Differential Spectrophotometric Determination of Rare Earths CHARLES V. BANKS, JOHN L. SPOONER, and JEROME W. O’LAUGHLIN Institute for Atomic Research and Department of Chemistry, Iowa State College, Ames, Iowa
b Differential spectrophotometric methods are applied to the determination of neodymium-erbium, praseodymium-erbium, and praseodymiumneodymium-samarium mixtures with a standard deviation of 0.17% for analyses made by the “general” method and a standard deviation of 1.2570 for analyses made by the “trace analysis” method. An accurate method of measuring small molar absorptivities is discussed. A statistical breakdown of the data obtained indicates optimum conditions may have to be determined experimentally rather than theoretically.
I
recent publication ( I ) , the application of the “general” method of Reilley and Crawford (2) to the spectrophotometric determination of neodymium-yttrium oxide mixtures was described. This method has been extended to the determination of other rare earth mixtures. In several cases it was necessary to make a correction for the absorbance of rare earths which absorbed a t the same wave length as the rare earth being determined. The nature of this correction can be seen by considering the fundamental relationship suggested by Reilley and Crawford (8) in their Equation 5 . When the appropriate Beer-Lambert relationships for the intensities are substituted into this equation, the following expression 10-(‘bC + 6 b C I ) - 10--EbCz 100 = 10-CbCI - 10-ebCz N A
]
[
(1)
is obtained, where E and E , are the molar absorptivities of the species being determined and any other species which absorbs a t the wave length used, respectively; C, C , CZ,and C, are the molar concentrations of the species being determined in the unknown, in the dilute reference solution, in the concentrated reference solution, and of any interfering species in the unknown, respectively; b is the optical path length; and R is the scale reading. A general working equation is obtained by rearranging and solving for C (Equation
2).
Reilley and Crawford (2) also described a method which they claimed suitable for trace analysis. This “trace
458
ANALYTICAL CHEMISTRY
analysis” method is that special case of the general method which exists when CI = 0-i.e., when the dilute reference is solvent. This method was used for
CNdis read us. these references, Equation 3 states the relationship between : : ’e and the scale reading, €3:;
=
1 c = - -log €b 10-c32?tchr]
the determination of constituents which were present in very low concentrations, The low concentrations and hence low absorbances of the solutions required the 5-cm. cells. The instability of the instrument made it necessary to balance the spectrophotometer a t 0 and 100 immediately before reading the samples, Because the cell compartment of the Beckman Model DE mould hold only two conventional cells longer than 1 cm., in the early work with the trace analysis method, the spectrophotometer was balanced a t 100 with air as a reference instead of solvent. dlthough the use of air as a reference was satisfactory, a modified cell compartment capable of holding three 5-cm. cells was constructed which also permitted the cells to be positioned more reproducibly. When the trace analysis method was used, a relatively high concentration of another rare earth which absorbed slightly a t the analytical wave length was usually present. It was therefore necessary to know the molar absorp tivities of the interferences with considerable accuracy. The determination of such small molar absorptivities by ordinary methods was subject to large errors, because, even when rather concentrated solutions and cells with long optical paths were used, small absorbances were measured. Accurate determinations of these molar absorptivities were made by using an adaptation of the trace analysis method. For the determination of the molar absorptivity of neodymium a t 3 i 9 mp, as an example, the followingscheme was followed. The spectrophotometer \vas balanced a t 0 and 100 with the erbium reference solution, C’2Er,Table 11, and solvent, respectively. When a neodymium solution of concentration
(3)
If the concentrations are known, the relative molar absorptivity of neodymium a t 379 mp t o that of erbium can be calculated. APPARATUS
The apparatus m s the same as previously described (1) except that three 5-cm. cells ivere employed in the trace analysis method which required the use of the modified cell compartment shown in Figure 1. A Beckman No. 4400 cell compartment assembly was fitted with a new cell rack machined from brass. The cells were held firmly in place and against the cell stops with fiber set screws. The cell rack assembly was sprayed with optical black paint. REAGENTS
Perchloric Acid. Baker and Adamson, reagent grade, 70 to 72% perchloric acid was used. A 10% (v./v.) perchloric acid solution was prepared and is referred to in this paper as solvent. Rare Earth Oxides. Freshly ignited Draseodvmium oxide, PrnOll: - __, neodymium oiide, Xd2O3; samarium oxide, Smz03; and erbium oxide, Er2O3 (Ames Laboratory stock, spectrographically free of other rare earths), were used in the preparation of solutions. Preparation of Solutions. The desired amount of rare earth oxide was weighed into a volumetric flask of suitable size and covered with i-iater, and a volume of 70 to 72% perchloric acid equal to 1,’1~ of the total capacity of the flask was added. If the samples did not dissolve immediately, as was the case for samples containing large amounts of praseodymium or erbium, the flasks were heated on a steam bath until dissolution was complete. The solutions were cooled to 25’ C., allowed to stand for 24 hours, then diluted to volume with water. Samples which contained praseo-
Figure 1.
Modified cell compartment
dymium reacted with perchloric acid with the evolution of chlorine. As chlorine absorbs a t 379 mp, the wave length a t which erbium was determined, it was necessary to remove all traces of chlorine from these solutions. This was best accomplished by dissolving the sample in a beaker with the appropriate amount of perchloric acid, diluting to about the final volume, adding about 1 ml. of 30% hydrogen peroxide, and boiling vigorously for about an hour. The solution was then allowed to cool, transferred to a volumetric flask, and diluted t o volume. EXPERIMENTAL
Determination of Molar Absorptivities. The molar absorptivity of neodymium a t its absorption band near 57’5 mp was reported t o be 6.95 h 0.03 liter mole-‘ cm.-l for a 0.025-mm. slit width (1). The molar absorptivities of praseodymium, samarium, and erbium a t their absorption peaks near 444, 401, and 379 mp, respectively, were determined by the “ordinary” spectrophotometric method and are shown in Table I. The molar absorptivities of the interferences were determined by the proposed adaptation of the trace analysis method. Values for the molar absorptivities of praseodymium a t 575, 401, and 379 mp; of neodymium a t 444, 401, and 379 mp; and of samarium a t 575 and 444 mp are also shown in Table I. Determination of Rare Earths. Solutions 1 to 4 (Table 111) and 69, 70, and 76 t o 79 (Table V), whose neodymium concentration lay between the reference solution C l N d and C z N d (Table 11) were analyzed for neodymium by the general method. Solutions 17 to 26 (Table 111) and 73 to 7’5 (Table V) were analyzed by the trace analysis method using C i ~ d(Table 11) as a reference. The wave length was set a t the neodymium absorption band near 575 nip. A 0.025-nim. slit width was used with a tungsten lamp. In all cases 1-cm. cells were employed with the general method and 5-cm. cells with the trace analysis method. Solutions 5 to 16 (Table 111) and 49 to 52 (Table IT’) whose erbium concentrations lay between the reference solution CIS, and C2Er(Table 11) were analyzed for erbium by the general method. Solutions 27 to 36 (Table
‘I
111) and 65 to 68 (Table IV) were analyzed by the trace analysis method using CiEI (Table 11) as a reference. The hydrogen lamp was used and a 0.04-mni. slit was employed. The wave length was set a t the erbium absorption band near 379 mp. Solutions 37 to 48 (Table IV) and 69 to 79 (Table V) were analyzed for praseodymium by the general method using CI,, and C2?,(Table 11) as references. Solutions 53 to 64 (Table IV) were analyzed by the trace analysis method using C2Pr (Table 11) as a reference. The tungsten lamp was
Table 1.
Wave Length,
40 1
Rare Earth Er Nd Pr Sm Pr Xd
444
575
RESULTS
The observed scale readings, R,, were corrected for differences in the lengths and absorbances of the cells used by means of Equation 4,
Experimental Molar Absorptivities
AIp
379
used and a 0.03-mm. slit width employed, The wave length was set a t the praseodymium absorption band near 444 mp. Solutions 69 to 72 and 74 to 77 (Table V) were analyzed for samarium by the general rnethod using CI,, and Czs, (Table 11) as references. Solutions 73, 78, and 79 (Table V) were analyzed by the trace analysis method using CiSm(Table 11) as a reference. The tungsten lamp was used and a 0.04-mm. slit was employed. The wave length was set a t the samarium absorption band near 401 mp. I n an effort to achieve greater accuracy, solutions 84 to 87’ and 80 to 83 (Table VI) mere analyzed for neodymium by the general method using the reference solutions CI,, and C b 6 N d j and C1.5Ndand CZ, (Table 11), respectively. Solutions 88 to 94 (Table VII) were read against the reference solutions CIS,, and CZaNd(Table 11). The same spectrophotometer settings given previously for neodymium were used.
Pr Xd Sm Er Nd Pr Sm
Slit Width, Mm. 0.040 0.040 0.040 0,040 0.040 0 040
Molar Absorptivitv, Liter Mole-1 Cm. 6 82 f 0.04 0 0164 f 0.0002° 0.001 or less” 3.26 =t0.02 0.0186 =I=0.0001a 0 0120 o o o o i a 10:34-- & 0.06.0.0210 i 0.0002a 0,142 f 0,001 a 0.300 f 0.002 = 6 95 =z! 0.003 0 082 i 0.001 = 0.001 or lessa
*
0.020
0.020 0.020 0.020 0.025 0 025 0.025
Determined by proposed adaptation of trace analysis method, Table II.
General Method G Solution CISd C?Sd CI. 5 S d h Clasd C?,SJ ClEr
C?Er
c,Pr C?P,
CIS, C2Srn a
Reference Solutions
Trans(R.E.)203/100 mittance, m1.a 1-cm. cells 0.8210 0.458 1.41’76 0.260 1,0919 0.355 2.1989 0.124 2.7411 0.740 0.9536 0.457 1.6062 0,267 0.4791 0.512 0.8000 0,327 1.5834 0.506 2.5889 0.328
Trace Analysis Method G. Trans(R.E.)203/100 mittance, Solution m1.a 5-cm. cells C’2Nd 0.1502 0.490 C’2Er 0,1530 0.533 C’*Pr C’2Srn
0.0715
0.3015
0.607
0.522
I n case of praseodymium, concentration is given in grams of PrsOl, per 100 ml.
* Used only for data in Table VII.
VOL. 30,
NO. 4, APRIL 1958
459
Solution 1
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 a
Table 111. Determination of Neodymium-Erbium Mixtures Nd Concn., G. Nd203/ Er Concn., G. Erz08/ 100 M1. 100 M1. x 100 Present Found Present Found 1.2483 1.2470 -0.11 0 5917 . . -0.06 i , io59 ... +0.39 1 ,7879 ... $0.17 2.8038 1.2970 1 ,2949 +0.17 1.2751 1.2730 1,2865 1.2907 1.3006 1.3036 4.0249 ... ... 3.9130 ... ... 1 2495 1 2482 6.1340 ... ... 1 3197 1 3229 8.6388 1 3223 1 3248 ... 1 0244 1 0227 ... 1 1097 1 1090 ... ... 1 ,2847 1.2847 ... ... ... 1.4095 1,4071 1.5160 1.5136 0. ii540 0.1'1'5'58~ $0.'16 $0.78 0.08692 0. 08757O 0.06213 0 ,06213a 0.00 0.03280 0.03280s 0.00 0.01500 0,015350 ... $2.30 0.ii466 0.ii54iO 0.5930 $0.70 0 . 08525a ... 0,08525 0.00 0.8160 1.0094 ... 0.06280a 0.06280 0.00 1,1580 0.038475 0.03860 -0.34 0.0167la -4.70 2.4705 0.01753 0. 13574O 0,13674 ... ... ... 0 09450 0.09484" ... ... ... 0,06639" ... ... 0 06580 ... 0,03986" 0.03946 ... 0.01734a ... 0 01734 0.12183° 0 12146 o.6ik ... 0,09320" 0 09270 0.8035 ... ... O.0668Oa 0.06543 ... 1 0167 ... 0.03188" 1 2033 ... 0 03110 ... 0,01590" ... 0 01590 2.2101
$
. I .
... ... ...
-0.17 -0.17 f0.33 $0, 28 -0.11 +O. 28 $0.22 -0.17 -0.06 0.00 -0.17 -0.17
... . , .
... ... -0.73 +O. 36 +o. 90 +1.02
0.00 +O. 31
$0.54 +2,07 1-2.51 0.00
Analyzed by trace analysis method.
Table IV.
Solution 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 ~~
58
59 60 61 62 63 64 65 66 67 68 a
$ x 100
Pr Concn., G. PrsOii/ 100 M1. Present Found 0,4967 0,4976 0,5595 0.5608 0.6161 0.6171 0,6981 0.6993 0.7899 0.7586 0.7810 0,7801 0.5017 0.4962 0.5559 0,5563 0.6141 0.6155 0.6576 0.6580 0.7545 0.7558 0.7745 0.7750 5.0500 4.0786 ... 2.8919 ... 2.0327 0,07045 O.OjO63O 0.062025 0.06220 0.04984O 0.05000 0.03665 0,03628. 0.027720 0.02752 0.01759O 0.01735 0.00955a 0,00992 0.06891° 0.06882 0.04400 0 , 0436g5 0.03425* 0.03425 0,02257~ 0.02282 O.0166ln 0.01692 0.8823 1,5615 2.0010 3.0466
Analyzed by trace analysis method.
460
Determination
ANALYTICAL CHEMISTRY
of Praseodymium-Erbium Mixtures
g x 100 $0.17 +0.23 $0.17 -0.17 -0.17 -0.11 $0.11 -0.06 -0.23 -0.06 -0.17 -0.06
... +0.26 -0.29 -0.32 -1.00 $0.74 +1.36 -3.64 +O. 13 -0.69 0.00 -1.14 -1.82
...
Er Concn., G. Erz03/ 100 M1. Present Found
--
... ... 1.4816 1.3655 1.3113 1 2084 1.0695 1.0221 0.9595 1.1178 1.1982 1.3190
...
...
...
...
... ...
...
...
...
... ...
, . .
...
0.9595 1.1165 1.1962 1.3149 ... ... ... ...
... 0,09588 0.31645 0.51391 0.78225 0.96915 0,1441 0.1034 0.06992 0.03306
% x 100
...
0.00 -0.11 -0.17 -0.28
...
...
... ...
...
...
0 . i433a 0.1031" 0.06992° 0,03349"
-0.56 -0.32 0.00 f1.32
Table V. Determination of Praseodymium-Neodymium-Samarium Mixtures Nd Conon., G. Ndz03/ Sm Concn., G. SmzO,/ Pr Concn., G. PrgOll/ 100 M1. 100 M1. 100 $11. Solution Present Found x 100 Present Found loo Present Found 69 0.6495 0.6502 $0.14 0.9823 0.9840 $0. 12 1 ,7466 1.7457 70 0.7348 0.7352 $0.07 1.0625 1,0643 +0.18 2.4100 2.4078 71 0.5965 0.5961 -0.07 5.0161 2.0171 2.0153 72 1.9784 ... 1,0617 1.0641 $0.24 2.0065 2.0076 73 0.07600 O.Oi600 0.00 0.1392 0.1398" $0. 46 0.2797 0 . 2802" 74 0.6009 0.5999 -0.21 0.1001 0,0996" -0.50 2.0470 2.0507 75 0.5864 0.5847 -0.28 0.05142 0.05095" -0.93 2.0036 1,9990 76 0.04938 0.04959" $0.42 1.0153 1.0135 -0.18 2.0341 2.0330 77 0.03580 0.03590" +0.28 1.0384 1.0366 -0.18 2.0679 2.0669 78 0.6359 0.6355 -0.07 1.0130 1.0118 -0.12 0,2024 0 .2027" 0.00 1.0003 0.9997 -0.30 0.1535 0.1535" 79 0.6198 0.6198 Analyzed by trace analysis method,
%
Table VI. Effect of Transmittance Span of References on Precision Nd Concn.. G.
Nd203/100'b11. Solution Present Found C 80 1.3501 1.3515 + O . l l 81 1.2890 1.2865 -0.22 82 1.2258 1.2283 +0.22 83 1 . 1 ~ 7 1 1 . 1 . ~ +n.ifi , 84 1.0325 i.0309 -0.16 85 0.9700 0.9676 -0.25 86 0.9181 0.9181 0.00 87 0.8729 0.8700 -0.33
Table VII. Precision with Reference Solutions of l o w Transmittance
Nd Concn.. G. Ndz03/100kIl. C $ Solution Present Found 88 2.7243 2.7256 +0.04 89 2.6455 2.6437 -0.07 90 2.5666 2.5646 -0.08 91 2.4993 2.4993 0.00 92 2.4045 2.4027 -n 07 93 2.32si 2.32si 0.00 94 2.2513 2.2500 -0.06
$
Table VIII.
Element Det. Nd Nd Nd Nd Er Er Pr Pr Sm Sm 5
Reference Solutions Cmd, C z N d C 1 N d j Ci.md, ClaSd, C h N d
Ci"d CIE~,CZE~
C;E~
CI,P,, CzPr CZCISrnt
C&rn
CZS,
Czxd
4G x C
100
-0.05 -0.09 +o. 09 $0.05 $0.17 $0.20 -0.23 -0.05 -0.09 0.14 0.00
+
Analytical Error
Transmittance Span Method 0.198 General 0,103"or 0,095' General General 0.049 Trace analysis 0.510 General 0.190 Trace analysis 0.467 General 0.185 Trace analysis 0.393 General 0.178 Trace analysis 0,478
KO. of Mean Standard Anal- Recovery, Deviayses % tion, s 12 100.03 0.21 8 99.94 0.21 0.011 7 99.97 1.60 13 99.84 16 99.97 0.21 0.95 14 100.53 20 99.96 0.17 14 99.59 1.22 0.14 8 99.98 3 100.10 0.089
Appropriate pair of reference solutions used,
given by Reilley and Crawford (2) was approximately half as large. NO increase in precision was noted, however. The data in Table VI1 obtained with reference solutions CIS,, and ckyd (Table 11), which had a transmittance span of 0.049, had a significantly lower DISCUSSION mean error. The smaller transmittance span would not make the error coReilley and Crawford ( 2 ) have shown efficient considerably smaller because, that the minimum error for the general R = 100 [Rd] (4) even though the numerator was much RM - RN method occurs when the transmittance smaller, the denominator begins to span of the reference solution is as which was derived in a previous publidecrease rapidly a t transmittances less small as possible and includes 0.368 cation (1). The general working equathan 0.100. transmittance. The practical limit is tion, Equation 2, was used to calculate The data given are too few to draw determined by the stability of the the concentration from the corrected any broad conclusions but indicate that instrument and the inconvenience of scale reading. maximum precision with the general too small a concentration span between The standard deviations obtained by method of Reilley and Crawford might the reference solutions. both the general and the trace analysis occur with reference solutions of lower Most of the data obtained (Tables method are summarized in Table VIII. transmittances than 0.368. This could 111, IV, and V) were with reference With the exception of the seven solube due to a breakdown of the assumpsolutions of transmittances in the range tions read against Cia,, and CINd, tion that AR is independent of T . 0.3 to 0.5. The transmittance span the standard deviation for the general The data presented were all obtained ranged from 0.178 for C1, and Czsm by one operator using the same spectromethod was about 0.17%. The stand(Table 11) to 0.198 for CINdand CZNd. photometer. Maximum care was exerard deviation for all the analyses by The data in Table VI, read against the trace analysis method was 1.25%. cised in all volumetric and spectroCI,,, c1.6~~~ and Czxd, were obtained Because the expected error by the photometric manipulations. At least with solutions of transmittance span trace analysis method for a constant three readings were taken on each 0.103 and 0.095. An approximately reading error, AR, is a function of R solution and the instrument rebalanced twofold increase in precision would be and increases rapidly with R ( d ) , the before each reading. The result reexpected, since the numerator in the calculated standard deviation for the ported in each case is the average. error coefficient analyses by the latter method is not It is believed that the data presented too significant. For very dilute sample experimentally verify the usefulness of TI - Tz solutions, a relatively high error must the general method of Reilley and CrawTs In Ts be expected. Inspection of the data in Tables 111, IV, and V shows that the relatively high standard deviation for the trace analysis method is largely due to the analysis of a few very dilute solutions.
VOL. 30, NO. 4, APRIL 1958
* 461
ford (I) A . significant increase in precision over that which could be obtained by ordinary spectrophotometric methods was observed. This Teason and those mentioned in an earlier publication (1) prove the useful-ness of the technique for the analysis of rare earth mixtures, and it should be generally useful for the analysis of substances which have a auitable absorption spectrum.
ACKNOWLEDGMENT
LITERATURE CITED
The authors wish to express their appreciation to F. H. Spedding for supplying the pure rare earth oxides used in this study. They also wish t o thank V. A. Fassel for spectrographically analyzing the various rare earth oxides for other rare earth impurities and F. \v. Sealock for her assistance in this work.
(1) Banks, C. V., Spooner, J. L., O'LaughEn, J. W., ANAL.CHEM.28, 1894 (1956). (2) Reilley, c* Crawford, c. M., Ibid., 27, 716 (1955). for review November 12, lg5&
Accepted January 3, 1958. Contribution N ~ 607, . Laboratory, U. 8, Atomic Energy Commission.
Differential Spectrophotometric Determination of Zirconium in Presence of Hafnium HARRY FREUND' and W. FLOYD HOLBROOK Northwest Nectrodevelopment laboratory,
A differential spectrophotometric method of sufficient accuracy has been developed to permit the determination of zirconium in the presence of hafnium. By applying the Alizarin Red differential method of Manning and White to a fixed sum of hafnium oxide and zirconium oxide, it i s possible to achieve this goal. The results are comparable in accuracy and precision to those obtainable b y gravimetric methods.
N
o
cHmuc.iL REACTION has ever been reported that permits a clear-cut distinction between zirconium and hafnium. Similar electronic configurations, coupled with essentially identical ionic radii, make it unlikely that any such simple quantitative separation will ever be achieved. Although essentially pure zirconium (or pure hafnium) has been prepared by ion exchange, chromatography, and solvent extraction, a significant middle fraction in each case nullifies the separations for quantitative analytical purposes. Chemical methods of analysis have, in general, been indirect. The change in weight accompanying the conversion of a pair of pure compounds, such as mixed selenites, into a second pair of pure compounds, such as mixed oxides, permits calculation of the hafnium-zirconium ratio. A spectrophotometric approach for detc,rminiiig zirconium in the presence of hafnium is described here. The method is based on the following considerations.
Both zirconium and hafnium react in 1 Present address, Department of Chemistry, Oregon State College, Corvallis, Ore.
462
ANALYTICAL CHEMISTRY
U. S.
Department o f the Inferior, Bureau o f Mines, Albany, Ore.
an identical manner with sodium alizarin sulfonate. The color intensity of the colored chelate formed between alizarin and mixtures of hafnium and zirconium depends on the ratio of these elements when the sum is constant. Differential spectrophotometry provides sufficient gain in sensitivity and accuracy to permit analysis by colorimetric means. Reilley and Crawford (3) have presented an excellent analysis of the principles of precision colorimetry. I n the ordinary spectrophotometric method, referred to as Case I, the absorbance of a solution is measured against a solvent blank. IVith this method the absorbance of two series of solutions, one containing increasing weights of hafnium alizarin chelate and the other increasing weights of zirconium alizarin chelate, can be plotted against concentration (n-eight RO,/volume). Two diverging straight lines are obtained, as shonn in Figure 1. As the molar absorbances of hafnium and zirconium are esscntially identical, the following derivation shon-s the relationship of the two curves. - 4 ~= ~ a0 x ~ C x (I/ZrOa) AHQ = u X C X (1/HfO,) where a = molar absorbance C = weight concentration Dividing, we obtain:
The absorbance of any mixture of zirconium and hafnium having a total concentration C1 must lie between the limiting values set by the pure components. The small range, coupled with the inherent errors of conventional spectrophotometry, could yield only
C3YCEhTRATI3N (weight oxide/vol~mel
Figure 1. Idealized calibration curves for zirconium and hafnium
a crude estimate of the zirconiumhafnium ratio. Khile a t a high concentration level (C,) the idealized calibration curves of Case I are sufficiently separated, the absorbance values are far beyond the practical instrumental limits. The use of a high concentration level of hafnium-alizarin chelate corresponding to C2 in the null cell yields a perfectly usable Case I1 calibration curve for hafnium. The absorbance of a zirconium solution of concentration Cz can be estimated as follows. Assume the zirconium solution of concentration C2 can be replaced by two zirconium solutions of equal volume. Xhen they are placed in series the fraction of light transmitted will be exactly the same as that transmitted by the original zirconium solution. Solution 1 contains sufficient zirconium to have an absorbance equal to that of the null hafnium solution of concentration
