Lengyel, B., Sammt, A., Z . physik.
(4) Delahay, P., Rlamantov, G., ANAL. CHEM.27, 478 (1955). (5) Delimarski. Y. K.. Csvekhi Khim. 23, 766 (1954). I
Chem. 181A, 55 (1937-8). Lyalikov, Y. S., Zhur. Anal. Khim. 5 ,
.
323 (1950); 8, 38 (1953).
Delimarski, Y. K., Markov, B. F., Berenblum, L. S., Zhur. Fiz. Khim.
Nachtrieb. M.. Steinberg. R l . . J . Am. Chem. hoc. ‘70, 2613-(19-18); 3558 (1950).
27, 1848 (1953).
Ferguson, W. S.,. Ph.D. thesis, University of Illinois 1956. Fernelius, W. C., “Inorganic Syntheses,” Vol. 2, p. 1, RlcGraw-Hill, New York, 1946. Flood, H., Forland, T., Acta Chem. Scand. 1, 592 (1947). Gierst, L., Juliard, A., J . Phys. Chem. 57, 701 (1953).
Gierst. L.. hlechelvnck. P. H., Anal. C h i h . Acta 12, f 9 (1955)
72,
M.M., Karchmer, J. H., ANAL.CHEM.27, 1095 (1955). (16) Osteryoung, R. A,, Ph.D. thesis, University of Illinois, 1954. (17) Porter. B.. Feinleib. M..J . Electrocheh. Sdc. 103, 300 (1956). (18) Reilley, C. S . ,Everett, G. W., Johns,
(20) Rochow, E., Didtschenko, R., J . Am. Chem. SOC.76, 3291 (1954). (21) Sand, H. J. S., Phil. Mag. 1, 45 (1901). (22) Van Artsdalen, E. R., Yaffe, I. S., J . Phys. Chem. 59, 118 (1955).
(15) Nicholson,
.
I
R.H.. ANAL. CHEM.27.483 (1955). (19) Reilley,‘C.N., Scribner, iV. G.; Ibid., 27, 1210 (1955).
RECEIVED for review June 22, 1956. Accepted September 29, 1956. Division of Analytical Chemistry, 130th Meeting, ACS, Atlantic City, N. J., September 1956. Taken from a thesis submitted by W. S. Ferguson to the Graduate College of the University of Illinois in partial fulfillment of the requirements for the Ph.D. degree in chemistry.
Thermochemical Titrations Enthalpy Titrations JOSEPH JORDAN and T.
G. ALLEMAN’
Department of Chemistry, Pennsylvania State Universify, Universify Park, Pa. Based on careful fundamental considerations, a method is described for the titrimetric determination of heats of reaction and a corresponding theoretical equation is derived. The change in temperature during the titration vs. mole ratio of reactants was recorded automatically with the aid of a high sensitivity thermistor bridge circuit, a potentiometer, and a synchronously coupled constant-flow buret. Under judiciously controlled experimental conditions the shape of the titration curves was a function of enthalpy differences between reactants and products. Extrapolated zero ordinate intercepts can be used for rapid and convenient analytical determinations. Potentialities and limitations of enthalpy titrations are illustrated with their applications to the determination of divalent cations with ethylenediamine tetraacetate. From the titration curves the relevant heats of chelation were evaluated. The method is applicable to concentrations as low as 5 X M, yielding an accuracy within 3%. A precision and accuracy within 1% can readily b e attained in 1 0-2M solutions. An enthalpymetric sensitivity index is defined and discussed.
E
is tentatively proposed as a designation for volumetric methods depending on heats of reaction. This terminology was selected in accordance with Mellon’s suggestion (7) that analytical nomenclature should consistently identify procedures by the characteristic property involved. Specifically, automatic titrations are presented which yield in an adiabatic system a plot of temPresent address, Procter and Gamble Co., Cincinnati, Ohio. NTHALPY TITRATIONS
perature change us. moles of added reagent. Similar techniques have previously been called “thermometric titrations” (5) and “thermal titrations” (3). However, the Committee on Nomenclature, Division of AnalyticalChemistry, AMERICANCHEMICALSOCIETY, has discouraged use of the adjective “thermometric” (4). A comprehensive review of relevant work published prior to 1953 is given by Linde, Rogers, and Hume (6)and additional references are summarized by Eming ( 3 ) . Enthalpy titrations have been applied to the accurate evaluation of heats of chelation of divalent cations with (ethylenedinitri1o)tetraacetate (ethylenediamine tetraacetate, EDTA). This is believed to be the first report of a successful titrimetric method for the determination of heats of reaction in aqueous solutions. Chelatimetric procedures for the quantitative determination by enthalpy titrations with EDTA of lead, cadmium, cupric, nickelous, calcium, zinc, cobaltous, and magnesium ions are applicable t o concentrations as low as 0.0005M, and to some binary mixtures, EXPERIMENTAL
Materials. Reagent grade chemicals were used throughout. I n all experiments EDTA mas supplied to the titration mixtures as a solution of the tetrasodium salt of ethylenediaminetetraacetic acid, which was prepared by adding stoichiometric amounts of carbonatefree sodium hydroxide to the disodium salt of EDTA. obtained from the Bersworth Chemical Co., Framingham, Mass. Apparatus. Instrumentation was similar to that recommended by Linde, Rogers, and Hume (6). Certain refinements were incorporated, which in-
creased by a factor of about 30 the sensitivity of the temperature-monitoring signal, compared to what has been reported hitherto in thermal titration studies. As shown in Figure 1, the titrations were carried out with a horizontal, constant-flow, automatic syringe-buret patterned after the one described by Lingane (6). The unit was driven by a Model SG15, 110-volt, 6O-cycle1 600r.p.m. synchronous motor supplied by the Merkle-Korff Gear Go., Chicago, Ill. With the aid of interchangeable precision-machined gears, flow rates between 0.2 and 0.6 ml. per minute were readily obtained, the latter being used exclusively in the studies described here. A volume of titrant not exceeding 1 ml. was used in each titration, with a view to minimizing variations in the heat capacity of the system. The buret was calibrated in terms of both a revolution counter geared into its mechanism, and the volume of liquid delivered in a given time. With either method, volumes of titrant of the order of 1 ml. (corresponding under the experimental conditions to a delivery time of about 2 minutes or to 1500 counts) were measured with a precision and accuracy within 10.0015 ml. (*0.15%). The syringe-buret was connected by a three-way stopcock either to a titrant reservoir or to a tube with a capillary delivery tip which was immersed under the surface of the solution titrated. The titrations were performed in a 250-ml. wide-mouthed Dewar closed with a stopper with appropriate holes for inserting the buret tip, a glass stirrer (operated a t 600 r.p.m. by a Sargent synchronous rotator, Type KYC-22), and a temperature-monitoring device. The latter was a Western Electric 14B thermistor which had the following characteristics: sensitivity in the 25’ C. temperature range, -0.04 ohm per ohm per ’ C.; approximate resistance a t 25’ C., 2000 ohms; thermal time lag, less than 1 second. VOL. 29, NO. 1 , JANUARY 1957
9
the literature have been obtained hy independent methods (d). RESULTS
Figure 1 .
Apparatus used for enthalpy titrations
The i l . t ~ r l ~ l i s l GWAS r incorpor:ttt 11 in
3
\ \ h e n t i t r , w bridgv circuit, mounted in 1l.e t m showti ~ '11 thr left in Figure 1.
The unbalance potential was fed-into a 2.5-mv., 28-cni. chart width Brown recordingpotentiometer (not shown on the figure) operated at a chart speed of 2.54 cm. per minute. The sensitivity of the bridge circuit, expressed in terms of unbalance potential per given change in thermistor resistance, was regulated by varying the electromotive force supplied to the bridge, as suggested by Muller and Stolten (8). The corresponding circuit diagram is sketched in Figure 2. The unbalance potential, 7 , is given by
where yns denotes the potential drop hetween points A and B (which is equal to the electromotive force supplied to the thermistor bridge), RCDis the resistance between points Cand D; and 7,R,, and Rz are the resistances of the thermistor and the corresponding fixed resistors, respectively. I n terms of temperature change vs. recorder deflection, the circuit was calibrated by direct comparison with a Beckman thermometer. At the maximum sensitivity setting in the 24" to 26' C. temperature range (yns = E = 1.5 volts; RI = R ~ = D Ra = r 2000 ohms) a change of 1 C. corresponded to an unbalance potential of 15.7 mv. or 176 em. on the recorder chart ordinate This is equivalent to about 0.15" C. per full scale deflection. The chart ordinate can be read with an accuracy within about =t0.03 cm., corresponding to 10.0002" C. Accordingly, the minimum temperature difference which could be estimated with an accuracy within 1% was 0.02" C. Sensitivities smaller than the maximum were readily obtained by varying the setting of P2. Within a temperature range of *lo C., a t a given sensitivity setting, the variation of the recorder ordinate deflection
-
10
ANALYTICAL CHEMISTRY
with temperature approximated linearity within 1% or better (8). As the recording potentiometer chart is powered by a synchronous motor, the thermistor bridge circnit yields an automatic plot of a temperature ordinate us. a time abscissa. The abscissa of the recorder chart can also he calibrated in terms of volume of titrant and represents a direct measure of the mole ratio (at any point of the titration curve) of the titrant and the reactant initially present in the Dewar. Procedure. In order to obviate volume corrections, the molar concentration of the titrant mas as a rule 100 times larger than that of the reactant in the Dewar flask. Twenty-five milliliters of solution was titrated in each experiment. Solutions of titrants and other reactants were generally allowed to approach equilibrium with room temperature (24' to 26' C.) for 15 to 20 minutes. No attempt was made to equalize the temperatures rigorously. Reactants in the Dewar and titrants at the start of the titration differed by possibly as much as *0.3" C. This affected the shape of the titration curves hut did not alter any of the quantitative results. For the chelatimetric determination of cations, standard EDTA solution was used as titrant, for general convenience in auantitative analysis. I n determining heats of reaction, 0.01M EDTA solution in the Dewar was titrated with 1M standard solution of the various cations. This procedure was selected in preference to the reverse technique, because it yielded conditions throughout the titration--e.g., constancy of pH-which were considered to be most similar to those under which comparable enthalpy data reported in
Typical enthalpy titration curyes of divalent cations with EDTA are shown in Figure 3. The initial parts of the curves (between points A and B) represent temperature-time curves prior to the addition of titrant. Their virtually perfect horisontality indicates either that the system was adiabatic within experimental error and that heats of stirring were negligible, or that heat losses and heats of stirring compensated each other. Point B corresponds to the beginning of the titration and C to the end point. The linear portion, BC, was ascending or descending, depending on whether the algebraic sum of the heat effects due to reaction enthalpies on the one hand and t o extraneous phenomena on the other was positive or negative. For instance, in curve I a net decrease in temperature is noted between B and C, because the relevant reaction was endothermic and accounted for a larger cooling effect than the increase in temperature that might have been expected, because the titrant was warmer than the reactant solution in the Dewar. Curves II and Ill represent titrations involving an exothermic reaction with a titrant warmer (curve 11) and colder (curve 111) than the reactant. In all cnrves portion DE corresponds to the addition of excess titrant and depends on such extraneous effects as heats of dilution and equalization of initial temperature differences between titrant and other reactants. An anomalously small slope was observed between points C and D on the curves. A similar anomaly was detected in enthalpy titrations of a strong acid with a strong base. Consequently, the phenomenon does not appear to he due to side reactions or unsatisfactory kinetics. The anomaly was a t times not observed when a new thermistor was used, hut showed up a few days later. The anomalous slopes are therefore attributed to a thermistor characteristic, the precise nature of which is not known. I n the absence of the anomaly, the titration curves in Figure 3 would he expected to exhibit only two discrete slopes-BC and DE. I n all instances, point C was considered to represent the titration end point and the distance on the abscissa between B and C was used as a measure of the corresponding volume of titrant. Stoichiometric results are calculated on the basis of Equation 2:
+ Y;$ e [MeYl:: + zH*O
(2)
where Me++ denotw a divalent cation and Y+ is the quadrivalent EDTA anion. Side reactions, which may take
place to a limited extent under the esperimental conditions, do not affect the over-all stoichiometric reaction ratio between EDTA and the divalent cations used in this study (2, 9). Quantitative data are summarized in Table I. on the titration of eight cations with EDTA.
Table I, Enthalpy Titrations with Standard EDTA Titrant
I n 10-*N
Ion=
I'h - Cd-' Cu-+c Si'' Caf' %n-+ Cofl ~
Lou-est concn.6 Solution Accessible to Pr e- Acc u- D et,ermination cision, racy, with Accuracy % of3%,hlmole/L. 1 0 4 0 3 0 4 0.4 0.7
0.1
0 5 1
1 0.4
..
1 1
0 5 1 0 8 0 1 0.5
1.5
2 2 1.5
AIg-+ o4 a Arranged in sequence of decreasing exothermicity of chelation with EDTA. ' Estimated to nearest 0.5 mmole/l. Used for calibrating abscissa scale on recorder.
As can be seen, a precision and accuracy within 1% were readily attained in 10-231 solutions. The lowest concentration which is accessible to determination with a given accuracy (Table I, column 4) depends on the heat of the relevant reaction, A H , which is
given by Equation 3 where AH is expressed in kilocalories per mole: AH =
AT X
Q
~
(31
A T , expressed in degrees centigrade, denotes the change in temperature corresponding to the formation of .I*, moles of product; and Q represents an "enthalpymetric cell constant'' expressed in kilocalories per degree centigrade and equal to the effective heat capacity of titration cell plus reactant solution. A graphical method was used to evaluate 42'from the titration curves, as indicated in dotted lines in Figure 3. It involves parallel displacement of the excess reagent line, DE, through the end point, C, and linear extrapolation to point F , which has the same abscissa as the initial point of the titration, B . Distance FB on the ordinate was taken equal to A T , The extrapolation procedure is considered adequate for correcting for heats of dilution and other extraneous effects. Under the experimental conditions, Q was evaluated by titrating 25 ml. of 0.01OOM hydrochloric acid with 1. O O M sodium hydroxide. Assuming AH for the neutralization of the 0.01.V acid to be equal to AH" = 13.4 kcal. per mole, a value of Q = 0.0360 kcal. per "C. was calculated. Keglecting possible differences between specific heats of the various 0.OlM solutions, the heats of chelation with EDTA of divalent cations in 0.01.11 solution were cal-
culated from enthalpy titration data on the basis of Equation 3. The difference between these values and the heats of reaction a t infinite dilution is negligible ( 2 ) . The AHo values determined by enthalpy titrations are listed in Table 11, together with corresponding data reported recently by Charles on the basis of direct calorimetric measurements by classical methods ( 2 ) . Table II. Heats of Formation at 25" C. of EDTA Complexes" A H " , Kcal./hIole
Ion Pb"
Detd. by enthalpy titration
Cd--
-12 8 - 9 2 - 8 2
Si-'
- i 4
Ca"Zn--
- 5 7 - 4 6 - 4 2 + 5 5
cu - co--
Reported
by Charles: (2) -13 1 - 9 1 - 8 2 - 7 6 - 5 8 - 4 5 - 4 1 1 3 1
hIg-0 Corresponding to Reaction 1
An enthalpy titration curve of a binary mixture of 5 X 10-3X calcium plus 5 x 10-3x magnesium with 1M EDTA is shown in Figure 4. C1 was taken as the end point corresponding to calcium (extrapolated) and Cp as the magnesium end point. Results obtained by this method for both cations were precise and accurate to 0.4 and 2%, respectively. DISCUSSION
S
L
TO RECORDER
Figure 2. E.
Thermistor bridge circuit
1.5-volt source 1 000-ohm potentiometer P1. 5 0 - o h m potentiometer 7. Thermistor Ri, Ra. 2000-ohm resistors RP. 1500-ohm resistor S. Single-pole single-throw switch 2. Zero adjuster ( 4 )
PI.
Titrimetric Determination of Heats of Reaction. As can be seen in Table 11,AHo values for the chelation of seven cations-Le., all except magnesiumwith EDTA, determined by enthalpy titrations, agreed within 3% with the best available calorimetric data. This agreement is considered satisfactory. as the values obtained by Charles have been reported to a number of significant figures which indicate the same order of accuracy. In general, it is believed that enthalpy titrations are methodologically preferable to the classical type calorimetric approach, on the basis of the following considerations: I n the titrimetric evaluation of AT (from which AH is calculated directly), heats of dilution are corrected by n simple graphical extrapolation method. In the direct calorimetric method, separate "blank" experiments are required. I n the case of reactions involving enthalpy differences exceeding 3 kcal. per mole, the titrations may be used to determine AH values with an accuracy of 3% in solutions as dilute as 2 X M , obviating corrections for infinite dilution. Katurally, enthalpy titrations have the disadvantage that the determination VOL. 2 9 , NO. 1 , JANUARY 1 9 5 7
11
TIME
I
TIME 1
PI MIN
I
1
i:
I
\
VOLUME
OF TITRANT
Enthalpy titration with 1.OOO
Figure 4.
M EDTA of mixture of 0.00506M calcium plus 0.00540 magnesium / F
/
k 0 57 M I 4 VOLUME
OF T I T R A N T
Figure 3. Enthalpy titration curves with 1.000M EDTA of divalent cations I.
0.01080M Mg++, titrant warmer than reactant 0.01 170M Cui+, titrant warmer than reactant 0.01 170M Cu++, titrant colder than reactant
11. 111. Curves shifted arbitrarily along vertical axis
of AH depends on the unambiguous knowledge of the relevant equilibria and presupposes favorable kinetics. According to the titrimetric determination, the chelation of magnesium ion with EDTA is endothermic to the extent of +5.5 kcal. per mole, while Charles reported AH" for this reaction to be f3.1 kcal. per mole. In contradistinction to both these results, Carini and Martell (1) have concluded on the basis of a study of the variation of the equilibrium contant of Reaction 2, that the chelation of magnesium with EDTA is exothermic and AH" = -2.9 kcal. per mole. There appears to be little doubt that the chelation of magnesium was endothermic under the experimental conditions described in this paper. This is particularly obvious from the titration curve of a mixture of calcium and magnesium ions (Figure 4). Analytical Significance of Enthalpy Titrations. The potentialities and limi-
tations of enthalpy titrations are inherent in the fact that they depend on free energies as well as on entropies, in accordance with the equation: A H = AF
+ TAS
(4)
Many titrimetric methods of analysise.g., potentiometric titrations-are 12
ANALYTICAL CHEMISTRY
based solely on favorable equilibrium constants of the reactions involvedi.e., they are pure "free energy methods." For an enthalpy titration the free energy requirements, while still essential, are less stringent. An enthalpy titration may be feasible whenever the entropy term in Equation 4 is of such sign and magnitude as to yield an over-all suitable AH situation (provided, naturally, that the relevant reactions are sufficiently fast and that the corresponding AF's are still adequately favorable). The chelatimetric titration of a binary mixture of calcium and magnesium ion furnishes an interesting illustration to this effect. In the conventional procedure [using Eriochrome Black T as indicator (IO)] only the sum of the two ions can be evaluated. By enthalpy titration (Figure 4) both calcium and magnesium can be determined, notwithstanding the fact that the two corresponding chelation constants differ by less than two orders of magnitude. The simultaneous enthalpymetric determination of the two cations is made possible by the difference in sign of their heats of reaction with EDTA. The relevant fundamental stochastic data are listed in Table 111. As can be seen, the endothermic character of the chelation of magnesium ion, as contrasted with the exothermic chelation of calcium, is primarily accounted
for by the very great difference in the entropies of chelation (column 4). Based on similar considerations, work is now in progress in this laboratory for exploring the applicability of chelatimetric enthalpy titrations to the determination of total hardness of water. The quantity P,
= IC, X
AH"I
(5)
provides a convenient criterion for characterizing accuracy limits of enthalpy titrations. I n Equation 5, C, denotes the minimum concentration (expressed in millimole per liter) with which a determination can be carried out with a given accuracy (expressed in per cent) indicated by the subscript n. P , (expressed in calories per liter) is called the "enthalpymetric sensitivity index" and depends solely on the characteristics of the experimental setup. For instance, under the conditions used to obtain the data reported in this paper, PI%was of the order of 8 cal. per liter. This implies that a strong base can be titrated acidimetrically with an accuracy within 3% in a concentration of the order of 5 X 10-4M (AH" = 13.4 kcal.) Rapid Quantitative Evaluation of
.
Concentrations Using Nonstandardized Titrants. Equation 3 can be
applied directly to quantitative estimations, if AH is known. The quantity
Table 111. Stochastic Characteristics a t 25" of Chelation Equilibria for Calcium and Magnesium Ions" 720 fir A
Ion
pK
Ca++
-11 0 9 1
hIg++
-
Kcal.)
AS"
-15.0 -12.4
$31 +60
Mole
E.UI
4 Calculated for Reaction 1 from AH' values listed in Table I1 and from free energy data given by Charles (2).
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. In 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 EDTA 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., Furman, 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
