seen in this experiment. We see no such clusters in AGV-1 and BCR-1 and can only conclude that the dispersion in U values previously observed is due probably to analytic uncertainties. For the rocks DTS-1 and PCC-1 all the investigators are in fairly good agreement. The technique is applicable to whole rock U determinations in a variety of rock types. At the present time, programs in progress at this laboratory are using the technique to study U distributions in hundreds of samples of deep sea sediments, ultramafic rocks, and stone meteorites.
ACKNOWLEDGMENT I am grateful to Ted Middleton and Bill Doss for laboratory assistance, to H. Hamaguchi, M. Tatsumoto, and F. J, Flanagan for the USGS samples, and to Hamaguchi and Tatsumoto for permission to quote unpublished data. RECEIVED for review November 10, 1969. Accepted December 22,1969. Work supported in part by the National Science Foundation, Grant GA-1702.
Spectropolarimetric Determinations of the Lanthanide Metals Propylenediaminetetraacetic Acid with D-(-) -1,2D. L. Caldwell, P. E. Reinbold, and K. H. Pearson Department of Chemistry, Texas A&M University, College Station, Texas 77843
SPECTROPOLARIMETRIC TITRIMETRY is a relatively new analytical technique, first described by Kirschner and Bhatnagar in 1963 (I), 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. To date applications have been made to both acid-base titrations and the determination of metal ions (I, 2). The titrant, ~-(-)-1,2-propylenediaminetetraacetic acid, was chosen because of its chelating strength and its stereospecific behavior with the transition metals, though its behavior with the lanthanides is not certain.
EXPERIMENTAL Instrumentation. A Perkin-Elmer Model 141 photoelectric polarimeter was used to monitor the optical rotation of the solution during the titration. The use of the flow-through cell, connected to the titration vessel with Tygon tubing, allowed continuous monitoring of the optical rotation as the titration proceeded. In all of the titrations, the optimum wavelength was determined to be 365 nm because of the greatest specific rotation of the complexes at this wavelength and the greatest difference between the rotations of the complexes and the free ligand. One of the titration vessels, previously described (3), was jacketed to facilitate both high and low temperature titrations when necessary. For those titrations where high or low temperature reactions were necessary, a Haake Model KT 41 Kryokool constant temperature circulator was used. It controlled the temperature of the titration vessel to an accuracy of =tO.O1 “C. The titration set-up, response time, and sensitivity have been previously described (3). A 5-ml microburet readable to ?=0.001 ml was used for the delivery of the titrant. Reagents. All solutions were prepared with demineralized water and stored in polyethylene bottles. D-(-)-1,2-propylenediaminetetraacetic acid was prepared by a modified method of Dwyer and Garvan (4). The specific rotation of (1) S. Kirschner and D. C . Bhatnager, ANAL. CHEM., 35, 1069 (1963). (2) K. H. Pearson and S . Kirschner, Anal. Chim. Acra, 48, 339 (1969). (3) R. J. Palma, Sr., P. E. Reinbold, and K. H. Pearson, ANAL. CHEM., 42, 47 (1970). (4) F. P. Dwyer and F. L. Garvan, J. Amer. Chem. Soc., 81, 2955 (1959). 416
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
0.5% aqueous solution of the acid was -47.2 degree-ml/ gram-dm at 589 nm. The standard solution of D(-)PDTA was prepared by dissolving 95 grams of the solid D(-)PDTA and 48 grams of analytical reagent grade sodium hydroxide in deionized water, filtering, and diluting to one liter. This solution was standardized by titration with standard zinc solution at pH 10, using Eriochrome Black T as the indicator. The standard solutions of EDTA were prepared from primary standard G . F. Smith disodium dihydrogen ethylenediaminetetraacetate dihydrate. Both of the buffer solutions were sodium acetate-acetic acid buffers. Most of the standard metal solutions were prepared by dissolving the appropriate amount of the rare earth oxide in perchloric acid and diluting it to one liter with deionized water. Aliquots of the metal solutions were standardized at a pH of 5 with standard EDTA, using xylenol orange as the indicator (5-7). Procedure. The spectropolarimetric procedure has been described previously (3). The only significant difference was that for europium the temperature jacketed cell was employed, and the titration was carried out at approximately 5 “C.
DISCUSSION The spectropolarimetric titrations of lanthanum with D( -)PDTA have a precision of =t0.15%; however, the deviation from the visual standardization was -0.99%. The lanthanum solution was also titrated with D(-)PDTA, following the procedure for the visual standardization; the results agreed with the visual EDTA standardization rather than with the spectropolarimetric titrations. The deviation is thus related to the method of analysis and not simply to the titrants used. The effect of the spectropolarimetric titrations of single metal ions give results which are usually lower than the visual standardization. This indicates that the solutions probably possess contaminants which are chelated in the usual visual procedures, leading to erroneously high results. This additive error would not be seen in the spectropolarimetric points, probably being lost in the roundoff at the equivalence point. (5) F. J. Welcher, “The Analytical Uses of Ethylenediaminetetraacetic Acid,” D. van Nostrand Co., Princeton, N. J., 1958. (6) J. Kinnunen and B. Wennerstrand, Chemist-Analyst, 46, 92 (1957). (7) J. Korbl and R. Pribil, ibid., 45, 102 (1956).
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OF
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Figure 3. Spectropolarimetric titration of 0.08583M samarium with standard D( -)PDTA at 365 nm and pH 5.0
Figure 1. Spectropolarimetric titration of 0.08704M lanthanum with standard D(-)PDTA at 365 nm and pH = 5.0
0.300 -0.100
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Figure 2. Spectropolarimetric titration of 0.09469M praseodymium with standard D( -)PDTA at 365 nm andpH = 5.0
I
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Figure 4. Spectropolarimetric titration of 0.09015M europium with standard D(-)PDTA at 365 nm, pH 5.0, and temperature of 5 "C
Figure 1 is a typical example of a large positive spectropolarimetric titration end point. The spectropolarimetric titration of praseodymium had excellent analytical precision and agreement with the visual standardizations. However, from Figure 2, it can be seen that the titration curve shows 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. Although the theoretical reasons for the two breaks in the praseodymium titration plots are not completely understood, the breaks occurred in each titration and extrapolation of the second portion gave excellent precision and accuracy. It does not seem probable, although the praseodymium solution was prepared from the mixed oxide (Pr6OI1), that the effect can be ascribed to mixed oxidation states in solution because of the high value of the electrode potential of the
Pr(IV)/Pr(III) couple and the rapid rate of reduction of Pr(1V)
to Pr(II1). The spectropolarimetric titrations of samarium are unique in that the samarium-D-(-)PDTA has a very small positive optical rotation, Figure 3, and therefore the end point determinations are highly dependent upon the exact slope of the excess D( -)PDTA curve, and thus upon the maintenance of a constant pH. The amount of pH 5 buffer used in these titrations was 50 ml and the analytical precision and deviation from the visual standardizations were excellent. Titrations of 10-fold dilution of samarium gave an observed rotation of 0.000 degrees until the end point and then the negative rotation of the excess D( -)PDTA was observed. Therefore it can be seen that even though the complex has essentially zero rotation, an adequate spectropolarimetric titration can be performed as long as the slope differential is sufficiently large.
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417
Table I. Molecular Rotational Dependence on Temperature of D( -)PDTA and the Eu-D( -)PDTA Complex
589 578 546 436 365
z 2 +
- 137 - 144
- 73 - 75
- 257 - 385
- 133 - 191
-45
- 50
- 84
-159
- 55 - 85
-114
- 30 - 43
-47 - 69 - 83
- 34 - 42 -47 -60 -67
- 122
- 127
- 144 - 239 - 368
- 124 -132 - 150 - 249 - 384
Table 11. Results of Spectropolarimetric Titrations Deviation Metal Mg taken Mg Percent Mg found La 130.9 129.6 i 0.2 -1.3 -0.99 133.4 Pr 133.6 f 0 . 2 +0.2 +0.15 122.5 f 0 . 7 Nd 122.8 -0.3 -0.24 Srn 129.0 129.2 =t0 . 2 +0.2 +0.15 Eu 136.6 136.4 f 0.4 -0.2 -0.15 Gd 154.2 153.0 f 0.9 -1.2 -0.78 Tb 33.56 f 0.10 Yb 20.09 20.12 f 0.19 +0.03 +0.15
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Figure 5. Spectropolarimetric titration of 0.01159M ytterbium with standard D(-)PDTA at 365 nm and pH = 3.0 Europium is unique in that the molecular rotation of the europium complex with D(-)PDTA is very temperature dependent. Table I shows the optical rotational dependence on temperature of D( -)PDTA and the europium-D( -)PDTA complex. In order to obtain sufficient slope differential between the complex and the excess D( -)PDTA, the titration was performed at 5 "C. From Figure 4, a large slope differential of the Eu-D(-)PDTA complex and the excess D(-)PDTA is readily apparent at 5 OC. In the spectropolarimetric titrations of gadolinium, it was difficult to obtain a sufficient slope differential for reproducible results, because of similar molecular rotations of D(-)PDTA and the gadolinium-D( -)PDTA complex. The rotation of the complex did not vary as markedly with temperature as did that of the europium complex. Of the rare earth metals studied, this was the one that had the greatest variance in analytical precision and the largest deviation from the standard visual methods. Visual standardizations of the solution of terbium at this concentration were unsuccessful. However, the spectropolarimetric titrations of terbium were successful. Because the dilute terbium solution could not be standardized by EDTA visual titrations, no estimate of the accuracy of the determination can be given; however, the analytical precision
418
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
of the spectropolarimetric titrations were of the same order of magnitude as that found for the other metals. Figure 5 shows the spectropolarimetric titrations of a dilute solution of ytterbium. This is a typical dilute solution spectropolarimetric titration showing a negative titration break. The analytical precision of the dilute solution was lower than that for the concentrated solutions, probably because of additional manipulations required and the use of small aliquots. However, the agreement with the visual standardization was good, with the percentage error in good agreement with the concentrated solutions. Results. Table I1 gives the results of the lanthanide metal ion titrations. Each spectropolarimetric value reported is the average of at least three individual titrations. The results obtained for the analyses of all of the lanthanide series ions determined gave an error of 0.37% compared to the conventional EDTA titrations. The deviations ranged from 0.15% to 0.99%. The maximum time required for the spectropolarimetric titrations and graphical evaluation of the data for each titration is less than 15 minutes. ACKNOWLEDGMENT
Appreciation is expressed to The Dow Chemical Company for its grant of a leave of absence for the summer of 1968 to D. L. C. RECEIVED for review July 30, 1969. Accepted December 17, 1969. This work was supported by The Robert A. Welch Foundation Fellowship Grant A-262. This study was presented at the 157th National Meeting of the American Chemical Society on April 18, 1969 at Minneapolis, Minn.
