appears in both phases (Equation 3).
+ Y4-
[Fe(SCN)n](3-n)f
-+
Fey-
+ nSCN-
Table I. Precision of Method in Terms of Relative Standard Deviation No. of Re1 std Iron taken, pg detn Iron found, pg dev 84.89 3 83.78 1.3 169.78 3 167.55 1.3 424.45 4 418.88 1.5
(3)
For amounts of iron below 1 mg, a microburet should be used. When the end point approaches, the amyl alcohol layer will be slightly pink. (A few more drops of the titrant will promote complete decoloration.) From this point on, after each drop of titrant, the flask must be agitated vigorously and then allowed to stand for 20 to 30 seconds and the color of the amyl alcohol layer observed. The end point is reached when this is colorless. The result of the determination is calculated from the volume of EDTA consumed.
Table 11. Levels of Interference from Various Cations Iron taken, Diverse ion Iron found, Difference, Pg added fig Pg 169.78 co 16 173.13 1.9 169.78 Ni 25 167.55 1.3 169.78 Mn 44 167.55 1.3 169.78 cu 36 167.55 1.3 Ag 80 167.55 1.3 169.78 169.78 Zn 41 167.55 1.3
z
RESULTS AND DISCUSSION
Good results were obtained with amounts of iron in the range 50 to 1000 pg with almost the same precision. The values of several determinations are presented in Table I. The interference due to the presence of phosphates, fluorides, and oxalates is eliminated by extraction with isoamyl alcohol (5). The small amounts of foreign ions such as Ag+, CuZf, Cozf, ZnZ+,etc., do not interfere as can be seen in Table 11. The method has been successfully used in the determination of iron in samples of coarse salt and calcinated bone meal used as supplements in cattle feeding. Thirty-nine ppm of iron could be determined in a sample of coarse salt. It was also used for the determination of iron in plants. In this case, the previous mineralization was made after Ward and Johnston (@.
The proposed method is probably suitable for determination of iron in foods, feeds, pharmaceuticals, and minerals. ACKNOWLEDGMENT
(5) G. W. Monier-Williams, “Trace Elements in Food,” Chapman & Hall, Ltd., London, 1949, pp 257-261. (6) G. M. Ward and F. B. Johnston, “Chemical Methods of Plant Analysis,” Canada Department of Agriculture; Publication 1064, Feb. 1960.
The author wishes to express his indebtedness to Dr. David Goldstein, Laboratbrio da Produ@o Mineral, Rio de Janeiro, for his experienced help in preparing the final manuscript, RECEIVED for review September 21, 1970. Accepted November 19, 1970.
Confirmation of the High Aromaticity of Anthracite by Broadline Carbon-13 Magnetic Resonance Spectrometry H. L. Retcofsky and R. A. Friedel U. S. Department of the Interior, Bureau of Mines, Pittsburgh Energy Research Center, 4800 Forbes Aue., Pittsburgh, Pa. 15213
ANTHRACITIC COALS are generally considered to be highly aromatic substances. The aromaticity of coal as well as of other carbonaceous materials is defined as the number of aromatic carbon atoms divided by the total number of carbon atoms and is designated fa. Literature values of J;1 for anthracites lie in the range 0.90 to 1.00 ( I ) . A large variety of physicochemical techniques including measurements of sound velocity, heat of combustion, proton magnetic resonance, and graphical densimetric methods have been used to arrive at these values. Detailed discussions of these and other methods of estimating fa for coals have appeared in several books ( 1 4 ) . The main objections to each of these
methods are that none of them yields a truly direct measure of and that all fa’s calculated from the data obtained from the particular measurement involved require a priori assumptions about the structure of coal. The present investigation was undertaken to explore the use of broadline carbon-13 nuclear magnetic resonance (13C NMR) spectrometry of solids in studies of coal structure. For this purpose, spectra of an anthracitic coal and the completely saturated hydrocarbon adamantane were obtained. NMR of solids, a relatively new technique, is potentially a method for the direct determination of aromaticities of coals and other organic materials of limited solubility.
fa
(1) D. W. van Krevelen, “Coal,” Elsevier, Amsterdam, 1961, p 447. (2) H. Tschamler and E. DeRuiter, “Chemistry of Coal Utilization,” SUPPI. v O l . 3 H.H.Lowry, Ed., Wih’, New York, 1963, P
EXPERIMENTAL
1 C JJ.
(3) W. Francis, “Coal,” 2nd ed., Edward Arnold, London, 1961,
720.
(4) H. Tscharnler and E. DeRuiter, “Coal Science,” R. F. G o ~ l d , Ed., Amer. Chem. SOC.,Washington, D.C., 1966, p 332.
NMR spectra were obtained using a Varian Associates Model ~ p - 6 0spectrometer operating at 15,085 M H ~and equipped with a time-averaging computer and Fieldial scanning unit. Broadline detection of the dispersion mode was employed throughout, thus all spectra produced are first derivative curves. The modulation amplitude was 0.156 G
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
485
D O R R A N C E A N T HR A C l T E i
Increasing f i e l d sweeps
Decreasing field sweeps
A D A M A NT A N E
at a sweep frequency of 40 Hz. Chemical shifts are referred to neat carbon disulfide and are designated 6,. In order to minimize errors due to sweep nonlinearities or peak asymmetries, all chemical shift and linewidth measurements are average values from two sets of time-averaged spectra: one obtained while scanning the magnetic field from low to high field, the other while scanning from high to low field. RESULTS AND DISCUSSION
Broadline I3C NMR spectra of Dorrance anthracite (moisture and ash-free analysis: 92.70z C, 2 S 3 z H, 2 . 9 5 z 0, 0.98% N, 0.84z S) are reproduced in Figure 1 (upper traces). Spectra obtained during both increasing field sweeps (738 scans) and decreasing field sweeps (564 scans) are shown. The chemical shift 6, was measured to be 53 ppm, however, errors of several ppm are probable. The spectral linewidth at one-half intensity AH is 1.5 G. Spectra of adamantane obtained during increasing field sweeps (277 scans) and decreasing field sweeps (571 scans) are also shown. For solid adamantane 6, = 163 ppm and AH = 0.8 G. A chemical shift correlation chart indicating the general spectral region of absorption by aromatic and saturated carbon atoms in selected hydrocarbons appears near the bottom of the figure. The chart is based on published data for methyl- and ethylbenzenes (5-7), hydroaromatic hydrocarbons (7), and aromatic compounds containing one fused partially saturated 5-membered ring (7). Chemical Shifts. The chemical shift of the anthracite sample (53 ppm) plus the fact that little if any absorption occurs in the spectral region attributed to resonances of saturated carbon atoms attest to the high aromaticity of anthracite. The purpose of obtaining spectra of solid adamantane was to extend the region of saturated carbon atom
absorption shown in Figure 1 to include the effects of at least some proton dipolar line broadening. Although some overlap with the anthracite resonance is apparent, the amount is small. These results, though qualitative, leave little doubt that anthracitic coals are highly aromatic substances. The present work also suggests that measurements made with more sophisticated spectrometers operating at higher magnetic fields should eventually lead to quantitative determinations of the aromaticity of coals and other solid substances. A second qualitative conclusion concerning the chemical structure of anthracite can be drawn if anthracite’s elemental constitution is viewed in light of the 13C NMR results. The high aromaticity of Dorrance anthracite together with its very low atomic H/C ratio (0.325) can be best explained by assuming the presence of large condensed polynuclear aromatic structures in the coal structure. The ordering of these structures in graphite-like layers may account for the anisotropy of both the magnetic susceptibility (8) and the electron spin resonance g value (9) found for this material. Spectral Linewidths. A large portion of the linewidth in the I3C NMR spectrum of solid adamantane is undoubtedly due to dipolar line broadening by protons in the molecule. Grant (10) has measured the chemical shifts of the two nonequivalent groups of carbon nuclei in adamantane in solution by high-resolution NMR and found them to differ by -9 ppm. No spin-spin coupling constants were measured since proton decoupling was used. Assuming reasonable values of 130 Hz for J13C--H, the total spread of the high-resolution adamantane spectrum can be estimated to be -330 Hz (-0.3 G) at 15.085 MHz. The observed linewidth in the solid is more than twice this amount. McCall and Douglas (ZI) have
( 5 ) P. C . Lauterbur, J. Amer. Chem. SOC.,83, 1838 (1961). (6) W. R. Woolfenden and D. M. Grant, ibid., 88, 1496 (1966). (7) H. L. Retcofsky and R. A. Friedel, “Spectrometry of Fuels,” R. A. Friedel, Ed., Plenum Press, New York, 1970, p 90.
486
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
(8) D. Bivins and S. Ergun, Science, 159,83 (1968). (9) H. L. Retcofsky, G. P. Thompson, and R. A. Friedel, U. S. Bureau of Mines, Pittsburgh, Pa,, unpublished work, 1969. (10) D. M. Grant, Abstracts 6th Experimental NMR Conf., Pittsburgh, Pa., 1965. (11) D. W. McCall and D. C. Douglas, J. Chem. Phys., 33, 777 (1960).
paired spins in the sample. The value of n for Dorrance anthracite has been reported previously (12). Substitution of this value (5.3 X l O I 9 g-l) into the equation yields a theoretical linewidth about twice that observed experimentally at room temperature. It is unlikely that unresolved chemical shifts play an important part since these are known to cover only a small range in the case of polynuclear aromatics (15-17). It is possible, however, that chemical shift anisotropy in the solid may be important. Although it is not possible to estimate this effect, it should again be noted that anisotropies do exist in other magnetic properties of anthracites.
shown the importance of proton dipolar line broadening in the proton spectrum of solid adamantane and found that motional narrowing of the adamantane resonance first - 130 “C. 13CNMR measurements at low temappears peratures would be needed to assess the importance of proton dipolar line broadening on the carbon resonance in adamantane. The linewidth in Dorrance anthracite is not easily explained. Possible contributors to the linewidth include proton dipolar interactions, unresolved carbon chemical shifts, chemical shift anisotropy, and line broadening by paramagnetic impurities (free radicals) which are known to be present in the anthracite (12). The paramagnetic contribution to the linewidth can be approximated using the Equation (13, 14)
-
AH
=
RECEIVED for review October 5, 1970. Accepted November 17, 1970. Reference to trade names is made for identification only and does not imply endorsement by the Bureau of Mines.
3.8yhn
where y is the magnetogyric ratio of the electron, h is the modified Planck constant, and n is the concentration of un(15) T. D. Alger, D. M. Grant, and E. G. Paul, J. Amer. Chern(12) H. L. Retcofsky, J. M. Stark, and R. A. Friedel, ANAL.
CHEM., 40, 1699 (1968). (13) A. Abragam, “Principles of Nuclear Magnetism,” Oxford Univ. Press, London, 1961, p 128. (14) M. N. Alexander,Phys. Reo., 172, 331 (1968).
Soc., 88, 5397 (1966). (16) H. L. Retcofsky, J. M. Hoffman, Jr., and R. A. Friedel, J . Chem. Phys., 46,4545 (1967). (17) R. J. Pugmire, D. M. Grant, M. J. Robins, and R. K. Robins, J. Arner. Chem. Soc., 91,6381 (1969).
New Method for Separation of Americium from Curium and Associated Elements in the Zirconium Phosphate-Nitric Acid System Fletcher L. Moore Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. To ACHIEVE THE SEPARATION of americium from curium in an all-inorganic system is the subject of a continuing study at this laboratory. Current methods based on ion exchange resins or organic solvents suffer from resin degradation, excessive gassing, and solvent instability. Although several inorganic systems (1-3) exist for this separation, they either provide inadequate decontamination, require excessive manipulations, or they are too time-consuming for analytical and process applications. At present the most widely-used method is based on the precipitation of Am(V) as the double carbonate (1). It is unsatisfactory because it requires multiple precipitations involving the use of concentrated potassium carbonate solutions. Various workers have found the subsequent removal of coprecipitated curium and large amounts of carbonate salts to be tedious and difficult. In addition, the carbonate precipitation method for americium cannot be used for the final removal of small amounts of americium from curium. The cation exchanger, zirconium phosphate, was selected as a promising candidate for the separation of americium from curium because it is stable in oxidizing media, strong (1) R. A. Penneman and T. K. Keenan, “The Radiochemistry of
Americium and Curium,” NAS-NS-3006 (1960). (2) F. L. Moore, ANAL.CHEM., 35,715 (1963). (3) H. P. Holcomb, ibid.,36,2329 (1964).
nitric acid, and elevated temperatures; moreover, it exhibits superior resistance to ionizing radiation over the organic ion exchangers and organic solvents. The new method described here is based on the negligible sorbability of Am(V) on zirconium phosphate from dilute nitric acid solutions. Under the conditions used, Cm(II1) and a number of other elements are strongly sorbed. For analytical applications the isotope dilution technique in conjunction with alpha spectrometry is used for an accurate determination of 243Am. EXPERIMENTAL
Apparatus. An internal sample methane proportional counter was used for fission, alpha, and beta counting at voltage settings of 2100, 2900, nnd 4300, respectively. A NaI well-type scintillation counter, 13/4 X 2 inches, was used for gamma counting. A silicon diode detector (3 cm2) coupled to a 256-channel analyzer was used for alpha spectrometry. A glass tube, 5 mm i.d. and 180 mm in length was drawn to a tip at one end. A small glass wool plug was inserted in the tube to retain the support. To prevent disruption of the column, a small glass wool plug was placed on top of the column. Reagents. “OB, 0.01M; “Os, 10M; and (NH&S20s, 0.5M were used. Zirconium phosphate cation exchange crystals, Bio-Rad ZP-1, 50-100 mesh, hydrogen form is available from Bio-Rad Laboratories, 32nd and Griffith Ave., Richmond, Calif. ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
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