the latter system is probably caused by increased contribution of two-point adsorption of the solutes at lower concentrations of dioxane, due to a decrease in the eluent strength below the critical valuez For complete two-point adsorption 2A
Z
so that the Rv = 2.0).
e AzZ; US.
K A ~= Z
xA%Z/xA2xZ
log X s relationship is more steep (slope
To estimate the relative amounts of adsorbent, solvent, and solute in a typical TLC system, 0.3-mm silica gel layers were developed with moderately volatile solvents (decalin, propanol, etc). The layers were then rapidly removed from
the carrier plates, weighed, dried, and weighed again. The following approximate composition was found for an area of 0.25 cm2(average area of a zone): Solvent: 6 mg. Assuming its molecular weight equal to 100, this corresponds to 60 fimoles. Silica gel: 3 mg. Assuming that its surface area is 100 m*/g, this corresponds to 3 pmoles of hydroxyl groups (13). 5 pg of solute corresponds to 0.05 pmoles if M = 100. The results indicate that the assumptions relating to the relative amounts of the components are reasonable, at least for some types of chromdtographic systems. RECEIVED for review October 30, 1967. Accepted August 21,1968.
Differential Spectrophotometric Determination of Europium in Yttrium Europium Vanadate Using an Expanded-Scale Absorption Slidewire Edward W. Laming The Bayside Laboratory, Research Center of General Telephone & Electronics Laboratories, Znc., Bayside, N . Y.
A SET OF europium-activated yttrium vanadate phosphor samples was recently submitted to our laboratories for europium analysis. The presence of yttrium and vanadium prevents the use of volumetric and gravimetric procedures without resorting to time-consuming separations. Therefore, instrumental techniques, atomic absorption, neutron activation, and X-ray fluorescence are usually employed. This note describes a new spectrophotometric method which provides a useful alternative where the other techniques are not available. The spectrophotometric determination of europium, along with other rare earth elements (I-3), is generally limited ia accuracy. For example, the molar absorptivity at 3945 A for europium in dilute acid solutions is approximately 3.0 (I, 2). This means that for a solution containing a practical amount 7 of 3.5 % europium in a volume of 50 ml of sample ( ~ 0 .g) (enough to fill a 10-cm cell), the absorbance due to europium is approximately 0.1. To obtain accurate data (within 1 % relative), it is therefore necessary to measure absorbances more accurately than 0.001 which is the readout limit of most spectrophotometers. In the technique described here, a commercially available slidewire for the Cary Model No. 14 permits absorbances of from 0.0 to 0.2 to be read at *O.OOOl. This expanded-scale slidewire and the use of appropriate reference solutions made it possible to extend the range of precise spectrophotometric analysis to small amounts of europium.
2
I
w
V
2
a
m l
a
am a
a (b)
\
VANADIUM 4 * ION
O
> 1500
A
-
p 4000 4500 WAVELENGTH
6)
i
O
4 5000
Figure 1. Spectral scans of vanadyl and vanadate ions Solutions contain 172 mg of vanadium in 50 ml of 20 H2!304. Vanadium in solution a was oxidized with KMn04 while that in solution b was reduced with SOZ. Solutions scanned in 1-cm cells
EXPERIMENTAL
Apparatus. A Cary Model No. 14 recording spectrophotometer equipped with slidewire No. 1480560 was used throughout. (1) T. Moeller and J. C. Brantley, ANAL.CHEM., 22, 433, (1950). (2) C. V. Banks and D. W. Klingman, Anal. Chim. Acta., 15, 356 (1956). (3) D. C. Stewart and D. Kato, ANAL.CHEM., 30, 165 (1958).
182
ANALYTICAL CHEMISTRY
Reagents. A standard europium solution (1 mg/ml) was prepared by dissolving sufficient Euz08(Kleber Laboratories) in HCl. The solution was then standardized by gravimetric analysis (oxalate precipitation). In addition, Fisher-purified ammonium metavanadate and europium-free yttrium oxide prepared by ion exchange at these laboratories were used. Procedure. Phosphor samples (700 mg) and four standards containing 396 mg of ammonium metavanadate and
0.10
Table I. Comparison of Spectrophotometric Results with Those Obtained by Other Instrumental Techniques 0.09 *
0.08 -
X-ray fluorescence
Neutron activation
Atomic absorption
2.79 2.74 2 . 7 6 2.14
2.76
2.81
2.73
3.16
3.13
3.21
3.14
i::: 3.17
3.11
3.36
3.17
3 . 7 6 3.77 3.78
3.66
...
3.72
;:::
W V
z
Spectrophotometric
0.07-
0 a lJY
m
a 0.06-
1
0.05
363 mg of yttrium oxide each were dissolved in 20 ml of HC1. To three standards, 15.0-, 20.0-, and 30.0-ml aliquots of europium standard solution were added. All solutions were fumed with 10 ml of sulfuric acid, diluted to 50 ml, and treated with SO2 gas for 20 minutes. After the solutions stood for 1 hour, excess gas was removed by boiling until the volume of the solutions was reduced to approximately 35 ml. They then were transferred to 50-ml volumetric flasks and diluted. All spmples and standards were s c a y e d twice from 4100 to 3900 A in 10-cm cells at a rate of 2.5 A/sec using the europium-free standard as a reference. A base line w?s drawn across the europium band from 4075 to 3900 A, and the average net absorbance (peak-base line) was plotted against milligrams of europium. The amount of europium in the samples was read from the curve. RESULTS AND DISCUSSION
Because of the narrowness of the Eu3+band and the need to measure small absorbance differences, it was felt that scanning through the band and using the base line technique (4) offered the best chance of obtaining the desired results. To reduce background absorptions resulting from the dissolution of samples, solutions were fumed with sulfuric acid. Of the remaining ions, the only significantly absorbing species was vanadium. In solutions which have been fumed with sulfuric acid and diluted with water, the vanadium exists in the 4+ and 5+ states. Figure 1 shows the absorption spectra of vanadyl and vanadate ions which clearly indicates that in order to minimize interference the vanadium should be entirely reduced. Two convenient methods for the reduction of vanadium are the addition of tartaric acid and the addition and subsequent removal of the excess SO2 gas (5). (4) A.S.T.M. Manual on Recommended Practices in Spectroplzotometry, First ed., Feb. 1966, p 42. (5) “Scott’s Standard Methods of Chemical Analysis,” N. H. Furman, Ed., Sixth ed., Vol. 1, Van Nostrand, New York, 1962, p 1206.
Figure 2 shows the absorption spectra for solutions reduced both ways. The additional absorption in the tartaric acid solution indicated that SO2 should be used. The absorption in Curve a was then compensated by the addition of a similar amount of vanadium to the reference solution as seen in Curve b. The absorption spectra obtained for a synthetic sample of phosphor containing 15 mg of europium using a reference solution containing yttrium and reduced vanadium showed only the absorption due to europium. A calibration curve was prepared as described in the procedure, and the plot was found to be linear from 15 to 30 mg of europium. Yttrium was included in the standards to prevent any change in the absorptivity of europium due to salt effect (6). To determine the precision of absorbance measurements made using the expanded-slidewire-scanning technique, a solution containing approximately 25 mg of europium was measured nine times. The values were as follows: 0.1028, 0.1030, 0.1029, 0.1032, 0.1029, 0.1030, 0.1029, 0.1033, 0.1033, = 0.1030. The standard deviation, O.OOOl9, indicates only the instrumental precision. In order to determine the overall precision of the method, the absorbance readings for four sets of duplicate samples were examined. The disagreement between the two samples in each set was always less than 0.53 of the total absorbance measured. Table I compares the spectrophotometric results with those obtained by other instrumental techniques. The duplicate results obtained by spectrophotometry indicate good precision, while the agreement with the other techniques indicates the overall validity of the proposed method. This technique is particularly useful in the analysis of rare earth elements because their small absorptivities require accurate measurement and the narrowness of their absorption bands permits the use of a base line technique.
x
ACKNOWLEDGMENT
The author thanks the following analysts: R. J. Kiefer, spectrophotometry; F. Durkin, atomic absorption; M. Hofstetter, neutron activation; and W. D. Shelby, X-ray fluorescence.
RECEIVED for review August 30, 1968. Accepted October 14, 1968. ( 6 ) A. W. Wylie, J . Sac. Cliem. Ind., 69,143 (1950).
VOL. 41, NO. 1 , JANUARY 1969
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