V O L U M E 2 7 , N O . 7, J U L Y 1 9 5 5
1073
to saniples of Hilger spectrographically purc iron of known phosphorus content, and proceeding exactly as set out in the method. T h e resultant curve indicates a slight departure from Beer's law beyond a concentration of 0.06% phosphorus.
Notes. T h e method is independent of temperature, but it is essential that both blank and color solutions be brought t o the same temperature prior to measurement. In the preparation of the calibration curve the difference in temperature between color and blank should not exceed 0.5' C., but slightly greater latitude may be alloned in the actual analysip of a steel.
Table I. Results of Analyses Phosphorns Content 5% _____ ._____ SO.
Type
l l d BOH 13e BOH 21d AOH 20e A O H 22c Bessemer 129a Bessemer 32d Nickel-chromium lORa Nitralloy G 33c Nickel Lead-bearing 130 156 Nichel-chroGieinolybdenum
Certificate value
Found
Temp., ' C.
NBS Standards 0 006 0 OOB 0 021 0 021 0 041 0 043 0 055 0 055 0 083 0 083 0 094 0 094 0 012 0 013 0 Olli 0 015 0 017 0 010 0 028 0 025 0 032
0 031
24
so
28 24 18 27 20 30
0 003 0 000 002 0 000 0 000 0 000 +o 001 -0 001 -0 001 c0 001
22
-0
If, 14 23
t o
ACKhOWLEDGMENT
The paper is published with the permission of the Chief Scientist, Department of Supply, Australia.
001 LITERATURE CITED
3
N P L Standard 0 029 0 028
2
1,50 213
BCS Standards 0 008 0 008 0 038 0 040
25 12
R o l l iron 0 carbon
Deviation
the nitric acid ammonium sulfate mixture, and proceed as outlined in the method. The temperature insensitivitv of the modified permanganate oxidation method has heen demonstrated using National Bureau of Standards steel S o . 22c. T h e results over the range 10' to 35' C. are shown in Figure 5 and are compared with the results obtained in the absence of sulfate. Separate calibrations were necewx?-, the results in the absence of sulfate being read from a calibration curve prepared a t 18.5' C. Both the accuracy of the method and its applicability t o a wide range of steels have been checked by analyzing a series of standards; the results, together with the temperatures a t Tvhich the optical measurements u ere made are set out in Table I. These results, obtained a t widely different temperatures from a single calibration curve, provide confirmatory evidence of the insensitivitv of the method to temperature. Furthermore, the close coirelation of the results with the certificate values suggests that the method is a t least as accurate :is the best of other available methods.
-0
Barton. C. J., A s k r , . CHEJI., 20, 1068 (1948). Center, E. J. and Killard, H. H., ISD.ENG.CHEM.,AXAL.ED,, 14,287 (1942). Harrison, T. S . , J . S ~ J C C .h o n . Ind. (London),63, 350 (1944). Harrison, T. Y., and Fisher. IT., Ibid.,62, 219 (194.3). Hanes, C. C., Akn.Inat. .\lining .\Iet. Engrs., Tech. Pub. 1794 (1945). Hill, E. T., h x a ~CHEM., . 19, 318 (1947). Kitson, 1%. E., and Melion. 11. G., ISD. ESG.CHEM.,ASAL. ED., 16, 379 (1944). llisson, G., Chem.-Ztg., 32, 633 (1908). llurray, W.>I., Jr., and .Ishley, S.E. Q., IXD.ESG. CHEM., AS.AL.ED.,10, 1 (1938).
001
0 000 +O 002
Acid concentration is not highly critical, but the volume of acid misture used t o dissolve the sample should be controlled within 1 2 ml. of the specified volume. Some steels contain (>arbidesresistant t o attack by the acid mixture used. I n such cases, dissolve the sample in 10 ml. of ( 2 to 1) nitric acid, evaporate almoPt to dryness, add 50 ml. of
RECEIVED for reviev November 22, 1954. Accepted March 7 , 1955.
Routine Energy Measurements of Soft Radiations M. L. CURTIS and 1. W. HEYD M o u n d Laboratory, Monsanto Chemical Co., Miamisburg, O h i o
Identification of one or more radioisotopes in a mixture frequently may be established by determination of the energy of emitted particles. Energy determinations are made by means of absorption studies, utilizing absorbers positioned inside of a proportional chamber . low energy levels can be having a geometry of 2 ~ Verymeasured, limited only by the thinnest absorbers which can be fabricated. Beta particles can be distinguished from 01 radiation by changing the voltage setting of the instrument. With this system the energy of numerous very soft @ particles, as well as hard @ particles, has been measured in the presence of 01 radiation-for example, the energy of the 29 1c.e.v. radium-D p in the presence of hard @ and 01 particles; the lead recoil atom from a: emission of polonium-210 has been identified by means of energy determinations; and K-capture x-ray-s in a mixture have been identified.
F
.
REQUENTLY, identification of individual isotopes of an element is required, particularly where the element is separated as the result of a chemical analysis. Usually, identification of a radioactive isotope can be made if the maximum range
of the emitted particles can be determined, and the maximum energy may be calculated from the range of the particle. Feather ( 7 ) , Glendenin ( 8 ) , and others have developed techniques for determination of @ energy by means of absorption studies using Geiger counters. Their methods require comparatively large amounts of activity and @ energies exceeding 0.1 m.e.v. for reliable results. A method capable of measuring the energy of very soft radiation in the presence of hard emission in samples of low total activity is of value (4,5 ) . Such a method has been developed, using a windowless absorption counter (1-3, 11) and absorbers (3, I d ) . COUNTER
The counter consists of a slide mechanism (Suclear Meawrements Corp.) with a Type CC-2, 2-inch hemispherical proportional chamber and a ?\lode1 P-2 piston, a conventional scaler, and a high-gain, wide dynamic-range amplifier (1, 2 ) . The piston was modified so that absorbers could be inserted into the sensitive volume of the chamber between the sample and t h e center wire. This modification consisted of a depression formed in the piston t o hold the sample and a ledge around the depression t o support the absorbers. During counting, P-10 gas, consisting of 90% argon and 10% methane, was circulated through the chamber over the sample; a: particles were counted a t a definite voltage
ANALYTICAL CHEMISTRY
1074 and (Y plus 0 particles and y rays were counted a t a higher voltage. A series of supporting rings and absorbers (12) was so made t h a t the total thickness of ring and absorber was 0.25 inch. The absorbers ranged in thickness from 0.02 t o 1700 mg. per sq. cm. The thinnest absorbers, 0.02 t o 0.10 mg. per sq. cm., were made from Formvar E film and rendered electroconducting by evaporating silver or aluminum onto the film. Electroscope foil, in single layers and in two, three, or four laminations, and commercially available aluminum foil were used for the range from 0.15 t o 35.0 mg. per sq. cm. The thin absorbers were cemented t o the s u p p o r t i n g ,00 rings. Thethicker a b s o r b e r s were m round a c h i naluminum e d from
51j,,,
stock 2 inches in diameter, the ring and absorber being formed as a complete assembly. The d i s t a n c e 100 from the bottom 4 of the absorber to 5the sample must a be maintained a t a constant value, a . and must be as I - small as possible W in order to minia mize g e o m e t r y L 10loss and gas absorption effects. A set of spacers 05was made, one s a a c e r for each a b s o r b e r , of a t h i c k n e s s such that a distance of 200 400 600 $0 0.05 i n c h mas ABSORBER THICKNESS IN MG/CM. m a i n t a i n e d beFigure 1. Absorption curve of tween the top of phosphorus-32 the saacer and the bottom of t h e absorber when these parts were assembled in the depression formed in the piston. Saxple mounts v-ere placed on top of the spacer, and when thin mounts were used, a shim Ras put under the spacer to bring the sample as close as possible t o the absorber.
;
PROCEDURE
Measurements are made by placing the sample in position, flushing the chamber with gas, and counting the samples with gas
flowing through the chamber. This procedure is repeated with selected absorbers in place over the sample. When p radiation is counted in the pre,sence of (Y e m i s s i o n , a n cy measurement is made followed by u anr ea-plus-p m e n t , meas~h~
eo
y
2 5
$
E
8
'1
~ M P T X I E N T I I 7 W I
Y difference in the two measurements is recorded as the B count. I The efficiency for 8 y radiation is very poor, being about 0 I00 200 xx) 400 ABSORBER THICKNESS IN MG./CM? 1% for a cobaltGO--/, 1.2 m.e.v. Figure 3. Beta absorption curve of The percentage radium D-E-F transmittance of e a c h p-plus--/ measurement with background subtracted is plotted against absorber thickness on semilog paper. The equivalent of the absorption of the gas between the sample and the absorber must be added to the aluminum-absorbei thicknesq. The maximum range is determined from this graph, and the mavimum energy is determined from the range, using the Glendenin range-energy relationships (10). I n determining the maximum range from the graphs, energy studies on standards with a maximum energy approaching that of the unknown are of material assistance. Carbon-14 (0.16 m.e.v.), radium E (1.17 m.e.v.), and phosphorus-32 (1.71 m.e.v.) are suitable for use as standards. The absorption curve of a single p emitter, as determined by the windowless ahsorption counter, approximates a straight line over most of the range (Figures 1 and 2). ,4 Feather analyzer ( 9 ) , prepared from the absorption curve of an appropriate standard, is placed along the vertical scale of the absorption curve of the unknown, with the zero reading on the analyzer coinciding with 1 0 0 ~ transmittance o on the absorption curve. The point on the vertical scale which corresponds to 1.0 on the analyzer is taken as substantially zero transmittance. The absorber thickness required to reach this point, as determined from the extrapolated absorption curve of the unknown, is the maximum range of the unknown. This technique has been tested with a number of emitters of known energy, and it has been found to be a sound procedure. If several components are present in a sample, the contribution of the hardest component may be subtracted from the curve and the energy of the second component determined. The contribution of this component may be subtracted from the remainder t o obtain softer components, until all components have been determined. Since there is no window for the radiation to penetrate, the energy levels which can be measured are very low, being limited
OBSERVED CURVE
w
-
e
5-
2
.
4o
4 .
g w
B 1.0-
4
20
U
+
ROE COMPONENT
4 9
OBSERVE0 CURVE MINUS ROE COMPONENT EQUALS Ra D COMPONENT
(-,003 MEX)
0
I
Figure 4.
i B S O R B E R ;HICKNESS:N
MG/CMz5
6
Beta absorption curve of radium D-E-F
7
V O L U M E 2 7 , NO. 7, J U L Y 1 9 5 5 only by the thinliest absorber which can be fabricated. The , that a very small amount of geometry is favorable ( 2 ~ ) so activity is required. RESULTS AND DISCUSSIOS
Figures 3 and 4 show the absorption of 0 particles from radium-D (-0.03 m.e.v.) and radium-E (1.17 m.e.v.) counted in the presence of the radium-F CY (5.3 m.e.v.). Figure 5 is the absorption curve of nickel-63 (0.07 n1.e.v.); Figure 6 is the curve of a mixture of sulfur-35 (0.1i m.e.v.) and nickel-63; Figure 1 is the absorption curve of phosphorus-32 (1.71 m.e.v.); and Figure 2 is the curve of carbon-14 (0.16 n1.e.v.). The absorption of K-capture s-rays from iron-55 is shown i n F i g u r e 7. The steep initial d r o p of t h e sample mounted on tantalum is not present in the same material mounted on g l a s s . This is attributed to ab2 05 Eorption of very J Iweak secondary E electrons formed K a 0.1; in t h e highatomic-number 0.05backing m a t e rial by the xrays. T h e s e 0 01 s-rays can be J! IO 20 30 40 50 60 distinguished ABSORBER THICKNESS IN MWCM~! from 'Oft p parFigure 5. Absorption curve of nickel-63 ticles, since a soft (3 absorption curve does not appreciably change with backing material. This method is much simpler than the customary one of changing the atomic number of the absorber as only one set of absorbers is required.
I\
10
A- NICKEL (0.07
VJ OBSERVED C%E MNUS SULFUR MINUS IMPURITY 8- IMPURITY
0.01
ABSORBER THIGKNWS IN MG/CK~
Figure 6. Absorption curve of nickel-63 and sulfur-35
1075 The windowless absorption counter was used to identify recoil atoms from CY emission. These a-ere being counted as p particles in samples of polonium-210 and plutonium-239 which were known to contain no appreciable amounts of B activity. I n sufficiently thin samples, one apparent p was counted for each CY, and the half life was found to be the same as that of the CY.
0
5
Figure 7.
IO I5 20 ABSORBER THICKNESS IN MG/CM?
Absorption curve of iron-55
" 4bsorption of secondary electrons formed i n t a n t a l u m by K-x-rays from Fe65
Absorption of the apparent p associated with the a decay of polonium is shon-n in Figure 8. It was assumed that the range of the recoil atom, kinetic energy 0.96 m.e.v., would be less than that of an a particle of the same energy, or 0.15 mg. per sq. cm. of aluminum. The masimum range of approximately 0.08 mg. per sq. em. in Formvar, calculated from Figure 8, corresponds to the expected value and confirms the identification of lead recoil atoms. The tail on the absorption curve of Figure 8 has not been satisfactorily explained, but it has been observed repeatedly. I t may be due to L x-rays from the recoil of excited lead atoms, but the authors are unable to give a positive esplanation. In measuring soft radiation, Eamples should be as nearly weightless as possible. The samples used in this study were prepared by pipetting c a r r i e r - f r e e solutions onto slides. M e t a l disks xere used because best results are obtained with electro-conducting mounts. The metal used for a given sample must not be attacked by solvent in which the isotope is dissolved. If the solution to be analyzed is not entirely carrierfree self-absorption is n o t a serious problem; b e c a u s e of the 501 favorable counter 0.1 0.2 0.3 0.4 FORMVAR FILM ABSORBER THICKNESS IN M ~ / C M ? geometry, a dilute Figure 8. Absorption curves of recoil solution can be used. atoms
ANALYTICAL CHEMISTRY
1076 This counting system has been used extensively in identification of unknown emitters. For example, chemical tests indicated the presence of radioactive iron in a solution. An absorption
OF
K - X RAYS
CY particles when plotted on linear paper is a straight line whose intercepts are the range of the particles and the absolute 01 count. This makes possible very precise energy measurements of CY emitters. The- method is also applied to determining the proportion of two CY emitters in a mixture. These techniques are not infallible in identifying unknown emitters in a mixture, but they can be extremely useful when used in conjunction with chemical analysis and half-life studies.
A - GAMMA
8- OBSERVED CURVE MNUS GAMMA MINUS 046 MKV. COMPONENT IO 26 ME.’.! COMPONENT) C OBSERVED CURVE MINUS GAMMb 1046 ME,V. COMPONENT
D- OBSERVED CURVE
001-
0
,
,
20
40 60 80 100 I20 ABSORBER THICKNESS IN MG./CM!
IkO
Figure 9. Absorption curve of mixture of iron-55 and iron-59
study of the iron separation was made. Analysis of the curve shown in Figure 9 clearly shows the y and the 0.26- and 0.46m.e.v. p particles from iron-59 and the K-capture x-rays from iron-55. A half-life study confirmed the identification. The application of this system to CY counting has been described (6). It is shown that the absorption curve of monoenergetic
LITERATURE CITED
(1) Baker, TI-. H., “Counting Systems for Pulses of TVide Dynamic
Range,” Mound Laboratory, Monsanto Chemical Co., Miamisburg, Ohio, MLM-618 (Oct. 15, 1951). ( 2 ) Baker, W. H., “Sonlinear Pulse Amplifier of Wide Dynamic Range,” Mound Laboratory, JIonsanTo Chemical Co., Miamisburg, Ohio, MLM-851 (June 9, 1953). (3) Baker, W.H., Curtis, M .L., Gnagey, L. B., Heyd, J. W., and Stanton, J. S., .Vc~cZeonics, 13 (Yo. 2), 40-3 (1955). (4) Curtis, h l . L., and Heyd, J. W.,“Absolute Alpha Counting. I. Determination of Back-Scattering Factors and Ranges,” Mound Laboratory, Monsanto Cheniical Co., Miamisburg, Ohio, MLM-834 (April 10, 1953). (5) Curtis, hl. L., and Heyd, J. IT., “Windowless Absorption Counter for Routine Energy Measurement of Soft Radiations,” Mound Laboratory, Monsanto Chemical Co., Xliamisburg, Ohio, MLM-842 (dpril 10, 1953). (6) Curtis, M. L., Heyd, 3. IT.,Olt, R. G., and Eichelberger, J. F., Sucleonics, 13, h-0. 5. 39 (1955). (7) Feather, N., Proc. Cambridge P h i l . Soc., 34, 599 (1938). (8) Glendenin, L. E., Sucleonics, 2 (No. 11, 12 (1948). (9) Ibid.,p. 21. (10) Ibid.,p. 26. (11) Gnagey, L. B., “Windowless Proportional Chamber for Absorption Measurements,” Mound Laboratory, Monsanto Chemical Co., Miamisburg, Ohio, MLM-857 (May 15, 1953). (12) Stanton, J. S., and Heyd, J. W., “The Preparation of Plastic and Metallic Absorbers,” Mound Laboratory, Monsanto Chemical Co., Miamisburg, Ohio, MLM-858 (June 17, 1953). RECEIVED for review December 23, 1954. Accepted March 17, 1955. M o u n d Laboratory is operated by l l o n s a n t o Chemical Co. for the U. S. Atomic Energy Commission under Contract No. AT-33-1-GEN-53.
Schiff Reagent Its Preparation and Its Use in the Determination of Formaldehyde in Cellulose Acetate Formal DAVID E. KRAMM and CHARLES L. KOLB Celanere Corp. of America, Summit,
N. J,
This work was undertaken to provide a method for the determination of small amounts of combined formaldehyde in cellulose acetate formal samples. A Schiff reagent of controlled sensitivity and improved stability has been developed and utilized. Sensitivity has been correlated to sulfur dioxide concentrations and both optimum and reproducible response are shown to occur in the range 2.8 to 4.8 millimoles of sulfur dioxide per 100 ml. of reagent. The formaldehyde content of cellulose acetate formals has been determined colorimetrically using Schiff reagent. The method developed shows a standard deviation of 0.02170 of formaldehyde for samples of cellulose acetate formal film containing less than 0.8% of formaldehyde. The improved performance of Schiff reagent has resulted in’an analytical tool of increased versatility. Sufficient stability and reproducibility have been obtained to make daily calibration unnecessary.
T
HE development of a colorimetric method for the determina-
tion of formaldehyde in cellulose acetate formal falls naturally into two parts: the preparation of a sensitive Schiff reagent which will give reproducible results against known amounts of formaldehyde] and the achievement of sample hydrolysis in such a way that the formaldehyde is quantitatively obtained in a form suitable for measurement with Schiff reagent. STUDY OF SCHIFF REAGENT
Blaedel and Blacet ( 1 ) found that Schiff reagent reacts with formaldehyde in strong sulfuric acid to give a blue color which reaches a maximum intensity after 2 to 2.5 hours. While other aldehydes may also produce initial colors, these colors fade completely in the 2- to 2.5-hour period prescribed. This makes the test selective for formaldehyde. Blaedel and Blacet applied it to the determination of formaldehyde in the presence of other aldehydes. Hoffpauir, Buckaloo, and Guthrie ( 8 ) adapted the
