about the sample. This light intensity a t the sample, and the resulting phosphorescence of the samples, could probably be increased by an order of magnitude. Second, the optimum excitation frequency could be determined and a strong light source of this frequency used. This would, of course, increase excitation of the ZnS, but should also minimize excitation of the glass container or any dirt in the sample. Third, the phosphorescence emission varies considerably between types of ZnS pigment, and no attempt was made to select the pigment which would optimize sensitivity. In addition to providing a new and unique method of analysis, this procedure has been used t o calibrate the alpha excitation instruments (8) (which measure ZnS directly on clean filters) and to demonstrate that their response is linear on clean “field-run” filters.
This technique could be applied to the measurement of particle distribution (using ZnS as a tracer) on ground surfaces in different types of terrain and under different meteorological conditions, since ZnS can be measured in the presence of dirt. In a more general way, this technique could be applied to the measurement of any material which has a sufficiently high phosphorescence yield and a sufficiently long decay rate. I t is approaching the ultimate in sensitivity, since the individual phosphorescence photons resulting from electronic excitation of single molecules are being counted. The technique could be further refined by using optical spectrometric techniques to separate the desired wave lengths prior to measurement. ACKNOWLEDGMENT
The authors express their apprecia-
tion to P. W. Nickola and C. L. Simpson for their cooperation and effort on this project. LITERATURE CITED
(1) Colgate, S. A., Rev. Sci. Insir. 30, 140-1 (1959). (2) Garlick, G. F. J., “Luminescent Materials,” Clarendon Press, Oxford, 1949. (3) Ludwick, J. D., ANAL.CHEM.32, 607
(iw,n\. \ - - - - I -
(4) Ludwick, J. D., Unpublished work. (5) Morton, G. A., RCA Rev. 10, 525
(1949).
(6) Nickola, P. W., Hanford M.eteorologi-
cal Site, personal communication. (7) Pringsheim, P.,,, “Fluorescence and Phosphorescence, Interscience, New York, 1949. (8) Rankin, M. O., Zinc Sulfide Particle Detector, HW-55917, May 1, 1958. (9) Sharpe, J:, Photomultipliers for Tritium Counting, Conference on Organic Scintillation Detectors, Albuquerque, N. M., August 15-17, 1960. RECEIVED for review December 19, 1960. Accepted June 6, 1961.
Absorption Spectra of the Lanthanides in Fused Lithium ChI o ride-Potassiu m ChIo ride Eutectic CHARLES V. BANKS, MERLYN R. HEUSINKVELD, and JEROME W. O’LAUGHLIN Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa
b Spectra are presented for solutions of praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, and ytterbium fluorides in fused lithium chloride-potassium chloride eutectic at 400’ C. These spectra are discussed and compared with similar results which have been obtained in the same and other fused salt media and in aqueous solutions. Molar absorptivities for selected absorption bands of the rare earth spectra which might b e useful in quantitative analytical determinations are also given.
the same form that Banks and Klingman (2) used for their results on aqueous solutions of rare earth mixtures. The absorbance of the solvents used was determined a t the same temperatures and in the same quartz cells used for
EXPERIMENTAL
T
nE ABSORPTION SPECTRA of fused salts have been of interest for some time as aids in studying the chemical constitution and electronic structure of these media both as pure salts (12, ld-l?’, 21) and as solutes in fused salt solutions (3, 6-9, 18, 19, 21-24, 26-98). This investigation is concerned with the determination of the unique absorption spectra of dilute solutions of various lanthanide fluorides in fused LiCI-KCl eutectic solvent, and the determination of the molar absorptivities for significant absorption bands from these spectra. The molar absorptiTities of the bands of analytical interest are tabulated in
recording the spectra of the rare earth solutions, and then was subtracted from the absorbance of the solutions to obtain the absorption spectra of the solutes.
POROUS DISK-
) (c
Y BALL JOINT
Figure 1. Eutectic purification and filtering apparatus
Apparatus and Reagents. The spectra were obtained with a Cary Model 12 spectrophotometer using an electrically heated cell-block furnace as the cell holder. The high temperature cell assembly was similar to the one used by Sundheim and Greenberg (22). The temperature fluctuated *0.5’ C. in the cell block and less than *0.1” C. in the fused salt solutions contained in the optical cells. The optical absorption cells used were onepiece quartz cells having 1-cm. light paths. These square cells were connected to a borosilicate glass ball joint by a graded seal. The ball joint could be connected to either an outer joint fitted with a vacuum stopcock for sealing the cell or to the outer joint of the filtering apparatus used in aling the cells with molten solutions. The graded seal and tubing between the cell and the ball joint provided the necessary capacity for the LiC1-KC1 eutectic when it was loaded in the solid state. The lanthanide fluorides used were prepared a t the Ames Laboratory of the Atomic Energy Commission and analyzed for fluoride and rare earth metal content. VOL. 33, NO. 9, AUGUST 1961
1235
The fluoride determintttion was made by the 'pyrohydrolysis technique of Banks, Burke, and O'Laughlin (1). The osides resulting from these determinations were weighed and the rare earth metal content of each fluoride calculated. These oxides were used to prepare 0.1M solutions of each rare earth in 1N perchloric acid. Reagent grade lithium chloride was vacuum ovendried at 110" C. with correction for residual water content being applied from the data of Burkhard and Corbett (4) in weighing the reagent for the preparation of the eutectic mixture. Reagent grade potassium chloride was vacuum oven-dried a t 110" C. No residual water correction \vas necessary. Procedure. The LiCl and KC1 were combined to yield a 59 mole % LiCl eutectic mixture and then fused and purified, using a n apparatus similar t o the one used by Gruen and McBeth (7). The small droplets of solidified eutectic obtained were stored under carbon tetrachloride until needed. The eutectic droplets did not adhere t o one another, thus varying quantities could easily be withdrawn from the storage flask. Because the eutectic was hygroscopic, it had to be handled and weighed in a dry inert atmosphere. The latter was obtained by using a dry box containing a nitrogen atmosphere. To further purify the eutectic, a filtering apparatus similar to the one used by Boston and Smith (3) was utilized. By maintaining a pressure differential across the fine porosity fritted disk of this apparatus, the melt could be supported above the disk or filtered through it. This apparatus, shown in Figure 1, was wrapped with heating tape from below the tapered joint to just above the outer ball joint. The optical cell or equivalent receiving tube was attached at the ball joint and enclosed in a n electric furnace, The eutectic wm removed from the storage flask in the dry box and approximately 6.7 grams weighed into a small bottle. A borosilicate glass receiving tube was attached to the filtering apparatus and the entire apparatus was dried by heating it to 150" C. and passing dry helium gas through it. The eutectic was transferred from the small bottle to the filtering apparatus by quickly opening and closing the chamber a t the upper tapered joint. A vacuum was applied above the disk and anhydrous HCl gas was allowed to enter
Ti -
0s:01' 07;-
p
os-
-T
02
1, ll
1
1 -I 4-
650
Figure 2. A. 8.
Praseodymium(lll) spectra
0.2M In LiCI-KCI at 400" C. 0.1M In 1N HClO, at 25' C.
OD
n
Figure 3. A. B.
k.
Neodymium(ll1)spectra
thereby filtered through the disk and flowed into the receiving tube below. After the transfer was completed, the filtering chamber was allowed to cool slowly to 200" C. while the receiving tube temperature was maintained a t 500" C. with a vt+cuum applied to the entire apparatus to remove any gas bubbles from the melt. Helium was again allowed to enter the apparatus. The receiving tube was disconnected from the apparatue while a positive pressure of helium was maintained. The rereiving tube was then connected
---
----_1
A
05-
1 :::
407 5m.
Figure 4. A. B.
1236
0
Samarium(lll) spectra
0.1 M in LiCI-KCI at 400' C. 0.1M in 1N HCIO, at 25' C.
ANALYTICAL CHEMISTRY
-1
0.1M in LiCI-KCI at 400' C. 0.1 M in 1 N HClOa at 25" C.
below it as the apparatus was slowly heated to melt the eutectic. The HC1 gas, dispersed as fine bubbles by the fritted disk, was passed through the melt as the temperature was raised to 500" C. and for about 10 minutes thereafter to remove any water, oxygen, and C C L Helium gas was then passed through the melt for about 30 minutes while the melt temperature was maintained at 500" C. to displace the HC1 from the melt. Nest, a vacuum was applied below the fritted disk and helium gas pressure above the melt. The melt was
- --r---- I--
I\
Figure 5. A. 8.
Europium(ll1) spectra
0.1M in LiCI-KCI at 400' C. 0.1M in 1N HCIO4 at 2 5 ' C.
04
03
-
NO A-ION 3 5 0 - 10Wnr
S O
B 0.
03 02 Nomcwrkm 01
00
I 250
1
215 WELENGTH
Figure 6. A.
B.
.
-
300
325-10oom, I 325
A.
uo
f YILLIYICRW)
0.5M in LiCI-KCI at 400' C. 0.1M in 1N HClO, at 25' C.
8.
Gadolinium(lll) spectra
molarity of each solution from the weight of the rare earth fluoride and the weight of the eutectic used. It was assumed that the density of the eutectic solvent was that of the solution. Because only the conventional cell holder was in the reference cell compartment of the spectrophotometer, the molar absorptivities are us. air as the reference standard. After obtaining the weight of the eutectic solvent placed either directly in the optical cell, or in the filtering apparatus for further transfer to the optical cell, the amount of lanthanide fluoride needed to yield a solution
0.1 M in LiCI-KCI at 400' C. 0.1M in 1N HClOd at 25' C.
to an outer ball joint fitted with a stopcock and re-evacuated. The tube was then removed from the furnace and cooled rapidly. Another receiving tube was connected to the apparatus and the process repeated for another quantity of eutectic. The cooled eutectic could then be accurately weighed in these sealed tubes. In the dry box the eutectic easily slipped out of the tubes and could be loaded into dry optical cells. Two procedures were used for the addition of the lanthanides to the eutectic mixture. In the first procedure, a weighed amount of the appropriate lanthanide fluoride was placed in the filtering apparatus with a weighed portion of the purified eutectic. The same procedure described above for the purification of the eutectic pellets was repeated. This ensured thorough mixing of the lanthanide and the eutectic and presumably converted the lanthanide fluoride and any traces of oxide to the chloride. The entire filtering apparatus had to be cooled and thoroughly cleaned between the preparation of each lanthanide fused salt solution. The absence of water and oxygen from the eutectic solvent and solutions prepared in this manner was evidenced not only by the complete absence of etch on the highly polished quartz cells even after 48 hours a t temperatures in exccss of 400' C., but also by the reproducibility of the absorption spectra. The pure eutectic was used to determine the base line for the spectral measurements of the solutions. In the second procedure, the lanthanide fluorides wcre accurately weighed and added directly to the eutectic while the cells containing the fused eutectic were a t temperature in the cell-block furnace. Several hours were required, however, to be certain that all of the added salt had dissolved in the eutectic and that the solution was homogeneous. The spectra of solutions
prepared in the cell block furnace by addition of the lanthanide fluorides, without any treatment to remove the fluoride, yielded the same reproducible spectra and molar absorptivities as the solutions prepared by the first procedure. Molar concentrations were calculated using the density values for the LiC1KC1 eutectic determined by Van Artsdalen and Yaffee (26). Their value of 1.6741 grams per cubic centimeter a t 400" C. was used in calculating the
Table 1.
Spectrum of Praseodymium in Various Solvents
LiCl-KC1 400'
448 473 483 486 522 597
LiF (98)
c.
900'
1.47 1.12 1.30 1.45
...
262 353 358 360 532 582 589 597 751.5 808
...
881
1.1
...
... ...
...
1.3
... ... ...
485
. .
590
444 469 482
...
9.49 4.30 3.95
... ...
, . I
589
900'
x
E
... ...
1.90
... ...
...
... ... ... ...
523 582
...
1.70
253 259 347 351 354 522 575 578
2.10 1.60 2.70 2.40 3.70 4.10 6.83 5.00
1 .oo 1.10
740 794 800 862
6.50 8.08 5.60 2.90
885
a74
...
e
0.87
... ...
590
HClOl 25' C.
c. (6)
x
747 803
360 535
184'
X
755 810
...
c.
E
1.40 2.70 I .90 1.89 1.87 6.90 11.90 7.00 2.30 2.55
...
LiNOa-KN03
LiF ($8)
(94)
...
447 471
...
Spectrum of Neodymium in Various Solvents
373" C.
...
HClOd 25' C.
1840 C. (6)
1.4
(522) 588
LiC1KC1
LiC1-KC1 400' C.
x
It.
444 467 479
...
522 585
0.35
Table
1.2
...
...
LiF-NaF-KF 550" C. (27)
c.
444 468 479
LiNG KNOs
350 356
...
...
... ... ...
...
... ...
... ...
... ... 585 ... 526
739 801
...
871
...
VOL. 33, NO. 9, AUGUST 1961
...
1237
0.1, 0.2, 0.3, or 0.5iM in lanthanide concentration was calculated, weighed, and transferred to the optical cell or filtering apparatus. By making the molar concentrations multiples of 0 . l U concentration, the molar absorpticities were easily determined directly from the absorption spectra recorded for each solute concentration a t the selected wave lengths.
07
1
RESULTS AND DISCUSSION
Figure 8.
The absorption bands occurring in the infrared region of the spectrum were not resolved due to the large slit widths required. A comparison of wave lengths and molar absorptivities observed in the LiC1-KC1 eutectic and HC10, solutions is presented for the principal absorption bands occurring in the spectral region studied. The narrow bands so characteristic of aqueous solution spectra of the lanthanides are largely preserved in the fused salt solution spectra. The charge transfer band studied extensively by numerous investigators and discussed theoretically by Orgel (23) was observed as a "cut-off absorption edge" in the ultraviolet region of most of the fused salt solution spectra. The red shift in wave length caused by the increased temperature and concentration of chloride ion was also observed. The shift toward longer wave lengths of the absorption bands between 400' and 500' C. was too small to be calculated accurately, but was observed to some extent in the spectra of all of the lanthanides. PraseodymiumFluoride. The wave lengths and molar absorptivities of the principal absorption bands found in the spectrum of praseodymium fluoride in the fused eutectic and of praseodymium oxide in perchloric acid, are presented in Table I (see also Figure 2), with the results which have been reported in other solvents. The absorption band occurring a t 522 mp in the fused eutectic has been reported ($8). However, the band a t 486 mp and the shoulder band a t 483 mp have not been reported by other investigators. The wave lengths of the absorption bands in the spectrum shown agree closely with those reported by Gruen (5) in LiN03-KN01 eutectic. Gruen added KC1 to the fused nitrate eutectic solutions studied until the solubility limit of the chloride ion was reached and observed only minimal changes in the spectra of trivalent praseodymium and trivalent neodymium. Neodymium Fluoride. The wave lengths and molar absorptivities of the principal absorption bands found in the spectra of neodymium fluoride in fused LiCI-KCl eutectic and of neo-
A.
123%
9
ANALYTICAL CHEMISTRY
E.
Dysprosium(ll1) spectra
0.5M in LiCI-KCI at 400' C. 0.1M in 1N HCIO4 at 25' C.
..
::b
n
i i
, 700'850
Figure 9. A. E.
8.
950
Holmium(lll) spectra
0.1M in LiCI-KCI at 400' C. 0.1M In 1 N HClOl at 25' C.
Figure 10. A.
900
Erbium(ll1) spectra
0.1M in LiCI-KCI at 400' C. 0.1M in 1N HCIO, at 25' C.
dymium oxide in HCIOl are presented in Table I1 (see also Figure 3), with the selected results which have been reported in other solvents. The neodymium fluoride fused salt solutions were blue-green. Samarium Fluoride. The spectra of samarium fluoride in fused LiClKC1 eutectic and of samarium oxide in HClO, are presented in Figure 4. Young and White (88) have reported absorption bands for samarium fluoride, in fused lithium fluoride a t 340, 358,
373, 390, 399 ( e = 0.63), 419, 441, 460, and 468 mp.
Europium Fluoride. The spectra of europium fluoride in fused LiClKC1 eutectic and of europium oxide in HClOd are presented in Figure 5. The spectrum of europium chloride in LiC1-KCI eutectic has been studied qualitatively ($4) but only the ultraviolet cut-off beginning a t 440 mfi with no superimposed fine structure was reported. Because of the large charge transfer
band, it was necessary to use solutions less concentrated than 0.1M t o observe the fine structure a t the shorter wave lengths. Gadolinium Fluoride. The spectra of gadolinium fluoride in fused LiClKC1 eutectic and of gadolinium oxide in HC10, are presented in Figure 6. The aqueous absorption band a t 252 mp splits into three absorption bands a t 248, 254, and 255 mp in the LiClKCl eutectic. The molar absorptivity value, 3.43 liters per mole-centimeter, given by Moeller and Brantley (11) for the 272.7-mp band in HC1 solution, agrees better with the 335 liters per mole-centimeter value obtained in HC104 solution in this study than the 2.04 liters per mole-centimeter value of Stewart (20). Other Lanthanides. The spectra of terbium, dysprosium, holmium, erbium, thulium, and ytterbium fluoride in fused LiCl-KC1 eutectic and their respective oxides in HC101, are presented in Figures 7 through 12 Cerium, Promethium, and Lutetium. The spectra of cerium, promethium, and lutetium are not presented for various reasons. Cerium has broad, intense absorption bands which occur in the ultraviolet region of the spectrum very near the fused LiC1-KC1 eutectic cut-off in quartz cells, as shown by the results of several investigators ( 2 , 10, 11) of aqueous solutions of trivalent cerium compounds. Only an absorption edge in the ultraviolet would be expected and would be of little analytical value other than a possible evaluation as to cerium interference in the determinations of other rare earths in its presence. Promethium was not available in any form to the authors for investigation in this study. Lutetium fluoride was dissolved in the fused LiCI-ICC1 eutectic and, as expected, there was no absorption found in the wave length region investigated in this study. SUMMARY
As can be seen from the spectra presented and the molar absorptivities determined, it would not be practical to determine the concentrations of europium, gadolinium, or terbium in fused salt media by the spectrophotometric technique used in this study. Other lanthanides, with the possible exception of dysprosium, could be determined in the presence of gadolinium and terbium but not in the presence of europium by this technique. Table I11 shows the main absorp tion bands of eight of the lanthanide fluorides in fused LiC1-KC1 eutectic a t 400" C., and the interference molar absorptivities of the other lanthanides
Figure 1 1.
Thulium(lll) spectra
A. 0.3M in LiCI-KCI at 400. C.
E. 0.1M in
01
.-
1N HCIOd at 25' C.
I 3 i
4W-850my m* S ~ I O N
o B L > x z 2 - 2%
_A_ 400 v 5 0 900 WAV€LENGTH I YILLIMa)ONSl
3%
3W
Figure 12, A.
9%
lOa,
0%
Ytterbium(ll1) spectra
0.1M in LiCI-KCI at 400' C. at 2 5 " C.
E. 0.1M In 1N HClOd
Table Ill. Molar Absorptivities of Analytically Significant Lanthanide Absorption Bands in Fused LiCI-KCI Eutectic a t 400" C.
x
Pr
289
...
353 ... 360 363 380 407.5 ... 460 0.87 483 1 .30° _1.45O 486 524 0.07 539 ... 0.28 589 644 694 ... 751,5 ... 808 911 975 a
Nd
Molar Absorptivities, Liters/Mole-Centimeter Sm Dy Ho Er Tni Cut-
0.09
0.90
0.285 0.08
0.20 0.80 4. 05a
Off
2.70 1.89= 1.10 0.20
...
0.50 0.50 0.93 0.90 2.6P 0.08 0.05 0.06 0.10
0.22 0.25 0.16 1.02 1 .oo 0.10 11.90" 0.05 ... 0.20 . ~. ... 2.30a 2.55" ... 0.20 0.05
0.04
0.05 0.02 0.05 0.03 0.02 ...
...
Yb cut-
0.68
0.25 0.30 9.55a ... 0.05 0.10 12.10" 0.12 0 12 0.15 0.20 0.25 6.30° 0.62" 0.40 ... 0.12 0.38" 0.15 0.06
...
...
0.05 0.08"
0.10
Off
...
0.25 0.27
0.70 0.38 0.30
...
0.10
0.05 0.07 0.04
..
...
...
... ...
...
0.37"
...
0.70
...
0.12
0.20 1 .25a
, . .
Analytically significant absorption bands.
listed in the table a t each absorption band. LITERATURE CITED
(1) Banks,
C. V., Burke, K. E., O'Laughiin, J. W., Anal. Chim. Acta
19,239(1958).
( 2 ) Banks, C. V., Klingman, D.W., I b i d .
15,356 (1956). (3) Boston. C.R..Smith. G. P.. J . Phvs. Chem. 62, 409 (1958). ' (4) Burkhard, W. J., Corbett, J. D., J . Am. Chem. SOC.79,6361(1957). (5) Gruen, D. M.,J . Inorg. & Nuclear Chem. 4,74 (1957), VOL. 33,
NO. 9, AUGUST
1961
0'
1239
(6) Gruen, D. M., Nature 178, 1181 (1956). (7) Gruen, 1). M., McBeth, R. L., J . Inoig. & Nuclear Chem. 9,290 (1959). ( 8 ) Gruen, D. M., McBeth, R. L., J . Phys. Chem. 63,393 (1959). (9) Gruen, D. M., McBeth, R. L., Kooi, J., Carnall, W. T., Ann. N . Y . Acad. Sci. 79,941 (1960). (10) Holleck, L., Hartinger, L., Angew. Chem. 67, 648 (1955). (11) Moeller, T., Brantley, J. C., ANAL. CHEM.22,433 ( 1950). (12) Mollwo, E., 2. Physik 124, 118 (1947). (13) Orgel, L. E., Quart. Revs. (London) 8, 422 (1954).
(14) Sakai, K., J. Phys. Chem. 61, 1131 (1957). (15) Sakai, K., Nippon Kagaku Zasshi 77, 1731 (1956). (16) Ibid., 78, 138 (1957). (17) Ibid., 78, 306 (1957). (18) Ibid., 78, 1257 (1957). (19) Silcox, N. W., Haendler, H. M., J . Phys. Chem. 64, 303 (1960). (20) Sewart, D. C. [Part I.], U. S. Atomac Energy dornm. AECD-2389, SeDtember 22. 1948. (21) ‘Sundheim,‘ B. R., Greenberg, J., J . Chem. Phys. 28,439 (1958). (22) Sundheim, B. R., Greenberg, J., Rev. Sci. Instr. 27, 703 i1956). (23) Sundheim, B.’ R., ‘Harrington, G.,
J . Chem. Phys. 31,700 (1959). (24) Sundheim, B. R., Harrington, G., U.S . Atomic Enerau - - C m m . NYO-7742, . . March 9, 1959. (25) Van Artsdalen. E. R.. Yaffde.’ I. S.. ‘ J. Phys. Chem. 59; 118 (1955). (26) Young, J. P., White, J.’ C., ANAL. CHEM.31, 1892 (1959). (27) Ibid., 32, 799 (1960). (28) Ibid., 32, 1658 (1960).
RECEIVEDfor review March 3, 1961. Accepted May 4, 1961. Contribution No. 992. Work performed in the Ames Laboratory of the U. S. Atomic Energy Commission.
Estimation of Copper by a Luminescence Activation Method RICHARD C. ROPP and
NELSON W.
SHEARER
Sylvania Electric Products Inc., Towanda, Pa.
b Copper may be determined in solution by adsorption on silver-activated zinc sulfide, followed by heating to induce copper activation of the phosphor. The intensity of the induced green fluorescence, observed along with the original blue fluorescence, is directly proportional to the copper content and may be compared to standards containing known amounts of copper. The method has been used to estimate copper accurately and with good precision, particularly in the 10-p.p.b.’range. The method is selective for copper and is little affected by the presence of other impurities.
be obtained by inclusion of suitable quantities of both silver and copper. The resulting phosphor has spectral properties due to both activators. It is, perhaps, less well known that copper activation has greater stability than silver activation and that copper may take over the activation function
T
determination of copper has been of considerable interest in the past, and literature on the subject is voluminous. Methods include gravimetric (4, electrolytic (IO,II), polarographic ( I S ) , and colorimetric (2, 9, 6) procedures, the method chosen depending on the copper concentration and the form in whieh the copper is encountered. The usual colorimetric procedures have the common characteristic that they are not reproducible when quantities of copper much below 0.1 p.p.m. are encountered. The neocuproine method (14) is more accurate, but time-consuming. The present work describes a method in which copper may be estimated in the range of 0.01 to 500 p.p.m. with speed, accuracy, and good reproducibility. Copper migrates into the zinc sulfide lattice even a t very low temperatures to produce a copper-activated phosphor (18). Double activation may HE
124
ANALYTICAL CHEMISTRY
4000
4500
6000
5500
W A V E LENGTH
,
6000
6500
A.
Figure 1. Fluorescence spectra some zinc sulfide phosphors
of
ical procedure was subsequently developed. REAGENTS AND APPARATUS
A silver-activated zinc sulfide phosphor (Sylvania CR-20) was employed and the potassium silicate solution was a commercial electronic grade (Sylvania PS-6). The desired amount of copper was added to the test systems by adding appropriate aliquota of a standard copper acetate solution which was allowed to age for 5 to 7 days before use, to assure homogeneity. The phosphors were excited by a Tesla spark coil in a spark discharge tube evacuated to an approximate pressure of 0.1 mm. of Hg, or by a source emitting 3650-A. radiation. The luminescent colors were examined using a green filter, Corning Sextant Green-4010, to compare the green intensities of the emission. Alternatively, a plaque tester ( 1 ) was employed, using the untreated ZnS:Ag phosphor as a standard of roniparison. RECOMMENDED PROCEDURE
from silver, if copper is somehow included with silver-activated. zinc sulfide and the mixture reheated. The spectral properties shown in Figure 1 indicate that any such process would produce greener phosphors. By comparison of the color to known standards, the amount of copper can be estimated. In the television tube industry, the slightest contamination by copper produces a “green screen” and great care is exercised to exclude copper in any form. Duplication of the settling system on an experimental scale was found to fulfill the requirements and an analyt-
Any sample containing copper can be analyzed by dissolving the sample and comparing the baked phosphor to standards containing known amounts of copper. The pH following dissolution prior to performing the analysis should be as near to 7.0 as possible, to prevent the silicate solution from reacting with the components. It is necessary to employ the same materials for the preparation of standards as for unknowns.
Five milliliters of O.IN acetic acid are added to 900 ml. of distilled water with stirring (or to the solution containing the copper to be estimated).
