A Radioisotope X-Ray Fluorescence Spectrometer with a High-Resolution Semiconductor Detector Analytical Sensitivity for Elements in Low Atomic Number Matrices Sachio Yamamoto N a d Radiological Defense Laboratory, San Francisco, Calif. 94135 The analytical sensitivity of an X-ray fluorescence spectrometer utilizing a radioisotope exciter source and a high-resolution, lithium-drifted silicon detector was evaluated for elements in low atomic number matrices. The spectrometer system which was studied used lZ51 as the exciter source and contained a 30-mm2 detector with a resolution of 512 eV full-width at half-maximum (FWHM) at 6.40 keV. Known molybdenum samples ranging from 10 ng to 1 mg, and nickel samples ranging from 0.1 pg to 1 mg, were prepared and analyzed. It was found that the detection limit for molybdenum was 35 ng with a CQ. 5-mCi exciter source; precision in the submicrogram range was 1CL20~0. The detection limit for nickel was considerably poorer and was found to be 1 pg for an exciter source strength of CQ. 10 mCi. An examination of the various factors that affect the analytical sensitivity of the system showed that the sensitivity was limited primarily by poor detector geometry resulting from the small size of the highresolution Si(Li) detector, and by background. A principal contributor to background was backscatter of the exciter source radiation by materials surrounding the source and detector. It was found that the radiation was being scattered mainly by air.
THISREPORT describes a study of the analytical sensitivity of a radioisotope X-ray spectrometer with a high-resolution semiconductor detector. The purposes of this study were to extend the applicability of this instrument to the analysis of submicrogram-size samples of nickel and molybdenum in low-atomic-number matrices and to study the factors which affect the analytical sensitivity of the instrument. The use of radioisotope X-ray spectrometers-that is, nondispersive X-ray fluorescence spectrometers with radioisotope exciter sources and gas proportional or scintillation detectors-has increased greatly in recent years. These instruments have advantages over the conventional X-ray apparatus in that they are simpler, less expensive, and much more compact. As a result, many investigators have taken advantage of the unique features of this tool and have found numerous applications for it ( I ) . Moreover, the recent development of high-resolution semiconductor detectors has extended the applicability of the radioisotope X-ray fluorescence spectrometric method (2, 3). The high resolution of these detectors now makes it possible to measure simultaneously elements heavier than calcium which differ by only one in atomic number (4). In addition to the advantage of high resolution, semiconductor detectors have a high efficiency for X-rays, excellent linear energy response, and high stability. If a capability for high-sensitivity analysis could be added to the above advantages, a (1) J. R. Rhodes, The Analyst, 91, 683 (1966). (2) H. R. Bowman, E. K. Hyde, S. G. Thompson, and R. C . Jared, Science, 151, 562 (19.66). (3) E. Elad and M. Nakamura, Nucl. Znstr. Methods, 41,161 (1966). (4) W. B. Jones and R. A. Carpenter, Oak Ridge National Lab-
oratory Report, ORNL-IIC-10, 465 (1967).
radioisotope X-ray spectrometer with a high-resolution semiconductor detector would be a very useful analytical tool. A number of workers have described radioisotope X-ray fluorescence spectrometers with high-resolution semiconductor detectors, None, however, have reported sensitivities in the submicrogram range. Hyde, Bowman, and Sisson ( 5 ) have described a spectrometer with a lithium-drifted silicon detector and an *4IAm exciter source. They applied this system to the quantitative analysis of milligram-size barium and yttrium samples. Hollstein and DeVoe (6) used a similar spectrometer for the analysis of gram amounts of tin and molybdenum present as minor constituents in copper-base alloys and steel, respectively, and to singleelement samples ranging from copper to dysprosium. They estimated detection limits for this range of elements to be 10 to 100 pg. Klecka (7) utilized an lza1exciter source and a lithium-drifted silicon detector for the quantitative analysis of yttrium samples ranging in weight from 2.5 mg down to 1 pg. In the present work, a spectrometer with a lithium-drifted silicon detector and an l2SI exciter source was applied to the analysis of low-weight molybdenum and nickel samples, which extended into the submicrogram range. EXPERIMENTAL.
Apparatus. DETECTION SYSTEM.The apparatus used was a Model 3311332 Photon Spectroscopy System manufactured by Technical Measurement Corp. (TMC). This system includes : a lithium-drifted silicon detector with a 30-mm2 area and a 3-mm depletion layer, a preamplifier, a vacuum ion-pump, a 10-liter liquid-nitrogen cooling tank, and a power supply. The detector-head window was made of 10-mil beryllium. The main amplifier was a TMC Model 341 linear pulse amplifier. A TMC 4096 multi-parameter pulse height analyzer operated as a single-parameter 1024 channel system was used for pulse-height analysis. Resolution values (full-width at half-maximum) obtained with this system were 512 eV for the 6.40-keV K , X-ray of iron and 678 eV for the 25.27-keV K, X-ray of tin. EXCITERSOURCE. 12jI was used as the exciter source. This radionuclide decays by electron capture to a 35-keV level in lz5Te,which in turn decays to the ground state by means of a highly-converted gamma transition. The principle radiations from this source are the 27.47-keV Te K, and 30.99 keV Te Kp X-rays. Thus, this radionuclide is well suited for excitation of K X-rays of elements below tellurium. The half-life of lZjIis 60 days. ( 5 ) E. K. Hyde, H. R. Bowman, and D. H. Sisson, University of
California, Lawrence Radiation Laboratory Report, UCRL16845, May 1966. (6) M. G. Hollstein and J. R. DeVoe, Oak Ridge National Laboratory Report, OKNL-IIC-IO,483 (1967). (7) J. F. Klecka, University of California, Lawrence Radiation Laboratory Report, UCRL-17144 Rev, October 1966. VOL. 41, NO. 2, FEBRUARY 1969
337
-I W
z z a
I
Y
c
v)
z
2
300
V
Figure 1. Sample holder and exciter source assembly
The exciter source consisted of seven lZ5Isources. These were each prepared by evaporation of a solution of carrierfree lZ5I(purchased from Oak Ridge National Laboratory) in a small depression in a a/l&ch X lid-inch X 11/4-inch tin holder. Tin was used because the energies of its characteristic X-rays, which would be excited by the lZsIradiations, are similar to the energies of the 1*51 radiations. Hence, the resultant spectrum of the radiation from the exciter source would remain simple. The evaporations were carried out in a nitrogen atmosphere to prevent air oxidation of the iodide. lZ5Iwas then sealed in the depression in the holder under an aluminum foil (cn. 1 mil thick) cemented by “Plastic Steel.” The aluminum foil cap was then coated with “QuickSet Epoxy” to protect it from punctures. The lZ5Isources were mounted on a Lucite ring in such a manner that the sources formed an annular array as shown in Figure 1. The Lucite ring was then slipped over the detector head. Because of the short half-life of 1251,the activity of the exciter source was not constant. During the period of this study the exciter source activity decayed from approximately 10 down to 2 mCi. Also attached to the Lucite ring of the exciter source assembly (Figure 1) was a collimator made of l/ls-inch-thick lead. A I/4-inch-high lead lip was placed around the ‘Izinch-diameter collimator hole and the hole and lip were covered with a sheet of tin. The purpose of the lead was to shield the detector from direct radiation from the exciter source and to reduce the amount of scattered radiation reaching the detector. The purpose of the tin was to prevent excitation of lead L X-rays by backscattered radiations from the exciter source. SAMPLE HOLDER. The sample holder is also shown in Figure 1. The holder was made of Lucite and was designed to achieve reproducible positioning of samples and to present minimum mass in front of the exciter source so that scattered radiation could be kept to a minimum. Sample Preparation. Sample mounts were prepared by fastening sheets of Mylar film onto aluminum frames, each frame consisting of a 3-inch X 3-inch piece of l/ls-inch-thick aluminum with a 21/2-inch-diameterhole in the center (Figure 1). Known samples of molybdenum and nickel were prepared by evaporation of aliquots of NazMoOd and Ni(N03)* solutions, respectively, onto these sample mounts. The evaporated samples were dispersed on the Mylar films over a n area approximately inch in diameter. Counting Procedure. For counting, the samples were placed in the first slot of the sample holder. In this position the source-to-sample and sample-to-detector distances were 338
ANALYTICAL CHEMISTRY
c 500
600
700
800
CHANNEL NUMBER
Figure 2. Molybdenum K, X-ray spectrum obtained from a 50-ng molybdenum sample The counting interval was 2850 min
0.75 and 1 inch, respectively. Because our principal objective was to analyze low-weight samples, no restrictions were placed on counting intervals and they ranged in general from 30 to 1000 minutes. The latter corresponds to an overnight count. The fluorescent X-ray spectra were measured on the pulse-height analyzer. The spectral data were printed out and the counts in the K, X-ray peaks (Mo 17.47 keV and Ni 7.47 keV) were summed and recorded. Measurements of background were made with a blank sample mount in the first slot of the sample holder. Background spectra were printed out and summed over the channels corresponding to the measured fluorescent X-ray peaks. RESULTS
Known nickel samples ranging in weight from 0.1 pg to 1 mg were analyzed and the results are shown in Table I. The measured values were corrected for exciter source decay, because the source decayed slightly during the period in which these measurements were made. The activity of the exciter source was approximately 10 mCi during the period these samples were measured. Only one measurement was made of each sample, and, hence, no precision estimates could be made. However, the error of the value for the 1-pg sample is probably greater than 50% because of the low count-to-background ratio. For a cu. 10 mCi source, the average background count rate in the Ni K, X-ray region was 9.6 cpm, and the detection limit for a single 1000-min determination is estimated to be about 1 pg. This detection limit has a significance level of 5 % for random errors of
the first and second kind (8, 9). It should be noted that errors in the preparation of the known samples have been assumed to be negligible. Known molybdenum samples whose weights ranged from 10 ng to 1 mg were also analyzed. An example of a K, X-ray spectrum (uncorrected for background) obtained from a 50-ng molybdenum sample is shown in Figure 2. The results of the molybdenum analyses and their standard deviations are presented in Table 11. Six to eight measurements were made of each sample. The measured values were corrected for exciter source decay, because the source activity diminished from about 5 to 2 mCi during the period in which these measurements were made. Precision of the measurement is better than for Mo samples of 1 pg or greater and between 10 and 20% for samples less than 1 pg. These data indicate that the detection limit for molybdenum is between 10 and 50 ng under the experimental conditions used. Based on the background count rate of 2.2 cpm in the Mo K, X-ray region for a ca. 5 mCi exciter source, the detection limit for a single 1000-min determination is estimated to be 35 ng (at a significance level of 5 % for random errors of the first and second kind) (8, 9). The data shown in Tables I and I1 show that the detection limit of this instrument for nickel analysis is significantly poorer than that for molybdenum analysis. Indeed, submicrogram size samples of nickel could not be detected. This difference in detection limit is due to the background, which was 2-4 times higher in the nickel K, X-ray region, and to a lower K , X-ray intensity for nickel. In Figure 3, the data shown in Tables I and I1 are plotted. For this plot, the kalues for nickel have been normalized to the exciter-source-strength used (later) for the molybdenum determination, so that the results for the two elements can be compared directly. The figure shows that an excellent linear relationship exists over a wide range between K, X-ray intensity and sample weight for both molybdenum and nickel. It shows also that the K, X-ray intensity of a nickel sample is 20 to 30 times less than that of a molybdenum sample of corresponding weight. The lower fluorescent X-ray intensity of nickel can be accounted for in part by the following factors: the fluorescence yield for nickel is one half that of molybdenum and the mass attenuation coefficients of nickel and molybdenum for a 27-keV X-ray are 12 and 32 cm2'g, respectively, so that for samples of comparably low surface density the fraction of 1251 radiation absorbed by Ni would be about one third as great as that for Mo. In the following section the various factors which affect the analytical sensitivity of the X-ray fluorescence method will be examined in greater detail.
a
Table I. K, X-Ray Count Rate of Nickel Samples (ca. 10 mCi lz5IExciter Source) Counting Weight of interval Net cpm Ni (8) (min) of K , X-ray. 10-3 100 285 10-4 180 31.5 10-5 lo00 3.96 10-6 2000 0.5 10-7 2000 Not detected Background count rate: 9.6 cpm.
5z
(8) B. Altschuler and B. Pasternack, Health Physics, 9, 293 (1963). (9) L. A. Currie, ANAL.CHEM., 40, 586 (1968).
Weight of Mo (8) 10-3 10-4 10-5 10-6 10-7
Let N be the total count rate of pulses whose amplitudes correspond to that of the characteristic K X-ray of a n element in a sample. Then (2),
N
=
Count rate of K X-ray emitted by element of interest
= IORskabsWKQDBD
where lo
+ Background
+B
(1)
intensity (photons per unit time) of exciter radiation Qs = source-to-sample geometry /cabs = fraction of the exciter photons, reaching the sample, which are absorbed by the K-electron shell of the atom of interest w K = fluorescence yield OD = sample-to-detector geometry e D = intrinsic detection efficiency for the energy of the K X-ray B = background count rate =
In the above equation, it is assumed that the sample is sufficiently thin so that no corrections need be made for absorption or enhancement effects. We shall now examine each of the factors in Equation 1. The fluorescence yield, w K , depends solely upon the element in question and is beyond the control of the analyst. For nickel and molybdenum the values are 0.38 and 0.77, respectively. The intrinsic detection efficiency, tD, of the silicon semiconductor detector for X-rays is very high and is one of the attractive features of this type of detector. For example, the photopeak efficiencies of the detector used in this work for the Mo K, (17.47 keV) and Ni K, (7.47 keV) X-rays are nearly 100%. The factor, kabs, is a function of the probability of K-shell absorption (given that a photon is absorbed), photoelectric absorption coefficient, and sample thickness. The proba-
Table 11. K, X-Ray Count Rate of Molybdenum Samples (ca. 5 mCi lz5IExciter Source) Counting interval No. of Net cpm (min) measurements of K, X-ray" Std dev (cpm) 30 6 4967 i60 100 7 509 c 10 100 200 loo0
x 10-8 1 ~ 3 0 0 0 1 x 10-8 2000 Background count rate: 2.2 cpm. 5
DISCUSSION
6 6 6 8 2
54.3
5.42 0.60 0.39 Not detected
i. 1 . 7
i 0.27 i 0.09 i 0.06
...
VOL. 41, NO. 2, FEBRUARY 1969
339
Figure 3. K, X-ray count rate cs. sample weight for molybdenum and nickel samples
WEIGHT OF SAMPLE ( g )
z
bility of K-shell absorption is 80 to 85 for both molybdenum and nickel for the lz5Iradiation. Because the photoelectric absorption coefficient depends on both the element and the energy of the excitation radiation, one can increase k a b s by a judicious choice of an exciter source. It is well known that absorption of the excitation radiation can be maximized by selection of an exciter source whose energy is just above the critical absorption edge of the element of interest. However, background due to backscatter will increase greatly under these conditions. In Figure 2, much of the background under the Mo K, X-ray peak is due to the low-energy tail of the backscattered Te K, X-ray. If the excitation photon energy had been closer to the critical absorption edge of molybdenum, the background would have been much greater. Equation 1 contains two geometry factors: source-tosample geometry, Q,, and sample-to-detector geometry, QD. In the system used in this work, they were 1 % and 0.35%, respectively. Both factors could be improved to some extent by improvement in the physical arrangement of the detector, exciter source, and sample holder. Use of a much smaller exciter source than the one used in this work would permit a more compact arrangement of the three parts and would increase both geometry factors. The poor sample-to-detector geometry, however, is due primarily to the small size (0.3-cm radius) of the high-resolution Si(Li) detector and represents one of the major limitations of the system. Even if the geometrical arrangement of exciter source and sample is optimized, Q D would not exceed a few per cent. Thus, detectors that are considerably larger than those currently available are needed if we are to obtain large geometry factors. 340
ANALYTICAL CHEMISTRY
Because high-resolution semiconductor detectors of large size are not available, the only way to increase signal strength -Le., count rate of the fluorescent X-ray-is to increase the intensity of the exciter source. Unfortunately, as Io is increased, so is the backscatter contribution to background. However, the detection limit is inversely proportional to S / d i , where S is the signal strength and B is the background. Because, in this case, signal and backscatter are both directly proportional to exciter-source intensity, the detection limit can be improved by increasing source intensity. The relationships between backscatter and source intensity, as well as the general problem of background, will be discussed next. Background is perhaps the most important factor which limits the sensitivity of this method for low-weight samples. As sample quantity is made smaller, count-to-background ratios become very small (see Tables I and 11) and errors become large. Furthermore, background depends in part on the materials, including sample components, in the vicinity of the detector, and because the presence of such materials, particularly those in the sample itself, are sometimes difficult to control, an accurate background estimate may be difficult to obtain. The X-ray spectral background was found to consist primarily of electronic noise, instrumental artifacts, and as discussed earlier, backscatter from the exciter source. Ambient radiation also contributes to background, but its contribution is negligibly small. Electronic noise generally contributes to background in the low-energy region of the X-ray spectrum and is independent of the exciter source; however, in the measurements which were made, background due
CWNNEL NUMBER
Figure 4.
Background spectrum obtained with l z 6 1 exciter source A . Location of Ni K, X-ray (channels 210-230) B. Location of Mo K, X-ray (channels 550-575)
to noise was found to fluctuate considerably. Instrumental artifacts, which include, for example, fluorescent X-rays from detector impurities and from materials used in the construction of the spectrometer, do depend on the exciter source. Backscatter from the exciter source contributes spectral characteristics which are determined by the radiation spectrum of the exciter source itself. Because the backscatter contribution is dependent on the exciter source, the intensity of this contribution would be expected to be proportional to the intensity of the exciter source. In these measurements, such proportionality was indeed observed. The backscatter contribution was observed to “decay” smoothly with time in concert with the decay of the source activity. A background spectrum of an lZ6Isource is shown in Figure 4. In the region above about channel 400 (ca. 12 keV) the background is due almost entirely to backscatter. The three principal peaks (above channel 750) are the backscattered Te K , and KO X-rays from the exciter source and the Sn K, X-ray from the tin placed in the collimator. The lowenergy continuum below channel 400 is due to electronic noise. There are broad peaks superimposed on the highenergy tail (channel 100400) of the noise continuum and we can only speculate about their origins. Their energies preclude their being Compton absorption events in the detector or escape peaks. They may be due to fluorescent X-rays from impurities in the detector or other materials comprising the detector system. From Figure 4 it can be seen that the nickel K, X-ray peak (channel 210-230) falls in the region of the noise tail. The higher background in this region detracted significantly from the ability to achieve a lower detection limit for nickel. The Mo K, X-ray peak (channel
550-575), however, falls in the region where the background spectrum is at its lowest and where the background contribution is due almost entirely to backscatter. Thus, the X-ray spectrometer system used in this work is more ideally suited to molybdenum analysis than to nickel analysis. Because detection limits for elements whose fluorescent X-rays fall in the backscatter region can be improved greatly if backscatter is reduced, it is of interest to determine the principal contributor to backscatter. It was found that the sample mount and sample holder contributed about 20 to 2 5 z to the backscatter. The remainder was believed to be due primarily to air. To determine if scattering in the air surrounding the detector does contribute significantly to backscatter, we made a background measurement in a helium atmosphere. For this measurement, the sourcedetector-sample holder assembly (Figure 1) was enclosed within a polyethylene bag (18 inches X 24 inches) which was purged continuously with helium as the background was being measured. The bag was large and not leak-tight; therefore the helium purge was undoubtedly inefficient and incomplete. Nonetheless, the backscatter intensity was reduced by more than 3 0 z by this method. Hence, backscatter can be reduced appreciably by operation of the spectrometer in a helium atmosphere or in a vacuum. CONCLUSIONS
From the results, one can conclude that radioisotope X-ray fluorescence spectrometers with high-resolution semiconductor detectors can be applied to the analysis of submicrogram samples of elements in low atomic number matrices, With an l25Iexciter source (ca. 5 mCi) the detection limit for molybVOL. 41, NO. 2, FEBRUARY 1969
341
denum was found to be 35 ng; precision in the submicrogram region was 10 to 2 0 z . The detection limit for nickel was considerably poorer and was found to be about 1 pg for an exciter source strength of approximately 10 mCi. An examination of the factors affecting the analytical sensitivity of the system showed that it is limited primarily by poor detector geometry and background. The poor detector geometry results from the small size of the detector. Background results from electronic noise, instrumental artifacts, and backscatter of the exciter source radiation. However, background in the region of the X-ray spectrum above about 12 keV was due almost entirely to backscatter. Sensitivity of the system can be improved by increase in source intensity and reduction in background. (High de-
tector geometries cannot be achieved because large highresolution semiconductor detectors are not currently available.) Although electronic noise and instrumental artifacts cannot be readily reduced, backscatter can be reduced by selection of an exciter source whose backscatter peak will be far removed from that of the measured X-ray and by reduction in the mass of material placed in front of the detector and source. A major reduction in mass can be accomplished by operation of the spectrometer in a vacuum or in a helium atmosphere. RECEIVED for review July 24, 1968. Accepted October 7, 1968. Division of Nuclear Chemistry and Technology, 155th National Meeting, ACS, San Francisco, Calif., April 1968.
~~
Rapid Extraction and Direct Spectrophotometric Determination of Copper with Thiothenoyltrifluoroacetone V. M. Shinde and S. M. Khopkar Department of Chemistry, Indian Institute of Technology, Bombay, Bombay 76, India A procedure is described for the extractive photometric determination of copper(l1) with thiothenoyltrifluoroacetone (STTA) in carbon tetrachloride. The olive brown Cu(lI)-STTA chelate solution in carbon tetrachloride obeys Beer’s law over the concentration range of 1.23 to 12.35 pg of Cu per ml. Between pH 2 to 5, quantitative extraction is feasible with 0.001M STTA in carbon tetrachloride. The color of the complex is stable to 72 hours. Copper(l1) can be extracted rapidly and determined in the presence of large numbers of cations as well as anions.
THE CHELATING AGFNT thiothenoyltrifluoroacetone (STTA) was used for the extraction of transition elements ( I ) . From slightly acidic solution it gives, with copper, an olive brown complex in carbon tetrachloride measurable spectrophotornetrically at 490 mp. Acetylacetone (2-4) forms a complex with copper at pH 2 to 5, whereas benzoylacetone (5) can extract copper between pH 4 to 9. Furoyltrifluoroacetone (6, 7) gives a green complex in hexone which can be measured at 660 mp. 2-Thenoyltrifluoroacetone (8-10) was capable of extracting copper in benzene at pH 3 to 4. Such a complex can be measured spectrophotometrically at 430 mp. Chaston et al. (11, 12) synthesized the thio derivative of thenoyltrifluoroacetone, whereas Berg (t) V. M. Shinde and S. M. Khopkar, Chem. Ind., 1967, 1785. (2) T. Shigematsu and M. Tabushi, Bull. Inst. Chem. Res., Kyoto Unic., 39, 35 (1961). (3) A. H . I. BenBassat and Kupfer G. Frydman, Chemist Analyst,
51, 44 (1962). (4) Zbid., 52, 8 (1963). (5) J. Stary and E. Hladk?, Anal. Chim. A s f a , 28,227 (1963). (6) E. W. Berg and M. C. Day, ibid., 18, 578 (1958). (7) R. T. McIntyre, E. W. Berg, and D. N. Campbell, ANAL. CHEM., 28, 1316 (1956). (8) S. M. Khopkar and A. K. De, Z. Anal. Chem., 171,241 (1959). (9) R. A. Bolomey and L. Wish, J . Anier. Chem. SOC.,72, 4483 (1950). 27, 195 (1955). (10) E. W. Berg and R. T. McIntyre, ANAL.CHEM., (11) S. H. Chaston and S. E. Livingstone,Proc. Roy. SOC.(London), 111 (1964). (12) S. H. Chaston, S. E. Livingstone, T. N Lockyer, V. A. Pickles, and J . S. Shannon, Aust. J. Chem., 18,673 (1965). 342
ANALYTICAL CHEMISTRY
and Reed (13) indicated the possibility of utilizing it as a chelating agent for the transition elements. Thiothenoyltrifluoroacetone can be used as the extracting and colorimetric reagent for the transition elements ( I ) , including copper. This paper describes systematic studies on the solvent extraction of copper with thiothenoyltrifluoroacetone. The method is simple and rapid, and effects clean-cut separation and simultaneous spectrophotometric determination of copper at tracer level. EXPERIMENTAL
A Type C44 quartz spectrophotometer, a Cambridge pH meter, and a wrist action flask shaker were used. Thiothenoyltrifluoroacetone (STTA) was synthesized from 2-thenoyltrifluoroacetone (Fluka A. G.)by the procedure of Berg and Reed (13). About 0.001M reagent was used in carbon tetrachloride and was usually preserved in a refrigerator. A stock solution of copper sulfate was prepared by dissolving about 0.9818 gram of copper sulfate pentahydrate (B.D.H. AnalaR) in 1 liter of distilled water. The solution was standardized iodometrically and contained 0.99 mg of copper per ml. The solutions of lower concentration were prepared by volumetric dilution of the stock solution. General Procedure. An aliquot of copper sulfate solution (containing about 49.4 pg of copper) was taken and adjusted to pH 4 with 0.01N sodium hydroxide and 0,OlN sulfuric acid in a 25-ml volume. The solution was then introduced into a separatory funnel and shaken with 10 ml of 0.001M S P A in carbon tetrachloride for 10 minutes, Layers were allowed to separate. The organic phase was withdrawn in a 10-ml volumetric flask and measured at 490 mp against the reagent blank. The amount of copper was then obtained from the calibration curve. Apparatus and Reagents.
RESULTS AND DISCUSSION
Absorption Curve. The absorption spectrum of a solution of the Cu(I1)-STTA complex (49.4 pg of copper) extracted at pH 4.0 against the reagent blank as a reference is shown in (13) E. W. Berg and K. P. Reed, Anal. Chim. Acta, 36,372 (1966).
