1017
V O L U M E 2 2 , NO, 8, A U G U S T 1 9 5 0 small. The presence of 12% boron BaOa in the borosilicate glass analyzed did not prevent accurate results from being obtained by this method. The gas used with air in these experiments was acetylene. Substitution of propane, using an appropriate burner, might be of some advantage, aa there would then be a lower rate of excitation of any nonalkali metals present. Sodium and potsssium excite more readily than most other commonly present metals. The photometer readings were made on the instrument without previously cleaning the burner which had been in use for some time. Although it is well always to keep the burner clean, the results obtained indicate that good work may be done even when the instrument is not in optimum condition. It is important that the flow of air and of gas do not fluctuate during the readings. To this end it is advisable to insert a ma-
nometer in each line. If the air is supplied from an air pump, the presence of a large vessel in the line will help to equalize pressure. LITERATURE CITED
Berry, J. W., Chappell, D. C., and Barnes, R. B., IND.ENQ. CHEM.,ANAL.ED., 18,19-24 (1946). (2) Bills, C. E., McDonald, F. G., Niedermeier, W., and Schwarta, M. C., ANAL.CHEM.,21, 1076-80 (1949). (3) Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganio Chemistry,” pp. 788-92, New York, John W h y & Sons, 1929. (4) Parks, T. D., Johnson, H. O., and Lykken, L., ANAL.CHEM.,20,
(1)
822-5 (1948).
Perkin-Elmer Corp., “Model 52A Instruction Manual.” (6) Pratt, P. F., and Larson, W. E., ANAL.CHEM.,21, 1296 (1949). (5)
RECEIVED January 19, 1950. Presented before the Meeting-in-Miniaturea SOCIETY, Newark, N. J., January North Jersey Section, AMERICANCHEMICAL 9, 1950.
Hafnium-Zirconium and Tantalum-Columbium Systems Quantitative Analysis by X-Ray Fluorescence L. S. BIRKS A N D E. J. BROOKS U . S. Naval Research Laboratory, Washington, D . C. The x-ray fluorescence analysis method was adapted to the determination of small amounts of hafnium in zirconium and tantalum in columbium. Curves of the relative intensity of spectral lines of tantalum and columbium or hafnium and zirconium were plotted against percentage composition by using prepared standard compositions. As an example of the accuracy attainable, with a counting time of 3 to 5 minutes on a 0.5 atomic 70 specimen of tantalum in columbium, the probable error in tantalum content due to statistical fluctuations of the Geiger counter was 0.029” or 4% of the amount present.
Q
UAXTITATIVE analysis of the hafnium-zirconium and tantalum-columbium systems is difficult by standard chemical methods. It is also somewhat difficult by spectrochemical means; however, Feldman (3)has recently published very good
-
/I\
/ I\ ’
II
t t
I I
Figure 1. Principles of X-Ray Fluorescence Analysis Method
data on spectrochemical analysis of samples containing less than 1% by weight of hafnium in zirconium. Most present interest lies in the low-hafnium or -tantalum end of these systems. Hafnium usually occurs as small impurity of about 1.5 weight % in zirconium deposits and is very difficult to separate from the zirconium. X-ray analysis has been used extensively for the analysis of the hafnium-rich end of the hafnium-zirconium system since the discovery of hafnium by Coster and von Hevesy (S). Its application to the hafnium-poor end of the system is shown in this paper. Hafnium was discovered by its x-ray L spectrum when a sample of zircon was placed on the target of an x-ray tube and excited by electron bombardment. Placing the specimen on the target of 8 demountable tube is inconvenient, however, With the develop ment of the x-ray fluorescence method (4) where the specimen is outaide the tube, the procedure is simpler and faster. The principles of x-ray fluorescence are shown in Figure 1. The method has proved valuable in measuring the small concentrations of lead and bromine in gasoline and in similar problems and would seem to be directly applicable to the small concentrations of hafnium in zirconium or tantalum in columbium. Usually there is no overlapping of the x-ray spectral lines from elements with similar chemical properties because they fall in the same column of the periodic table and the difference in atomic number is such that the x-ray spectra are well separated. However, in the hafnium-zirconium and tantalum-columbium systems, the difference in atomic number is such that the K series spectrum of airconium dzracted in the second order by the crystal overlaps somewhat the L series spectrum of hafnium. The same is true of columbium and tantalum. Therefore, the method of analysis waa more complicated than with most systems. The wave lengths of
ANALYTICAL CHEMISTRY
1018
the spectral lines of the elements in the two systems are shown in Figure 2. Recently Cauchois and Mac Taggart ( 1 ) described a method for analysis by x-ray absorption where the absorption was measured on both sides of the absorption edge of hafnium. They did not give the results of the experiment or indicate thc accuracy
2ND OiDER
zi
KSERII?~
Cm K wilts
'
I!L, .i :I .i .b LO
I&
IL
1.3
1.4
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TI, L SEIIILS (6
TANTALUM-COLUMBIUM SYSTEM C . P . filings of tantalum and columbium metals were converted to the entoxides by heating. The columbium contained less of tantalum and the tantalum contained even less than columbium. Standards were prepared by adding tantalum to columbium in amounts of 0.5, 1, 2, and 4.5 atomic 70and the x-ray spectra were plotted from 6 to 19 degrees 8 with a sodium chloride crystal.
lh
ZR- HF S Y S T E M )ST ORDER
power supplies or tubes. The wave length of HfKa is 0.22 A,, which would require an analyzing crystal with a spacing of the order of 1 A., such as one of the weaker planes in quartz.
IIT ti '
X ~ A .
Figure 2. Spectra of Elements in HafniumZirconium and Tantalum-Columbium Systems
Figure 3 shows the spectrum of the columbium standard and of the mixture with 4.5 atomic % tantalum. The curve in Figure 4 was obtained by Method B by taking the ratio of the area of the unresolved doublet of TaLB plus ChKB to the area of the first order of CbKa-that is, the ratio of A / B in Figure 3. Figure 4 is the standard curve for the tantalum-columbium system and depends on the fact that all other elements which would give spectral lines near A or B have been removed. Such a separation is not difficult and would be necessary for almost any method of analysis. In fluorescence analysis, the metal atoms fluoresce independently of the compound in which they are present. The nature of the compound therefore does not affect the line intensity or position, except for a very small second-order effect on the intensity due to the small absorption of the oxygen or other nonmetallic atoms in the compound. Thus the same calibration curve would be equally valid for the sulfide, nitride, or silicate, because it is based on the atomic per cent of tantalum in columbium. The accuracy of the data depends on the number of counts measured by the Geiger counter. Other causes of inaccuracy, such as voltage fluctuations and circuit irregularities, have been reduced to well below the statistical accuracy of the Geiger
Langthm of linea are only a n approximate representation of relative intennities.
-C8-4
possible. Zemany, Winslow, Poellmitz, and Liebhafsky have also reported on the use of x-ray absorption for the analysis of a hafnia-zirconia mixture (6). However, they were working with percentages of hafnia above 20%.
r
----.Ca
IATOMIC ' L T A ADDED STANDARD
APPARATUS
The x-ray fluorescence analysis unit was .essentially that described by Friedman and Birks ( 4 ) ,with slight modification in the method of specimen mounting and replacement of the spectrometer arc with a diffraction-type spectrometer in which the Geiger counter was geared to move at twice the speed of the crystal. As is shown in Figure 1, the beam from the x-ray tube strikes the specimen, causing it to fluoresce the characteristic x-ray spectra of the elements. A beam of parallel fluorescent radiation coming off the specimen is assed through the collimator and diffracted according to its wave7ength by the crystal and the intensity is measured by the Geiger counter with standard scaling circuits.
e I N DEGREES
Figure 3. Representative Spectra of TantalumColumbium System with Collimator 4 Inches Long 8. Brags angle for diffraction from NaCl crystal A.
B.
Unremlred doublet of TaLB liner, plus mwnd-order CbKB line Firet-order CbKa line
1
EXPERIMENTAL METHODS
The overlapping of the spectra complicated the experimental procedure, but three methods of attacking the problem were tried with comparable accuracy of results: Method A, fine collimation to resolve the overlapped hafnium and zirconium lines; Method B, comparison of the integrated intensity of an unresolved tantalum-columbium doublet with the integrated intensity of a resolved columbium line; Method C, lowering the x-ray tube voltage to excite the hafnium L spectrum without exciting the zirconium K spectrum. It might seem advantageous a t &t thought to raise the tube voltage high enough to excite the K spectrum of hafnium and so avoid the overlap with zirconium. However, this would require power of the order of 80 kv., which is not attainable with the commercial diffraction type of x-ray
Figure 4. A / B Represents Ratio of Areas under Curves A and B from Spectra Such as Those in Figure 3 Ratio plotted against atomio per cent Ta added t o Cb standard
1019
V O L U M E 22, NO. 8, A U G U S T 1 9 5 0 counter. The general rule for determining the statistical accuracy of the Geiger counter data is that the standard deviation in counts is the square root of the total number of counts. The probable error is two thirds of the standard deviation. The areas used in calculating the ratios in Figure 4 were obtained by planimetering experimental curves such as those in Figure 3. To calculate the probable error in the doha it was easier to add up the total counts obtained a t 11 points on line A , Figure 3, and the total number of counts at 11 points on the background and treat these according to the usual method of handling Geiger counter data. The probable error in B line intensity measurement is given by P.E. = ' / J (-Vg ~VTS)'" (1)
In Table I for the tantalum-columbium specimens are listed the total number of counts, NsI and the background count, N B , the standard deviation in each of these values, and finally the relative probable error from Equation 2.
+
where SSis the total number of counts of the line plus background and N B is the total number of counts of the background. The relative probable error of the line above background measurement is then
0
o
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a
3
4
s l1OYlC %
i
;
i
o
l
o
Mi
Hr
Figure 7 . Ratio of Peak Intensities of Unresolved HfLB Plus Second-Order ZrKB Doublet and ZrKa Line with Collimator 4 Inches Long Tube voltage decreased t o 30 kv. to reduce Zr apectrum preferentially HAFNIUM-ZIRCONIUM SYSTEM
8
IN
Method h was tried for the hafnium-zirconium system. The HfL& line was resolved from the ZrKB plus HfL& doublet by using a collimator 16 inches long with nickel tubes '/I& inch in diameter. Samples were prepared for the calibration curve from three specimens of zirconium with known hafnium content loaned by the Bureau of Mines, Albany, Ore., and from C.P. zirconium oxide (from the Fansteel Products Company, North Chicago, Ill., listed aa C.P. Zr02)with known hafnium content to which hafnium metal (Varlacoid Chemical Company, New York, X. Y., listed. as 98-99% pure Hf) was added. The metallic zirconium obtained from the Bureau of Mines was converted to the oxide chemically to make all the samples as nearly alike as possible.
DEGREES
Figure 5. Representative Spectra of Hafnium-Zirconium System with Collimator 16 Inches Long A . HfLpn line. B. Unresolved HfLBi Ius secondorder ZrK@doublet. For a Zr stantard with no €It, A should not be y e n t . Ita appearance indicated that Zr stan ard contained nome Ht impurity
1614-
L2Y)-
%
86-
4-
ATOMIC
X Hr
Figure 6. A / B Represents Ratio of Peak Intensities of Lines A and B from Data Such as Those in Figure 5 for Standard Specimens
Table I. Probable Error Data for Lines A, Figure 3, Using Equation 2 Atomio '7
Counts, Ns
Counts, N B
Background
Std.Dev.
of N s
5td.Dev. of N B
0
17 100 22:700 24,600 27,050 33,000
6200 7240 6540 6920 9040
131 151 157 164
79 85 81 83 95
TaAddecf 0.6
1
2 4.6
181
Re1.P.E. of N s - NB,
% 0.93 0.75 0.65 0.61 0.57
Figure 5 shows the spectra for the C.P. zirconium oxide (hafnium content 1.1 atomic %) and the mixture of C.P. zirconium oxide with 10 atomic % ' of hafnium added. The ratio of peak intensities A / B is plotted for the various samples in Figure 6. Because peak intensity A represents the amount of hafnium present, the curve should pass through the origin. Figure 6 is the calibration curve for the hafnium-zirconium system and is independent of the compounds in which they are present, just as for the tantalum-columbium system. Data were also obtained on the hafnium-zirconium system by Method C, that is, lowering the x-ray tube voltage to reduce the intensity of the zirconium K spectrum relative to the intensity of the hafnium L spectrum. It was found impractical to reduce the voltage below 30 kv. because of the decrease in intensity of the hafnium lines. However, a t 30 kv. the intensity of the zirconium spectrum was reduced preferentially and the results shown in Figure 7 were obtained. The ordinate represents the ratio of the peak intensity of the unresolved doublet of HfLB plus secondorder ZrKB to the peak intensity of the fistrorder ZrKa line. Either Method A or C seems to be satisfactory for hafnium-zirconium with approximately the same accuracy, CONCLUSION 9
Any of Methods A, B, or C could be applied to the hafniumzirconium system, but in the tantalum-columbium system it waa not possible even with the 16inch-long collimator to resolve the TaLpp line from the CbKB second-order line. Therefore, only Methods B and C were applicable to tantalum-columbium, The accuracy of any of the methods depends on the intensity of the spectral lines-Le., on the counting rate as measured by the Geiger counter and the Geiger counter statistics rather than experimental limitation. Greater accuracy is obtained by determining the integrated intensity of the lines than by merely determining the peak intensity when the lines are not bell-shaped.
1020
ANALYTICAL CHEMISTRY
Once the relation of area to peak intensity had been established, greater accuracy would be obtained by counting for the full time at the peak of the line. In the tantalum-columbium system, the TaLp plus CbKp doublet was determined by counting for 32 seconds a t each of 11 points on the line and 11points on the background. From Table I for the 0.5 atomic % tantalum specimen, the relative probable error was 0.75%. The probable error in the area under the CbKa line was negligibly small compared to this. Therefore, the probable error in ratio A / B was about 1%. In Figure 4, a t 0.5 % tantalum, ratio A / B was about 0.11. The probable error was 1% of this or 0.001. From the curve, it is seen that ratio A / B changes by 0.06 in going from 0 to 1% tantalum. Therefore, 0.001 represents 0.02% tantalum. The measured value for the tantalum content in this region has B statistical fluctuation of 0.02/0.5 or 4% of the amount present. In the hafnium-zirconium system, the accuracy was not as great by Method A because of the lower counting rate with the long collimator. For the zirconium standard, the accuracy was between 5 and 10% of the amount present for counting times of
32 seconds. However, Method B should give the same accuracy for hafnium-zirconium as it did for tantalum-columbium. The limit of the minimum amount of hafnium or tantalum detectable would depend on the length of the counting time, but considering the data above would probably be of the order of 0.1% for reasonable counting times. ACKNOWLEDGMENT
The authors wish to thank Herbert Friedman for helpful discussion during the course of the work and R. T. Seal for assisting in obtaining the experimental data. LITERATURE CITED
(1) Cauchois, Y.,and Mac Taggart, K., Compt. rend., 12, 1003 (1949). (2) Coster, D., and Hevesy, G., von, Nature, 111, 79 (1932). (3) Feldman, C., ANAL.CHEM.,21, 1211 (1949). (4) Friedman, H., and Birks, L. S., Rev. Sci. Instruments, 19, 323-30 (1948). ( 5 ) Zemany, P. D., Window, E. H., Poellmitz, G. S., and Liebhafsky, H.A,, ANAL.CHEM.,21, 493 (1949).
RECEIVED December 23,1949.
Phenoldisulfonic Acid Method of Determining Nitrate in Water Photometric Study MICHAEL J. TAMS Department of Water Supply, Detroit, Mich.
Optimum conditions for nitrate analysis include absenoe of chloride ion and presence of a neutral or' slightly alkaline medium during sample evaporation. Reproducible noninterference is possible at nitrite nitrogen levels below 0.2 p.p.m. All reagent volumes must be equalized in both visual colorimetric standards and samples. In many cases use of ammonium hydroxide for color development obviates necessity for final filtration.
F
OR many yearn, nitrate determination haa been difficult in this laboratory. Scrupulous adherence to the standard procedure ( 1 )yielded an off-color yellow which taxed the imagination when compared against standards prepared in the accepted manner. At first, it waa thought that the fault lay in the synthe& of the phenoldisulfonic acid, and extreme care was exercised in preparation of the reagent. This precaution failed to improve the situation in any material way. A search of the literature revealed the Detroit difficulty to be no isolated caae. Burke's recommendation (3)that 1 ml. of phenoldisulfonic acid be added to the standards waa found to produce a better match between the standards and sample. However, the bulk of the studies (4-7) to date have centered on the nitrate range above 1.0 p.p.m. A thorough photometric investigation of the nitrate reaction disclosed that factors which could be overlooked in the higher nitrate ranges must be controlled very carefully in the lower ranges; doubly so, because phenoldisulfonic acid is one of the few reagents available for the determinations of the relatively small nitrate concentrations preaent in many municipal water suppliea. &vera1 improvements suggested themselves during this period of trouble-shooting. It waa found that the success of the visual determination depends on the equalization of all reagent volumes
in standards and samples alike, and that significant nitrate losses arise from the presence of chloride ion and the neutralization of the total alkalinity. Accordingly, a procedure embodying these findings was developed. PHENOLDISULFONIC ACID REAGENT
The color of the phenoldisulfonic acid reagent depends on the initial color of the solid phenol and the acids used in the preparation. The whiter the crystals and solutions, the clearer and paler is the h a 1 reagent, Oftentimes, the reagent possesses a faint color and reacts with strong alkalies to give a characteristic hue. This positive interference can be compensated for by the addition of identical volumes of reagent to samples and standards. This precaution nullifies the considerable error which may otherwise occur a t the lower nitrate levels (under 0.5 p.p.m.), and also improves the visual color match where Nessler standards must be employed. The practice of introducing equal volumes of all reagents into the blank and the visual and calibration standards is supported by the spectral transmittance curve obtained with a Beckman Model DU spectrophotometer. At wave length 410 mp, the point of maximum absorption, the reagent blank records a
