A N A L Y T I C A L CHEMISTRY
880 Table 111. Analysis of Epoxide-.4ldehyde-WaterMixtures Composition, % Observed, Mixture Ethylene oxide-acetaldehyde
Epoxide
Aldehyde
Epoxide
Aldehyde
9 0
0.97
8 9
0.94
6 9
Ethylene oxide-acetaldehyde Propylene oxide-propionaldehyde Propylene oxide-propionaldehyde
0 36
6 9
0 36
5 5
0 21
5 :
0 21
8 8
1 09
8 4
1 09
Isobutylene oxide-isobutyraldehyde
8.G
0.61
7.5
0.67
1,2-Epoxybutane-n-butyraldehyde
5 5
0.36
5.5
0.33
Obviously, samples taken for analysis must be essentially neutral or a correction must be made for acidity or alkalinity This may be conveniently done by adjusting the sample directly, or an aliquot ma)- he taken and titrated to provide thc required correc-
tion for subsequent calculations. The same end point that is used in the epoxide analysis should be observed. LITERATURE CITED
Eastham, A. M., and Latremouille, G. A,, Can. J . Research, 28B, 264 (1950). King, G., J. Chem. SOC., 1951, 1980. King, G., Nature, 164, 706 (1949). Lubat,ti, 0. F., J . SOC.Chem. Ind.,51T,361 (1932). Nicolet, B. H., and Poulter, T. C., J . A m . Chem. SOC.,5 2 , 1186 (1930). Ripper. M.,Monatsh., 21, 1079 (1900). Ross, W. C. J., J . Chem. SOC.,1950, 2257. Schenck, R. T. E., and Kaiserman, S., J . A m . Chem. SOC.,75, 1636 (1953). Siggia, S., “Quantitative Organic Analysis Via, Functional Groups,” p. 108, Xew York, John Wiley & Sons, Inc., 1949. Swern. D.. Findley, T. W., Billen, G. N., and Scanlan, J. T., As.ir.. (.HEM., 19, 414 (1947). Walker, J. F.. “Forrnaldehyde,” p. 757, Xew York, Reinhold Publishing Cor[)., 1944. RECEIVED for review December 2 1 ,
1Hj3.
Accepted February 23, 1954.
Determination of Tritium by Ion Current Measurement K. E. WILZBACH, A. R. VAN DYKEN, and LOUIS KAPLAN 111.
Chemistry Division, Argonne N a t i o n a l Laboratory, Lemont,
The increasing use of tritium in chemical and biological research has led to a need for convenient and reliable methods for its determination. Since the measurement of ion current with the vibrating reed electrometer has proved highly satisfactory for carbon-14 assay, the applicability of this method to tritium determination has been investigated. Relative ion currents collected from several gases in a Borkowski chamber have been determined under various conditions. The results establish that the current is proportional to the disintegration rate over a wide range of tritium concentrations and is not sensitive to changes in specified conditions of filling pressure and collecting voltage. 4 s few as 10 disintegrations per second in 250 cc. of gas can be determined with an accuracy of 1%. The charge collected per disintegration has been determined in methane, propane, and hydrogen by calibration with a known sample of tritium to permit conversion of ion currents to disintegration rates.
T
RITIUM, the radioactive isotope of hydrogen, is potentially an extremely useful tracer in chemical and biological research. Its determination, however, suffers from the handicap imposed by the low energy of the beta particle emitted in its disintegration. The maximum beta energy of 18.9 e. kv. (ZO), corresponding to a calculated (16) range of 0.9 mg. per square cm., precludes the use of end window counters, and makes the counting ( 4 ) of thick samples in windowless counters inefficient. Radioautography (6), although useful for many purposes, is likewise very inefficient with samples of finite thickness. Measurement with high efficiency is more readily attained if the disintegrations take place within the active volume of the detector. Of the available methods for achieving this condition, scintillation counting of solutions (5, 11) containing liquid scintillators offers considerable promise as a means of rapid analysis, but much development appears to be required before it can be relied upon to give accurate results in routine analyses of a wide variety of materials. More widely used, a t present, are proce-
dures in which the tritium is introduced in the form of a gas into the detector, the disintegration rate being measured by the radiation-induced ionization of the gas. Depending on the field strength employed and on the esternal circuitry, the disintegrations may be recorded as discrete pulses by a counting tube operating either in the Geiger (8, 9, 18) or proportional (4, 20, 23) region or as the total ionization collected during a given time in an ionization chamber (1, 1 2 ) . I n this laboratory, the measurement of ion current, with equipment routinely used for carbon14 analysis (2, s),has been found to be a convenient method for the determination of tritium in gases which are readily prepared from water or organic compounds. The factors which affect the measured ion current have been investigated and conditions have been established for the determination of 3 X 10-’0 to 0.01 curie of tritium with an accuracy of 1%. THEORY
The chamber used for ion current measurement (24)in gas ssmples consists of a gas-tight conducting shell having an insulated electrode and a guard ring to shield the insulator. The ions of a given sign produced by radiation in the gas contained in the chamber are collected on the electrode by applying to the shell a potential low enough so that multiplication of ions does not occur. The rate of ion collection is measured by an electroscope or an elpctrometer. The measured current includes not only that
Table I.
Ionization Produced by Tritium Radiation in Several Gases
Ion Pairs,/ Gas Rel. Ionization ( 2 8 ) Disintegration’” 150 Hydrogen 0 78 159 Nitrogen 0 81 Air 163 0 86 0 92 175 Helium 177 Oxygen 0 93 Methane 1 00 188 1 11 211 Argon a Based on a value of 2 7 . 0 e v . per ion pair in argon and 5.69 e . h . for the average energy of tritium radiation.
V O L U M E 2 6 , NO. 5, M A Y 1 9 5 4 from the radiation in the sample, but also a background, arising from instrumental drift and extraneous radiation, which must be subtracted. The net current has a limiting value which would be obtained if all of the energy of radiation in the sample were absorbed in the gas and all of the ions so produced %-erecollected. This value is equal to the product of the disintegration rate of the sample, the average energy per disintegration, and the number of ion pairs produced per unit of energy expended. The first two of these are fixed by the activity being measured, u-hile the last varies with the gas. The number of ion pairs produced in various gases by tritium radiation, calculated from the data (22) of Valentine and Curran, using 5.69 e.kv as the average energy (14) of disintegration, is shown in Table I. I n practice, the net current is less than the limiting value because of the loss oi energy to the walls of the chamber and the loss of ions by recombination. Some loss of ions by recombination occurs in the volume of the chamber not exposed to the electric field; the fraction lost is determined by the design of the chamber. The loss of ions by recombination in the active volume is a function of the field strength and of the nature, concentration, and mean free path of the ions; a t constant field strength it increases with the pressure and rate of ionization of the filling gas. The loss of energy to the walls is decreased by increasing the dimensions of the chamber, the pressure. or the number of electrons in each molecule of the gas. EQUIPMEYT
881 cept for statistical fluctuations; abrupt increases occasionally caused by alpha particles must be subtracted. Measurement is continued until the rate of charge has been determined with the desired precision. A current greater than ampere is measured by the voltage drop across a resistor. An adapter is mounted on the electrometer head and the resistor which will give the greatest voltage drop measurable on the electrometer is connected between the input terminal and the feedback circuit. The shorting switch is opened and the deflection is recorded until a satisfactory visual average of the statistical fluctuations can be made; occasional larger pulses caused by alpha particles are disregarded. The voltage drop can be determined with greater accuracy if the electrometer is operated as a null instrument by applying from the external potentiometer an opposing voltage. The background, determined similarly with the chamber filled with inactive gas, is subtracted from the observed reading. The reading is converted either to a relative or to an absolute ion current, which, as shown below, is proportional to the tritium content. Factors for converting readings to relative ion currents are established by intercomparison: An intercomparison of two resistances differing by a factor of 10 is best obtained from the ratio of the voltage drops produced across them by a single sample; the rate of charge produced by a suitable sample is compared with the voltage drop across the loz1 ohm resistor. Absolute ion currents may be calculated if the value of a resistance or of the capacitance is known. The tritium content may be determined from the relative ion current if the ionization chamber has been calibrated with a sample containing a known amount of tritium. RESULTS
Ionization Chamber. ,I cylindrical ionization chamber 7 cm. in diamrter with a volume of approximately 250 cc. was constructed of welded stainltw steel using a design by Borkowski ( 2 ) . il chamber, Model 3095, having the same dimensions and efficiency but differing in the materials and details of construction is available from the Applied Physics Corp., Pasadena, Calif. -4 diagram of the chamber is given by Brownell and Lockhart ( 3 ) . Voltage Source. Dry cells (90-volt) are connected in series to give the desired voltage. Electrometer. A vibrating reed electrometer (19), -4pplied Physics Corp., Model 30, is used for the ion current nieasurements. Resistors. Glass-covered, vacuum-sealed resistors, supplied by Victoreen Instrument Co., Cleveland, Ohio, are used to measure currents larger than 10-13 ampere. A set of four having resistances of 10s to 10" ohms permits the determination of quantities of tritium up to 10 millicurie.. .In insulating and shielding adapter, Model 3096, for mounting the resistors can be supplied with the electrometer. Recorder. The rlectrometer output is recorded on a Brown Elektronik strip chart potentiometer, Model 153x12, with a pen speed of 4I/*seconds for full scale travel, a chart speed of 20 inches per hour, and a full scale range of 25 mv., graduated from 0 to 100. Potentiometer. A Rubicon potentiometer having ranges of 0 to 160 mv. and 0 to 1600 mv. is convenient if it is desired to operate the electrometer as a null-point instrument. PROCEDURE
For a determination of tritium, the chamber is evacuated to a pressure of mm. or less and a sample of the gas to be assayed is introduced. The entire sample can be transferred t o the chamber with a Torpler pump or a known fraction of the gas can be introduced by expansion. Small amounts of a condensable gas can be introduced quantitatively by expanding from a small U-tube if diluent gas is added through the same tube. I n routine measurements, the chamber is filled to a pressure slightly greater than atmospheric, adding diluent gas if necessary. The chamber is mounted on the electrometer and a voltage negative with respect to ground is applied to the shell. current less than ampere is measured by the rate of charge of a capacitance of about 12 X 10-12 farad within the electrometer. The shorting switch of the electrometer ip opened and a range is selected such that the charge on the condenser, which is continuously recorded, increases a t a suitable rate. The rate of charge may change initially until extraneous charges, such as those produced by insulator strain, have been dissipated. The charge produced by the sample should increase uniformly ex-
The ion current collected from a given quantity of tritium in an ionization chamber depends upon the nature of the filling gas, the filling pressure, and the collecting voltage. The effect of these factors has been investigated to provide information on the significance and reproducibility of ion current measurements \\-ith the described chamber under various conditions. The dependence on collecting voltage and filling pressure was studied in hydrogen, methane, and propane, which are gases likely to be used for tritium assay because convenient methods are available for converting the hydrogen in water or organic compounds to hydrogen gas (1, 4,9, id, 18) or a hydrocarbon (8, 20,23, 85). The chamber has been calibrated with a standard tritium sample to permit the conversion of ion current measurements to absolute disintegration rates. Table 11.
Ionization Collected from Tritium Radiation in Several Gases Gas
Rel. Ionization 0.685 0.82 0.84 0.84 1.00
Hydrogen Nitrogen Helium Carbon dioxide Methane 1.06 Argon 1.13 Propane Based on oslibration described in text.
Ion Pairs/ Disintegration" 132
158 162 162
193 205
218
The ion currents collected in several gases from small amounts of hydrogen-trit'ium mixture in chambers filled to atmospheric pressure were measured a t saturation voltage. The ionization. relative to that in methane, is given for each gas in Table 11, together x i t h the number of ions collectrd per tritium disintegration. Comparison with the values in Table I indicates that escept for the gases of lowest stopping power, hydrogen and helium, the efficiency of the chamber is greater than 95%. The variation of ion current with collecting voltage is shown in Table I11 for chambers filled to atmospheric pressure with hydrogen, methane, or propane containiiig various quantities of tritium in small amounts of carrier hydrogen. I t is seen that the 1 0 s ~of ions by recombination increases viith the tritium content,
ANALYTICAL CHEMISTRY and, a t a given concentration, increases in the order: hydrogen, methane, propane. Routine measurements are made using a potential of 180 volts for hydrogen, 360 volts for methane, and 450 volts for propane. The current measured under these conditions is essentially the saturation value for quantities of tritium up to 0.1 mc. in propane and a t least 1 mc. in methane and 5 mc. in hydrogen.
Table 111.
Effect of Collecting Voltage on Ion Current
Tritium Content, hlc.
90 volts
0.78 5.30
99.1 96.4
99.7 99.4
99.8 99.8
99.9 99.9
Methane
0.01 1.00
99.2 92.6
99.8 98.6
99.9 99.6
100.0 99.9
Propane
0.01 0.11 0.93
97.0 54.7
99.2 95.7 82.1
99.6 98.4 92.7
99.9 99.5 97.5
Gas Hydrogen
85,l
Relative Ion Current= 180 volts 270 volts 360 volts
of hydrogen were generated from each of a series of dilutions of tritiated water, as well as from distilled water blanks, by reaction with zinc in a sealed tube ($5). The samples were expanded into the chamber, diluted to atmospheric pressure with methane, and the ion currents were determined. The results, summarized in Table V, show that, except a t the lowest tritium concentration, the standard error of a single determination is 1% or less. Over the entire range of concentrations, the factor relating ion current to disintegration rate varies by less than 1%, with no indication of a trend. A better estimate of the precision of assay of small amounts of tritium mas obtained in a separate experiment using more samples. The results in Table VI indicate that, with a measuring time of 1 hour each for sample and background, 25 disintegrations per second can be measured with a standard error of 1.3%.
Current collected a t 450 volts defined as 100.
Table IV.
Variation in Ion Current with Filling Pressure
Relative Ion Current= Gas 100 mm. H g 400 mm. Hg 600 mm. Hg 700 mm. Hg Hydrogen 55.4 92.0 97.6 99.3 Methane 85.0 97.3 99.2 99.8 Propane 92.5 99.3 99.8 100.0 a Current with filling gas a t atmospheric pressure is defined as 100.
The current a t various pressures, relative to that a t atmospheric pressure, is shown in Table IV. For convenience the chamber is filled to atmospheric pressure for routine measurement. Only in the case of hydrogen are larger currents obtained a t higher pressures, but even with this gas the variation of current with pressure is small a t l atmosphere. The variation of ion current with composition has been studied for some mixtures of gases which might be introduced into the chamber deliberately, to analyze a reaction product or to permit the reisolation of the active gas from the diluent, or accidentally, as by leakage of air. The relative currents a t saturation voltage in mixtures of hydrogen with methane and propane a t a total pressure of one atmosphere are shown in Figures 1 and 2, respectively. The effect of air in hydrogen and in methane is also khown in Figure 1. Ion current measurements in both methane and propane are relatively unaffected by the presence of other gases; readings in hydrogen are considerably more sensitive to impurities. The data presented show that reliable ion current measurements can be obtained in all the gases studied, even under conditions which differ considerably from those routinely employed. There are, however, some advantages in the use of methane as a filling gas wherever practicable. I n this gas, loss of ions by recombination is much less serious than in propane and the effects of pressure and of impurities are less than in hydrogen. The precision of tritium assay in the range of 10-15 to 10-9 ampere was determined in an experiment designed also to test the proportionality of ion current to tritium content. Samples
6 ; z F U 3
\ 1
0.80
o
:
loa
IIR
0 = 211
cn4
IN
AIR IN RZ
0.70
0
20
10
30
40
50
60
70
80
90
1.00 100
Hz, %
Figure 1. Ion Currents at Saturation Voltage in Hydrogen-Methane-Air Mixtures at 1 Atmosphere
Table VI. Sample Size, Mg. Ha0 4.81 4.75 4.84 4.73
Precision of Low-Level Tritium -4ssay
Ion Current, Amp. X 10lna Total Blank Net' 11.04 10.83 11.19 10.83
2.90 2.97 2.94 2.87
8.14 7.86 8.23 7.96
Spec. Current, Amp. X 101% Mg. H20 1 692 1 655 1 705 1 683 1.684
Mean Std. dev. 0.021(1.3%) a I n methane, 10-16 ampere = 3.23 disintegratiors per second, all currents were measured for l hour, starting 20 minutes after filling the chamber.
The value of the factor for converting ion current to disintegration rate was determined ( 7 ) for methane using a sample with a known disintegration rate. Hydrogen gas of known tritium content was obtained by dilution of a mass s p e c t r o m e t r i c a l l y analyzed sample of essentially pure tritium gas. Table V. Proportionality of Ion Current to Disintegration Rate The ion currents from small samples of Measured this gas in a chamber filled with methRelative Ion Current, Time/ ane to a pressure of 1 atmosphere were Amp. x io'no N o . of Measurement, Rel. Current per Dev. from Concn. Mg. Ha0 Analyses Min. Errorb, % Disintegration Mean. % of T measured a t saturation voltage using re1 x 108 1.734 X 108 4 5 0.35 1 -0.9 sistors calibrated by the method of 0.980 X lo' 1.720 X lo' 3 5 0.35 1.012 +0.3 3 15 I .03 1,010 t o 1 0.949 X 102 1.663 X 102 Lynch and Wesenberg (17). The mean 9.50 16.63 3 25 0.22 1.010 +o. 1 1.049 1.824 2 50 2.61 1 ,003 -0.6 values obtained in two independent exWrichtrd - ...~ .-periments, based on a half life (14, 15) of mean 1.009 Background current about 6 X lO-!e,ampere,; chamber contained hydrogen from about 5 rng. 12.4 years, are 3.04 X 1O-l' and 3.14 X water: in methane, 10-18 ampere = 3.23 disintegrations per second. lo-'' coulomb per disintegration. From b Standard error of single determination. the average of these values for methane Q
883
V O L U M E 26, NO. 5, M A Y 1 9 5 4 Table VII.
Relation of Ion Current to Disintegration Rate
Gas
Coulombs/Disintegration x 1017
Curies/Ampere
Hydrogen Methane Propane
2.12 3.09 3.49
1.275 0,874
I00
90
80
I
1.0
x 10-0 0.775
70
60
50
40
30
I
I
I
I
I
20
10 I
0 J
- 1.60 0
0 ”
that 13 tritium disintegrations per second (4 X 10-16 ampere in methane) can be determined with an accuracy of 1% for a time of measurement of 1 hour each for sample and background. The results in Table VI show that samples containing 25 disintegrations per second can be analyzed with this precision. The maximum quantity of tritium which can be measured is that above which recombination cannot be prevented without using a collecting voltage so large as to cause leakage of current or multiplication of ionization. The data in Table I11 indicate that in hydrogen or methane saturation currents are obtained at moderate voltages for quantities of tritium up to 10 me. LITERATURE CITED
-
-
6
0.9 -
I I
-
o 0 c z Y z Y
-
0
-
a a 0.8
1.50
2
I N
u- 1.40 & W
-
- 1.30
3
2
1.20
?
1.10
0.6 0
I
I
I
I
I
I
1
10
20
30
40
50
60
70
Hp,
.
\
1.00
A
I
80
90
100
%
Figure 2. Ion Currents at Saturation Voltage in Hydrogen-Propane Mixtures at 1 Atmosphere
and from the relative ion currents given in Table 11, the factors in Table VI1 have been computed. The method of tritium assay which has been described is useful over a t least a 108-fold range of tritium activities. The lower limit is determined by the magnitude and reproducibility of the background current. I n the authors’ experience this can be easily maintained near 5 X ampere, of which 2 X 10-16 ampere is caused b y penetrating radiation, including cosmic rays, and 3 X 10-lB ampere arises from soft radiation. If i t is assumed that each ionizing event produces 300 ion pairs, a value between the 450 ion pairs produced ($1) by a cosmic event and the 200 produced by a tritium disintegration, i t can be calculated (13)
(1) Biggs, M. W., Kritchevsky, D., and Kirk, XI. R., A N ~ LCHEY., . 24,223 (1952). (2) Borkowski, C. J., U. S. Atomic Energy Commission, Doc. MDDC 1099 (declassified June 12, 1947). (3) Brownell, G. L., and Lockhart, H. S., Nucleonics, 10, No. 2, 26 (1952). (4) Eidinoff, M.L., and Knoll, J. E., Science, 112, 250 (1950). (5) Farmer, E. C., and Berstein, I. A., Ibid., 117, 279 (1953). (6) Fitegerald, P. J., et al., Ibid., 114, 494 (1951). (7) Flotow, H., Weinstock, B., and Wilabach, K., drgonne Natl.
Lab. Rept., to be issued. Glascock, R. F., Nucleonics, 9, KO.5, 28 (1951). Grosse, A. V., et el., Science, 113, 1 (1951). Hanna, G., and Pontecorvo, B., Phys. Reu., 75, 983 (1949). Hayes, F. K.,and Gould, R. G., Science, 117, 480 (1953). Henriques, F. C., Jr., and Margnetti, C., IND.ENG. CHEY., ANAL.ED.,18,420 (1946). (13) Janney, C. D., and Noyer, B. J., Rev.Sei. Instr., 19, 667 (1948). (14) Jenks, G. H., Ghormley, J. A., and Sweeton, F. H., Phys. Rev., (8) (9) (10) (11) (12)
75,701 (1949). (15) (16) (17) (18) (19)
Jones, W. RI., Ibid., 83, 537 (1951). Libby, W. F., IND.ENG.CHEM.,ANAL.ED.,19, 2 (1947). Lynch, F., and Wesenberg, C., Rev. Sci. Instr., in press. Melander, L., Acta Chem. Scand., 2, 440 (1948). Palevsky, H., Swank, R. K., and Grenchik, R., Rev. Sei. Instr.,
18,298 (1947). (20) Robinson, C. V., Ibid., 22,353 (1951). (21) Stranathan, J. D., “The Particles of Modern Physics,” Philadelphia, Blakiston Co., 1942. (22) Valentine, J. hl., and Curran, S. C., Phil. Mag., 43, 964 (1952). (23) White, D. F., Campbell, I. G., and Payne, P. R., Nature, 166, 629 (1950). (24) Wilkinson, D. H., “Ioniaation Chambers and Counters,” London, Cambridge University Press, 1950. (25) Wilabach, K. E., Kaplan, L., and Brown, W. G., Science, 118, 522 (1953). RECEIVED for review October 27, 1953. Accepted February 15, 1954.
Cupferron and Neocupferroa Complexes of the Rare Earth Elements ALEXANDER I. POPOV and WESLEY W. WENDLANDT D e p a r t m e n t o f Chemistry, State University of Iowa, Iowa City, Iowa
Cupferron and neocupferron show definite promise as precipitating reagents for the rare earth ions. The exact conditions of precipitation and the properties of the resulting complexes have not been reported in literature. In this work it was found that precipitation seems to be quantitative, but the precipitates are somewhat contaminated by coprecipitation. Ignition of the cupferrates and the neocupferrates to corresponding oxides yields analytical results which compare favorably with the oxalate method. The precipitates are somewhat soluble below pH 2.0 and the optimum pH for precipitation seems to be around 3.5. Conductimetric and high-frequency titrations indicate formation of an intermediate 1 to 1 complex MCup++.
A
LTHOUGH cupferron (ammonium nitrosophenylhydroxylamine) and neocupferron (ammonium nitrosonaphthylhydroxylamine) have been used extensively as precipitants for various inorganic cations during the past 40 years (3, 6), very little work has been done so far on their reactions with the rare earth elements. The existence of cerium(III), cerium(1V) (I), and neodymium ( 8 ) cupferrates, as well as that of neodymium neocupferrate ( a ) , has been reported in the literature. It was determined that the complexes were relatively insoluble in water and in organic solvents, but beyond that their properties were not described. This investigation was undertaken in order t o obtain a more detailed picture of the types of complexes formed and of some of their properties.
