t o a great extent by the target life. As the target life decreases, the neutron output decreases also. A greater variation can be expected due to the decreased activity measured. Errors due to neutron flux inhomogeneity were kept to a minimum by using standards having the identical geometry as the samples. In addition, the matrix material and form of the sample were the same as that of the standard. Rotation of sample and standard during irradiation and flux monitoring using a counter close to the irradiation station further reduced the errors caused by flux variation. No interferences were encountered during the analysis. However, chromium, if present in large concentrations, can cause some interference in the vanadium analysis through a
W r (n, cr)"Ti reaction. However, 54Crhas a low abundance and the cross section for the reaction is small, so that this interference is unlikely to be serious. ACKNOWLEDGMENT
The authors thank E. W. Lanning and R. P. Weberling for performing the comparative chemical analyses.
RECEIVED for review March 7, 1968. Accepted April 16,1968. Presented a t the 19th Pittsburgh Conference o n Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1968.
Determination of Parts-per-Million Quantities of Plutonium-236 in Plutonium-238by Alpha Pulse Height Analysis Mary Lou Curtis Monsanto Research Corporation, Mound Laboratory, Miamisburg, Ohio
P L U T O N I U Mproduced -~~~ by irradiation of neptunium-237 contains small amounts of plutonium-236, the decay product of neptunium-236, produced by (n,2n) or (7,n) reactions in the neptunium-237. The plutonium-236 (2.86 yr) decays to uranium-232 (72 yr) and eventually to thallium-208 (3 min) which emits a 2.6-MeV gamma ray. The possible build-up of this high energy gamma emitter over a period of time is of concern where plutonium-238 is used in a power source, as in a n artificial heart. A method of determining ppm (partsper-million) quantities of plutonium-236 in plutonium-238 is therefore required. This can be accomplished directly, quickly and simply by alpha pulse height analysis as described in the subsequent sections. The general principles involved in the method are well known; however, the specific techniques described resulted in the high sensitivity of the measurements. These techniques are applicable for the determination of parts-per-million concentrations of impurities in any alpha emitter, provided the trace impurity is higher in energy than the major component ( I ) .
Channel Number L
Figure 1. Alpha spectrum of plutonium-238 containing plutonium-236
EXPERIMENTAL
Apparatus. An Ortec Model SCCJ surface barrier detector, Industrial Development Products vacuum chamber, power supply, and amplifier system, and a Packard Instrument Co. 400-channel analyzer were used for the work. Resolution was 25 keV at full width, half maximum. Method. The system was calibrated to about 5 keV per channel, covering the range from 4.5 to 6.5 MeV. The alpha spectrum of approximately one part per million plutonium236 in plutonium-238 is shown in Figure 1 . The favorable half-life ratio of the two plutonium isotopes (87.4 yrj2.86 yr) ensures more counts by a factor of 30.82 per unit weight of plutonium-236 than from plutonium-238. The less energetic component of plutonium-236 is 221 keV more energetic than the highest energy alpha from plutonium-238. This represents a 44-channel separation. (1) An indirect and another direct technique are discussed by W. H. Smith, F. K. Tomlinson, D. W. Eppink, and G. R. Hagee, "Determination of Parts-per-Million Quantities of Plutonium236 in Plutonium-238," AEC Report MLM-1486 (to be pub-
lished). 1352
ANALYTICAL CHEMISTRY
Samples were prepared by electroplating the solutions onto polished slides to get a uniformly thin sample deposit ( 2 ) . Activity levels were kept low to reduce the probability of pulse pile-up (the combination of two pulses occurring nearly simultaneously to form one larger pulse simulating that from a higher energy alpha), a n effect which increases with counting rate. With the counting systems used for this work, no pulse pile-up was observed when a maximum of 5 x 103 alpha particles per minute was allowed to impinge o n the sensitive area of the detector. To get the desired statistical accuracy for plutonium-236 at this counting rate, a minimum of five 200-min measurements were made with each sample. The analyzer memory was not cleared between measurements. If the sample counting rate exceeded 5 x lo3 cpm, part of the sample deposit was covered with cellophane tape. This reduced the counting rate without distorting the spectrum. Alpha particles emitted by the portion of the sample which was covered were totally absorbed in the tape. (2) M. Y . Donnan and E. K. Dukes, ANAL.CHEM., 36, 392 (1964).
Table I.
Sample 1
Counting system Pl
e2 Total 2
3
4
5
Counting time, min 1300 400 1700
Direct Plutonium-236 Determination by Alpha Pulse Height Analysis
-
C236counts
counts 6.670 X lo6 2.042 X lo6 8.712 X lo6
217
120 122 242 =k 1 2 . 6 Z U 111 67
c233
80
297 =I=11.4%;"
#2 Total
2000
4.377 x 106 4.083 X lo6 8.460 X lo6
+1 #2 Total
1000 1000 2000
4.382 X lo6 2.618 X 106 7.000 X lo6
178 =k 14.6%"
$1 #2 Total
1400 1400
4.303 X lo6 4.436 X lo6 8.739 X lo6
346 313 659 =k 7.6%"
5.251 x 106 4.848 X lo6 10.099 X lo6
156 154 310 i 11.1%.
#1
$1
P2 Total
1000 1000
2800
2200 1600 3800
Maximum possible 227Th 223Ra contribution counts 70 21 91 or 30.6% of c236 57 49 106 or 43.8% of c 2 3 6 55 32 87 or 48.6% of C238 35 60 95 or 14.4% of c 2 3 6 60 41 101 or 32.6% of
6
7
C1 42 Total =f1
@ Total
1200 2200
3.324 X lo6 2.069 X lo6 5.393 x 106
1000 1200 2200
6.257 X lo6 6.384 X lo6 12.641 X 106
1000
84 66 150 & 16.OZa
35 12 4701'31.3% of C236
240 =!=
8.7Za
60 or 40% of
a
Limits of error, %
f15, -40
0.90 0.98 0.94
$15, -50
0.83 0.84 0.83
$15, - 5 5
2.63 2.30 2.47
$15, -25
0.97
1.04 1.00
+15, -40
0.83 1.04 0.91
$20, -40
1.40 1.22 1.31
$10, -40
c236
27 1
511
236Pu,
rJpm by weight 1.08 1.28 1.12
c236
Statistical error, 95 % probability.
CALCULATIOKS
I n calculating the plutonium-236/plutonium-238 ratio, the numerical sum of counts in channels 0 through E4 (Figure 1) was used for the plutonium-238 component, hereafter designated as C238. The sum of the counts in the shaded area from Es to E8 comprised the plutonium-236 component, shown o n a n expanded scale. This was corrected for plutonium-236 pulses appearing in lower channels due to energy degradation by self-absorption. This was accomplished by choosing a point, El, a t five channels (25 keV) below E2,and determining the fraction of the counts in channels 0 t o E4 which appear below El. The counts attributed to plutonium-236 were increased by this fraction. The resulting count was designated Points E4 C23.5. The point E5 is five channels below E6. and E8 a r e seven channels (35 keV) above E3 and E7. Points Ez and E3,E6 and E7 a r e the peak channels for plutonium-238 and plutonium-236, respectively. The amount of plutonium-236 expressed as parts-permillion by weight is given by:
Americium-241 cannot be distinguished from plutonium238 by alpha pulse height analysis; however, gamma analysis (3)showed that the amounts present were insignificant. Alpha impurities with energies close to those of plutonium-236 could increase the apparent plutonium-236 count, which would result in a high ratio. The decay of thorium-227 (from actinium-227) to radium-223 emits alphas in the plutonium-236 energy range; the maximum possible contribution to & was calculated from the counts for higher energy components of these isotopes. This figure was used in determining the limits of error. The spectra were also examined for the presence of actinium-225 (from the decay of thorium-229 to radium-225) and for radium-224, and the limits of accuracy were adjusted accordingly. Curium-243 and curium-244 are close in energy to plutonium-236 but are unlikely contaminants in this material. The cumulative errors, except for counting statistics, result in a high plutonium-236/plutonium-238 ratio. Hence, any error would be o n the safe side. RESULTS
ACCURACY OF THE METHOD
Errors due to pulse pile-up and counting statistics a r e functions of counting rate and counting time, and were controlled as described. Certain alpha-emitting impurities comprise another source of error.
Duplicate measurements made on seven samples, with two different detector-analyzer-amplifier systems (see Table I), verified that results were reproducible. Sample-detector (3) K. Watanabe, E. Sakai, and K. Minami, J . Nuclenr Sci. Techno/., 1,No. 6, 197 (1964). VOL. 40, NO. 8, JULY 1968
1353
geometry varied; therefore, counting rates o n successive runs are not comparable. The limits of error listed take into account all possible factors; the results in all probability were more accurate than these limits suggest. The peaks of plutonium-236 fell in the channels predicted from calibration data, and the proportions of high and low energy components agreed with the literature values (69% and 31 %) (4). Sample No. 7 had previously been assayed a t Savannah River Laboratory by a radiochemical method. [The plutonium was separated from uranium. After a three-week grow-in period for uranium-232 and uranium234 from plutonium-236 and plutonium-238, respectively, another uranium-plutonium separation was performed. The ratio of the uranium daughters after the second separation was determined by alpha pulse height analysis, and the relative amounts of plutonium-236 and plutonium-238 were calculated from this ratio and the known grow-in time (5).] The result (4) D. Strominger, J. M. Hollander, and G. T. Seaborg, Rev. Mod. Plzys., 30,823 (1958). (5) E. L. Albenesius, Savannah River Laboratory, private communication, January 30, 1967.
of this indirect determination was 1.26 ppm plutonium-236 Thus the simpler, quicker, direct method described herein gave results (1.31 ppm) which agreed closely with the independent determination by the indirect method.
(a with a n estimated accuracy of 10%.
ACKNOWLEDGMENT
The author acknowledges with gratitude the suggestion of G. Matlack, Los Alamos Scientific Laboratory, that the analyses could be made by alpha pulse height measurements, and the contribution of R. H. Lambek in the preparation of the slides used in the study.
RECEIVED! for review February 2, 1968. Accepted March 27, 1968. Mound Laboratory is operated by Monsanto Research Corp. for the U. S. Atomic Energy Commision under Contract No. AT-33-1-GEN-53. (6) M. G. Linn, Savannah River Laboratory, private communication, August 28, 1967.
Spectrophotometric Determination of Cyanide Ion with tris( 1,lO-Phenanthrol ine) Iron( II)-Triiodide Ion Association Reagent Jack L. Lambert and David J. Manzo Department of Chemistry, Kansas State Uniuersity, Manhattan, Kan.
THEPROCEDURE described here is presented as a useful, rapid method for the determination of small concentrations of free, uncomplexed cyanide ion in neutral solution in the absence of strong oxidizing or reducing agents. It is also presented as a n example of a solid phase colorimetric reagent, consisting of a n insoluble ion association compound, one of whose member ions reacts selectively with a species in solution and thereby releases the colored counter ion into solution. The very insoluble tris(1,lO-phenanthroline)iron(II) triiodide reacts rapidly with cyanide ion to release the red complex cation for spectrophotometric determination. Most ions found in the concentrations normally encountered in natural waters d o not interfere. Cyanide ion is usually determined colorimetrically by modifications of the Konig method (1, 2) developed by Aldridge (3, 4 ) and Epstein (5). The method of Aldridge involves the reaction of cyanogen bromide with a pyridine-benzidine mixture, while the Epstein method used the reaction of cyanogen chloride with pyridine, l-phenyl-3-methyl-5-pyrazolone, and bis-l-phenyl-3-methyl-5-pyrazolone.Other workers (6-9) (1) W. Konig, J . Prcrkt. Cliem., 69, 105 (1904). (2) W. Konig, 2.Agnew. Cliem., 69, 115 (1905). (3) W. N. Aldridge, Analyst (London), 69,262 (1944). (4) Ibid.,70, 474 (1945). (5) J. Epstein, ANAL.CHEM., 19,272 (1947). (6) . , G. V. L. M. Murtv and T . S . Viswanathan. Anal. Chim.Acta. 25, 293 (1961). (7) L. S. Bark and H. G. Hieson. Talarzta., 11., 621 (1964). . (8j A. Niwinski and H. DuGykowa, Przemysl Ferment. Rolny, 10, 352 (1966). (9) M . Simon, Tech. Eau (Brussels),239, 17 (1966). ,
1354
e
ANALYTICAL CHEMISTRY
have reported variations of these procedures, notably substituting barbituric acid in place of benzidine (6). Specific and highly sensitive fluorescent methods for cyanide ion have been reported by Guilbault and Kramer ( I O , 11), which involve the production of fluorescent compounds by the reaction of quinones and quinone derivatives with cyanide ion. Guilbault and Kramer (12) have also described a specific and extremely sensitive spectrophotometric method in which cyanide ion catalyzes the reduction of o-dinitrobenzene by p-ni trobenzaldehyde. Analytical use of a n insoluble ion association compound as a colorimetric reagent has been reported previously at least one time. G. Bouilloux (13) proposed a method for silver ion in which methylene blue dye cation was released from the insoluble compound formed between methylene blue cation and tetraiodomercurate anion by the reaction of silver ion with the complex anion. EXPERIMENTAL
Reagent. Porous porcelain chips (30-60 mesh) served as a support for the reagent. Unglazed porcelain plates (Fisher Scientific Co. catalog No. 13-750) were ground and sieved to the desired size range, and repeatedly washed. Subsequent study has shown that more effective washing can be (10) G. G. Guilbault and D. N. Kramer, ANAL.CHEM.,37, 918 (1965). (11) Ibid., 1395. (12) Zbid.,38, 834 (1966). (13) G. Bouilloux, Bull. Soc. Chim., 7,184(1940).
