the half lives come closer together, the separation rapidly becomes more inaccurate and makes the time selections correspondingly more critical. Curves B, C, D, and E demonstrate how the determination is improved when determinant A is partially diagonalized with energy multipliers. Table I shows how RT varies as one activity decreases until the mixture can be accurately determined with fewer count intervals than originally necessary. It should be noted that the asymptotic behavior of RT for five-count intervals is due t o the mathematical formulation and computer treatment of the problem which doesn’t allow a count interval t o be set equal t o zero since the equations then become invalid. GENERAL EXAMPLE OF PROGRAM APPLICATIONS
Programs CERES and CIRCE may be used t o evaluate the statistical feasibility of analyses. Table I1 presents the computer results of considering the analysis of 10 ppm vanadium in the presence of 10 ppm each of aluminum, copper, and magnesium in an inert matrix. These elements were chosen because upon irradiation with neutrons they all produce radioactive nuclides with half lives between 2 and 10 minutes. Radiochemical separations become very difficult with such short half lives and data resolution must be accomplished either instrumentally or by very careful selection of counting times. For the purposes of this calculation the values of the N*’s are assumed to be exact. The optimized relative stan-
dard deviations computed by program CIRCE, where the interferences are being determined, indicates that by time selection alone the vanadium may be analyzed to a relative standard deviation of 3.91 X lO-*zand with energy discrimination the relative standard deviation is improved to 1.27 X Program CERES, where the interferences are known, shows that this data case is so favorable that a relative standard deviation of about 1.25 X is obtained by either sequential counting method, with a slight improvement t o a relative standard deviation of 1.10 X for the simultaneous counting case, I t must be remembered, of course, that the values computed in this manner are theoretical lower limits o n accuracy and that sampling procedures, flux variations, and other problems would have t o be considered t o attain such accuracies experimentally. However, without proper treatment of the statistical problems, the accuracy attained by instrumental and technique development in activation analysis is unnecessarily limited in a rather arbitrary fashion. ACKNOWLEDGMENT
Acknowledgment is gratefully given to V. Schomaker for his helpful suggestions regarding this work. RECEIVED for review March 27, 1967. Accepted July 25,1967
Determination of Trace Metallic Impurities in High Purity Sodium Using Atomic Absorption Spectrometry James M. Scarborough, Carleton D. Bingham, and Paul F. DeVries Atomics International, A Division of North American Aviation, Inc., P. 0. Box 309, Canoga Park, Calif. 91304 A method for determining trace level concentrations of Cr, Mn, Fe, Co, and Ni in the 0- to 10-ppm range in a oneg r a m sample of sodium using atomic absorption spectrometry is presented. Carrier precipitation on lanthanum hydroxide is employed to remove the difficulty associated with aspirating a solution of high salt content. At the 1-ppm level, 95% confidence limits for precision of i 1 0 % (relative) a r e attained. Time required for these five elements in a one-gram sample, including sample dissolution and standard preparation, is approximately five hours.
ALKALIMETALS are used in nuclear and aerospace applications as high temperature heat transfer fluids. Such high temperature service, requiring a high systems operational reliability, places severe materials compatibility requirements on a system. A program designed to furnish fundamental information relative to the properties of sodium metal as a solvent for high temperature materials in general, and in particular potential fast reactor materials, is under way in our laboratories. To study the effect of interstitial impurities on these solvent properties required that the initial sodium be of ultra-high purity. I t was further required that methods be available for measuring slight changes in small concentrations of solute elements with both good sensitivity and precision. It soon became evident that the sodium prepared by hot trapping and distilling reactor-grade starting material was 1394
ANALYTICAL CHEMISTRY
of higher purity with respect to metallic constituents than the material then being used to prepare standards for emission spectrographic determination of impurities in sodium. Because of this, methods yielding improved precision and sensitivity were sought. Bordonali et a1 (1) had earlier applied atomic absorption spectrometry to the determination of Cu, Mn, Fe, Pb, Ni, and Zn in the range of 10 to 1000 ppm in sodium. Atomic absorption has been used to analyze solutions containing Fe, Ni, Co, Mn, and C r at the 0.1-pp-n level (2, 3). These elements-the principal constituents of stainless steel, commonly used as container or structural material in a high temperature alkali metal system-in ti-e range of 1 to 10 ppm in a one-gram sample c r 0.1 to 1 r p m in solution were those of initial interest in tbis study. Preliminary experiments in our l a b x atory, however, showed that the desired precision using atomic absorption spectrometry could not be obtained without a preliminary separation of the impurities from the bulk of the sodium even with the 3-slot Boling burner (a high solids burner). (1) C . Bordonali, M. Biancifiori, and G. Besazza, Clzirn. hid. ( M i l ~ t ?47, ) 397-401 (1965). (2) K . Fuwa and B. L. Vallee, ANAL.CHEM., 35, 942 (1963). (3) W. Slavin, “Atomic Absorption Instrumentation and Techniques-A Review,” “Analytical Instrumentation”, L. Fowler, D. K. Roe, and R. G. Harman, Plenum Press, Eds., New York,
1964.
Bordonali and his associates ( I ) used vacuum distillation to remove the bulk of the sodium from a 15-gram sample, N o solvent extraction technique capable of separating all five impurity elements in a single extraction-a requirement imposed by the small samples available-was readily apparent. The technique suggested by Malissa and Shoffmann ( 4 ) using ammonium pyrrolidin dithiocarbamate did not appear applicable because of high salt concentrations. The application of a simple carrier precipitation technique, commonly used in radiochemical separations, was felt worthy of investigation. A technique was envisioned requiring minimum handling, using simple chemistry in which the impurities of interest are kept in cation form and wherein the carrier does not interfere with atomic absorption spectrometric measurement and is available in high purity form. Lanthanum behaves well in atomic absorption spectrometry and La(OH)3 should be a good scavenger, carrier for the hydroxides of the elements of interest; further, lanthanum oxide is available in high purity. All the hydroxides of interest are readily soluble in dilute hydrochloric acid so that chloride solutions can be those analyzed for impurities. EXPERIMENTAL Calibration Curves. Individual solutions over the range 0.0 to 1.0 pprn of each element in 0.01N HC1 were prepared from stock solutions and transferred to polyethylene bottles. Absorption measurements were made over these concentration ranges for each element using a Perkin-Elmer Model 303 spectrophotometer with a Boling burner head. Optimum flame and wave length parameters were established for each element (Table I). All absorption readings were taken relative to a n instrumental zero set without aspiration. An aspiration rate of 4 ml/min was maintained throughout the study. Similar data were obtained for standard solutions containing a mixture of the five impurities in 0.01N HC1 in the solution concentration range of 0.0 to 1.0 ppm and in similar mixtures containing 0.5, 1.0, and 1.5 mg Lalml. Data were also collected for similar standard solutions containing 1 mg La/ml plus 50, 100, 200, and 500 ppm Na. The absorption for each element in the various standard mixtures was compared over the concentration ranges with the absorption for the single element in solution. Sample Analysis. Sodium metal is dissolved in a Vycor crucible using argon saturated with water vapor then neutralized with HC1. LaCI3, prepared from high purity L a 2 0 3 , is added to the resulting chloride solution. The hydroxides are precipitated (ammonia and sodium hydroxide as precipitants are compared later). The precipitate is separated and dissolved in dilute HCI, made to volume, and analyzed by atomic absorption spectrometry. The solution which is aspirated into the flame of the atomic absorption spectrophotometer contains the impurity elements as well as lanthanum, sodium (no attempt is made to remove all the sodium from the precipitate), and hydrochloric acid. It is necessary to determine whether the impurities mutually interfere and whether lanthanum and/or sodium interfere. Chloride content can be controlled. In the ammonia precipitation, the presence of excess N H 3 could prevent quantitative recovery of Co and Ni and an excess of NaOH precipitant could affect the recovery of Cr. A p H of 8.5-9 is required for manganese precipitation; therefore all precipitations were made at pH 9. In this study, high purity sodium chloride solutions were used in place of sodium samples. Synthetic sample solutions containing 100 mg Na ml plus
(4) H. Malissa and E. Shoffmann, Mikrochinr. Acta, 1, 187 (1955)
Table I. Spectrophotometer Operating Conditions Spectral Waveslit Lamp length, width, current, Element mp Fuel/air Scale mm mA Fe 248.3 1019 10 2 40 co 919 10 2 30 240.7 Ni 919 10 2 20 232.0 Cr 357.9 1119 10 2 20 Mn 919 5 2 15 279.8
Element Fe co Ni Cr Mn
Table 11. Calibration Curve Data Standard deviation of concentration estimated from Equation of regression linea regression curveb C = 0.0180 ( A - 4.84) 0.010 ppm C 0.0195 ( A - 5.29) 0.010 ppm C = 0.0147 ( A - 6.21) 0.007 ppm C 0.0103 ( A - 0.57) 0.008 ppm C = 0.0169 ( A - 0.66) 0.008 ppm
C = solution concentration-ppm(wt). A sorption. Evaluated at 0.10 ppm. Q
=
per cent ab-
mixed impurities in the range from 0 to 1.0 ppm (equivalent to 0-10 ppm relative to sodium) in 0.01N HCI were prepared. To 100-ml aliquots of the above (a) N H 3 PRECIPITATION. solutions in a 250-ml centrifuge bottle are added 100 mg of La carrier with thorough mixing. Reagent grade ammonia gas diluted with argon is bubbled through the solution until pH 9 is attained. Separation of the resulting precipitate is made by decantation. The precipitate is dissolved in hN HC1 and the solution transferred by washing with 0.01N HC1 to a 100-ml volumetric flask. After diluting to volume, the solution is transferred to polyethylene bottle. Analysis is made in accordance with the procedure outlined for obtaining calibration data. An aliquot (100 ml) of sodium (b) NAOH PRECIPITATION. chloride standard containing five impurities is pipetted into polyethylene bottles and 100 mg of La are added and mixed. 3N NaOH (0.750 ml) and 50 ml of water are added to a 250ml centrifuge bottle and mixed. The standard mixture is poured rapidly with stirring into the centrifuge bottle. The resulting p H is nearly 9. Dilute NaOH is added dropwise until p H 9 is attained. Separation of the resulting precipitate and further analysis is as described in (a) above. RESULTS AND DISCUSSION
From the experimentally observed variation of absorption with concentration of the individual elements, a least squares regression line, Le., a calibration curve, was established for each element. Analysis of variance was used to determine the standard deviation of a value estimated from the regression lines and values for the standard deviations of a concentration estimated from the regression curve are given in Table 11. Any effects caused by the other impurity elements and/or the variation in La and/or content of the final chloride solutions of the standard mixtures should be manifested by a statistically significant variation from the absorption data obtained for the single element. These results are summarized in Tables 111-VII. Data from the analyses of synthetic standard samples are given in Table VIII. Absorption data for solutions containing all five impurities show no statistical differences from data obtained from soluVOL. 39,
NO. 12, OCTOBER 1967
1395
tions of a single impurity, thus verifying that there exists n o mutual interference in determining these elements by atomic absorption spectrometry. The presence of varying quantities of sodium-up t o 500 ppm-in the analyte has n o significant effect at the 95% confidence limit on the absorption values for the elements studied. Higher sodium concentrations have not been encountered with the techniques employed
here but larger amounts (up to the point where light scattering, viscosity effects, and burner clogging become troublesome) can be tolerated. Lanthanum has n o effect on Fe, Co, and Ni absorption value at concentrations up to 1.5 mg/ml in the analyte (1 mg/ml is recommended). Lanthanum probably causes slight enhancement in chromium absorption but its contribution is not considered significant in this study.
Table 111. Effect of Experimental Variables on Determination of Iron Per cent absorption ( X 10) Mixed Mixed Mixed Impurities Impurities 95 % impurities Confidence Mixed impurities impurities +50 ppm $100 ppm $0.5 mg impurities +1.5 mg Na $1.0 Na 1.0 range $1.0 mg Lalml only La/ml La/ml mg Lalml mg La/ml of A (X10) ~~
Solution concentration, ppm
x
0 0.05 0.10 0.20 0.40 0.60 0.80 1.o
4 8 1 1 6 1.6 7.6 I 10.4 rt 1 . 6 16.0 i 1 . 6 27.1 i 1.6 38.2 rt 1.6 49.3 i 1.7 60.5 11.8
Solution concentration, ppm
of % A (X10)
0 0.05 0.10 0.20 0.40 0.60 0.80 1.o
5.3 1 1 . 2 7.9 i 1.2 10.4 -c 1.2 15.6 i 1 . 2 25.8 i 1 . 2 36.1 rt 1 . 2 46.4 rt 1 . 2 56.7 i 1 . 2
Solution concentration, pprn
1.0
1396
5.4 7.7 10.3 16.0 25.3 36.0 47.2 55.9
10.0 16.1 37.2 59.8
4.7 7.9 10.1 17.0 28.0 39.0 51.5 59.7
+
mg La/ml
+
mg La/ml
5.4 10.3 16.0 36.2 56.5
5.4 7.7 10.3 16.0 24.8 36.2 48.2 56.5
+
4.4
4.3
4.5
4.2
4.7
10.4 16.4
10.4
9.9
9.7
10.3
37.5
37.6
37.3
37.3
37.5
60.1
59.8
60.0
60.9
59.8
Impurities
Impurities
+200 ppm Na +1.0
+500 ppm Na $1.0
mg La/ml
mg La/mg
5.4
5.6
5.4
5.4
5.4
10.3 15.8
10.3
10.5
10.3
10.3
37.8
37.1
36.9
36.6
36.6
56.5
56.5
56.5
56.4
56.6
~~
Table V. Effect of Experimental Variables on Determination of Nickel Per cent absorption ( X 10) ____-_ 95 Mixed Mixed Mixed Impurities Impurities +50 ppm +lo0 ppm Confidence Mixed impurities impurities impurities $0.5 mg $1.0 mg $1.5 mg Na t 1 . 0 Na $1.0 range impurities of % A ( ~ 1 0 ) only La/ml La/ml La/ml mg La/ml mg La/ml
z
6.2 i 1.2 9.6 i 1.2 13.0 d= 1 . 2 19.8 11 . 2 3 4 . 4 1 1.2 4 7 . 0 3 ~1 . 3 60.6 i 1 . 4 74.2 i 1.5
0 0.05 0.10 0.20 0.40 0.60 0.80 1.o
0 0.05 0.10 0.20 0.40 0.60 0.80
~~
4.7
Impurities +500 ppm Na 1.0
Table IV. Effect of Experimental Variables on Determination of Cobalt Per cent absorption (~. X 10) Mixed Mixed Impurities Impurities Mixed Mixed impurities impurities impurities $50 ppm + l o 0 ppm impurities $0.5 mg Na f1.0 +1.5 mg Na $1.0 $1.0 mg only La/ml mg La/ml mg La/ml La/ml La/ml
95 % Confidence range
~~
Solution concentration, PPm
4.7 7.5 9.9 17.1 29.0 38.6 51.2 61.2
Impurities +200 ppm Na 1.0
6.0 9.4 13.2 19.7 34.2 46.5 60.4 74.0
6.0 13.8 19.7 46.5 74.3
6.0 9.4 13.4 19.7 34.2 46.5 60.4 74.3
ANALYTICAL CHEMISTRY
0 4.6 10.2 20.2 40.0 59.0 77.9 95.4
1.5 10.2 20.3 61.1 100
1.4 4.7 10.2 20.2 39.5 60.1 80.6 100
Impurities +500 pprn
mg La/ml
mg La/ml
Na 4-1.0
5.9
5.9
5.8
5.7
5.7
13.4 19.7
13.4
13.5
13.4
13.5
46.5
46.5
47.0
46.5
47.4
74.3
74.3
74.3
74.3
74.3
Table VI. Effect of Experimental Variables on Determination of Per cent absorption ( X 10) __95 % Mixed Mixed Mixed Impurities confidence Mixed impurities impurities impurities $50 ppm range impurities + 0.5 mg $1.0 mg $1.5 nig Na 4-1.0 mg La/ml La/ml L.a/ml La/ml of A(X1O) only 0 . 6 1 2.0 5 . 4 t 2.0 10.3 i 2 . 0 19.9 I2 . 0 39.3 i 2 . 0 58.61. 2.1 18.0 i 2.2 97.3 t 99.5
Impurities +200 ppm Na 4-1.0
Chromium
Impurities
Impurities
Impurities
$100 ppm Na +1.0
+200 ppm Na $1.0
+500 ppm Na +l.O
Mg La/ml
mg La/ml
mg La/ml
2.6
2.0
1.o
2.1
2.8
12.0 20.4
10.2
10.2
10.4
10.5
60.6
60.8
60.8
60.6
60.6
98.0
97.9
97.9
97.8
100
Solution concentration, ppm 0 0.05 0.10
0.20 0.40 0.60 0.80 1.0
Table VII. Effect of Experimental Variables on Determination of Per cent absorption ( X 5) Mixed Impurities Mixed Mixed 95 Mixed impurities impurities impurities Confidence +50 ppm +1.5 mg Na +1.0 impurities range +0.5 mg $1.0 mg La/ml mg La/ml La/ml9 La/ml only of A ( X 5 ) 0 0 0 0 0 . 7 ?C 1.1 0 3.8 3 . 3 i: 1.1 3.4 6.8 6.8 6.8 6.8 6 . 6 =k 1 . 1 6.6 12.6 12.8 12.8 12.4 12.5 + 1 . 1 25.0 24.4 =t1.1 24.4 38.8 38.9 38.3 38.0 36.2 =t1 . 1 36.3 50.2 48.1 =k 1 . 2 48.3 62.7 62.8 59.9 i: 1.2 62.7 62.7 59.5
z
z
Table VIII.
t
6oo
Manganese
Impurities +lo0 ppm Na +1.0 mg La/ml
Impurities +200 ppm Na +1.0 mg La/ml
+500 ppm
Impurities
0
0
0
6.6
6.8
38.0
38.0
38.0
62.7
62.7
62.1
Na +1.0 mg La/ml
.
Analysis of Synthetic Samples
Amount of Amount of impurity recovered ___ impurity By NaOH BY 3" ( p g ) added precipiReprecipiReper gram tation, covery, tation, covery, Impurity of sodium pg. Pg Fe 1.0 1.3 130 1.3 130 2.0 2.0 100 1.9 95 4.0 ... ... 3.9 97 100 5.6 93 6.0 6.0 10.0 10.0 100 9.4 94 co 1.0 1 .o 100 1.0 100 2.0 ... ... 1.6 80 4.0 ... ... 3.1 78 6.0 6.0 100 5.8 97 10.0 9.6 96 7.6 76 84 0.70 70 Ni 1.0 0.84 2.0 2.0 100 0.80 40 4.0 ... ... 0.80 20 6.0 6.0 100 2.8 47 10.0 9.8 98 2.9 29 Cr 1.0 0.95 95 0.97 97 2.0 2.0 100 2.0 100 4.0 ... ... 4.1 102 100 6.2 103 6.0 6.0 10.0 9.9 99 9.7 97 Mn 1.0 1.0 100 0.80 80 2.1 105 2.0 100 2.0 4.0 ... ... 3.9 97 6.0 6.0 100 5.9 98 10.0 9.6 96 9.9 99
z
0 ," 4001
%
a
300
1
2001
100
J
I-
v-
0
020 040 060 000 CONCENTRATION (ppm in solution)
1.0
Figure 1. Absorption of manganese showing effect of lanthanum
Manganese absorption is enhanced by the presence of L a t o the extent that a slight, statistically significant difference is observed (cf. Table VI1 and Figure 1). For very precise work, it is recommended that C r and Mn standards used for calibration purposes contain the same amounts of La as corresponding samples. In any carrier precipitation technique, a homogeneous precipitant is desirable. To this end, urea was originally used as the precipitating agent. The release of ammonia o n heating produced a well-formed precipitate with good recoveries for Fe, Co, Ni, and Cr. M n did not precipitate until p€-I 9 was reached. I t is impractical to use urea to achieve this pH. If the determination of Mn is not required, urea can be used. Bubbling argon-diluted ammonia gas through the solution offered something resembling a homogeneous precipitation. A p H of 9 was easily attained but difficult t o control. Higher concentration resulted in nonquantitative recovery of C r and Ni (cf. Table VIII). Digestion of the ammonia precipitate improves the recovery of Co and Ni, but in view of the excellent recoveries obtained with NaOH, the added time is not justified.
6.8
z
The recoveries listed in Table VI11 are, within 95% confidence limits of the experimental error, quantitative. This method using atomic absorption spectrometry is rapid and reliable and is suitable for determining a minimum of five impurities in the 1- to 10-ppm range in a 1-gram sample of sodium. The time required t o analyze a single sample for five impurities, including sample dissolution and preparation of standards which must be run with each set of samples, is approximately five hours. RECEIVED for review April 21, 1967. Accepted July 31, 1967. Presented in part at the Tenth Conference o n Analytical Chemistry in Nuclear Technology, Gatlinburg, Tenn., September 1966. Work done under the auspices of U. S. Atomic Energy Commission Contract AT(04-3)701.
VOL. 39, NO. 12, OCTOBER 1967
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