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Spectrophotometric determination of formation constants of 1:1 complexes of lanthanides with 4-(2-pyridylazo)resorcinol (par)
0039-9140/84 $3.00 + 0.00 Copyright 0 1984 Pergamon Press Ltd
Talanra, Vol. 31, No. 12, pp. 1129-1132, 1984 Printed in Great Britain. All rights reserved
SPECTROPHOTOMETRIC DETERMINATION OF FORMATION CONSTANTS OF 1: 1 COMPLEXES OF LANTHANIDES WITH 4-(2-PYRIDYLAZO)RESORCINOL (PAR) EMIKO OHYOSHI Yatsushiro
College
of Technology,
Hirayamashinmachi
2627, Yatsushiro,
Kumamoto,
866 Japan
(Received 7 February 1984. Revised 23 April 1984. Accepted 25 May 1984) Summary-Complex formation (1: 1) between lanthanides(II1) and 4-(2-pyridylazo)resorcinol (PAR or H,R) has been studied by spectrophotometry. The method is based on indirect estimation of the protonated (MHR) and the normal (MR) complexes by measuring the absorbance at the peak for the ligand, which decreases with increasing metal concentration at a constant pH. Similar experiments were made at various pH values. Both MHR and MR complexes were found to be formed in the pH range determined by graphical analysis, ranged from 5-6. Their formation constants, log&,, and log&,, 3.78 + 0.02 (Ce) to 4.39 + 0.02 (Lu), and from 9.61 k 0.06 (Ce) to 10.70 k 0.05 (Lu), respectively. The acidity of the MHR complexes parallels the order of stability of the MR complexes.
4-(2-Pyridylazo)resorcinol (PAR or H,R) forms intensely coloured complexes with many metal ions and is widely used in analytical chemistry.‘,* The formation of these complexes is known to be very dependent on pH but little information on the complexation equilibria has been obtained. Such information will aid in choosing experimental conditions so as to increase the selectivity. The system is rather complicated because of simultaneous formation of the protonated and normal type of complexes with many metal ions. In a previous paper3 we proposed a method for facilitating the spectrophotometric analysis of these two types of 1: 1 complex, and applied it to the zinc and lanthanum PAR complexes. The results showed that the method was more applicable to the lanthanum system because a larger variation in absorbance with change in metal concentration could be obtained, owing to the lower absorptivity of LaHR at the absorption peak for the free ligand, PAR. This suggests that the method should also be applicable to the system of PAR and the other lanthanides(II1). Sommer and Novotna4 have studied the PAR complexes of several lanthanides(II1) and found no simple relation between the stability of the complexes and the atomic number or radius of the lanthanide ions. To establish such a relationship the complexation should be studied for all the lanthanides(II1). In the present work, we investigated the complexation of PAR with all the lanthanides(II1) by applying the method developed previously,3 and have compared the results with those reported by Sommer and Novotna.“ EXPERIMENTAL Reagents All chemicals used were of analytical-reagent grade. PAR was obtained from Dojindo Chemical Co., and purified by
recrystallization. The purity was checked by thin-layer chromatography.5 The concentration of PAR in its aqueous stock solution was calculated from its weight. The lanthanide(II1) nitrate solutions, except for cerium(III), were prepared by dissolving the calculated amounts of the respective oxides in nitric acid. The ccrium(III) nitrate _ solution was prepared by dissol;ing Ce(NO,),. 2(NH,NO,).4H,O in 0.01 M nitric acid. and standa;dizez’by‘ED?A &at&. Sodium acetate and’ hexamine solutions (0.02M) were used to maintain the pH in the range 4.8-6.3. The ionic strength was maintained at 0.1 with sodium nitrate. Procedure The metal solution was added in small increments (50 PI), by ~1 pipette, to 10 ml of buffered PAR solution (2.0-3.0 x 10e5M), to give concentrations increasing from lo-” to 5 x IO-jM. Before and after each addition of metal the absorbance at the wavelength for the absorption peak of PAR (which depends on the pH, and varies from 390 to 413 nm) was measured with a Shimadzu UV 200s spectrophotometer. No precipitation occurred under these experimental conditions. RESULTS AND DISCUSSION Absorption
spectra
of the complexes
The absorption spectra of PAR solutions containing excess of each lanthanide(II1) were measured over the range 35&600 nm. The variation in the spectrum with metal concentration (CM) was investigated at constant pH values in the range 4.8-6.3. The absorbance at the peak for PAR was observed to decrease with increasing C,. After addition of sufficient metal there was no observable change in the spectra, indicating practically complete formation of the I: I complexes. The spectra varied according to the lanthanide(II1) used and the pH. The spectra obtained for different lanthanides(II1) at pH 4.83 showed two maxima, at 410 and 495 nm. The apparent molar absorptivities at 410 nm decreased from 1.48 to 1.28 x IO4 l.mole-‘.cm-’ and those at 495 nm in1129
ANALYTICAL
1130
creased from 0.95 to 1.15 x 10“ l.molee’.cm-’ with atomic number of the lanthanide. The isosbestic points also varied from 428 to 442 nm. As the pH increased, the peak at 410 nm decreased and the one at 495 nm (which was slightly shifted to 500 nm at pH 6.3) increased remarkably with atomic number. To study the latter peak more extensively we further increased the pH by adding a small amount of 1.OM sodium hydroxide. As a result, the peak was further shifted to 505 nm and showed nearly the same limiting absorptivity of 3.0 x lo4 l.molee’.cm-’ for all the lanthanides, though this value was reached at different pH values for the different lanthanides, ranging from pH 6.7 (Lu) to 7.5 (Ce). From these facts, the peaks at 410 nm and 495 nm are attributed to the MHR complex and that at 505 nm is attributed to the MR complex.
DATA
the formation constants, KMHR= [MHR]/[M] [HR] and KM,, = [MR]/[M] [RI, the acid dissociation constants of H,R, K, = [H] [HR]/[H,R] and Kg = [H] [R]/[HR], and the Ringbom cc-coefficient,6 c(~ = [R’]/[R], equations (1) and (2) can be converted into the following expressions:
[Ml(f&JGWl + &&&Y(MVG) A /CR,=
= GAR’I - 1
(3)
[R/l{1 + (Q~RKMHRKI[HI
Combining equation (3) with equation (4) and substituting CM for [M] (since C, $ C,), and A0 for CR.+, we obtain
Determination of the formation constants of MHR and MR As described previously,3 the stability constants of the metal complexes of PAR (H,R), MHR and MR, can be indirectly estimated by utilizing the ligand absorption peak. The absorbance at the peak for the free ligand, measured in the absence (A,) and the presence (A) of the metal, under conditions where C, + C, (C, = ligand concentration), can be related to the formation constants, KhnHRand KMR,of the 1: 1 complexes MHR and MR assumed to be formed as follows (charges are omitted for simplicity): CR = [MHR] + [MR] + [R’] A = ahlHRIMHR] + s&MR]
E
Ra RKK I 2
A
= (A,IA - 1)/G,
(5)
From equation (5) a linear relation between (A,/A - 1)/C, and A,/A should be obtained at constant pH (where Ed. remains constant). The product of the intercept y0 on the y-axis b = (A,/A - 1)/C,], at a fixed pH, and mRK,KZis a function of [HI:
(1) ++[R’]
(2)
where R’ is the free ligand not combined with any metal, and sMHR, .ahlRand sR. are the molar absorptivities of MHR, MR and R’ respectively, at the wavelength of the free-ligand peak. With the aid of
A,
- +wwAvdC[Hl+ %wC,,K,K~X- 4,
/A
Fig. 1. Plots according to equation (5) for the system of PAR and different lanthanides(II1) at a constant pH of 5.70. (1) Lu, Yb; (2) Tm; (3) Er; (4) Ho; (5) Dy; (6) Tb, Cd, Eu, Sm; (7) Nd; (8) Pr; (9) Ce.
According to equation (6), a plot of the left-hand side of equation (6) us. [H] should be linear in a pH-range where no hydrolysis of the metal ion occurs. From the slope and the intercept of the line, both KMHRand KMR can be determined. From the measured absorbance decreases at the peak for PAR with increasing metal concentrations and constant pH, we plotted the relation expressed by equation (5). All the plots at different pH values for the different lanthanides(II1) were linear with a negative slope as expected. The intercepts on the y-axis, which indicate the degree of complex formation, increased with either increasing pH or atomic number. There is no observable difference in the values for Lu(II1) and Yb(II1) and for Tb(II1). Gd(III), Eu(II1) and Sm(II1) at any pH studied, and for Er(III), Ho(II1) and Dy(II1) at relatively low pH values in the range studied. The intercept ,u, on the x-axis (Au/A) at pH 4.83, which might be nearly equal to the ratio E~./E#“~ at 390 nm at such a low pH, slightly increased with atomic number (from 1.5 to 1.7). This is in accord with the decrease in the molar absorptivities of the MHR complexes at 410 nm with atomic number (1.48-1.28 x lo4 l.molee’.cm ‘). At the higher pH of 5.70, as illustrated in Fig. 1, both the x0 and y, values increase with atomic number to a greater extent, to give some detectable differentiation between Er(III), Ho(II1) and Dy(II1). This would be
ANALYTICAL
1131
DATA
Table 1. Formation constants of lanthanide_PAR and the proton dissociation constants of MHR,
complexes, KMHR and KMR, KL,, (O.lMNaNO,, 25°C)
log 4,~ Sommer
log &.,mr Element Ce(II1) Pr(II1) Nd(II1) Sm(II1) Eu(iIIj Gd(III) Tb(IIIj Dy(III) Ho(II1) Er(II1) Tm(II1) Yb(II1) Lu(II1)
this work
3.78 f 3.95 f 4.07 * 4.28 f 4.28 + 4.28 k 4.28 f 4.29 + 4.29 + 4.31 * 4.34 + 4.39 + 4.39 *
0.02 0.02 0.02 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.02 0.02 0.02
This work
9.61 f 9.78 f 10.02 + 10.25 + 10.25 +
10.25 + 10.25 z 10.36 + 10.47 f 10.52 k 10.57 * 10.70 * 10.70 +
0.06 0.06 0.06 0.07 0.07 0.07 0.07 0.06 0.06 0.06 0.05 0.05 0.05
and
Novotna4 9.3 9.8 10.1
10.6 10.1 10.2
P%,
>
this work 6.47 6.47 6.35 6.33 6.33 6.33 6.33 6.23 6.12 6.09 6.07 5.99 5.99
our results mostly agree with those reported by Sommer and Novotna. Although they offered no explanation for the unusual relation between complex stability and atomic number, this could result from -hdkdG[W + &MRK~RK,K*)I(~R.~RK,KZ) varies the different values of the molar absorptivity of MR (1.484.1 x 10“ l.mole-‘.cm-’ at 530 nm) estimated with the KM,, value if it can be assumed that for the different lanthanides(II1). From our results, eMnR +eENR and eMHR5 eH.uR (M and M’ being different lanthanides), suggesting that the KMHR however, the E values of MR at 505 nm are nearly the same (3.0 x lo4 l.molee’.cm-‘) for all the lanvalues for Sm(III)-Tm(II1) are not so much different thanides(II1). from each other. To determine the formation constants of the MHR Comparison of complexation trends of HR and R2and MR complexes we plotted the relationship expressed by equation (6) for various lanthanides(II1). The dependence of the formation constants, KMHR The values, K, = 10m5.50and K, = 10-‘2.30,7 were used and KMR on the reciprocal of the ionic radius r is for calculation of a,K,K,. All the plots were linear, plotted in Fig. 2. The two curves show a similar with intercepts indicating the presence of both the complexation trend for the light lanthanides(II1). MHR and MR complexes. It was generally observed For the heavy lanthanides(III), however, there that the greater the slope of the line the greater the are different trends between the HR- and the R2intercept, and that both increased with atomic numligands. These two trends are often observed in ber, though the extent of the two increases was not lanthanide chemistry. According to Moeller et aL9, the same. The lanthanides heavier than Sm(II1) who divided the ligands into three types showing showed greater difference in the intercept than in the different complexation trends, the R2- and the HRslope. The slopes for Sm(III)-Er(II1) were very similigand belong to the first and the second grow, lar, as is to be expected. Table 1 shows the formation respectively. Although the similarity among the liconstants of KMHRand KMR obtained from the slope 2 and intercept of the plot of equation (6) and the proton dissociation constant of MHR, Kc,,, calculated from Ki,, = KZKMRIKMHR.The values were obtained by the least-squares method. For comparison the values reported in the literature4 are included. Sommer and Novotna4 assumed existence of the protonated complexes, MRH* where H* denotes the proton of thep-hydroxy group of PAR, and calculated the K’,,, values by using K, as an approximation to K;, where K, is the dissociation constant of H3R+. The resulting values for K;,, were larger than those for K,,. However, Chalmers* has stated that the use of k, is preferable to use of K3 for the approximation, because the a -hydroxy group should be regarded intrinsically as at least as acidic as the Fig. 2. Plots of the formation constants of the lanp-hydroxy group, its lower acidity being due to thanide(III)-PAR complexes, MHR and MR, us. the reciprocal of the radius of the metal ions. internal hydrogen-bonding. For the MR complexes, due to formation of MR complexes as well as the MHR complexes. The stabilities of the MR complexes of the heavy lanthanides(II1) must differ more than those of the MHR complexes. The slope given by
1132
ANALYTICAL
gands in the respective groups has not been clearly indicated, the ligands in the first group are capable of forming at least one chelate ring in the complexes. In an attempt to elucidate the co-ordination structure of the protonated and the normal type of PAR complexes, detailed studies on extraction of the 1: 2 Ni-PAR complexes were made by Hoshino et ai.” of They suggested that in the chelation Ni(HR),.2H,O and NiRi-, the ligands, HR- and R*-, are acting as bidentate and terdentate, respectively. They stated that the basic change of the chelate structure from Ni(HR),.2Hz0 to NiRgand deprotonation of HR- ligand both caused a remarkable increase in the absorptivity (from 3.73 x lo4 to 8.08 x lo4 l.mole-‘.cm-I). Although the lanthanide-PAR complexes (1: 1) differ in type from the Ni-PAR complexes, a considerable increase in the absorptivity (from 0.95-1.15 x lo4 to 3.0 x lo4 1.mole-‘.cm-‘) was similarly observed with increasing pH. The LnR complexes (Ln = lanthanide) may have a more stable chelate structure which gives rise to a larger difference in stability between the heavy lanthanides(II1) than that for the LnHR complexes. The acidity of the LnHR complexes parallels the stability order of the LnR complexes (Table 1).
DATA
CONCLUSIONS
The complexes MHR and MR can be differentiated by spectrophotometric means and their formation constants, &,a and I&k, determined. Both values regularly increase with increasing atomic number up to Sm(II1). For the heavy lanthanides(III), the KMR values also increase but the &a values are nearly the same. REFERENCES 1.
R. G. Anderson and G. Nickless, Analyst, 1967,92,207.
2. S. Shibata, 3. 4. 5.
6. 7. 8. 9.
10.
in Chelates in Ana&aiChemistry, A. J. Barnard Jr. and H. Flaschka (eds.). Vol. 3. Dekker. New York, 1972. E. Ohyoshi, Anal. Chem., 1983, 55, 2404. L. Sommer and H. Novotna, Talanta, 1967, 14, 457. F. H. Pollard, G. Nickless, T. J. Samuelson and R. G. Anderson, J. Chromaiog., 1964, 16, 231. A. Ringbom, Complexation in Analytical Chemistry, Interscience, New York, 1963. W. J. Geary, G. Nickless and F. H. Pollard, Anal. Chim. Acta, 1962, 27, 7 1. R. A. Chalmers, Talanta, 1967, 14, 527. T. Moeller. D. F. Martin. L. C. Thomoson. R. Ferrus. G. R. Feisiel and W. J. Randall, Chek. Re;., 1965, 65; 1. H. Hoshino, T. Yotsuyanagi and K. Aomura, Anal. Chim. Acta, 1976, 83, 317.
Talanra, Vol. 31, No. 12, pp. 1129-1132, 1984 Printed in Great Britain. All rights reserved
SPECTROPHOTOMETRIC DETERMINATION OF FORMATION CONSTANTS OF 1: 1 COMPLEXES OF LANTHANIDES WITH 4-(2-PYRIDYLAZO)RESORCINOL (PAR) EMIKO OHYOSHI Yatsushiro
College
of Technology,
Hirayamashinmachi
2627, Yatsushiro,
Kumamoto,
866 Japan
(Received 7 February 1984. Revised 23 April 1984. Accepted 25 May 1984) Summary-Complex formation (1: 1) between lanthanides(II1) and 4-(2-pyridylazo)resorcinol (PAR or H,R) has been studied by spectrophotometry. The method is based on indirect estimation of the protonated (MHR) and the normal (MR) complexes by measuring the absorbance at the peak for the ligand, which decreases with increasing metal concentration at a constant pH. Similar experiments were made at various pH values. Both MHR and MR complexes were found to be formed in the pH range determined by graphical analysis, ranged from 5-6. Their formation constants, log&,, and log&,, 3.78 + 0.02 (Ce) to 4.39 + 0.02 (Lu), and from 9.61 k 0.06 (Ce) to 10.70 k 0.05 (Lu), respectively. The acidity of the MHR complexes parallels the order of stability of the MR complexes.
4-(2-Pyridylazo)resorcinol (PAR or H,R) forms intensely coloured complexes with many metal ions and is widely used in analytical chemistry.‘,* The formation of these complexes is known to be very dependent on pH but little information on the complexation equilibria has been obtained. Such information will aid in choosing experimental conditions so as to increase the selectivity. The system is rather complicated because of simultaneous formation of the protonated and normal type of complexes with many metal ions. In a previous paper3 we proposed a method for facilitating the spectrophotometric analysis of these two types of 1: 1 complex, and applied it to the zinc and lanthanum PAR complexes. The results showed that the method was more applicable to the lanthanum system because a larger variation in absorbance with change in metal concentration could be obtained, owing to the lower absorptivity of LaHR at the absorption peak for the free ligand, PAR. This suggests that the method should also be applicable to the system of PAR and the other lanthanides(II1). Sommer and Novotna4 have studied the PAR complexes of several lanthanides(II1) and found no simple relation between the stability of the complexes and the atomic number or radius of the lanthanide ions. To establish such a relationship the complexation should be studied for all the lanthanides(II1). In the present work, we investigated the complexation of PAR with all the lanthanides(II1) by applying the method developed previously,3 and have compared the results with those reported by Sommer and Novotna.“ EXPERIMENTAL Reagents All chemicals used were of analytical-reagent grade. PAR was obtained from Dojindo Chemical Co., and purified by
recrystallization. The purity was checked by thin-layer chromatography.5 The concentration of PAR in its aqueous stock solution was calculated from its weight. The lanthanide(II1) nitrate solutions, except for cerium(III), were prepared by dissolving the calculated amounts of the respective oxides in nitric acid. The ccrium(III) nitrate _ solution was prepared by dissol;ing Ce(NO,),. 2(NH,NO,).4H,O in 0.01 M nitric acid. and standa;dizez’by‘ED?A &at&. Sodium acetate and’ hexamine solutions (0.02M) were used to maintain the pH in the range 4.8-6.3. The ionic strength was maintained at 0.1 with sodium nitrate. Procedure The metal solution was added in small increments (50 PI), by ~1 pipette, to 10 ml of buffered PAR solution (2.0-3.0 x 10e5M), to give concentrations increasing from lo-” to 5 x IO-jM. Before and after each addition of metal the absorbance at the wavelength for the absorption peak of PAR (which depends on the pH, and varies from 390 to 413 nm) was measured with a Shimadzu UV 200s spectrophotometer. No precipitation occurred under these experimental conditions. RESULTS AND DISCUSSION Absorption
spectra
of the complexes
The absorption spectra of PAR solutions containing excess of each lanthanide(II1) were measured over the range 35&600 nm. The variation in the spectrum with metal concentration (CM) was investigated at constant pH values in the range 4.8-6.3. The absorbance at the peak for PAR was observed to decrease with increasing C,. After addition of sufficient metal there was no observable change in the spectra, indicating practically complete formation of the I: I complexes. The spectra varied according to the lanthanide(II1) used and the pH. The spectra obtained for different lanthanides(II1) at pH 4.83 showed two maxima, at 410 and 495 nm. The apparent molar absorptivities at 410 nm decreased from 1.48 to 1.28 x IO4 l.mole-‘.cm-’ and those at 495 nm in1129
ANALYTICAL
1130
creased from 0.95 to 1.15 x 10“ l.molee’.cm-’ with atomic number of the lanthanide. The isosbestic points also varied from 428 to 442 nm. As the pH increased, the peak at 410 nm decreased and the one at 495 nm (which was slightly shifted to 500 nm at pH 6.3) increased remarkably with atomic number. To study the latter peak more extensively we further increased the pH by adding a small amount of 1.OM sodium hydroxide. As a result, the peak was further shifted to 505 nm and showed nearly the same limiting absorptivity of 3.0 x lo4 l.molee’.cm-’ for all the lanthanides, though this value was reached at different pH values for the different lanthanides, ranging from pH 6.7 (Lu) to 7.5 (Ce). From these facts, the peaks at 410 nm and 495 nm are attributed to the MHR complex and that at 505 nm is attributed to the MR complex.
DATA
the formation constants, KMHR= [MHR]/[M] [HR] and KM,, = [MR]/[M] [RI, the acid dissociation constants of H,R, K, = [H] [HR]/[H,R] and Kg = [H] [R]/[HR], and the Ringbom cc-coefficient,6 c(~ = [R’]/[R], equations (1) and (2) can be converted into the following expressions:
[Ml(f&JGWl + &&&Y(MVG) A /CR,=
= GAR’I - 1
(3)
[R/l{1 + (Q~RKMHRKI[HI
Combining equation (3) with equation (4) and substituting CM for [M] (since C, $ C,), and A0 for CR.+, we obtain
Determination of the formation constants of MHR and MR As described previously,3 the stability constants of the metal complexes of PAR (H,R), MHR and MR, can be indirectly estimated by utilizing the ligand absorption peak. The absorbance at the peak for the free ligand, measured in the absence (A,) and the presence (A) of the metal, under conditions where C, + C, (C, = ligand concentration), can be related to the formation constants, KhnHRand KMR,of the 1: 1 complexes MHR and MR assumed to be formed as follows (charges are omitted for simplicity): CR = [MHR] + [MR] + [R’] A = ahlHRIMHR] + s&MR]
E
Ra RKK I 2
A
= (A,IA - 1)/G,
(5)
From equation (5) a linear relation between (A,/A - 1)/C, and A,/A should be obtained at constant pH (where Ed. remains constant). The product of the intercept y0 on the y-axis b = (A,/A - 1)/C,], at a fixed pH, and mRK,KZis a function of [HI:
(1) ++[R’]
(2)
where R’ is the free ligand not combined with any metal, and sMHR, .ahlRand sR. are the molar absorptivities of MHR, MR and R’ respectively, at the wavelength of the free-ligand peak. With the aid of
A,
- +wwAvdC[Hl+ %wC,,K,K~X- 4,
/A
Fig. 1. Plots according to equation (5) for the system of PAR and different lanthanides(II1) at a constant pH of 5.70. (1) Lu, Yb; (2) Tm; (3) Er; (4) Ho; (5) Dy; (6) Tb, Cd, Eu, Sm; (7) Nd; (8) Pr; (9) Ce.
According to equation (6), a plot of the left-hand side of equation (6) us. [H] should be linear in a pH-range where no hydrolysis of the metal ion occurs. From the slope and the intercept of the line, both KMHRand KMR can be determined. From the measured absorbance decreases at the peak for PAR with increasing metal concentrations and constant pH, we plotted the relation expressed by equation (5). All the plots at different pH values for the different lanthanides(II1) were linear with a negative slope as expected. The intercepts on the y-axis, which indicate the degree of complex formation, increased with either increasing pH or atomic number. There is no observable difference in the values for Lu(II1) and Yb(II1) and for Tb(II1). Gd(III), Eu(II1) and Sm(II1) at any pH studied, and for Er(III), Ho(II1) and Dy(II1) at relatively low pH values in the range studied. The intercept ,u, on the x-axis (Au/A) at pH 4.83, which might be nearly equal to the ratio E~./E#“~ at 390 nm at such a low pH, slightly increased with atomic number (from 1.5 to 1.7). This is in accord with the decrease in the molar absorptivities of the MHR complexes at 410 nm with atomic number (1.48-1.28 x lo4 l.molee’.cm ‘). At the higher pH of 5.70, as illustrated in Fig. 1, both the x0 and y, values increase with atomic number to a greater extent, to give some detectable differentiation between Er(III), Ho(II1) and Dy(II1). This would be
ANALYTICAL
1131
DATA
Table 1. Formation constants of lanthanide_PAR and the proton dissociation constants of MHR,
complexes, KMHR and KMR, KL,, (O.lMNaNO,, 25°C)
log 4,~ Sommer
log &.,mr Element Ce(II1) Pr(II1) Nd(II1) Sm(II1) Eu(iIIj Gd(III) Tb(IIIj Dy(III) Ho(II1) Er(II1) Tm(II1) Yb(II1) Lu(II1)
this work
3.78 f 3.95 f 4.07 * 4.28 f 4.28 + 4.28 k 4.28 f 4.29 + 4.29 + 4.31 * 4.34 + 4.39 + 4.39 *
0.02 0.02 0.02 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.02 0.02 0.02
This work
9.61 f 9.78 f 10.02 + 10.25 + 10.25 +
10.25 + 10.25 z 10.36 + 10.47 f 10.52 k 10.57 * 10.70 * 10.70 +
0.06 0.06 0.06 0.07 0.07 0.07 0.07 0.06 0.06 0.06 0.05 0.05 0.05
and
Novotna4 9.3 9.8 10.1
10.6 10.1 10.2
P%,
>
this work 6.47 6.47 6.35 6.33 6.33 6.33 6.33 6.23 6.12 6.09 6.07 5.99 5.99
our results mostly agree with those reported by Sommer and Novotna. Although they offered no explanation for the unusual relation between complex stability and atomic number, this could result from -hdkdG[W + &MRK~RK,K*)I(~R.~RK,KZ) varies the different values of the molar absorptivity of MR (1.484.1 x 10“ l.mole-‘.cm-’ at 530 nm) estimated with the KM,, value if it can be assumed that for the different lanthanides(II1). From our results, eMnR +eENR and eMHR5 eH.uR (M and M’ being different lanthanides), suggesting that the KMHR however, the E values of MR at 505 nm are nearly the same (3.0 x lo4 l.molee’.cm-‘) for all the lanvalues for Sm(III)-Tm(II1) are not so much different thanides(II1). from each other. To determine the formation constants of the MHR Comparison of complexation trends of HR and R2and MR complexes we plotted the relationship expressed by equation (6) for various lanthanides(II1). The dependence of the formation constants, KMHR The values, K, = 10m5.50and K, = 10-‘2.30,7 were used and KMR on the reciprocal of the ionic radius r is for calculation of a,K,K,. All the plots were linear, plotted in Fig. 2. The two curves show a similar with intercepts indicating the presence of both the complexation trend for the light lanthanides(II1). MHR and MR complexes. It was generally observed For the heavy lanthanides(III), however, there that the greater the slope of the line the greater the are different trends between the HR- and the R2intercept, and that both increased with atomic numligands. These two trends are often observed in ber, though the extent of the two increases was not lanthanide chemistry. According to Moeller et aL9, the same. The lanthanides heavier than Sm(II1) who divided the ligands into three types showing showed greater difference in the intercept than in the different complexation trends, the R2- and the HRslope. The slopes for Sm(III)-Er(II1) were very similigand belong to the first and the second grow, lar, as is to be expected. Table 1 shows the formation respectively. Although the similarity among the liconstants of KMHRand KMR obtained from the slope 2 and intercept of the plot of equation (6) and the proton dissociation constant of MHR, Kc,,, calculated from Ki,, = KZKMRIKMHR.The values were obtained by the least-squares method. For comparison the values reported in the literature4 are included. Sommer and Novotna4 assumed existence of the protonated complexes, MRH* where H* denotes the proton of thep-hydroxy group of PAR, and calculated the K’,,, values by using K, as an approximation to K;, where K, is the dissociation constant of H3R+. The resulting values for K;,, were larger than those for K,,. However, Chalmers* has stated that the use of k, is preferable to use of K3 for the approximation, because the a -hydroxy group should be regarded intrinsically as at least as acidic as the Fig. 2. Plots of the formation constants of the lanp-hydroxy group, its lower acidity being due to thanide(III)-PAR complexes, MHR and MR, us. the reciprocal of the radius of the metal ions. internal hydrogen-bonding. For the MR complexes, due to formation of MR complexes as well as the MHR complexes. The stabilities of the MR complexes of the heavy lanthanides(II1) must differ more than those of the MHR complexes. The slope given by
1132
ANALYTICAL
gands in the respective groups has not been clearly indicated, the ligands in the first group are capable of forming at least one chelate ring in the complexes. In an attempt to elucidate the co-ordination structure of the protonated and the normal type of PAR complexes, detailed studies on extraction of the 1: 2 Ni-PAR complexes were made by Hoshino et ai.” of They suggested that in the chelation Ni(HR),.2H,O and NiRi-, the ligands, HR- and R*-, are acting as bidentate and terdentate, respectively. They stated that the basic change of the chelate structure from Ni(HR),.2Hz0 to NiRgand deprotonation of HR- ligand both caused a remarkable increase in the absorptivity (from 3.73 x lo4 to 8.08 x lo4 l.mole-‘.cm-I). Although the lanthanide-PAR complexes (1: 1) differ in type from the Ni-PAR complexes, a considerable increase in the absorptivity (from 0.95-1.15 x lo4 to 3.0 x lo4 1.mole-‘.cm-‘) was similarly observed with increasing pH. The LnR complexes (Ln = lanthanide) may have a more stable chelate structure which gives rise to a larger difference in stability between the heavy lanthanides(II1) than that for the LnHR complexes. The acidity of the LnHR complexes parallels the stability order of the LnR complexes (Table 1).
DATA
CONCLUSIONS
The complexes MHR and MR can be differentiated by spectrophotometric means and their formation constants, &,a and I&k, determined. Both values regularly increase with increasing atomic number up to Sm(II1). For the heavy lanthanides(III), the KMR values also increase but the &a values are nearly the same. REFERENCES 1.
R. G. Anderson and G. Nickless, Analyst, 1967,92,207.
2. S. Shibata, 3. 4. 5.
6. 7. 8. 9.
10.
in Chelates in Ana&aiChemistry, A. J. Barnard Jr. and H. Flaschka (eds.). Vol. 3. Dekker. New York, 1972. E. Ohyoshi, Anal. Chem., 1983, 55, 2404. L. Sommer and H. Novotna, Talanta, 1967, 14, 457. F. H. Pollard, G. Nickless, T. J. Samuelson and R. G. Anderson, J. Chromaiog., 1964, 16, 231. A. Ringbom, Complexation in Analytical Chemistry, Interscience, New York, 1963. W. J. Geary, G. Nickless and F. H. Pollard, Anal. Chim. Acta, 1962, 27, 7 1. R. A. Chalmers, Talanta, 1967, 14, 527. T. Moeller. D. F. Martin. L. C. Thomoson. R. Ferrus. G. R. Feisiel and W. J. Randall, Chek. Re;., 1965, 65; 1. H. Hoshino, T. Yotsuyanagi and K. Aomura, Anal. Chim. Acta, 1976, 83, 317.