Q A T / AH yields directly the number of moles of reactant from the value of the cell constant, Q, and the measured temperature difference, AT. I n solutions sufficiently dilute to warrant the approximation AH = AH’ (and if AH’ is known), it is thus possible to carry out quantitative determinations using a titrant solution of unknown concentration. The corresponding accuracy attained in titrations with unstandardized E D T A of the various divalent cations was within 5% in the millimolar concentrated range. ACKNOWLEDGMENT
Acknon-ledgment is made to the
Research Corp. for a grant in support of this work. Thanks are due 31. L. Willard, and to W. L. Chambers and H*Dumbaugh for preparing figures* LITERATURE CITED
(1) Carini, F. F., Martell, A. E., J. Am. Chem. Soc. 76, 2153 (1954). (2) Charles, R. G., Ibid.7 76,5854 (1954). (3) Ewing, G. W., “Instrumental Meth-
ods of Chemical Analysis,” pp.
311-13. McGraw-Hill. New York. 1954. (4) Hallett, L. T., Graham, R. P., Fur-
man, N. H., Diehl, H. C., Ashley, S. E. Q., Churchill, H. V., ANAL. CHEM.24, 1348 (1952). ( 5 ) Linde, H. W., Rogers, L. B., Hume, D. N., Ibid., 25, 404 (1953).
(6) Lingam, J. J., Ibid., 20, 285 (1948).
(7) Mellon, Rf. G., “Quantitative Analysis,” p. 555, T. Y. Crowell Co., New York, 1955. (8) Mfiller, R. H., Stolten, H. J., ANAL, CHEW25, 1103 (1953). (9) Schwarzenbach, G., “Die Komplexometrische Titration.” D. 8. Ferdi-
nand Enke Verlag, Stittgak, Germany, 1955. (10) Schrarzenbach, G., Biedermann, W., Bangerter, H., Helv. Chim. Acta 29, 811 (1946).
RECEIVED for review April 12, 1956. Ac-
cepted October 4, 1956. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, February 28, 1956. Based on a thesis by T. G. Alleman.
Liquid Scintillation Counting of Carbon-1 4Labeled Organic Nitrocompounds SAMUEL HELF and CECIL WHITE Chemical Research laboratory, Picatinny Arsenal, Dover,
b Carbon-1 4-labeled organic nitro compounds in liquid scintillators exhibit a quenching of scintillation which results in a reduction of counting efficiency. This effect is dependent on concentration and is shown to b e due to absorption by the nitro compound of a portion of the light spectrum emitted by the scintillator. By the use of suitable wave length shifters in the liquid scintillator medium, the quenching effect can b e greatly offset and high counting efficiencies obtained.
C
methods of counting low-energy beta emitters. in addition to their usual disadvantages, are even more unattractive when applied t o radioactive materials which are highly volatile and/or explosive. The advent of the liquid scintillation counting technique (2, 3) offered a highly efficient detection method which minimized the disadvantages associated with the counting of materials of this nature. Most of the previous references on liquid scintillation counting have dealt with an evaluation of either solutes or solvents for use in liquid scintillator systems (4-9). Very little general information has been published thus far regarding the effects of adding small amounts of particular radioactivelabeled materials such as would be required for tracer or analytical work. Such information was desired in this laboratory with regard to carbon-14labeled organic nitro compounds. Hayes and coworkers (6) evaluated a large number of scintillation solutes and demonstrated that the addition ONVENTIONAL
N. 1.
of a nitro group to an excellent scintillator such as 2,5-diphenyloxazole almost completely quenched the scintillation process. As a result, the potential application of this technique to the counting of nitro compounds appeared rather doubtful a t first. However, a few preliminary experiments indicated that for small concentrations of material containing a reasonable amount of specific activity, sufficiently high counting rates were obtained to justify further study. This investigation is thus an attempt to extend the liquid scintillation counting technique to a class of materials which does not normally lend itself to this method. To obtain a quantitative evaluation of tlie behavior of nitro compounds in a liquid scintillator system, the following materials were studied in a 2,5-diphenyloxazole-toluene (3 grams per liter) solution: 0-, p - , and m-nitrotoluene, 2.4 - dinitrotoluene, 2,6 - dinitrotoluene, 2,4,6trinitrotoluene, nitrobenzene, 1,1, 1-trinitroethane, and nitromethane. 2,5-Diphenyloxazole in toluene was selected as the liquid scintillator because experience has proved it to be one of the most efficient light producers n-hen combined with low-energy beta emitters and sufficiently soluble in most organic solvents a t the reduced temperatures required for low background counting. It has thus become the standard liquid scintillator to which all other systems are compared. EXPERIMENTAL
A Tricarb liquid scintillation counter (Packard Instrument Co., La Grange,
Ill.) was used to obtain all of the counting data in this investigation. The two Dumont 6292 photomultiplier tubes comprising the coincidence counting arrangement exhibit an S-9 response-Le. , the wave length a t maximum response is 4800 & 500A. The phototubes and sample system are housed in a liphttight commercial freezer maintained a t 20” F. The toluene used,as the solvent was tagged with carbon-14 in the methyl position to provide the source of radiation and to eliminate the necessity for tagging each individual nitro compound. ii 30-ml. sample of the 2,5-diphenyloxazole-toluene-C14 solution was first counted over a 700-volt range. To this original sample were added successive small increments of nitro compound being studied; the same scan was made of counting rate us. voltage. The lower and upper discriminators of the twochannel pulse height analyzer were arbitrarily fixed a t 10 and 50 volts, respectively, to maintain a small background (about 80 c.p.m.) and to obtain a reasonably accentuated intensity distribution peak. All samples were counted in cylindrical Kimble Opticlear vials (7-dram capacity) fitted with polyethylene caps. A Beckman D R monochromator, Model 15800, was used for the light absorption determinations; all solutions were measured in silica cells, 1 mm. thick. The organic nitrocompounds used for the determinations were of the highest purity obtainable, with special attention directed toward obtaining them as nearly colorless as possible. The toluene used as a solvent was C.P.grade, dried over anhydrous magnesium sulfate. VOL. 29, NO. 1, JANUARY 1957
13
The scintillation grade solutes were obtained from the following sources: 2,5-Diphenyloxazole, Pilot Chemicals, Inc., Waltham, Mass. p-Terphenyl, Eastman Organic Cheniicals, Rochester, N. Y. 1,6-DiphenylhexatrieneI {The Matheson Co., Inc., East Rutherford, N. J. Tetraphenylbutadiene; 2-(1-naphthyl)-5-phenyloxazole; 1,4-bis [2-(5phenyloxazolyl) ] benzene, Arapahoe Chemicals, Inc., Boulder, Colo. RESULTS A N D DISCUSSION
Figure 1 illustrates the family of curves obtained with several concentrations of p-nitrotoluene. The uppermost curve represents the pure 2,sdiphenyloxazole-toluene-C14 scintillator. As the concentration of the nitro compound is increased, there is a gradual shift of the spectrum toward the region of higher voltage with a corresponding loss in peak intensity. This loss is due to absorption by the nitro compound of a portion of the light emitted by the pure scintillator (as discussed below). This produces a redistribution of the light spectrum such that higher potentials have to be supplied to the photomultiplier tubes to obtain sufficient amplification of the resultant lower energy pulses. All of the nitro compounds were subjected to this same experimental treatment. ii convenient way of comparing
,002 004 ,006 DM
SCINTILLATOR
1630 MG. p-NITROTOLUENE +SCINTILLATOR *)
P x 12 +-w I)
f
at 0 w
I-ln $8
48 89 MG. p - NITROTOLUENE -tSCINTILLATOR
90.00 MG. p-NITROTOLUENE SCINTILLATOR
+
14
a
800
ANALYTICAL CHEMISTRY
900
.om
DIO D Z O D Z P 014 [MOLE
me
PER ML]
Figure 2. Quenching of scintillation by nitro compounds in 2,5-diphenyloxazole-toluene-C~4
,-PURE
700
DIO DIZ DM
CONCENTRATION
1000
1100
1200
1300
1400
the data obtained was to plot on semilog paper 1/10 vs. concentration for each compound. Values of I were taken as the peak counting rate for a particular concentration of nitro compound and 10as the peak counting rate for the pure scintillator (2,5-diphenyloxazole-toluene-C14). These curves are reproduced in Figure 2. Each nitro compound exhibits a characteristic attenuation of the counting rate of the pure scintillator solution. Moreover, the linearity of the data indicates B strong adherence t o a n exponential relationship, especially for the nitro aromatic compounds. The quenching of the scintillation process, as exhibited by the nitro conipounds, is strongly suggestive of simple light absorption. To substantiate this, the ultraviolet and near-visible spectrum of each compound in 2,5-diphenyloxazole-toluene solution was determined using the 50% extinction concentration for each material (as determined from Figure 2). All of the compounds exhibited a definite absorption peak in the ultraviolet region. These spectroscopic data are listed in Table I. To fully correlate these absorption data with the curves in Figure 2, consideration must be given to the emission characteristics of the pure scintillator. 2,5-Diphenyloxazole-toluene (3 grams per liter) has an emission range of 3400 to 4300 A. with a maximum at 3800 A. (1). The absorption overlap of the nitro com-
Table 1.
toluene sj-stein in which l,4-his [a( 5 - phenyloxazoly1)lbenzene produces more than a sixfold increase in counting rate. The presence of impurities possessing light-absorbing characteristics diffeient from those of the compound being counted may affect the accuracy of analytical results. However, since for any radioanalytical procedure, more than the usual precautions should be taken to ensure high purity, the effect of impurities should be no more critical with this counting technique than any other.
Absorption Data for Nitro Compounds
Concn., Absorption Mmole Peak, Compound per Ml." A. Trinitrotoluene 0.0040 3500 2,6-Dinitrotoluene 0.0052 3470 2,4-Dinitrotoluene 0,0060 3475 o-Nitrotoluene 0.0072 3510 p-Nitrotoluene 0.0078 3510 m-Nitrotoluene 0.0096 3500 Nitrobenzene 0.0120 3480 Nitromethane 0.0485 3150 l,l,l-Trinitroethane 0.0115 3625 Solutions contained in silica cells, 1 nim. thick.
pounds is plainly evident. The position and slope of each curve in Figure 2 can be qualitatively correlated with the absorption range and per cent transmittance at the peak for each compound. Kitromethane, which absorbs more at shorter wave lengths and exhibits the least overlap of the 2,5-diphenyloxazoletoluene emission spectrum, accordingly has the least effect on the counting rate of the pule scintillator. Having established that the action of nitro compounds in reducing the counting rate of liquid scintillation systems is due mainly to the absorption of light, attention was then directed toward finding the most efficient scintillatoi system for the counting of such materials. The action of secondary scintillation solutes in shifting the emission spectrum of the primary solute has been discussed by other authors (4, 9 ) . For the particular case of organic nitro compounds, a shift of the emission spectrum away from the ultraviolet region toward the longer wave lengths n-ould certainly be beneficial. The following compounds hare been mentioned as among the most efficient secondary solutes in toluene scintillator systems (10): diphenylhexatriene, tetraphenylbutadiene, 2 - (1 - naphthyl)5-phenyloxazoleJ and 1,4-bis [2-(5-phenyloxazolyl) ]benzene. Figure 3 demonstrates the increase in counting efficiency effected by the addition of 1,4 - bis 12 - ( 5 - phenyloxazolyl) ]benzene (100 mg. per liter) to the 2,5-diphenylosazole-toluene-C~4 scintillator in the presence of p-nitrotoluene It is obvious that the addition of the secondary solute has shifted the emission spectrum considerably into the visible region where absorption effects are not so pronounced. To obtain a comparison of the Tvave length shifters cited above, a standard concentration of p-nitrotoluene 10.008 nimole per nil.) was selected and its counting rate determined in the presence of these secondary solutes. These experiments were performed in a saturated solution (20" F.) of p-terphenyl i n toluene-Cl4 (the second most corn-
Absorption Range, A.
33804250 3380-4250 3380-4250 3400-4250 3390-4250 3400-4200 3420-4250 2900-3850 3550-4100
%
Transmission at Peak 53 54 53 60 66 68 66 98 97
nion liquid scintillator foi loiv-energy beta counting) as well as in 2,5-diphenyloxa~ole-toluene-C~~. These results are listed in Table 11. The voltage a t the peak counting rate and the efficiency relative to pure 2.5-diphenyloxazole-t01uene-C~~are reported for each system. K i t h the exception of tetraphenylbutadiene, each wave-length shifter caused a marked increase in counting efficiency over the simple scintillator system, with 1,4-bis [2-(5phenyloxazolyl) ]benzene yielding the highest efficiency in combination with both primary solutes. The beneficial effect of adding !a wave-length shifter is best demonstrated in the p-terphenyl-
CONCLUSIONS
Appreciable quantities of organic nitro compounds, normally considered as strong quenchers, can be counted in liquid scintillators with relatively high efficiency TT-hen labeled with carbon-14. The loss of counting efficiency due t o light absorption can be partially and sometimes greatly offset by using secondary solutes to shift the emission spectrum of the primary scintillator toward longer wave lengths. For oiganic nitro compounds. 1,4-bis[2-(5phenyloxazolyl) ]benzene in combination n-ith 2,5-diphenyloxazole or p-terphenyl is very effective for this purpose. For chemical, analytical, or tracer
10 9.
.a7-
6-
.5.
.a0
x
3-
.2-
I
I-
,002 ,004 ,006 DO8 ,010
,012 ,014
016 ,018
CONCENTRATION
[MMOLE
,020 ,022 ,024 ,026
PER MLJ
Figure 3. Quenching of scintillation by p-nitrotoluene in 2,5-diphenylo~azole-toluene-C~~with and without wave length shifter VOL. 29, NO. 1, JANUARY 1957
15
Table II. Evaluation of W a v e Length Shifters with Simple Liquid Scintillator Systems in Presence of p-Nitrotoluene
.
Vnlta
System"
p-Kitrotoluene (0.008 mmole None per ml.) in 2,5-diphenyloxa- lJ6-Diphenylhexatriene (10 mg./ zole-toluene-C14 (3 grams per liter) liter) 2-(l-Naphthyl)-5-phenyloxazole (50 g./liter) Tetraphenylbutadiene (100 mg./ liter) 1,4-bis [2-(5-phenyloxazolyl)]benzene (100 mg./liter)
1100
0.49
1100
0.63
1100
0.61
1100
0.45
1040 1200
0.81 0.12
pNitrotoluene (0.008 mmole per None ml.) in terphenyl-toluene-C14 1,6-Diphenylhexatriene (10 mg./ (saturated solution a t 20" F.) liter) 1140 ' 2-(l-Naphthyl)-5-phenyloxazole 1100 (.50 g./liter) Tetraphenylbutadiene (100 mg./ 1220 liter)
-
1,4-bis[2-(5-phenyloxazolyl)] benzene (100 mg./liter) a
1050
0.46 0.56 0.18 0.79
Total sample, 30 ml.
* Efficiency relative to pure 2,5-diphenyloxazole-toluene (3 grams per liter). applications, the effect of absorption can be neglected by counting the same concentration of labeled material for all experiments. Where different sample Feights of the same absorbing compound have to be radioassayed, a suit-
ACKNOWLEDGMENT
_^"I
a t Peak Relative Counting EffiRate ciency"
Wave Length Shifter
light attenuation primarily by ultraviolet absorption.
able correction can be made by first performing a study of 1/10 us. concentration, as illustrated in Figure 2. These conclusions should be applicable to the liquid scintillation counting of all colorless materials that exhibit
The authors wish to thank James R. Arnold, Princeton University, for his helpful discussions. Acknowledgment is given to T. C. Castorina and F. S. Holahan for preparation of the toluene0 4 . The authors also are indebted to the Ordnance Corps for permission to publish this manuscript. LITERATURE CITED
(1) Haves, F. N., U. S. Atomic Energy Commission, Rept. No. LA-1639,
May 1954. (2) Hayes, F. N., Gould, R. G., Science 117, 480 (1953). (3) Hayes, F. N., Hiebert, R. D., Schuch, R. L., Zbid., 116, 140 (1952). (4) Hayes, F. N., Ott, D. G., Kerr, V. N., Nucleonics 14, No. 1, 42 (1956). ( 5 ) Hayes, F. N., Ott, D. G., Kerr, V. N., Rogers, B. S., Ibid., 13,No.13, 38 11955). (6) Hayes, 8. N., Rogers, B. S., Sanders, P. C., Ibid., 13, No. 1, 46 (1955). ( 7 ) Kallman, H., Furst, M., Zbid., 8,KO. 3, 32 (1951). ( 8 ) Kallman, H., Furst, M., Phys. Rev. 79. 857 (1950). (9) Ibid.; 81, 853 (1951). (10) Tracerlog, No. 67, Tracerlab, Inc., Boston, Mass., February, 1955. RECEIVEDfor review June 8, 1956. Accepted September 29, 1956.
Photometric Determination of Aliphatic Amines HERBERT M. HERSHENSON' and DAVID N. HUME Department o f Chemistry and laboratory for Nuclear Science, Massachuseffs Institute of Technology, Cambridge
b Two modifications have been developed of a procedure for the determination of aliphatic amines, based on the characteristic absorption band from 750 to 950 mp of the complex which forms when the amine solution is added to an alcoholic solution containing excess cupric chloride. The first is a direct procedure for the determination of aliphatic amines in alcoholic solution or in aqueous solution of sufficiently high amine concentration to permit dilution with alcohol to a point below the limit where water interferes. The second is an extraction procedure in which chloroform is used to effect a separation from other interfering basic substances and from water.
of absorption (Figure 1) in the near infrared, about 750 to 950 mp ( 1 ) . The formation of the absorbing complex is independent of the nature of the aliphatic amine, and the very near infrared absorption occurs only when the cupric chloride is maintained in excess. The structure of the reaction product is not known, but it is definite that the reactants combine in the proportions of two cupric ions. four chloride ions, and one basic molecule or ion. This reaction has been made the basis of a method for the determination of aliphatic amines. All aliphatic amines produce the same product, and a single calibration curve serves for all of them in direct determinations.
T
Photometric measurements mere made with a Beckman Model B spectrophotometer, using 1.000-em. Corex cells. Appropriate cell corrections were applied to the observed readings whenever required. The absorbance measurements were made against P reference solution containing all the components except one (usually the basic substance).
APPARATUS AND REAGENTS
of aliphatic amines and certain other basic substances to an alcoholic solution containing a n excess of cupric chloride produces a visible color change from green to yellow, and the appearance of a characteristic zone HE ADDITIOK
Present address, Prntt and Whitney Aircraft Co., East Hartford, Conn. 16
ANALYTICAL CHEMISTRY
39, Mass.
This meant that the reference solution, as well as the sample, was normally highly colored. All inorganic chemicals were the best grade obtainable, usually reagent grade. Commercial absolute ethyl alcohol was used as the solvent without further purification. The amines used were obtained from a variety of sources and mere of varying purity. Solutions were made by weighing out quantities of the amine and diluting to the appropriate volume with solvent in a volumetric flask. These solutions were standardized by titration with hydrochloric acid, using phenol red indicator. Alcoholic amine solutions were standardized after dilution with an equal volume of water. The cupric chloride reagent solution was prepared to contain 50 mmoles (8.525 grams) of cupric chloride dihydrate per liter of absolute ethyl alcohol. METHOD
Direct Procedure. Dilute the sample with ethyl alcohol so t h a t a 1- t o 5ml. aliquot contains between 10 and 80 pmoles of amine. Place a 5-ml. portion of stock cupric chloride in a 25-ml. volumetric flask and add 15 ml. of absolute
.
