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Determination of Microgram Quantities of Potassium by X-Ray Emission Spectrography of Ion Exchange Membranes P. D. ZEMANY, W. W. WELBON, and G. L. GAINES, Jr. N. Y.
General Electric Research Laboratory, Schenectady,
b Potassium in the range 5 to 150 y has been extracted from aqueous potassium chloride solutions and from mica surfaces (where it is present as an exchangeable ion) b y ion exchange membranes and determined by x-ray emission spectrography. A precision to about 15% is attainable.
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extraction of microgram quantities of several elements from solution by means of ion exchange membranes and their subsequent determination by x-ray emission spectrograph3 has been described (4). Potassium, both because of its relatively less favorable distribution in the ion exchanger-solution system and because of the difficulties in determining its characteristic x-rays, presents a more difficult problem. Other considerations, coupled with the paucity of satisfactory methods for determining potassium in the microgram range, honever, led the authors to investigate the possibility of applying the technique to this element. The other methods available (5) offer greater precision and sensitivity in some cases, but concentration by ion exchange and determination in the membrane itself by x-ray emission have important advantages of simplicity and ease of manipulation. Potassium ions are liberated in s o h tion by ground mica ( 3 ) . I n the present investigation the authors were concerned with determining the amounts of potassium derived from various mica samples, presumably by an ion exchange with ions on the surfaces of the crystals. The ion exchange membrane technique provides a method for the determination of interest without subjecting the mica t o severe chemical attack and, as such, forms part of a more extensive study. The details of that investigation are described elsewhere ( 2 ) . HE
EXPERIMENTAL
Potassium chloride solutions &ereprepared by weight from recrystallized, analytical reagent grade potassium chloride and distilled, deionized water (resistivity > 106 ohms) through several dilutions. Polyethylene containers were used to avoid the adsorption which oc-
curs in glass with the dilute solutions used. KO evidence for exchange or adsorption with the polyethylene could be found. The calibration samples contained 0 to 150 y of potassium in 75 ml. of water. Details of preparing the mica samples are given elsewhere ( 2 ) . The muscovite used contains 2.5 meq. of potassium per gram. When ground and graded to 40 to 60 mesh, about 1 sq. meter of surface is exposed, and about 4 peq. of potassium are exchangeable. Appropriate samples of mica were weighed out into polyethylene bottles and 75 ml. of distilled deionized water were added. Rectangles of Salfilm-1 (National iiluminate Corp.), a cation exchange membrane 3.5 mils thick, were cut 22 X 35 mm. to fit the sample holder of the spectrograph. The capacity of these pieces was estimated a t about 58 peq. (measured 0.8 meq. per cc.). The XalElm was equilibrated with hydrochloric acid or cesium chloride solution, and repeatedly washed with the distilled deionized water. To carry out an extraction, one of the rectangles of membrane was placed in the sample for a t least 24 hours. Longer equilibration times (up to 4 days) gave results identical to those obtained after 24 hours. The films were removed, dried in an evacuated desiccator a t room temperature, and suspended in the sample holder of the apparatus by narrow strips of Scotch tape along the edges. The x-ray spectrograph n-as of conventional design ( 1 ) . A Philips tungsten target Type FA-60 tube was used for excitation a t 50 kv. 50 ma. The radiation n-as diffracted from an E D D T crystal, through a parallel plate collimator 20 em. long with 20-mil spacing into a gas flow proportional counter tube (GE SPG-4) using P-10 gas (90% argon, 10% methane). A lj4-mil Mylar window was used on the counter tube. Helium displaced the air in the optical path. A pulse height discriminator (reverter) was used in conjunction with a decade scaler (Berklry No. 2200-2). Counting times were in the range of 100 to 400 seconds, and the accumulated counts ranged from 2000 (for background) to 30,000 counts-i. e., the standard deviation of the statistical counting error was less than 2.3% for any single counting interval. Two sample holders which differed slightly in geometry were used, giving slightly aifferent calibrations.
RESULTS AND DISCUSSION
As an indication of the order of magnitude of the numbers, the background determined was about 22 counts per second (two degrees in 28 above the K , peak, sufficiently higher in wave length to avoid interference from the potassium radiation), and the counting rate for potassium averaged 2.7 counts per second per y above background in the hydrogen membranes. The blank membranes exhibited count rates corresponding t o about 5 7 of potassium. Beyond about 2 peq. the exchange was not complete with hydrogen membranes-Le., some potassium n as left in solution in the calibration standards, 17-hich could be removed by a second membrane. Thus a t 3y0 saturation of the membrane, extraction is not complete. For a typical calibration point in this range, a solution containing 100 y of potassium was extracted with a piece of the membrane. A second extraction with a fresh membrane showed that about 16 y of potassium had remained in the solution after the first extraction. This mould indicate that the selectivity coefficient for the potassium-hydrogen exchange is about 0.2, if it is assumed that all the hydrogen ion in solution is that exchanged from the membrane by the potassium. Actually, this gives only a loner limit for the hydrogen ion concentration; hence this is a lower limit for K D . Potassium-hydrogen exchanges on strong acid exchangers have been studied by several workers (6). The selectivities observed, which are strongly dependent on resin cross linking and other factors for which the present authors have no estimates for their system, range from 0.8 upward. The results obtained for the extraction of potassium from potassium chloride solutions with hydrogen Nalfilm-1 are shown in Figure 1. In Figure 2 are some data for the extraction of potassium from mica samples. The solid line indicates the theoretical amount of exchangeable potassium ( 2 ) . In addition, some extractions were carried out using membranes saturated with cesium to avoid the liberation of free acid in the solution from the memVOL. 30, NO. 2, FEBRUARY 1958
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Figure 1. X-ray emission of potassium extracted from potassium chloride solutions by hydrogenNalfilm- 1
0
1
I
50
loo PO K
1 IM
Figure 4. X-ray emission of potassium extracted from potassium chloride solutions by hydrogen Amberplex C-1
f
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20
(UWTAYINATION?
P 0
V
0
10 I
K1
I
20
1
30
p i K+ mUN0
Figure 2. Potassium ion extracted from 40- to 60-mesh mica by hydrogen Nalfilm-1 as determined by x-ray emission
brane. Figure 3 shows the calibration data using this variation. Here we obtain only about 1.1 counts per second per y of potassium, as might be expected, because of the presence of cesium as a n absorber. Disregarding the point in Figure 3 marked “contamination” (such contamination might readily occur-e.g., from cigarette smoke in the air during preparation of this sample) it is apparent that the precision of the determination is well within 5 1 5 % . The capacity can be increased, using this ion exchange material, by increasing the thickness, but at the expense of sensitivity. At 3.73 A., the wave length of K a for potassium, the absorption coefficient of carbon, which is the chief absorbant in the membrane,
0
10
20
30
was lower (-0.5 count per second per y) because of the increased thickness, but, as shown in Figure 4, the linear range extended to somewhat higher amounts of potassium. The greater thickness and attendant lower tendency t o curl makes this membrane somewhat easier to handle in the spectrograph, and may contribute to the higher relative precision of the results by reducing variations in geometry.
PP K
Figure 3. X-ray emission of potassium extracted from potassium chloride solutions by cesium Nalfilm-1
is approximately 62; the thickness required to attenuate the potassium x-rays by is 0.01 em. Hence a second layer would have half the responsethat is, about 0.85 counts per second per y of potassium-but would increase the background. Another alternative is to use membranes of higher capacity, with the same thickness, which would increase the useful range of the method. Some experiments were made with a higher capacity membrane, Amberplex C-1 (Rohm & Haas Go., Philadelphia), whose capacity is about 1.8 meq. per cc. and which was available a t a thickness of about 27.5 mils. The sensitivity
ACKNOWLEDGMENT
The authors are indebted t o W. T. Grubb for providing the membrane samples and to C. P. Rutkowsi for assistance in some of the experiments. LITERATURE CITED
Friedman, H., Birks L. S., Brooks, E. J., A.S.T.M. d p e c . Tech. Pub. 157. 2 (195.1).
Gain€ 140 Gardi
(4) Grubb, W.T., Zemany, P. D., Nature 176, 221 (1965). (5) Kirk, P. L., “Quantitative Cltramicroanalysis,” p. 143, Wiley, S e w York,
1950. (6) Reichenberg, D., MacCauley, D. J., J. Chem. SOC.1955, 2741. RECEIVED for review May 4, 1957. Accepted August 26, 1957.
Chemical Identification of Halide and Sulfate in Submicron Particles BARBARA J. TUFTS and JAMES P. LODGE, Jr.’ Cloud Physics Project, Department of Meteorology, University o f Chicago, Chicago 37, 111.
b Individual halide particles down to 50 A., and sulfate particles down to 1000 A., may be identified specifically by modified spot tests in the electron microscope. Each particle gives a characteristic spot reaction on the substrate. The size of the resulting spot i s approximately a linear function of the size of the original particle.
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ANALYTICAL CHEMISTRY
Hence, the particle size distribution, as well as particle number, may be determined for each species. The proportionality factor for each i s determined.
creasing interest in recent years. Because the action of whitecaps in the sea produces large numbers of halide particles, the halide content of air has been taken as an index of its maritime or con-
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1 Present address, Robert 4.Taft Sanitary Engineering Center, U. S. Public
of determining quantitatively the particulate constituents of the atmosphere has been of inHE PROBLEM
Health Service, Cincinnati, Ohio.
