Table 111. Comparison of the Analysis of Gold Alloys Samole nominal gold content in karats
Mathematical treatment and NAA
10 12 14 16 18 20 22
41.97 f 0.39 46.56 + 0.55 61.13 f 1.40 62.62 f 0 . 9 4 73.46 f 0.72 79.18 f 0.86 87.63 f 0 . 3 8
~
-
_
Au, _ Z _
Standard comparison, NAA 40.79 46.64 60.53 62.64 72.80 79.02 88.14
Ag,
-
f 0.34 f 0.34 f 0.68 f 0.81 f 0.90 =t 0.49 f 0.36
Mathematical treatment and NAA 13.41 f 0.45 10.39 =t 0.61 16.00 f 0.27 12.79 f 0.27 16.33 f 0.34 9.34 f 0.94 0.16 =t0.04
between two identical samples. The comparison of a n average specific activity of the samples with the specific activity of the standard diminishes some of the effects due to flux depression and flux inhomogeneity. CONCLUSIONS The method developed in this work can be applied to the quantitative analysis of macroconstituents when their identities are known and provided they can be activated by n,y reactions. Moreover, the method can be generally applied to determine the concentration ratio between two elements present in any matrix with the aid of Equations 9 and 10. Once the concentration of one of these constituents is known, it is possible to render quantitative data about their concentrations. Thus, it is not necessary, as is often done in conven-
Z
cu, %
Atomic absorption
Mathematical treatment and NAA
Atomic absorption
14.20 f 0.34 11.06 i 0.26 17.62 f 0.30 14.52 f 0 . 3 5 17.28 f 0.51 10.03 i 0.78 0 . 0 8 + 0.03
44.61 f 0.41 43.05 f 0.99 22.86 f 1.55 24.59 i 0.96 10.20 f 0.49 11.47 =t 0.81 12.20 f 0.36
45.36 f 0.41 44.52 f 1.02 21.73 f 0.30 24.10 =I= 0.86 9.83 f 0 . 3 3 11.62 f 0.61 11.91 =t0.20
tional NAA, to prepare standards for each of the constituents to be analyzed. The method is amenable for the treatment of the spectral data with computers. For this reason, a large number of k values of the majority of elements normally present in alloys are given in this work. The method is rapid and precise since errors due to neutron irradiation are minimized. ACKNOWLEDGMENT
Thanks are due to E. GonAlez for the atomic absorption analysis. The authors are also indebted to T. Monsalve for technical assistance. RECEIVED for review April 18, 1972. Accepted August 22, 1972.
Determination of Fluorine and of Oxygen in the Presence of Fluorine by Selective Neutron Activation Using Californium-252 and a 14-MeV Generator J. J. Lauff, E. R. Champlin, a n d E. P. Przybylowicz' Research Laboratories, Eastman Kodak Company, Rochester, N . Y. 14650 A method has been developed for neutron activation analysis of fluorine using a californium-252 neutron source. In addition, a differential method has been developed for the analysis of oxygen in the presence of fluorine by the complementary use of a californium-252 neutron source and a 14-MeV neutron generator. The sensitivity for the fluorine determination is 0.4 mg fluorine in a 10-gram sample; that for oxygen i s 0.04 mg. Both methods are nondestructive, interferencefree, and applicable to either organic or inorganic matrices. A precision of 1% is obtained for the macrolevel fluorine determination and 2% or more for the differential oxygen measurement. Fluorine analyses of the same materials by a spectrophotometric method and by neutron activation with californium-252 provide a basis for comparison of the two methods. The californium-252 procedure yields results with better precision and accuracy based on fewer determinations, although a much larger sample (1 gram) is required.
SEVERAL UNIQUE physical properties make californium-252 attractive as a maintenance-free, continuous neutron source for activation analysis. Its high specific activity (2.34 X lo'? To whom correspondence should be addressed. 52
neutrons per second per gram) coupled with a relatively low alpha yield allows fabrication of neutron sources with significantly smaller physical dimensions than was previously possible. An equivalent americium-beryllium neutron source must be over l o 4larger in volume to accommodate the helium produced by alpha decay. A californium-252 source with a yield of 5 x 10'0 neutrons/sec dissipates only 0.8 W of thermal energy to its environment (as. 750 W for Am-Be), thus essentially eliminating the need for heat-dissipating surfaces (e.g., fins, heat exchangers). These characteristics greatly lower source fabrication and shielding costs and permit analytical samples to be placed very close to an intense, encapsulated isotopic source of neutrons. Two of the strongest advantages of neutron activation analysis are that the technique is nondestructive, and sample solubility is not required for analysis. Several wet chemical methods for the determination of macro-level fluorine exist, but, in addition to being prone to interference from a number of common elements, they require complete dissolution of the sample for an accurate determination. Activation analysis of fluorine with a californium-252 source has been investigated by the National Bureau of Standards and a preliminary report
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
Table I.
Reaction l9F(n,p)lSO lgF(n,a) 16N 19F(n,2n)18F 19F(n,y)zaF
Neutron Reactions with
Threshold (e),MeV -3.99 -1.49 -10.43
Emitted y-radiation, MeV 0.2
6.1, 7 . 1
0.51
1.63
I9F
tli2
29 sec 7.2sec 110 min 11 sec
(1)
(2) (3)
(4)
of the work has been published (1). Ricci and Handley ( 2 ) have demonstrated negligible activation of oxygen by neutrons from californium-252 but did not investigate the possibility of fluorine activation. Two wet chemical methods are used widely for the analysis of fluorine. A spectrophotometric method based on the work of Hansley and Barney (3) utilizes the metathetical reaction of fluoride ion with thorium chloranilate in ethyl alcohol, buffered a t p H 4.5. The absorbance of the chloranilate released is measured a t 330 n m and a calibration curve ranging from 0.2 t o 1 mg of A uorine is linear. Interferences include many common cations such as aluminum, copper, zinc, and nickel. Sulfur does not interfere; phosphorus does, but it can be separated prior to reaction with the chromogenic reagent. A potentiometric titration method for fluoride similar to that reported by Light and Mannion ( 4 ) is also commonly used. The titrant is0.005M thorium nitrate and the end-point detector is a n Orion specific ion electrode, Model 94-90. The p H of the alcoholic solution containing from 1 to 10 mg of sample must be held a t 5.0 or above to prevent complexation of the fluoride. Interferences include most polyvalent cations, phosphorus, and sulfur. These can be controlled by use of high p H or prior separation. Common to both procedures is a sample-preparation step involving burning the sample in a quartz oxygen flask as described by Schoniger (5). In addition to these classical methods, several neutron activation techniques have been reported in the literature. Neutrons interact with fluorine in four different ways as shown in Table I. As the thresholds in Table I show, the first three reactions require high-energy neutrons, whereas the fourth is a neutroncapture reaction requiring a relatively intense source of thermal neutrons. Thus, it can be used only by analysts having access to a nuclear reactor (6). The remaining three reactions can be observed by means of 14-MeV neutron activation using a n accelerator. The use of reaction 1 is inconvenient because of the high Compton background and possible interferences found in the low-energy region of the principal gamma ray. Although a coincidence method employing the less intense 1.37-MeV gamma ray also emitted by this reaction should alleviate this problem, its sensitivity is not sufficiently high to give precise counting statistics (7). Methods employing reaction 3 also suffer from matrix effects since any positron emitters other than I8Fproduced by (1) “Californium-252 Progress,” No. 7, United Slates Atomic Energy Commission, April, 1971, p 32. (2) E. Ricci and T. H. Handley, ANAL.CHEM., 42, 378 (1970). (3) A. L. Hansley and J. E. Barney, ibid., 32,828 (1960). (4) T. S. Light and R. F. Mannion, ibid., 41,107 (1969). (5) W. Schoniger, Mikrochim. Acto, 1956, 869. (6) G. J. Atchison and W. H . Beams, ANAL.Cmai., 28, 237 (1956).
(7) L. C. Nelson, Jr., and H. Russell, “Annual Progress Report,” No. NBL,-230, United States Atomic Energy Commission, Dec., 1965, p 23.
Figure 1. Layout of Eastman Kodak neutron activation analysis facility; cavity and labyrinth are below ground and outside of building the neutron irradiation will interfere with the analysis (8, 9 ) . Reaction 2 can be used, but since 14-MeV neutron irradiation of oxygen produces the same I6N isotope, its utility with accelerator-produced neutrons is restricted to specific matrices in which oxygen is absent or remains a t a constant level. The use of californium-252 neutrons in reaction 2 results in a practically matrix-independent fluorine analysis method. Since the neutron energy spectrum of californium-252 exhibits a negligible flux at energies above 10 MeV, the threshold energy for the oxygen (n, a) reaction, its activation by californium-252 irradiation is small, being only about 1/200 a s intense as that of the fluorine reaction. If the sample contains less than 1 0 % oxygen, no correction is necessary since the counts attributable to oxygen activation are much less than the counting error. If a sample contains more than 10 oxygen, only a n order-of-magnitude estimate of the oxygen content suffices to correct for its contribution to the gross counts in the l6N photopeaks. EXPERIMENTAL
Instrumentation. The Cockroft-Walton 14-MeV neutron generator is a n Accelerator I manufactured by Accelerators, Inc. of Austin, Texas. A t an acceleration potential of 150 kV and the maximum deuteron beam current of 3.5 mA, yields as high as 3.5 x 10” neutrons per second can be realized when 5-Ci tritium targets are used. Useful target life is about 4 hr a t the normal operating level of 0.6 mA; the reaction involved is 3H(d,n)4He. The californium-252 sources are contained in a 40- by 40by 60-inch distilled-water-filled biological shield erected in the corner of the sub-basement cavity which also houses the 14-MeV neutron generator (Figure 1). The tank is l/q-inch wall, type 316 stainless steel; all welded seams were passivated to prevent corrosion. Sixteen inches of concrete block were cemented in two 8-inch-thick ofTset rows around the outer walls of the tank (Figure 2 ) to provide added shielding. A 12-inch-diameter, cylindrical inner core was welded to the far corner of the large tank. The eight californium-252 (8) G. L. Priest, F. C. Burns, and H. F. Priest, ANAL.CHEM.,39, 110 (1967). (9) E. A. M. England, J. B. Hornsby, W. T. Jones, and D. R. Terrey, Anal. Chim. Acta, 40, 365 (1968).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
53
Corner of Room
-4
I Borated
Polyethylene Plug Plug
Disposoble sample insert (Volume 10 cc)
Plug
Figure 4. Sample carrier for neutron activation analysis
Figure 2. Californium-252 irradiation facility
--
50
25-1-400
--
Figure 3. Holder for californium-252 neutron sources sources were arranged in a circular array at the bottom of the cylindrical inner core (Figure 3). The outer tank is always completely filled with distilled water, but a stainlesssteel pumping system provides for transferring the water from the inner core to a storage tank from which it can be returned by gravity. The eight californium-252 sources, ranging in weight from 582 to 758 pg, were fabricated at the Savannah River Laboratory of the United States Atomic Energy Commission and are a slightly modified type AA, 100 series. The total 5.47 mg of material yields a flux of about lo7 n cm-2 sec-I at the sample site when the inner tank is filled with water. The axes of the sources are approximately 11i3inches from the axis of the sample tube. A pneumatic transfer system controlled by Picker-Nuclear electronics, as modified by Reactor Experiments, is used to deliver samples to and from the irradiation sites and the counting station. A routing switch operated from the control room selects either the line to the californium-252 source or the branch to the biaxial sample rotator at the 14-MeV neutron generator head. Samples travel to the californium-252 source through a 3/4-inch i.d. stainless-steel tube which penetrates the polyethylene plug and carries them to the geometric center of the circular source array. The counting station consists of two 3- by 3-inch sodium iodide crystals arranged face-on around the sample counting position in order to provide maximum counting efficiency. 54
0
A '/*-inch Lucite (DuPont) /%shield is used in front of each crystal. The pair of thallium-activated NaI crystals are Harshaw "integral line" assemblies having better than 7 pulse-height resolution, matched to within 0.05 %. The current output of each photocathode is fed to a n Ortec Model 113 preamplifier. Bias voltage for the photomultipliers is provided by Fluke Model 412B high-voltage power supplies. The outputs from both preamplifiers are combined and fed into an Ortec 440A linear amplifier. The amplifier output then goes through a single-channel analyzer (Ortec Model 406A) to the multiscaling input of a Nuclear Data Model 2200, 1024 channel analyzer. Visual data display is provided by a Tektronix type R M 503 oscilloscope, and hard copy of data is obtained from an IBM Selectric typewriter or a Tally paper-tape punch (Model 420). Sample Preparation. Since sample rotation is not provided in the californium-252 irradiation facility, sample packing as well as reproducible irradiation and count positioning have proved to be critical factors affecting the precision of the fluorine analysis. Originally, anomalous results involving unacceptably large precision errors (2-4 %) were obtained and traced to nonuniform packing. Because of the point-source nature of the eight californium-252 sources, an anisotropic neutron flux distribution results, causing the flux to drop by approximately 30% at the ends of the sample carrier from that at the center. Consequently, any variation of sample length o r of packing density over the length of the sample can result in large errors. The samples are packed in 1/4-inch diameter, heat-shrinkinches long. Since the sample able polyethylene tubing, 113/16 is transferred under 90 lb of pressure, it is necessary to use a die and press to pack the sample tightly enough that n o further compacting o r distortion arises during the analysis. Polyethylene plugs '/?-inch long are sealed into the ends of the tubing such that the sample is 13/16-inChlong and positioned in the center of the tube. The packed sample is then placed inside a polyethylene carrier (Figure 4) specifically designed for fast transfer in the pneumatic transfer system. The sample tube contains between 0.7 and 1.5 grams of sample depending on the density of the materials to be analyzed. Method. FLUORINE DETERMINATION. Analysis of fluorine is accomplished by measuring the induced 16Nactivity using a modified multiscaling technique. The 6- to 7-MeV gamma rays are selectively counted by routing the output pulses from the dual crystal detection system through the single channel analyzer such that only input pulses of 4- to 8-MeV energy enable an output pulse. This output is then accumulated in the multiscale recur mode of the 1024 channel analyzer using a dwell time of 40 msec per channel. An exponential decay curve results, which is totalized over channels 1 to 1023, giving the net number of counts. This number, after appropriate background correction, is proportional to the weight of fluorine in the sample. Each fluorine determination with californium-252 as the neutron source comprises four individual cycles, each cycle consisting of a 4-sec delay to transfer the sample pneu-
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
Compound
zF, theor.
CaFz
48.7
Teflon (DuPont)
76.0
4,N,Rr-Dimethylaminobenzene-
38.9
Table 11. Fluorine Analysis F, spectrophotometric
z
76.2 75.8 76.7 76.7 77.6 77.7 77.4 73.6 75.5 77.6 77.6 76.4 AV = 76.6 i 1.4*
diazonium hexafluorophosphate Fluorobenzenesulfonyl chloride
9.74
9.76 9.30 9.74 9.62 9.14 9.64 8.80 9.60 8.87 9.31 9.14 9.95 AV = 9.41 f 0.37 30,14 30.22 29,58 Trifluoroacetanilide 29,72 29.26 30.35 29.65 39.41 30.02 29.32 29.16 28.90 30.17 AV = 30.45 =t0.44 13.56 13.47 13,22 Fluorobenzoic acid 13.94 13.26 13.21 13.01 13.57 13.01 13.03 13,61 13.14 13.42 AV = 13.32 i 0.29 Values in parentheses are estimates of standard deviation for individual determinations. * Estimated standard deviation of the average.
matically to the irradiation station, a 40-sec irradiation, a 2-sec delay for return of the sample, and a 38-sec counting period. Each determination (4 cycles) requires 5 . 5 min. The total counts received in all four cycles are visually displayed as an exponential decay curve covering the entire 1024 channels of the analyzer, allowing inspection of the determination for any anomalies (improper decay or nonexponential behavior). Each analysis consists of four replicate determinations (16 cycles) and thus, requires about 25-30 min excluding time for sample preparation and packing. This technique is a direct comparator method in which the Alfa Inorganics Ultrapure LiF (87628) serves as the primary stundard. The number of counts per gram of fluorine is calculated by using a standard capsule of LiF in the analysis procedure. The percentage of fluorine in the unknown sample is then determined by comparison with the standard result after appropriate background corrections have been applied to both standard and sample. As will be discussed below, it is necessary to make a correction for a high level of oxygen in the sample to obtain the best accuracy in the fluorine determination. OXYGENDETERMINATION IN THE PRESENCE OF FLUORINE. As stated earlier, the activation of fluorine in the californium252 facility is 200 times greater than that of oxygen, whereas the activation of fluorine by the 14-MeV accelerator is onethird as great as that of oxygen. Consequently, the determination of both oxygen and fluorine content is possible by complementary use of the californium-252 and 14-MeV facilities. First, the apparent fluorine content of a sample is
zF, californium-252a 48.8 (0.26) 49.3 (0.28) 48.6 (0.28) 49.0 (0.25) 48.8 (0.28) 48.6 (0.28) 48.6(0.28) 48.9 (0.28) AV = 48.82 i 0.42b 76.3 (0.41) 75.4 (0.40) 75.7(0.40) 75.7 (0.41) 76.4 (0.41) 75.1 (0.41) 75.5 (0.40) 75.3 (0.41) AV = 75.68 i 0.48 38.6 (0.24) 38.9 (0.24) 38.5 (0.24) 38.9 (0.22) 38.3 (0.24) 38.0 (0.26) 38.3 (0.24) 39.1 (0.28) AV = 38.57 f 0 . 3 7 9.67 (0.08) 9.80 (0.07) 9.65 (0.08) 9.62 (0.09) 9.84 (0.08) 9.71 (0.08) 9.62(0.09) AV = 9.70 f 0.09 29.5 (0.22) 30.1 (0.19) 29.6 (0.19) 29.9 (0.19) 30.5 (0.17) 30.3 (0.19) 29.6 (0.19) 29.6 (0.19) AV = 29.89 i 0.38 13.6 (0.10) 13.8 (0.23) 13.3 (0.13) 13 . 6 (0,14) 13.6 (0.13) AV = 13.58 =k 0.18
determined by the method described in the preceding section. Then, the sample is transferred to the 14-MeV neutron generator where it is irradiated simultaneously with a 10-gram nylon flux monitor. The IBNactivity induced in the sample during the 30-sec 14-MeV neutron irradiation is the sum of 160(n,p)16N and 19F(n, a)16N. After a 4-sec transfer delay, the sample is counted for 17.2 sec. Next a 13.6-sec flux monitor count is made at a n elapsed time of 30.9 sec after irradiation. The electronics used are the same as described earlier, except that both sample and flux monitor activity are determined in a single multiscaling pass in channels 10 t o 410 and 612 to 1012, respectively. The I6N activity in the sample and flux monitor are each automatically totalized and data from four replicate determinations are processed with an IBM 1130 computer program “NGEN” (IO), which subtracts the environmental background, calculates the count ratio of sample to the flux monitor (the decay ratio), determines the standard deviation, averages the results, and tests against a X * / V criterion at the 95% confidence level. A discussion of the statistics involved is given by Dixon and Massey (11). Nonstatistical behavior of the data set is signaled by the program, and thus systematic errors are readily detected.
Nargolwalla, Scintrex, Ltd., 222 Suidercroft Rd, Concord, Ontario, Canada, personal communication, 1972. (11) W. J. Dixon and F. J. Massey, Jr., “Introduction to Statistical Analysis,” McGraw-Hill, New York, N.Y., 1957.
(10) S. S.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
55
Table 111. Determination of Oxygen in the Presence of Fluorine l/r-Inch-Diameter Samples
Z 0 by
Compound Fluorobenzenesulfonyl chloride Fluorobenzoic acid
Sample wt, g 1,1143
0.8417
0 theory 16.0
22.9
Trifluoroacetanilide
0.8571
8.5
CaFl
0.9042
0.0
Teflon (DuPont)
1.2144
0.0
Li F
1.1775
0.0
a
14-MeVcalifornium-252 differential 15.9 15.8 15.0 16.4 15.1 AV = 15.6 i.0 . 6 a 22.3 22.9 22.8 22.9 23.7 21.9 22.6 22.3
W, W,
C, C, C,
8.7 8.5 8.4 8.7 AV = 8.6 i 0 . 2
W,,
0.5
0.8
0.08
(2)
(4)
Count ratio of oxygen standard by 14-MeV ac, tivation = Count ratio of fluorine standard by 14-MeV ac. tivation = Count ratio of sample standard by 14-MeV ac tivation = Weight of oxygen in oxygen standard (grams) = Weight of fluorine in fluorine standard (grams) = Net counts in sample by californium-252 activatior = Net counts in fluorine standard by californium. 252 activation = Net counts in oxygen standard by californium 252 activation = Fluorine weight in sample by californium-25; activation (grams) = Oxygen weight in sample =
RESULTS AND DISCUSSION
0.008
0.3 0.1 0.6 AV = 0.30 f 0.25 1.1 0.06 0.8 0.02 0.06 AV = 0.41 i 0.44
(3)
56
R,
W,,
In the computational sequence for the differential oxygen determination (Equations 1 4 ) , three count ratios are determined by 14-MeV activation as described above: R,, the count ratio for the oxygen standard (NBS certified benzoic acid-S.R.M. 350); R,, the count ratio for the fluorine standard (LiF) and R t , the sample count ratio itself. I n addition, activation of sample and standard by californium252 yields the apparent fluorine content ( W,,). The fluorine contribution to the count ratio obtained for the sample by 14-MeV activation is then subtracted, yielding a count ratio (R,)due only to oxygen content. By comparison with the previously determined count ratio for the oxygen standard (R,)and its weight ( E',), the net weight of oxygen ( Woz)in the sample is determined. Since the original counts obtained from californium-252 activation (C,) contained a small contribution from oxygen activation in the sample, C, is corrected by subtracting the counts attributable to the weight of oxygen found by the difference method in the sample. This corrected count (Cz') is then used in the computational sequence (1-3) to yield the final weights of fluorine and oxygen in the sample. This single iteration normally suffices to correct the fluorine and oxygen determinations to the precision expected from counting statistics. Since this is a difference method, the per cent standard deviation and accuracy of the macro-oxygen determination are expected to be no better than 2%, dependent on both oxygen and fluorine content.
c, - wo,
R,
23.1 AV = 22.7 f 0 . 5
0.7 0.11 -0.02 Av = 0.33 i 0.38 Estimated standard deviation of the average.
C,' =
R,
The californium-252 source used in this work activates fluorine 200 times as intensely as it does oxygen. For exam ple, the activation of fluorobenzoic acid by californium-25: without a n additional correction for the oxygen content woulc rathei result in a calculated fluorine content of 13.7 (i0.1 than 13.6 %. However, a correction based on an approximatt value of oxygen content as 20% (true value 22.9%) removes any statistically significant bias from the data [i.p., (13.7 '/ZOO X 20) = 13.61. If a sample with a macro-level of fluorinc contains less than 10% oxygen, the correction for the oxyger content would be less than the counting error and therefore unnecessary. Although high-energy y-rays might be expected from chlorine or sulfur, these have not interfered with the analysi: for fluorine. Moreover, since no neutron or gamma attenuation has been observed, the method appears equally ap. plicable to organic or inorganic matrices of widely varying types. Since the precision of the analysis is statistically controlled the instrumental technique described was chosen as a corn. promise to obtain adequate counting statistics for good preci. sion and a reasonable analysis time. The procedure yields a specific activity of 65,000 counts per gram of fluorine per de. termination, each analysis being the average of 4 determinations. The sensitivity, calculated as the weight of fluorine which will produce a n activity equal to three times the square root of the background activity, is 0.4 mg for a 10-gram sam. ple. For a 1-gram sample, a precision and accuracy of 12 are found at the 1 u confidence level if the compound contain! 15 or more fluorine. The results of fluorine analyses by the californium-251 method (column 4) compared with the theoretical amount of fluorine present (column 2) and the results from a number of determinations by the spectrophotometric method (column 3) are listed in Table 11. The californium-252 results were obtained over a two-week period by measurements on duplicate samples of each compound. The spectrophotometric results are duplicate analyses run over a period of six days. The precision for each californium-252 analysis is listed in parentheses in column 4. All but one of the individual californium252 analyses show a precision of 1 % or better. Moreover. a chi-square analysis test (11) was run to detect any skew in the data sets and within the 1 cr limit, none was found. Finally? the average deviation for all determinations on a given COIIIpound is recorded, Comparison of this with the precision of individual determinations indicates some day-to-day variations in the method. Comparison of the average percentage
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
z),
bund for each compound with the theoretical percentage ;ives an indication of absolute accuracy. In all cases, the .alifornium-252 method shows greater accuracy and precision han the spectrophotometric method. Data from the differential oxygen method are presented in ’able 111. Agreement with the theoretical values for oxygen ontent is very satisfactory. The sensitivity of an oxygen ietermination by 14-MeV activation with a fast neutron flux If lo8 n/cm2/secis 0.04 mg for a 10-gram sample. However, ince the oxygen determination with fluorine in the sample is omputed by subtracting one number with an uncertainty rom a comparable number with another uncertainty, the nethod is limited in most cases t o samples containing several dligrams of oxygen. Previous experience at our facility iith the determination of oxygen by 14-MeV neutron activa!on has shown that samples packed under atmospheric condi.ons can adsorb as much as 0 . 2 x oxygen as gas or water apor. This fact, coupled with the possibility of low-level xygen contamination in the samples (e.g., CaO in C a F J and degree of uncertainty in the oxygen blank values for indiidual polyethylene sample carriers may account for the slightly ositive values obtained for non-oxygen-containing comounds. SUMMARY
A method has been developed for the nondestructive analyis of fluorine by neutron activation with a californium-252 ;atopic source. About 1 gram of sample is required and the nalysis time is about 30 min per sample. A comparative
study has demonstrated the method t o be superior in precision and accuracy t o an established spectrophotometric method although a much larger sample is required (1 gram us. 10 mg). If one can accept a factor 2 lower precision, the required sample size could be reduced to 250 mg. The sample size may be further reduced by another factor of 3 by modifying the Californium-252 source holder so that sources are closer to the sample. Although analysis time for the classical method is equivalent, the method consumes the sample. Finally, in conjunction with 14-MeV neutron activation, an oxygen analysis method has also been developed, which eliminates the fluorine interference in routine oxygen determinations. Agreement of the results with theory has been established for a series of compounds with known composition. ACKNOWLEDGMENT
The authors wish to express their appreciation to these people for their help in the acquisition and installation of californium-252: R. L. Griffith for his many suggestions; C. J. Kolb for aid in the design and coordination of construction of the biological shield and source holder; the United States Atomic Energy Commission for loan of the sources under contract AT(38-1)-672. Technical assistance was given by M . C. Ferringer, R. K. Cross, and J. L. Pietruszewski. The comparative fluorine analyses were provided by D. F. Ketchum and G . N. Meyer. RECEIVED for review March 31, 1972. Accepted August 14, 1972.
h e a r and Nonlinear System Characteristics of zontrolled-Potential Electrolysis Cells
. E. Harrar’ and C. L. PomernackiZ awrence Lioermore Laboratory, University of California, Liuermore, Calif. 94550 detailed study was made of the characteristics of iree-electrode controlled-potential electrolysis cells s components of control systems. I n the absence of ignificant faradaic current, these cells can be repreented as linear, bridged-T type networks. Important arameters that determine cell response are the refernce electrode resistance and a parasitic capacitance i a t couples the cell input to the output. Cells whose lectrodes are arranged for optimum dc potential dis4 bution were also found to have the minimum phase hift for a given attenuation, and on the basis of the ircuit model the phase shift will not exceed ,90 Cells 4th poor geometry exhibit excessive phase shift in ieir transfer functions. In the presence of significant nradaic current, the fundamental frequency transfer dnction is altered considerably and at applied potenals near the current-potential waves the cells are onlinear. Negative-admittance reactions can cause i e cell phase shift to be more negative than -go”, iut most faradaic reactions cause the cell to exhibit sss phase shift than the background solution value. ufficient conditions for system stability, taking into ccount the time-varying, nonlinear, and other comlicating characteristics of the cells, can be rigorously btained using the circle criterion. Several aspects of lectrochemical system design and measurement are iscussed.
’.
Chemistry Department. Electronics Engineering Department.
THE FULL ACCURACY and significance of electrochemical experiments involving the control of the potential of a working electrode are realized only when the characteristics of the potentiostat and electrolysis cell are carefully analyzed as components of a feedback control system. Reviews of the development of this concept by various investigators, including a detailed discussion of the approach using classical control theory in electrochemical experiments ( I ) as well as in chemical laboratory problems generally ( 2 ) , have been published. In applying the methods of control theory to potentiostatic control problems, the electrolysis cell has been characterized as a linear, time-invariant system in terms of its transfer function (TF). An equivalent-circuit two-port network is usually invoked to correlate changes in cell parameters with variations of the TF. The cell T F then provides the basis for tailoring the potentiostat response or modifying the cell to obtain the desired stability and response for the closed-loop cell-potentiostat system.
(1) R. R. Schroeder and I . Shain, Chem. Instrum., 1,233 (1969). ( 2 ) D. E. Smith, C. E. Borchers, and R. J. Loyd in “Techniques of Chemistry,” Vol. I, Part IB, A. Weissberger and B. W. Rossiter, Ed., Wiley-Interscience, New York, N.Y., 1971, Chap. VIII. ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
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