal ions (Received 13 June 1977) The rates of spontaneous and catalysed aquation of malonatopentaamminecobalt(lll) by H +, A13+, Ga 3+ and In 3+ have been reported earlier[I,2]. Catalysis of the metal ions has been ascribed to the reactivity of the binuclear complexes, [(NH3)sCoCO2CH2CO2M] ~+. The spontaneous and acid catalysed aquation of (o)phthalatopentaamminecobalt(III) has also been investigated[l]. The present communication describes the kinetics of the acid aquation of malonato- and (o)phthalatopentaamminecobalt(III) in the presence of Fe 3+, Cu 2+, Zn 2+ and Fe 3+ respectively. This work provides further opportunity to examine the metal ion catalysis of the aquation of carboxylatopentaamminecobalt(III) complexes as a function of the complexing ability as well as the steric and electronic properties of the carboxylate ligands. EXPERIMENTAL The complexes, [(NH3)sCoCO2CH2CO2H](CIO4)2, [(NH3)sCoCO2(o)C6H4CO2H](CIO4) 2 and [(NH3)sCoOH2](CIO4)3, were prepared by the published methods[l]. The purity of the samples was checked by estimating cobalt which agreed within
0.2% of the calculated values. The molar absorption coefficients of malonato, phthalato and aquo-pentaamminecobalt(Ill) ions at the A.... measured in 0.1 M HCIO4 medium, were 75.0, 79.0 (505 nm) and 48.0 M -] cm -J (490 nm) respectively which agreed well with the published data[I, 2]. AnalaR or extra pure quality chemicals were used. Perchloric acid and sodium perchlorate were used to adjust acidity and ionic strength of the reaction medium respectively. Stock metal perchlorate solutions were prepared and analysed for the metal and free acid contents as described earlier[3]. A Beckman DU 2 spectrophotometer was used to obtain the kinetic and the spectral data. The details of the kinetic procedure adopted have been described elsewhere [2]. RESULTS AND DISCUSSION
The pseudo-first-order rate constants (kobs) of aquation of the malonato complex in the presence of Fe 3+, Cu 2+ and Zn2+ are collected in Table 1. The rate data for Fe 3+ and Cu 2+ fit the rate law, - d In [complex] dt
ko~
k~ + k2[H÷] + k3K[Mn+]/[H+] 1 + K[Mn+]/[H ÷]
(1)
Notes
J. inorg, nucL Chem.. 1978, Vol. 40, pp. 131-132. Pergamon Press. Printed in Great Brilain
Some physico chemical studies on synthetic calcium arsenate hydroxyapatites (Received 2 September 1976; received for publication 22 June 1977) Due to isostructural and isoelectronic nature, arsenate can be incorporated in calcium hydroxyapatite, Cato(PO4)6(OH)2, (HA) which is an important inorganic crystalline component of bone[l] by PO~---, ASPS- exchange reaction resulting in the formation of solid solutions of calcium arsenate hydroxyapatites according to the equation: Cal0(PQ)6(OH) 2 + n AsO~-Ca,o(PO4)s_,,(AsQ),(OH)2 + nPO 3-. When this exchange reaction is made to progress till completion, pure arsenate hydroxyapatite, Calo(AsO4)6(OH)2, (AsHA) is resulted. Though the presence of isomorphously incorporated arsenate in naturally occurring phosphatic minerals is well known[2] and the possibility of similar substitution at high temperatures has been shown[3], solid solutions of calcium arsenate hydroxyapatite have not been prepared in aqueous media. The present paper deals with determination of lattice constants and assignment of the major frequencies observed in the IR spectra of the samples obtained by Coprecipitation. The solubility trend of the prepared samples on (i) the pH of the medium in biologically important range 5.0 to 8.0 and (ii) on the arsenate content at a given pH were investigated. EXPERIMENTAL The conditions used for the preparation of samples were reported earlier[4]. The samples used for analyses were acetone washed and dried at 100°C for 6 hr. Calcium and phosphorus in the samples were determined complexometrically[5]. The combined arsenate and phosphate in solid solutions was precipitated as magnesium ammonium phosphate and arsenate which was dissolved in the minimum volume of 0.1 M hydrochloric acid and subsequently determined complexometrically[5]. In another experiment containing phosphate and arsenate, the latter was reduced to arsenite by sulphur dioxide and arsenic sulphide, As2Ss was precipitated by hydrogen sulphide in 9 M hydrochloric acid after removal of excess of sulphur-dioxide[6]. The precipitate was separated by filtration through a IG 4 crucible. The phosphate content alone in the filtrate was then determined. The arsenate content was then known by difference. The average weight percent errors were found to be: -+0.05, -+0.08 and -+0.1 for calcium, phosphate and arsenate respectively. The X-ray powder patterns were obtained on a Debye Scherrer Unicam Camera using CuK~ radiation. The IR spectra was recorded as KBr pellets with Perkin-Elmer model 137 E spectrophotometer. The solubility studies of the samples were made at 31 -+0.5°C by equilibrating 0.2g of the powdered sample to 200
mesh with 100ml of suitable buffer combinations[7]. Calcium content in the medium after filtration was determined after the attainment of saturation determined by preliminary experiments. RKSULTSANDDISCUSSION The results of chemical analyses of the samples were given in Table 1. Based on the analytical data formulae for these compounds were proposed. The calculated g atom ratio of Ca/P + As approached the theoretical value of 1.6718]; the observed divergence could be attributed to the experimental errors involved in the determinations. The X-ray diffraction patterns of all the samples contained broad and diffused lines consequent upon the colloidal dimensions of the crystals. The values increased with increase in arsenate content of the samples. These results confirmed that the samples prepared were homogeneous solid solutions. Assignments of the major frequencies [9] observed in the IR spectra of the samples have been made in Table 2. The effect of substitution of PO43- by ASPS- resulted in shift of v~ (980cm-~) and vs (1080cm-') vibrations of PO~ and that of OH- (3600cm -~) to lower frequencies. This is due to the reinforcement of increased mass by the diminshed force constants of bonds. Graphical representations of the solubilities of the samples as a function of arsenate content in a mole of the sample was made in Fig. 1. Each curve represents the trend in the solubility behaviour of the samples at a chosen pH. In general (i) reduction of solubility of a given sample with increase in pH of the medium and (ii) increase in solubility with increase in arsenate content of the samples at a given pH were observed. The solubility of HA is due to the hydrolysis and the following equilibria are established: Calo(PO4)6(OH)2 ~ 10Ca2+ + 6HPO2 + 8OH
Table 2. Assignment of observed frequencies (cm ') in IR spectra
PO~Compound A B D F
980 sh 970 s 965 br 955br
G
--
vI
v3
1080 s 1063 s 1060 s 1057-1060 s --
AsO~ OH-
-855 m 850 m 836m
3600 m 3600m 3500m 350(Im
836 m
3500m
sh, shoulder; s, strong; br, broad; m, medium.
Table 1. Results of chemical analyses and lattice constants of calcium arsenate hydroxyapatites
Compound
Ca
Wt% P As
A B C D E F G
39.19 38.20 40.00 39.60 39.60 39.60 38.20
17.34 -14.30 4.00 13.50 12.09 10.50 1 8 . 0 0 5.89 2 9 . 1 8 3.72 3 5 . 2 5 -43.00
Proposed formula Calo(PO4)6(OH)2 Cag.s(PO4)4.7(AsO4)l.3(OH)2 Calo(PO4)4.1(AsO4)t.9(OH)2 Cag.9(PO413.4(AsO4)z6(OH): Cag.9(PO4)I.9(AsOa)4.1(OH)2 Ca9.9(POa)l.z(AsO4)4.s(OH)2 Cag.s(AsO4)5.s(OH)2
g atom ratio, Ca/P + As 1.63 1.63
1.66 1.65 1.65 1.65 1.68
Lattice constants (,~) a c 9.42 9.44 9.48 9.68 9.69 10.01 10.16
6.95 7.17 7.26 7.40 7.40 7.45 7.50
(1)
132
Notes more to the right resulting in a decrease in calcium content in solution and the observed pH dependence is understandable. At a given pH, the increase in solubilities of the samples with increase in arsenate content may be due to reduction of lattice energy of HA consequent upon introduction of bigger arsenate ion in the lattice.
900 -
700--
Acknowledgements--We thank Dr. B. R. Sant, Deputy Director, Regional Research Laboratory, Bhaubaneswar for the powder patterns and Mr. B. S. Bisht, Scientist, CDRI, Lucknow for the IR spectra of the samples.
~'soo -
Department o.f Chemistry Khallikote College Berhampur-760 001 India
°9 _E 3oo
8
7.
~"
I 0 0 HA
20
40
1
t
I 60
I 80
Mol percenL x
IOO AsHA.
Fig. 1. Dependence of solubility of samples of calcium arsenate hydroxyapatites on arsenate content at the following pH values of the medium of equilibration: (1) 5.0, (2) 5.5, (3) 6.0, (4) 6.5, (5) 7.0, (6) 7.5, (7) 8.0. 6HPO~- + 6Ca 2+ ~ 6 CaHPO4
(2)
4Ca 2+ + 8OH- ~ 4Ca(OH) 2.
(3)
Analogoues equations can be written for AsHA. With increase in pH of the medium the equilibrium (3) shifts
N. S. CHICKERUR P. P. MAHAPATRA
REI~_,RENCES 1. W. F. Neuman and M. W. Neuman, Chem. Rev. 1, 55 (1953). 2. J. W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. 9, p. 171. Longmans, London (1957). 3. M. Masseys, J. C. Troube, G. Boal and G. Montel, Bull. Soc. Chim. (France) 3, 2308 (1969). 4. N. S. Chickerur and P~ P. Mahapatra, J. Inst. Chemists 47, 13 (1975). 5. R. P. Singh, N. S. Chickerur and T. S. B. Narasaraju, Z. Anal. Chem. 117, 237 (1968). 6. A. I. Vogel, A Text Book of Micro and Semimicro Qualitative Inorganic Analysis. Longmans, London (1962). 7. M. Donbrow, Instrumental Methods in Analytical Chemistry-Their Principles and Practice, Vol. 1. Pitman, London (1966). 8. J. R. Van Wazer, Phosphorus and its Compounds, Vol. 1. Interscience, New York (1958). 9. K. Nakamoto, Infrared Spectra o/Inorganic and Coordination Compounds. Wiley, New York (1971).
Z inorg, nucl. Chem., 1978, Vol. 40, pp. 132-134. Pergamon Press. Printed in Great Britain
Kinetics of catalysed aquation of malonato- and (o)phthalatopentaamminecobalt(lll) complexes by metal ions (Received 13 June 1977) The rates of spontaneous and catalysed aquation of malonatopentaamminecobalt(lll) by H +, A13+, Ga 3+ and In 3+ have been reported earlier[I,2]. Catalysis of the metal ions has been ascribed to the reactivity of the binuclear complexes, [(NH3)sCoCO2CH2CO2M] ~+. The spontaneous and acid catalysed aquation of (o)phthalatopentaamminecobalt(III) has also been investigated[l]. The present communication describes the kinetics of the acid aquation of malonato- and (o)phthalatopentaamminecobalt(III) in the presence of Fe 3+, Cu 2+, Zn 2+ and Fe 3+ respectively. This work provides further opportunity to examine the metal ion catalysis of the aquation of carboxylatopentaamminecobalt(III) complexes as a function of the complexing ability as well as the steric and electronic properties of the carboxylate ligands. EXPERIMENTAL The complexes, [(NH3)sCoCO2CH2CO2H](CIO4)2, [(NH3)sCoCO2(o)C6H4CO2H](CIO4) 2 and [(NH3)sCoOH2](CIO4)3, were prepared by the published methods[l]. The purity of the samples was checked by estimating cobalt which agreed within
0.2% of the calculated values. The molar absorption coefficients of malonato, phthalato and aquo-pentaamminecobalt(Ill) ions at the A.... measured in 0.1 M HCIO4 medium, were 75.0, 79.0 (505 nm) and 48.0 M -] cm -J (490 nm) respectively which agreed well with the published data[I, 2]. AnalaR or extra pure quality chemicals were used. Perchloric acid and sodium perchlorate were used to adjust acidity and ionic strength of the reaction medium respectively. Stock metal perchlorate solutions were prepared and analysed for the metal and free acid contents as described earlier[3]. A Beckman DU 2 spectrophotometer was used to obtain the kinetic and the spectral data. The details of the kinetic procedure adopted have been described elsewhere [2]. RESULTS AND DISCUSSION
The pseudo-first-order rate constants (kobs) of aquation of the malonato complex in the presence of Fe 3+, Cu 2+ and Zn2+ are collected in Table 1. The rate data for Fe 3+ and Cu 2+ fit the rate law, - d In [complex] dt
ko~
k~ + k2[H÷] + k3K[Mn+]/[H+] 1 + K[Mn+]/[H ÷]
(1)