Spectropolarimetric Titrations of the Lanthanide Metals with D- ( - ) - T ~ u ~ s 1,2=Cycl ohexanediaminetetraacetic Acid Karl H. Pearson’ Department of Chemistry, Cleueland State University, Cleueland, Ohio 44115 Joyce R. Baker Department of Chemistry and Physics, Middle Tennessee State University, Murfreesboro, Tenn. 37130 Paul E. Reinbold Department of Chemistry, Bethany NazareneICollege, Bethany, Okla. 73008 THE USE OF D-(-)-1,2-propylenediaminetetraacetic acid (D-( -)PDTA) for the spectropolarimetric titrations of the transition metals (I), lanthanide metals (2), and other metals (3-7) has been well characterized. The Group IIA metals (8) have been successfully titrated spectropolarimetrically using the stereospecific titrant D( - ) - t r a w l ,2-cyclohexanediaminetetraacetic acid (D-(-)CDTA). This paper reports the successful spectropolarimetric titrations of lanthanum and praseodymium through ytterbium, using D-(-)CDTA as the titrant. Fundamental to this technique is the use of a photoelectric polarimeter to monitor the change in optical rotation of the solution as the titration proceeds. Parameters must be selected so that a recognizable change in the optical rotation occurs in the vicinity of the end point. The reagent D(-)CDTA was chosen as the titrant because of its strong chelating ability with the lanthanide ions (9) and because it forms stable optically active 1 :1 complexes. Furthermore, at the pH and wavelength chosen for the spectropolarimetric titrations, the lanthanide-D( -)CDTA complexes have positive molecular rotations, whereas the titrant has a negative molecular rotation. Thus, the large differences in the molecular rotations of the free ligand and the lanthanide-D( -)CDTA complexes permit very sensitive’ and precise spectropolarimetric determinations. The stereospecificity of D(-)CDTA has been discussed previously (10) for the heavy metals, transitions metals, and octahedral complexes, although its stereospecific behavior with the lanthanide ions is uncertain. EXPERIMENTAL
Apparatus. A Perkin-Elmer Model 141 photoelectric polarimeter, modified as described by Reinbold and Pearson 1
To whom all correspondence should be addressed.
(1) R. J. Palma, P. E. Reinbold, and K. H. Pearson, ANAL.CHEM.,
42,47 (1970). (2) D. L. Caldwell, P. E. Reinbold, and K. H. Pearson, ibid., p 416.
(3) R. J. Palma and K. H. Pearson, Anal. Chim. Acta., 49, 497 (1970). (4) D. L. Caldwell, P. E. Reinbold, and K. H. Pearson, ibid., p 505. (5) R. J. Palma, P. E. Reinbold, and K. H. Pearson, ibid., 51,
329 (1970). (6) R. J. Palma, P. E. Reinbold, and K. H. Pearson, Anal. Lett., 2, 553 (1969). (7) D. L. Caldwell, P. E. Reinbold, and K. H. Pearson, ibid., 3, 93 (1970). (8) J. R. Baker and K. H. Pearson, Anal. Chim. Acta., 50, 255
(1970). (9) “Stability Constants of Metal-Ion Complexes,” 2nd ed., The Chemical Society, London, 1964. (10) P. E. Reinbold and K. H. Pearson, Talania, 17, 391 (1970). 2090
Table I. Conditions for Spectropolarimetric Titrations of the Lanthanide Metal Ions WaveMolecular length, rotation, [MI nm (deg ml dm-1 mole-’) Metal PH 300 1801 La 4.94 300 1713 Pr 4.94 300 1561 4.94 Nd 300 1415 4.94 Sm 340 1060 Eu 4.94 300 1112 4.94 Gd 300 978 4.94 Tb 853 300 4.94 DY 300 592 4.94 Ho 300 421 4.94 Er 300 284 4.94 Tm 300 107 4.94 Yb
(II), was used to monitor continuously the optical rotation of the solution during the titration. These modifications of the polarimeter provided an increase in the range of available wavelengths and increased the sensitivity of the analyses. The polarimeter cell used in the titrations was a 5-ml 1-dm flow-through cell with optically inactive quartz end plates. The titration vessel was constructed from a 250-ml glass beaker with one glass tube attached at the bottom center and another one attached about one-fifth up the side, and was connected to the flow-through polarimeter cell with Tygon tubing. The titration vessel was positioned over a magnetic stirrer and a magnetic stirbar, placed in the vessel, both mixed the solution and pumped it through the polarimeter cell. With this apparatus, the response time was less than 5 sec. All pH measurements were made with an Orion Model 801 digital pH meter. Reagents. All solutions were prepared with demineralized water and stored in polyethylene bottles. D-(-)CDTA was prepared by the method of Reinbold and Pearson (IO). A 0.5% aqueous solution of the D(-)CDTA had a specific rotation of -53.5” at 589 nm; literature values (IO, 12) for the anhydrous acid are -53.4’ and -53.0°, respectively. The standard solution of D(-)CDTA was prepared by dissolving 22 grams of anhydrous D(-)CDTA and 4.8 grams of sodium hydroxide pellets in deionized water and diluting to 100 ml. The Na2D(-)CDTA was standardized against a standard lead solution at pH 5.0,hexamethylenetetramine buffer, with Xylenol Orange as the indicator. The standard lead solution was standardized against standard solutions (11) P. E. Reinbold and K. H. Pearson, ANAL.CHEM.,43, 293
(1971).
(12) F. P. Dwyer and F. L. Garvan, J. Amer. Chem. Soc., 83, 2610 (1961).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972 , LT
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Figure 3. Spectropolarimetrictitration of 0.09332M europium(II1) at pH 4.94 and 340 nm with 0.5905M D( -)CDTA
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Figure 4. Spectropolarimetric titration of 0.08127M ytterbium(II1) at pH 4.94 and 300 nm with 0.5905M D( -)CDTA tion curve. The precision and deviation for the D(-)CDTA titrations of lanthanum(II1) are far superior to those of D( -)PDTA titrations. Second, because of the much larger positive molecular rotations of the lanthanum-D( -)CDTA complex, it would be possible to determine this ion in the concentration range of 10-3 to 10-4M. As the wavelength decreases, the molecular rotations of the lanthanum-D( - )CDTA complex, in general, become more positive, while the D( -)CDTA itself has more negative molecular rotations. Consequently, for very dilute solutions of lanthanum(III), spectropolarimetric titrimetry offers increased sensitivity and detectability at wavelengths below 300 nm. 2092
Figure 2 illustrates a typical spectropolarimetric titration of praseodymium with D( -)CDTA. Pearson and coworkers (2) reported that the titration curve of praseodymium with D(-)PDTA showed two distinct breaks. The results were obtained from the intercept of the second line segment with the excess D( -)PDTA curve, rather than from extrapolating the first segment to intersect the excess D(-)PDTA curve. When using D( -)CDTA as the titrant for praseodymium, only one end-point break was observable and the precision and accuracy of the spectropolarimetric titration compared excellently to the visual titrations. Thus, D(-)CDTA has the following two major advantages over D(-)PDTA as the spectropolarimetric titrant for praseodymium : an easier end point to detect since there is only one possible intersection of the extrapolated line segments; and the larger positive molecular rotation of the D(-)CDTA complex compared to the molecular rotation of the D( -)PDTA complex would permit lower limits of detection. The spectropolarimetric titrations of neodymium and samarium give similar titration curves to that of praseodymium. As seen from Table I, the molecular rotations of the neodymium- and samarium-D( - )CDTA complexes are slightly less possitive than the molecular rotations of the praseodymium-D( -)CDTA complex. The end points for both the neodymium(II1) and samarium(II1) spectropolarimetric titrations with D(-)CDTA were very sharp and just 10 ml of acetate buffer was needed. The titration curves for samarium with D(-)PDTA (2) required 50 ml of acetate buffer since the end-point determinations were highly dependent on the exact slope of the excess D(-)PDTA line segment and thus upon the maintenance of a very constant pH. Both of these lanthanide metal ion determinations can be performed with excellent precision and accuracy compared to visual titrimetry. The advantage of D( -)CDTA for the spectropolarimetric titration of the lanthanides can be dramatically seen in the determination of europium(II1). Figure 3 shows a positive titration graph for the titration of europium(II1) with D( -)CDTA at ambient temperatures. The europium-D( -)PDTA complex is very temperature dependent and the titration had to be performed at 5 "C to obtain sufficient slope differential between the complex and the excess titrant (2). The second advantage with D(-)CDTA is that the intersection of the positive extrapolated straight line segments for the europium-D(-)CDTA complex and the excess D( -)CDTA gave a much sharper end point than the extrapolated end point in the spectropolarimetric titration of europium (111) with D(-)PDTA. This is the only lanthanide metal ion that was not titrated at 300 nm because of the high absorbance of the europium-D( -)CDTA complex in this spectral region. The optimum wavelength was 340 nm where the absorbance of the complex was low and the molecular rotation was still very highly positive. Thus, with D(-)CDTA as the titrant, excellent accuracy and precision can be obtained at ambient temperature. From Table I it can be seen that with D(-)CDTA, the spectropolarimetric titrations curves obtained from gadolinium(III), terbium(III), dysprosium(III), holmium(III), erbium(III), and thulium(II1) would give very sharp positive end points. With D(-)PDTA it was difficult to obtain sufficient slope differential for reproducible results in the spectropolarimetric titration of gadolinium(II1) because of the similar molecular rotations of the ligand D(-)PDTA and the gadolinium-o(-)PDTA complex (2). Of the rare earth metals titrated spectropolarimetrically with o(-)PDTA,
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
gadolinium(1I:t) had the greatest variance in the analytical precision and the largest deviation from the standard visual method. The spectropolarimetric titrations of dysprosium(III), holmium(III), erbium(III), and thulium(II1) with D( -)PDTA at 365 nm were not feasible since it was impossible to obtain sufficient slope differentials for reproducible results, because of the similar molecular rotations of the metal-D(-)PDTA complexes and D(-)PDTA (16). All of these lanthanide metal ions when titrated spectropolarimetrically with D( -)CDTA gave excellent end-point breaks and the precision and accuracy were excellent when compared to visual titrimetry. Ytterbium(III), the lanthanide metal ion studied which had the lowest positive molecular rotation for its D( -)CDTA complex, exhibits a very sharp positive titration end point as can be seen from Figure 4. This is a distinct advantage over the spectropolarimetric titration of ytterbium(II1) with D(-)PDTA which exhibits a negative titration end point (2). Although the ytterbium-D( -)CDTA complex has a molecular rotation at 300 nm which is approximately twenty times less than the molecular rotation of the lanthanum-D( -)CDTA complex at the same wavelength, the accuracy and precision are comparable in this concentration range. Thus, Figure 4 shows that with rotational values of these magnitudes, the spectropolarimetric technique gives excellent precision and accuracy. Therefore, any concentrations of the lanthanide ions that would give positive rotational values of this magnitude with D( -)CDTA could be easily titrated by this spectropolarimetric technique using D( -)CDTA as the titrant. From Table I and Figures 2 and 4, it can be seen that the titration curves for neodymium(III), samarium(III), gadolinium(III), dysprosium(III), holmium(III), erbium(III), and thulium(II1) are intermediate between those for praseodymium(II1) and ytterbium(II1). Table I1 gives the results of the lanthanide metal ion titrations with D(-)CDTA. Each reported spectropolarimetric value is the average of at least three individual titrations. The accuracy of the spectropolarimetric titrations ranged from 0.00 to 0.38% with an average deviation of 0.19%. The precision of the analyses ranged from 0.00 to 0.2 mg. The (16) P. E. Reinbold and K. H. Pearson, Texas A&M University, unpublished work, 1970.
Table 11. Results of Spectropolarimetric Titrations Milligrams Deviations Metal Taken Found Mg 147.9 j=0.0 -0.4 -0.27 La 148.3
z
Pr Nd
Sm
Eu Gd
Tb DY Ho Er Tm Yb
134.1 135.8 121.4 142.1 148.6 106.1 161.2 171.8 165.6 185.3 140.9
133.9 + 0.1 135.7 zk 0.1 121.8 j= 0.0 142.3 j= 0 . 1 148.8 1: 0.1 106.5 i 0 . 1 160.61: 0.2 171.5 + 0 . 1 165.6 i 0.1 185.4 1: 0.1 140.6 =t0 . 2
-0.2 -0.1 $0.4 $0.2 +0.2 $0.4 -0.6 -0.3 0.0 $0.1 -0.3
-0.15 -0.07 $0.33 $0. 14 +0.13 $0.38 -0.37 -0.17 0.00 $0.05 -0.21
maximum time required for the titration and graphical evaluation of the data was in no case greater than 20 minutes. CONCLUSIONS
The advantages of spectropolarimetric titrimetry for the lanthanide(II1) metal ions with D(-)CDTA are its versatility, simplicity, rapidity of analyses, and applicability in solutions of high electrolyte concentrations. Each lanthanide metal ion can be titrated with the exception of europium(II1) with exactly the same analytical procedure. The only change in the analytical procedure for europium(II1) was that the titration had to be performed at 340 nm instead of 300 nm. Thus, the twelve lanthanide ions can be determined with excellent accuracy and precision by spectropolarimetric titrimetry with D( -)CDTA and only one buffer. ACKNOWLEDGMENT
The authors wish to thank El Paso Products Co. of Odessa, Texas, for the technical grade racemic rrans-l,2-cyclohexanediamine. RECEIVED for review December 21, 1971. Accepted May 26, 1972. Presented in part at the 18th Detroit Anachem Conference, October 1970. This work was supported by The Robert A. Welch Foundation Grant A-262 and a CSU Research Grant. Appreciation is expressed by J. R. B. for a N.D.E.A. Graduate Fellowship. Part of this study was performed at Texas A& M University.
Determination of Tra,cesof Phosphorus by a Radiomolybdenum Method J. E. Kenney' and M. P. Menon Department of Chemistry, Savannah State College, Savannah, Ga.
INSTRUMENTAL METHODS of analysis are based on the measurement of a physical property such as absorption, fluorescence, optical activity, etc. of either the sought substance or its reaction product which is proportional to the concentration of the substance in solution. However, the radioactive I Present address, Medical School, Rutgers University, N~~ Brunswick, N.J.
property of the product resulting from a reaction of the sought element with a radioactive reagent has been seldom used as an analytical tool. Such radioreagent methods of analysis are feasible only if the sought element, either in the elemental form or its combined state, reacts with a radioreagent to give a reaction product which is separable from the reagent in excess. This method has been used on a limited scale for the determination of cesium, fluoride, oxygen, pro-
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