0 II
COOH
COO-
Conformity to Beefs Law. This system obeys Beer’s law very nicely from 3.3 X to 1 X 10-3 moles/liter. The e is 1.47 X 10-3at580mp, Interfering Ions. None of the 59 ions tested reacted with MMPCA to form a color or a precipitate and therefore should not interfere. However, several of the ions form hydroxides or hydrated oxides in the basic solutions used and the precipitates may absorb some of the Mn04-. While such an effect was not readily apparent, it was not evaluated in detail. Conclusion. This work shows that M n 0 4 - can be very rapidly reduced quantitatively to Mn04- with 6-methoxy-2methylthio-4-pyrimidi1ie carboxylic acid in 25 NaOH solutions. Because of the speed of the reaction this system could
be used not only for the spectrophotometric determination of manganese but also titrimetrically. by taking advantage of the navy blue to green color change at the end point. The system is stable for at least one day, and the procedure does not require an exceedingly large excess of reagent. ACKNOWLEDGMENT
The authors thank C. C. Cheng of the Midwest Research Institute, Kansas City, Mo., for his supply of the pyrimidine derivatives used in this study. RECEIVED for review September 6, 1966. Accepted January 19,1967.
Internal Standard Techniques for Determination of Oxygen in Magnesium, Steel, and Titanium by Activation Analysis Bryce L. Twitty and Kenneth M. Fritz National Lead Co. of Ohio, Cincinnati, Ohio
45239
OXYGENHAS BEEN DETERMINED in many matrices (1-3. Several approaches fclr correcting for flux variations have been used (4, 6, 7). I n each case the l8N activity, after flux correction, is compared to a known sample weight. If the sample matrix is suitaklle for 14-Mev activation, its activation product can be used as an internal standard for both the flux and sample weight corrections. Therefore, no weighing of samples or lengthy standard calibrations are required. The requirement of it known element limits the use of this internal standard approach, but in many instances the concentration of the primary matrix material is known. Normally, the matrix material is in high abundance; therefore, its activation properties do not need to be ideal. If the sample is homogenous, there is no effect due to neutron shadowing and a less effect than with other methods due to self-absorption of y-ra ys. The ?-ray self-absorption is dependent on relative variation in the linear absorption coefficients of the matrix for the 7-ray energies involved. If the oxygen content of the sample is large enough that a correction (1) 0. U. Anders and D. W. Briden, ANAL.CHEM., 36,287 (1964). (2) Zbid.,37, 530 (1965). (3) J. T. Bryne, C. T. Illsley, and H. J. Price, “An Automatic System for the Determination of Oxyen in Beryllium Metal Components,” Proc. Int. Conf. Mod. Trends in Act. Anal., 1965, Texas A.&M. Univ. Press. (4) E. L. Steele and W. W. Meinke, ANAL.CHEM.,34, 185 (1962). (5) J. R. Vogt and W. D. Ehmann, Radiochim. Acta, 4, 24 (1965). (6) W. E. Mott and J. M. Orange, ANAL.CHEM.,37, 1338 (1965). (7) J. E. Strain, W. J. Hampton, and G. W. Leddicotte, “The ORNI, Analytical Chemistry Division’s 150-KV CockcroftWalton Generator,” U. S.At. Energy Comm. Rept. ORNL-TM362 (1962).
for the oxygen content of the rabbit is not required, then no weighing is necessary in routine determinations. EXPERIMENTAL
The activation facility design (8) and the programmer design (9) have been previously reported. Operation. Samples are prepared and sealed into the rabbits while in a nitrogen atmosphere. The rabbit and sample are weighed when required for flux-weight calculations or rabbit-weight corrections. During activation, the flux is monitored using a He-3 tube. The average count rate from this instrument is used where calculations are made on a weight-flux basis; otherwise, it is used merely to monitor the basic flux level of the neutron generator. The oxygen (I6N) activity is recorded by the discriminatorscaler with the discriminator set for a 4.7-Mev equivalent cutoff. The internal standard activity is obtained from the multichannel analyzer by printing the integrated total area under the reference peak via the data processor. Background corrections are necessary for iron due to the Compton continuum from significant higher energy gamma rays. For this background correction, activity is taken from a suitable number of channels before and after the reference peak. This background activity is averaged and subtracted from the total absorption (photo) peak activity. Standardization. The experimental cross section of oxygen was determined from repeated analysis of a National Bureau
(8) B. L. Twitty, “The Neutron Activation Facility at the National Lead Company of Ohio,” Ibid.,NLCO-955 (1965). (9) J. Kramer and M. R. Bailey, “A Versatile Sequential Pro-
gramming Timer,” Zbid.,NLCO-955 (1965). VOL 39, NO. 4, APRIL 1967
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Table I. Summary of Results for Determination of Oxygen in Magnesium Chips Technique Statistical Results Av. sample No. of samples Flux, Std. dev. of Rel. Sample form wt., G analyzed Atmoyphere n/cm2/sec measurements, 0 precision, %” Air Loose 1.2 12 5 x 107 10.024 15 Air Pellet 2.2 13 5 x 107 10.012 8 Nitrogen 5 x 107 10.021 13 Loose 1.2 17 Nitrogen Pellet 2.2 7 5 x 107 10.013 8 Air Pellet 2.2 14 1 x 10s f0.012 8 1.2 8 Nitrogen 1 x 108 10.011 7 Loose Pellet 2.2 17 Nitrogen 1 x 108 10.010 6 Nitrogen Loose 1.2 16 1 x 100 *0.007 4 Nitrogen Pellet 2.2 18 1 x 109 2~0.006 4 Calculated on an average oxygen content of 0.156%.
of Standards steel sample (No. 1045) and of purified ferric oxide. A 7.35-second half life was used for 1SN. For the activation to 68Mn, a half life of 2.576 hours and a crosssection of 0.110 barn were used. An apparent cross section of 0.073 barn was obtained. Using this cross section for oxygen, experimental cross sections of 0.088 and 0.63 barn were determined for 48Ti and 26Mg, respectively. For the experimental conditions used, these values were verifieL by comparison of analyses to those by other methods. In the case of magnesium, which is routinely analyzed, the comparison samples were then used as standards. The relatively few steel and titanium samples were calculated on an absolute basis, ratioing the relative amounts found of each. In the steel analysis, it was necessary to deduct 0.079 mg/gram of
Table 11. Per Cent Oxygen Determined in Magnesium Samples by Various Methods
Sample Form Solid alpinch bar Homogeneous chips +8 mesh chips -8 + 14 mesh chips - 14 + 25 mesh chips -25 mesh chips
Grignard volumetric, av. of four aliquots 0.028 0.144 0.239 0.271 0.392 0.719
Table 111.
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Sample 1
Vacuum fusion av. 0.260
2
0.148
3
0.121
4
0.060
5
0.054
ANALYTICAL CHEMISTRY
Activation method Internal standard, Weight-flux, individual individual aliquots aliquots 0.036 0.026 0.144 0.148 0.236 0.240 0.280 0.264 0.388 0.392 0.718 0.716
oxygen in each rabbit, the average rabbit weight being 2.150 grams. Magnesium Analysis. This study was made using magnesium chips having a purity greater than 99% magnesium. The total oxygen content including that on the surface was desired. The 0.97-Mev gamma of sodium-25 [26Mg(n, p ) NaZ6]was chosen as the reference y-ray of the internal standard. To optimize the method, the effects of sample form, atmosphere and flux were investigated. Except when weights were required to calculate results on a weight-flux relation, no weighing was performed after calibration. Chips were either poured loosely into rabbits or compressed into pellets for insertion into the rabbits. The effects of the variables on precision are shown in Table I. A series of magnesium samples was analyzed by activation, using both internal standard and weight-flux calculations ( l e ) that had previously been run in quadruplicate by the Grignard method of Anderson and Thieman (11). The samples were analyzed in a nitrogen atmosphere by a flux of 1 x IO9 n/cm2/sec. The chip samples had been separated into mesh sizes for better homogeneity and then pelletized. The comparative results are shown in Table 11. A comparison of the oxygen-to-standard count ratio obtained by the internal standard technique to the average oxygen concentration of the three methods was linear.
(10) B. L. Twitty and K. M. Fritz, “Rapid Determination of Oxygen in High Purity Magnesium Chips by Neutron Activation,’’ Zbid.,NLCO-973 (1966).
Oxygen Content of Titanium, Comparison of Various Methods Activation method Weight-flux Internal standard Individual Av. Individual 0.289 0.268 0.265 0.273 0.274 0.288 0.264 0.142 0.144 0.140 0.143 0.141 0.140 0.150 0.145 0.126 0.126 0.131 0.123 0.139 0.130 0,063 0.063 0.067 0.061 0.057 0.060 0.066 0.179 0.059 0.055 0.060 0.059 0.184 0.063
Av.
0.283 0.142 0.138 0.061 0.141
Titanium Analysis. ‘Thetitanium analyzed was in the form of high purity wire. This surface was treated to remove oxygen prior to activation. A summation of the three y-rays of scandium-48 [‘BTi (11, p ) 48Sc]was chosen as the internal standard. These samples, approximately 1 gram per rabbit, were activated by a flux of approximately 1 X l O Q n/cm2/sec and calculated by both the internal standard and weight-flux techniques. The high flux was required for sensitivity. The results are compared with the averages of multiple vacuumfusion results in Table HI. Using the method of pooled variance, the standard deviation was found to be Zk0.005 oxygen for the internal standard technique and =tC1.032 % for the weight-flux technique (Zk0.006x if sample 5 is ignored). Sample 5 was reanalyzed by the weight-flux method with no improvement in results; no explanation can be offered for this occurrence. Based on an average oxygen content of 0.132 %, the relative precision for the internal standard technique was 3 %. Steel Analysis. The activation method was investigated for oxygen concentrations as low as 10 ppm in steel. Three pieces of steel bar stock, that had been repeatedly analyzed by a vacuum-fusion method, were used after removal of surface oxygen. The 0.85-Mev y-ray of manganese-56 [56Fe (n, p ) 56Mn] was chosen as the internal standard. Approximately 5.1 grams of steel were used per rabbit. The samples were activated at a flux near 3 X IOQn/cm2/sec to obtain the best sensii;ivity possible. The results, shown in Table IV, were corrected for the rabbit oxygen content and calculated by the internal standard technique. A standard deviation of 3 ppm was found by pooled variance techniques. When an average value of 60 ppm oxygen was used, the relative precision was 5
x
x.
DISCUSSION
The internal standard method has significant advantages. No weighing is required, which greatly reduces analytical time. The preparation and calibration of standards are minimized since they are required only for determining changes within the instrument system. The system required is much simpler, less expensive, and easier to adjust than a dual sample system, especially when the generator must be used for thermal as well as fast activations. Since both isotopes measured are in the same matrix, there is identical selfshadowing of the flux. Although the relative self-absorption between the y-rays of W and the internal standard vary from matrix to matrix, it is constant for a fixed matrix. Relative self-absorption values were calculated from an equation derived from the relationships of Etherington (12) and found to be 0.995, 0.975, and 0.9E;l for magnesium, steel, and titanium, respectively. The principal problem in using the internal standard technique is due to the flux lime spectra. When the two isotopes (11) D. J. Anderson and €1. W. Thieman, “Determination of Magnesium Oxide in Magnesium Metal,” Zbid., MCW-1475 (1962). (12) H. Etherington, Ed., “Nuclear Engineering Handbook,” pp. 7-107, McGraw-Hill, New York, 1958.
Table 1V. Oxygen Content of Steel, Comparison of VacuumFusion and Activation Analyses Activation Vacuum-fusion Individual, Sample av., ppm PPm Av., ppm 140 1415 143a 1 140 148 2 28 29 25 22 24 3 10 13 11 12 9 Corrected for 90% iron count.
measured have different half lives, they are not equally affected when the flux varies during the exposure time. The 16N activity is affected, primarily, only during the last 14 seconds of the activation time, while the much longer-lived matrix materials are affected by the total activation period. This limitation is reflected in the precisions obtained. The advent of a square wave neutron pulse would completely eliminate this source of error. The optimum results should be obtained by using an internal standard technique in a dual sample system. Precision. The precisions found for titanium, 3% relative at an average level of 0.132%, and steel, 5 % relative at an average level of 60 ppm, correspond to the counting statistical deviation. The magnesium precision, 4 % relative at an average of 0.156%, was 1.4 times that attributed to counting statistical deviation. The better counting statistics of magnesium compared with those of titanium were due to larger sample weights and a larger reference count. The 0.4 excess with magnesium is attributed to the nonhomogeneity of oxygen within each rabbit. This nonhomogeneity is indicated by the variation of oxygen with particle size as shown in Table 11. Steel Analysis. While analyzing the steel samples for oxygen under the activation cycle listed, the activation products of chromium, manganese, and silicon were seen and could be analyzed simultaneously. Wood and Roper (13) have analyzed for silicon in steel using an internal standard approach with an activation similar to the one listed above.
RECEIVED for review July 11, 1966. Accepted February 2, 1967. The work reported herein was performed for the U. S. Atomic Energy Commission under Contract No. AT (30-1)-1156. Presented at Society for Applied Spectroscopy, National Meeting, Chicago, Ill., June 1966. (13) D. E. Wood and N. J. Roper, “Fast Neutron Activation Analysis for Silicon in Iron,” Kaman Nuclear Corp. Rep. KN-65140(R) (1965).
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