(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
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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 a n 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).
r . 1~ i i gritnis of silver-activated zinc sulfide phosphor are added with stirring. Then, 54 nil. of potassium silicate solutioii ar(' added and stirring is continued for 10 minutes. The phosphor is allowed to settle for 20 minutes and the supernatant liquid partially decanted. The phosphor is filtered, washed with 100 ml. of deionized water, and baked for 1 hour a t 525' C. in a 30-ml. covered silica crucible. The baked phosphor is visually compared in an evacuated demountable tube to a standard sample by placing a small amount of the test sample on the standard and pressing with a sniootli steel spatula.
The concentration limits of the procedure are 0.01 to 500 p.p.m. of copper, based on phosphor weight. Copper standards may be prepared a t any concentration within the limitations of the procedure by following the recommended method. When a prolonged analysis has been planned, it has been found desirable to increase proportions to assure an adequate supply of phosphor standards. Standards have been prepwed by careful weighing and thorough blending of high and loit- copper standards. The blend mixture method :illons the preparation of relatively fen. standards which can be used to prepare a n incremental series over the entire copper concentration range, if so desired. RESULTS
The coilcentration of constituents in solution affected both precision and :muracy of the method. Potassium Silicate and Acetic Acid.
Tests which involved the omission of each of the components in turn during the settling and filtering step showed t h a t the presence of the potassium silicate is essential t o obtain copper activation of the phosphor, whereas omission of acetic acid results in a partial activation. In the absence of potassium silicate, no copper activation was obtained. A minimum concentration of silicate was needed, but too high a eoncentration inhibited the removal of copper from solution. This is illustrated in Table I. Analysis of the solution after removal of the suspended phosphor gave varied results. In the presence of silicate, no copper was detected in the decanted solution; in the absence of silicate, practically all of the added copper was detected. It was concluded that copper adsorbs on the phosphor surface only in the presence of silicate; however, the combination of both silicate and acetic acid is required to obtain quantitative results. Effect of Heating. Copper activation of ZnS:Ag may be obtained a t very low temperatures (12). Tests
Table I. Effect of Settling System Components on Copper Removal 0.1N
Potassium Silicate] 54
Acetic Acid, M1. ...
54 125
5.0 5.0 5.0
~ 1 . 4
F I R I N G TIME, M I N U T E S
...
Figure 2. Effect of firing time on relative green brightness a
+
where I is increase in green intensity and c is concentration in parts per million of copper. Accuracy and Reproducibility. The neocuproine method of determining
%b
86.3 0 98.0 83.3
Specific gravity. 1.270 at 15" C.
yo KzO = 9.5; yo Si02
a t a constant copper level (0.1 p.p.m.) showed that the luminescent response was most reproducible when the ZnS: Ag (Cu) was ignited a t 525' C. (980' F.). The effect of temperature on fluorescence was gradual, and only a t the higher temperatures was a deviation noted due to overfiring. The firing time was somewhat more critical, as shown in Figure 2. Although higher green brightness n-as obtained a t 15 minutes, the results were more reproducible a t 40 to 70 minutes. A firing time of 45 to 60 minutes a t 525" C. was finally chosen and remained constant in all succeeding tests. Effect of Copper Concentration. When copper was added in the range 0.1 to 1000 p.p.m. (based on the solution) a continuous increase in green intensity \vas observed. The blue component of the emission tended to decrease with increasing copper concentration, n hile the green component tended to increase. The relative increase in green response using 3650-A. irradiation is shown in Figure 3, compared to green response of the original phosphor. Below concentrations of 1 p.p.m. of copper, the increase in green response is nearly linear on a loglog plot. The ielation may be represented by: log I = 0.787 log c 1.016 (1) (1.0 > c > 0.01 p.p.m.)
cu Removed,
* As
= 20.1.
found in hosphor (10 p.p.m. Cu originally addel). Table II. Comparison of Neocuproine and Luminescence Activation Methods for Recovery of Copper
CU Added, Cu Found, 70 X "/o X 10-6 1st day 2nd day' Luminescence Activation 41)
4- .1. )
3.5 3.0 2.5
3 .O 3.0 2.0
2.0
4.0 3.5 3.0
2.0
2.0
1.5
Neocuproine 3.9 3.3 2.6 2.2 1.9
3.3 2.6 2.1 2.1 2.1
4.0 3.5 3 .O 2.5
2.0
copper is a quantitative method for copper determination by which more than 98% of the copper can be extracted from a known copper solution (14). The copper is then determined colorimetrically. In Table I1 are shown results comparing the luminescence-activation method with the more timeconsuming neocuproine method for copper in commercial lots of potassium silicate solution. I t may be concluded that the luminescence activation method is as reproducible as the neocuproine metbod, if not more so. The sensitivity of the procedure is estimated to be about 0.007 p.p.m. of
Figure 3. Relative green plaque brightness as a function of copper content
.'
01
1.0
IO
100
1000
C u ADDED, P.P.Y.
VOL. 33, NO. 9, AUGUST 1961
1241
copper and the reproducibility is estimated to be 97%. The precision is about 2 to 3 in the ranges of 0.01 to 500 p.p.m. Effect of Interfering Ions. The estimation of copper by this method is unique, since as far as it’ is known no other elements or ions will activate the phosphor a t the conditions used. However, oxidizing agents in solution would destroy the phosphor by being absorbed and acting on the phosphor during the firing step. The presence of iiilrate, brcliilidc: chloride, sulfate, and hj-drogen proside below 0.5% (based on weight of silicate in solution) could bc tolerated, but above this level, noticeable interference occurred. DISCUSSION
Copper may be estimated in the rttnges of 0.010 to 500 p.p.m. by the use of the act,ivation analysis procedure. The advantages of t’his test lie not primarily in its ektreme accuracy at any given copper content’, but in its sensitivity at copper concentrations lower than the sensitivity of the usual colorimetric methods a,nd in its utility and convenience. Copper is removed from solution during the settling and filtering process, only in the presence of silicate, and not quantitatively, except in the presence of both silicate and acetic acid. I t is possible that the mechanism of copper removal from solution follows
that proposed for the formation of television tube screens, particularly since a duplication of the settling system has been employed. Hazel and coworkers (6, 7 , 8) have shown that the silicate adsorbs on the phosphor surface, which acquires a negative surface charge. The silicate-bearing phosphor particles settle to form a screen, but polymerization and bonding of the silicate to form zt stable film do not occur until the zeta potentials of the glass and phosphor surfaces are lowered to a sufficient degree by the presence of a n ion such as acetate. The removal of copper from solution in the presence of silicate could be explained by formation of a coppersilicate complex which is adsorbed on the phosphor surface. Experimentally, no soluble copper was found in the solution after removal of the phosphor particles. However, copper was detected in the settling solution, where no silicate was employed: The luminescence-activation analysis was developed and applied as a routine method for analyzing a commercial grade of potassium silicate solution. It has, however, been demonstrated that the method may be used to determine soluble copper in any solution, provided the copper concentration and/ or electrolyte concentration is not too high.
ACKNOWLEDGMENT
The authors are particularly indebted to A. B. Davis (deceased) and G. V Potter for their advice and encouragement. REFERENCES
(1) Butler,
K. H., Mooney, R. W., Sylvania Technologist 9, 121 (1956). (2) Chilton, J. M., ANAL. CHEM. 25, 1274 (1953); 26, 940 (1954). (3) Delavault, R. E., Zbid., 24, 1229
(1952) (abstryt). (4) DuvaI, C., Inorganic Thermogravimetric Analysis,” p. 237, Elsevier, Amsterdam, 1953. (5) Edelberg, R., Hazel, F. Trans. Electrochem. SOC.96, 13 (1949). (6) Feigl, F., Caldas, A., Anal. Chim. Acta 8 , 117 (1953). (7) Hazel, F., Schnable, G. L., J . Electrochem. SOC.100, 65 (1953). (8) Hazel, F., Schnable, G. L., J . Phys. C h a . 5 8 , 812 (1954). (9) Leverenz, H., “Introduction to the Luminescence of Solids,” p. 246, W h y , New York, 1950. (10) Norwitz, G., A N A L . CHEM. 21, 523 (1949). (11) Parks, T. D., Lykken, L., Zbid., 22, 1503 (1950). (12) Pringsheim, P., “Fluorescence and Phosphorescence,” p. 522, Interscience, New York, 1949. (13) Reynolds, C. A,, Rogers, L. B., AXAL.CHEM.2 1 , 176 (1949). (14) Smith, G. F., McCurdy, W. H., Zbid., 24, 371 (1952). for review December 9, 1960. RECEIVED Accepted May 5, 1961.
Spectrochemical Detection of Nonmetallic Elements C. E. HARVEY C. E . Harvey Associates, P . 0. Box 175, Pullman, Wash.
J. W. MELLICHAMP
U. S.
Army Signal Research and Development laboratory, Fort Monmouth,
A basic technique is given that will permit the simultaneous detection of the halogens, carbon, sulfur, phosphorus, and selenium in a single sample. Strong emphasis is placed on the use of standard instrumentation and on holding auxiliary equipment to a minimum of cost and complexity. In addition, a minimum amount of sample preparation and over-all operational time is sought. The method involves high-voltage spark excitation at reduced pressures. The sample is pressed into a pellet with silver powder and a silver rod is used for the counter-electrode. The technique is adaptable to both macro samples for trace impurities ond micro samples for major constituents.
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ANALYTICAL CHEMISTRY
N
N. 1.
ELEMENTS present in chemical compounds, usually as a constituent of the negative radical or anion, can be determined by spectrochemical techniques. However, these elements are not determined by conventional spectrochemical methods and some of these methods require e q u i p ment not normally found in the laboratory. Other preparations and conditions are necessary before these elements can be excited and their emitted spectra identified and evaluated. An important consideration of all methods is the selection of proper excitation conditions sufficient to obtain emission spectral lines in a n accessible analytical region of the spectrum. Sparks with high average excitation (6,8), powerful low-tension sparks (7, 10, 11), hollow cathode excitation (f, 9), and highfrequency excitation at low pressures ONMETALLIC
(3, 13) have all been used successfully. Spectrographs operated in a vacuum or inert atmosphere permit the use of sensitive spectral lines below 2000 A. (6, 12). A general discussion of the determination of nonmetallic elements has been given (2). Elements considered to be nonmetallic from the standpoint of spectroscopy are those with excitation potentials greater than 6 e v. Those commonly encountered are the halogens, carbon, sulfur, selenium, phosphorus, and gaseous elements such as hydrogen, oxygen, and nitrogen. I n the technique described in this article the additional equipment needed is a relatively simple system whereby sample materials can be excited at reduced pressures. Sample preparation has been kept to a minimum. The practicality of this method makes
