Table I. Recovery of Selenium from Synthetic Standards Se Se Foreign found, recovered, Se present, % elements, % % % 95.0 100.00 5.0Na 95.00 99.86 30.0 Na 69.9 70.00 95.0 100.00 5.0 A1 95,OO 95.0 100.00 5.0Si 95 .OO 98.89 5.0Ca 94.9 95.00 100.11 5.0Cr 95.1 95.00 95.1 100.11 5.0Mn 95.00 95.1 100.11 5.0Fe 95.00 95.0 100.00 5.0Co 95.00 94.9 99.89 5 . 0 Ni 95.00 100.00 5.0 Cu 95.0 95,OO 95 .O 100.00 5.0Zn 95 .OO 100.21 5.0 As 95.2 95.00 90.1 100.11 10.0 As 90.00 94.9 99.89 5.0Sb 95.00 99.89 5.0Te 94.9 95.00 97.8 99.80 2.0 Hg 98.00 99.89 5.0Pb 94.9 95.00 97.9 99.90 2.0 Bi 98 .OO Average % recovery: 99.98 Table 11. Precision. Comparative Results by Differential Reduction Atomic Absorption Determination and Classical Gravimetric Method Gravimetric Differential reduction deterA.A. determination mination Material No. of determinations, 24 IZ = 24 High purity selenium, 99.99% Mean Se %, 99.98 102.1 Std dev, 0,077 0.449 Crude Se, Lot No. 41-E No. of determinations, 24 II = 24 Mean Se %, 98.82 100.5 Std dev 0.095 0.660
Silver and tin were the only interfering elements after purification by hydrazine sulfate. The interference from silver was eliminated by precipitation with a small excess of hydrochloric acid before hydrazine reduction. Tin interferes by forming metastannic acid, which coprecipitates with selenium prior to hydrazine reduction. Small amounts of tin salts (up to 10 mg) and metallic tin (which does not dissolve) can be tolerated. The atomic absorption measurement of selenium in the final 5 v/v sulfuric acid solution is interference-free because all foreign salts with the exception of tellurium have been removed during purification. Tellurium up to 500 pg/ml causes no interference. Accuracy and Precision. Accuracy of the procedure was evaluated by recovery experiments on synthetic samples. Recovery was established by dissolving high purity selenium to which calculated amounts of impurities were added, and treating the samples as described (Table I). The average recovery was 99.98 %. Precision was evaluated on selenium samples of different purity (Table 11). The samples were also analyzed by the classical gravimetric procedure (5). In precision, the recommended method is superior to the gravimetric procedure. Standard deviation for sets of 24 results obtained in 12 separate runs made during two months was 0.095% for the differential procedure compared to 0.660 for gravimetry (Table 11). Other Applications. The procedure was applied successfully to selenium dioxide, sodium selenite, and selenium mixtures. The use of high purity selenium standards makes the procedure applicable to the indirect determination of the purity of hydrazine sulfate. ACKNOWLEDGMENT
The author is indebted to I. Krzisnik who conducted most of the experimental work. The direct differential reduction may be applied only to selenium materials free from substances reducible by hydrazine sulfate. In all other cases purification is necessary. A set of synthetic samples, each containing 95% selenium of a foreign element were analyzed by the proposed and procedure. The elements studied were Na, Al, Si, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Ag, Sn, Sb, Te, Hg, Pb, and Bi.
5z
RECEIVED for review February 18, 1971. Accepted May 12, 1971. (5) W. W. Scott. “Standard Methods of Chemical Analysis.” ‘Fifth Edition, Vol. I, D. Van Nostrand Company, Inc.,-New York, N.Y., 1939, p 787.
Filters for X-Ray Spectrometry Prepared by Thin-Layer Electrodeposition Basil H. Vassos, Roland F. Hirsch, and Donald G . Pachuta Chemistry Department, Seton Hal[ University, South Orange, N . J . 07079
FILTERS serve several purposes in analytical methods utilizing X-rays. When the filters are placed in the primary beam of an X-ray spectrometer, they help to improve sensitivity by reducing the background (1-3). Filters are used in energydispersive X-ray spectrometry to simplify mathematical anal(1) R. Jenkins and J. L. de Vries, “Practical X-Ray Spectrometry,” 2nd ed., Springer, New York, N. Y., 1969, p 175. (2) J. T. Gilmore, ANAL.CHEM., 40,2230 (1968). (3) S . Caticha-Ellis, A. Ramos, and L. Saravia, Advan. X-Ray Anal., 11, 105 (1968).
ysis of the spectra (4). Balanced pairs of filters permit monochromatization of X-ray beams for diffractometry (5,6). These filters must be thin if a satisfactory amount of the radiation of the desired wavelength is to be transmitted. Furthermore, the thickness of the filters must be carefully controlled if they are to serve as balanced pairs. Among the ~~
(4) E. P. Berth, “Principles and Practice of X-Ray Spectrometric Analysis,” Plenum Press, New York, N. Y.,1970, pp 258ff. ( 5 ) W. Bol, J. Sci. Instrum., 44, 736 (1967). (6) M. Berman and S . Ergun, Rev. Sci. Insfrum., 41, 870 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, S E P T E M B E R 1971
1503
Figure 1. Thin layer electrolysis cell A . pyrolytic graphite cathode; B, phase separation paper mask; C, filter paper; D,anode of metal being deposited; E, electrolyte
methods for preparation of X-ray filters described in the literature are pack-rolling metal foils (9,spreading a film of a dispersion of the powdered filter material in a plastic binder (7), and pressing the filter material with a binder into a slab (6). These methpds of preparing X-ray filters have several disadvantages. Some metals are brittle and others are difficult to roll to the desired thickness. The range of compounds which can be incorporated into films or slabs with binders is greater, but there still is a problem in obtaining uniform films (7). This report describes a new method for obtaining X-ray filters: plating the desired element onto a thin disk of pyrolytic graphite. The filters prepared by this method are reproducible and uniform in thickness and stable. The support material is transparent to X-rays, unlike other plating materials. The use of the new filters in X-ray spectrometry is discussed. EXPERIMENTAL
Preparation of Plated Filters. Thin sheets of pyrolytic graphite were cleaved from 2.5 X 2.5-cm blocks. The sheets were further reduced in thickness by polishing them on fine sandpaper. The final thickness was about 0.25 mm. The metal was deposited by thin-layer electrolysis in the arrangement shown in Figure 1. The electrolysis cell consists of a sheet of filter paper (Schleicher and Schuell No. 589 Black Label) impregnated with electrolyte. Around the edges of the cathode is a piece of phase separation paper (Schleicher and Schuell No. 2498). This paper has a hole cut in it in the shape desired for the deposit. Being water repellent, it provides a sharply defined boundary around the deposit and, at the same time, prevents any electrolyte from leaking out of the cell. The assembly was laid upon a graphite block serving as contact for the anode and a stainless steel rod was pressed against the graphite electrode serving as electrical contact for the cathode. The electrolytes used were: 100 g/l. CuSOa.4Hz0 and 20 g/l. concentrated HzSOa for copper, and 300 g/l. NiSO,, 7Hz0, 50 g/I. NiC12.6Hz0, and 1 g/l. boric acid, adjusted to pH 4, for nickel. The metal was plated at constant current, using a Sargent Model IV Coulometric Current Source. During electrolysis an equal amount of the metal dissolves from the anode. An electrodeposition rate of 0.1 microequivalent/cm2/sec was usually found to be satisfactory. After deposition was completed, the graphite sheet was rinsed with deionized water and air dried. The deposit was protected from abrasion by coating it with a layer of Krylon 1301 spray (8). This coating material does not attenuate the X-ray beam noticeably. The filters were tested in a General Electric XRD-5 X-ray spectrometer. The standard sample holders were modified, as shown in Figure 2, to accept the graphite-metal filters. (7) F.0.Halliday et ul., J. Appl. P h p . , 38, 1874 (1967). (8) B. H. Vassos, F. J. Berlandi, T. E. Neal, and H. B. Mark, Jr.,
ANAL.CHEM., 37,1652 (1965). 1504
Figure 2. Modified sample holder for General Electric XRD-5 X-ray spectrometer A , clip to hold primary beam filters; B, aluminum support
RESULTS AND DISCUSSION One of the advantages of electrodeposition as a means of preparing X-ray filters is that the thickness of the metal film or foil can be adjusted to any desired value by choosing the proper current and time of electrolysis. The thickness ( T , in mg/cm2) of the deposit is given by
T
=
M / A = Wtl/nFA
(1) where M is the weight of metal deposited in milligrams, A is the area of the deposit in cm2, W is the gram atomic weight of the metal, t is the deposition time in seconds, I is the (constant) current in milliamperes, n is the number of electrons required to deposit one metal atom, and F is the faraday (96,500 coulombs/equivalent). The length of time required. to yield the desired thickness of the filter is t = TnFA/WI (2) For example, to obtain a 1.00-cm2 copper filter with a thickness of 25.0 mg/cm2,at a current of 10.0 mA (about 0.1 microequivalent/sec) requires t =
25.0 X 2 X 96,500 X 1.00 63.5 X 10.0
=
7600 seconds
(3)
The validity of these calculations depends on two factors. First, the current efficiency (the fraction of the current which actually produces the desired electrolysis reaction) must be 100%. This is true for the metals studied under the conditions given in the Experimental section of this paper, as well as for many other metals, for example, cobalt, gold, and silver (8). Second, the deposit must be uniform in thickness, with no significant variation from one point to another. The thinlayer electrolysis cell used in this work assures uniformity of the deposit. To show this, a 1.5 mg/cm2 electrodeposited copper filter was placed in the sample position in the spectrometer under a mask with a 4-mm2 opening. Ten-second readings at 28 = 45.0 taken with various portions of the filter exposed to the X-ray beam gave 12,310 f 260 counts. This is a relative standard deviation of 3~2.1%,compared to the counting precision of about 3~0.9%. The variation in the filter thickness is thus at most about i1.5 %. (A thick piece of copper sheet in the same sample holder gave 150,600 counts in ten seconds. The filter tested for uniformity was thus far from the critical thickness beyond which any filter would appear uniform regardless of how much the thickness actually varied.) Discussion of the Electrodeposition Technique. For thin deposits (under 50 microequivalents/cmz), one can dispense
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
41.8
37.5 28
Figure 4. Effect of pyrolytic graphite on zinc X-ray spectrum
40
39
38
37
36
A , no graphite in beam; E , 0.25 mm thick graphite disk in beam
35
20 Figure 3. Effect of copper primary beam filter thickness on X-ray spectrum Numbers indicate the thickness of the filter in mg/cm2. The spectra are for a 0.001Mmercuric nitrate sample solution
with the phase separation paper mask. The simple sandwich of metal-paper-graphite is very easy to handle, and the deposits are homogeneous. When thicker deposits are made, they tend to include fibers from the filter paper. In this case, the use of the mask is essential. For very thick deposits evaporation of the electrolyte is a problem, since only a few drops of electrolyte are used. An inverted plastic cup was used successfully to cover the electrolysis cell to minimize evaporation. In some cases, however, it was advisable to demount the cell and add a drop of electrolyte every two hours or so. If the current efficiency is not loo%, some water may be electrolyzed. This happens when high current densities are used. For nickel, the current had to be reduced to 0.01 microequivalent/cm*/sec in order to eliminate this problem entirely. The occurrence cf water electrolysis can easily be detected by the presence of bubbles and constitutes a satisfactory test of the current efficiency. For those metals which cannot be readily deposited, there is another alternative to the conventional techniques of X-ray filter preparation. An ion of the element which will act as the X-ray absorber can be exchanged onto a cation or anion exchange membrane or filter paper. Campbell, Green, and coworkers (9, 10) have used ion exchange papers to collect traces (9) W. J. Campbell, E. F. Spano, and T. E. Green, ANAL.CHEM., 38, 987 (1966). (10) T. E. Green, S. L. Law, and W. J. Campbell, ibid., 42, 1749 (1970).
of ions for determination by X-ray spectrography. If the ion exchanger is loaded with an ion and then placed in the primary or secondary beam, it acts as an effective filter for the X-radiation. We have obtained satisfactory results with zinc and copper filters prepared from Ionac MC-3470 cation exchange membrane. Since the thickness of the filter is limited to those values that result using one or two or three layers of membrane, the technique is less versatile than electrodeposition. It may be useful when filters include elements such as Rb, Cs, Ba, Th, U, or Se, which are difficult to deposit electrochemically. An example of the use of electrodeposited filters is given in Figure 3, which shows the determination of mercury using its La, line. The copper filters in the primary beam serve to reduce the amount of scattered radiation from the X-ray tube. The degree of attenuation depends on the thickness of the filter. The ease of preparing a filter of exactly the desired thickness by the electrodeposition technique is therefore a significant advantage when trying to achieve the optimum signal-to-background ratio for an analysis. The same advantage holds for the preparation of balanced filters for monochromatization of X-ray beam. For example, copper-nickel filters were successfully prepared for use with zinc K a radiation. Graphite as a Filter Support. Pyrolytic graphite has several advantages as a support for the metal film which acts as the filter. It is easy to prepare flat plates of pyrolytic graphite by cleavage and a small amount of polishing. Thin plates (0.25 mm in this work) of the graphite are strong, chemically inert, and unaffected by the X-ray beam. As shown in Figure 4, the graphite itself reduces the X-ray beam intensity only slightly, hardly by 5 for the zinc radiation. If this loss were significant, thinner plates (0.1 mm) could be cleaved and used as filter supports with only
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
1505
slightly greater care in handling needed to prevent breakage. In addition, pyrolytic graphite is highly pure carbon and therefore does not itself add any background radiation to the X-ray spectrum. These properties are all in sharp contrast to those of other possible support materials for electrodeposited filters.
RECEIVED for review April 12,1971. Accepted May 28,1971. This work was supported by Undergraduate Research Participation Grant No. GYO-7516 from the National Science Foundation, D. G . P. was a National Science Foundation Undergraduate Research Participant, 1970, at Seton Hall University, while a student at Montclair State College.
Hydrated Porosijr of Macroreticular Cation Exchange Resins via Nuclear Magnetic Resonance L. S . Frankel Rohm and Haas Company, 5000 Richmond Street, Philadelphia Pa. 19137
CONVENTIONAL GEL ION EXCHANGE resins consist of a continuous network of quasi-homogeneous copolymer (1). Macroreticular resins have two discrete phases, a gel phase as described above and a phase composed of large pores or voids which are occupied in the hydrated state by water molecules (2). One of the most important physical characteristics of a macroreticular ion exchange resin is its porosity, or the fraction of the total volume of the resin occupied by the pores. The porosity is conventionally obtained in the dry state via a mercury porosimeter or a helium densitometer (2). However, of greater interest is the porosity of the hydrated resin and a subsequent comparison with the porosity of the dry resin. There is to our knowledge no simple way of determining the hydrated porosity. The determination of the hydrated porosity requires knowledge of the distribution of the total water between the gel phase and the pores. The only significant contribution to this problem we are aware of utilizes a mercury porosimeter and studies the porosity as a function of hydration. These results have been interpreted to indicate that the hydration of the gel phase is complete prior to any water entering the pores via capillary action (2). Our attention was called to this problem during our study of the nuclear magnetic resonance (NMR) spectra of ion exchange resins (3-16). We wish to (1) W. Riernan 111 and H. F. Walton, “Ion Exchange in Analytical Chemistry,” Pergamon Press, New York, N. Y., 1970, p 13. (2) K. A. Kun and R. Kunin, J . Polym. Sci., Part C , 16, 1457 ( 1967). (3) J. E. Gordon, J. Phys. Chem., 66, llSO(1962). (4) D. Reichenberg and I. J. Lawrenson, Trans. Faraday SOC.,59, 141 (1963). (5) R. H. Dinius, M. T. Emerson, and G. R . Choppin, J. Phys. Chem., 67, 1178 (1963). (6) J. P. devilhers and J. R. Parrish, J . Polym. Sci., Parr A , 2, 1331 (1964). (7) T. E. Gough, H. D. Sharma, and N. Subramanian, Can. J. Chem., 48, 917 (1970). (8) R. W . Creekmore and C. N. Reilley, ANAL. CHEM.,42, 570 (1970). (9) L. S.Frankel, Can. J. Chem.,48,2432 (1970). (10) R. W. Creekmore and C. N. Reilley, ANAL.CHEM.,42, 725 (1970). (11) A. D&covA, D. Dosko&lovB, g. SwvEik, and J. Stamberg, J. Polym. Sci., Part B, 8, 259 (1970). (12) L. S. Frankel, ANAL.CHEM.,42, 1640 (1970). (13) D.G. Howery and M. J. Kittay, J. Macromol. S i . , Part A , A(4), 1003 (1970). (14) D. G. Howery and B. H.Kohn, Anal. &IC., 3, 89 (1970). (15) H. Sternlicht, G. L. Kenyon, E. L. Packer, and J. Sinclair, J. Amer. Chem. Sac., 93, 199 (1971). (16) H. D. Sharma and N. Subrarnanian, Can. J . Chem., 49, 457 (1971). 3506
show how the hydrated porosity may be obtained from the NMR spectra and the resin moisture holding capacity. Data for a gel resin and a macroreticular resin, both of which contains 5 % DVB cross-linking, are reported. EXPERIMENTAL
All measurements were made on a Varian H.R. 60 spectrometer operating at 56.4 MHz. The true density or skeletal density of the dry macroreticular copolymer was obtained on a helium densitometer. The apparent density or the density of the entire mass of dry macroreticular resin was determined in a modified mercury porosimeter (2). Experimental bead copolymers of both the gel and macroreticular type were prepared using commercial grade monomers. The divinylbenzene contained 58.2% DVB as a mixture of ca. 70% m- and 30% p-isomer. The major impurities in the DVB were isomers of ethylvinylbenzene. A stock monomer solution containing 5 % by weight of DVB and 95% by weight of styrene and ethylvinylbenzenes was used for the synthesis of all polymers. The copolymers and resins were prepared by standard techniques (17) DISCUSSION AND RESULTS
The NMR spectra of ion exchange resins will generally show sets of peaks. One peak originates from solvent or counter ions inside the ion exchange resin (interior peak); the other (exterior peak) is from the molecules in the volume of the NMR tube not occupied by the resin beads (void volume of column). The solid resin backbone does not contribute any peaks to the spectrum since its molecular motion is highly restricted. In several previous reports on gel resins, the chemical shift of the hydrogen and sodium form have been shown to be linear functions of the internal molality, f%, of the resin, fir = (QW) (100 - % H20)/% H20. QW is the dry weight capacity of the resin and H?O is the weight per cent water or moisture holding capacity. Molal chemical shifts in the hydrogen form of 0.287 ppm (6), 0.321 ppm (7), and 0.337 pprn (8) have been reported. These values are slightly less than that reported for the effect of hydrogen ion on water, 0.344pprn (18). The molal chemical shift for the 5% DVB gel resin was 0.339 ppm and for the 5 % DVB macroreticular resin, 0.345 ppm. A similar analysis is applicable to the sodium form although the analytical sensitivity is much less. (17) J. A. Marinsky, “Ion Exchange,” Marcel Dekker, New York, N. Y., 1969, Chapter 6. (18) J. C. Hindman, J. Chem. Phys., 36, lo00 (1962).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
