800
ANALYTICAL CHEMISTRY
sity curves with the heated-core furnace, high precision source high voltage section and Production Control Quantometer showed that, contrary to previous experience with solid electrodes, intensity increases gradually with time for most elements throughout the exposure period. Relatively volatile elements such as sodium or alkaline earth metals tend to burn out preferentially, although not so rapidly as to preclude their determination. Of the melting procedures investigated, induction heating appears to provide the most generally satisfactory results and appears to combine best with modern direct-reading spectrometers. The principal weakness of the method, as with existing solid electrode methods, is in the sample itself. However, in this new method, physical structure in the solid state has been eliminated as a variable and the sampling problem has been reduced to that of obtaining a compositionally representative sample. The method is comparable in speed with existing solid-electrode methods, the complete analytical cycle of melting, sparking, and recording requiring less than 2 minutes. Precision is
comparable or superior to alternative methods available, and the method can be executed by laboratory personnel meeting typical requirements of routine analysts. In summary, the preliminary work reported in this paper indicates that the use of molten electrodes in metallurgical analysis provides a practical means of circumventing the structural effects which have sharply limited the application of spectrochemical methods in the past. Much further work is necessary to bring the method into general use, including the development of more suitable induction heating equipment, a more exhaustive study of excitation parameters, and the improvisation of sampling techniques. LITERATURE CITED
(1) Churchill, J. R., Iron A g e , 168,97 (1951).
RECEIVED for review November 16, 1953
Accepted December 24. 1953. Presented in part a t t h e Pittsburgh Conference on Analytical Chemistry and Spplied Spectroscopy, March 1953.
Quantitative Analysis of Niobium and Tantalum in Ores By Fluorescent X-Ray Spectroscopy WILLIAM J. CAMPBELL and HOWARD F. CARL Eastern Experiment Station, Bureau
o f Mines, College Park, Md.
The application of fluorescent x-ray spectroscopy to the analysis of niobium (columbium) and of tantalum in their ores was undertaken to provide a rapid yet accurate method for the determination of both of these elements. Three techniques are described: the first is the determination of the niobium-tantalum ratios in oxides chemically separated from ores. The second method consists of several applications of internal standard techniques and the third method is essentially an additive technique. The accuracy of the final results of these methods is believed to be superior to existing chemical methods. Between ten and fifty samples per day can be analyzed, depending on the type of material.
T
H E demand for niobium (columbium) and tantalum for various industrial uses has increased considerably in recent years. The available sources of supply of high-grade ores are not adequate for present requirements. To alleviate this shortage new deposits of niobium and tantalum are being investigated. A reliable method of analysis was needed to evaluate the untreated ores and also to follow the niobium and tantalum concentration during mineral-dressing processes. Various chemical and instrumental techniques had been employed to perform this type of analysis. They were all time-consuming or unreliable whenaccurate values of both niobium and tantalumivere required. For the past few years the x-ray laboratory a t the Eastern Experiment Station of the Bureau of Mines has been developing techniques of fluorescent x-ray spectroscopy for the analysis of niobium and tantalum. This paper briefly discusses the basic theory, considers several methods employed for niobium and tantalum determinations, and evaluates the accuracy and the time required for these analyses. The characteristic x-ray spectra which are used in fluorescent x-ray spectroscopy are the K and L series. The K a and KP series are used for elements titanium(22) to barium(56), and the more complex L series is used for barium(56) and all elements of higher atomic number. (Number in paTentheses is the atomic number of the element.) Elements below calcium(20) cannot
be detected by commercially available equipment because of the air absorption of their spectra of long wave lengths. By the use of a vacuum system and a specially constructed Geiger tube, elements calcium(20) down to aluminum(l3) may be detected. Such equipment has been described in a recent article by Birks (4). INSTRUMENTATION
The first commercially available model of a fluorescent x-ray spectrograph, manufactured by North American Philips Co., has been used in these studies. This company has subsequently incorporated its wide-range goniometer as the wavelength measuring device ( 3 ) . Figure 1 shows a top view of the instrument. It consists of a basicx-ray power supply, -4,which can be operated a t a maximum output of 60 kv. and 50 ma. The x-ray tube, B, is a Macklett AEG-50 tungsten target model which will permit operation a t 50 kv. and 50 ma. The primary x-rays strike the sample in holder C that is positioned immediately adjacent to the beryllium window of the x-ray tube. The x-ray tube and eample are enclosed in a lead-lined box, D. The secondary or fluorescent x-rays from the sample are collimated by a hesagonal close packing of fine nickel tubing, E, 0.80 mm. in diameter and 0.051 mm. in wall thickness. This collimated beam is diffracted by the analyzing crystal, F. The intensity of the diffracted beam is measured by a Geiger-counter tube, H mounted on a 90' goniometer, G. The Geiger tube is driven on the goniometer by a motor, I . The output of the Geiger tube is applied to a scaling circuit and then to a rate meter. Its intensity can be measured either by a mechanical register or by recording the output on a chart record. TKOpapers by Friedman and Birks (6, 9) present an excellent introduction to instrumentation of the fluorescent x-ray spectrograph. Fluorescent x-ray spectrographs capable of recording two or more wave lengths simultaneoudy have been described by Adler and k e l r o d ( 1 ) and by Gillam and Heal (IO). EXPERIMENTAL
In the applications of fluorescent x-ray spectroscopy to the quantitative analysis of minerals variations in x-ray absorption properties of the eample are a most important consideration. Since most samples have considerable variations in this property, it is necessary to apply a systematic correction. Three methode
V O L U M E 26, NO. 5, M A Y 1 9 5 4
F i y r e 1. Top View of Fluorescent X-Ray Spectrograph A . Besic x-ray power supply E. C. D. E.
X-raytube Sample holder Lead-lined box Nickel tubing F. Analyzing ~ r y n i d 6. Goniometer If. Geiger-oounter tube I . Motor 0. Holder for oxide sample P. Tin mask for ~ l ~ e i i sample ng
arc outlined for making analyses of minersls. Method A is a determination of the ratio of niobium to tantalum in their oxides separated chemically from their ores. Method B describes Beyera1 techniques for the use of an internal standard. Method C is substantially an additive technique. Method A. The difficulty of a chemical analysis for niobium and tantalum is discussed in a recent paper by Atkinson, Steigman, and Hiskey ( b ) , wherein i t is stated that an analysis of an ore sample by a competent chemist requires from 5 to 15 days, depending nn the method. These authors also presented in tabular form the results of analyses of the same sample by v x i OUB laboratories. It was indicated that chemical methods of determining t,he value of the combined niobium and tantalum oxides are much more accurate than the determination of the ratio of these oxides in a sample. This suggests that sfter the chemical laboratory bas quantitatively extracted the combined niobium-tantalum oxides from an ore, the niobium-tantalum rnt,io can be det.ermined by fluorescent x-ray spectroscopy. Thir results in an accuracy superior to chemical methods alone. Two chemical prooedures have been used at this station for extracting the combined oxides from ores. Originally t.he &andard Sohoeller method (13) was used; more recently the simplified Woods procedure has been employed (14). I n the Scboeller process the combined oxides of niobium, tantalum, and titanium are extracted. The x-ray laboratory then determines the niobium-tantalum ratio of the combined oxide nondestruotively and returns the oxide to the chemists far a colorimetric determination of the titanium. The new Woods method consists of the extraction of the niobium and tantalum as fluorides, with methyl ethyl ketone containing hydrofluoric acid, by passing the solution through an sctivated cellulose column. After removal of the solvent, the niobium and tantalum are precipitated with tannin, ignited, and weighed as the oxides. With the Woods technique titanium is not extracted with the combined oxides. When the Scboeller extraction method was used, a detailed study was made to determine the effect of titanium dioxide on the niobium-tantalum ratio as determined by the x-ray tech-
801 nique. For concentrations of titanium dioxide up to about 5% the effect is less than the experimental error associated with the x-ray method. If the concentration of titanium dioxide is much higher, the value of the niobium-tantalum ratio as determined will be slightly higher than the true value. As the titanium dioxide content is determined independently, a correction can be made if necessary. Calibration standards are prepared by thoroughly mixed varioua proportions of high-purity niobium and tantalum oxide powders. Intensity data. obtained from these standards are used in constructing the necessary calibration curves. The sample of oxide is packed by hand into a suitable holder (0, Figure 1) in such a manner to obtain a flat surface. As the amount of available oxides varies a mask of tin (P,Figure 1) is placed over the sample while i t is exposed to the primary x-ray beam to ensure that all samples will have equal surface areas exposed to the beam. The determination of niobium in the presence of tantalum is not complicated by overlapping spectra. This determination is made by measuring the N b K a intensity at 15.25' 28, using a sodium chloride analyzing crystal (2 d = 5.6 A,). The x-ray tube is operated a t 35 kv. and 35 ma. As a portion of the total radiation recorded at 15.25" is due to background, a determination of background intensity is made a t 16.2' 28 and this is subtracted from the total intensity to give a value of corrected intensity. All intensities are obtained by positioning the Geiger tube a t the proper angle and determining the time required to record a fixed number of counts. A calibration curve is prepared by plotting the log of the corrected int,ensity of NbKa, in counts per second, versus the log of weight per cent niobium oxide. Figure 2 shows such a plot. The niobium concentrations in the extracted oxides are then obtained from such a curve, using the values of NbKa measured under identical conditions.
PERCENT NIOBIUM
__
OXlOE
Figure 2. Log I n t e n s i t y NhKa US. Log Weight Niobium Oxide i n T a n t a l u m Oxide
%
Calibration curve for undiluted oxide samples
/.
The deter minatiou of tantalum oxide is complicated b y the ninon t n +hn f i m + . m d o second-order X T L r . . 1:"- $-lL.n.:m.. v=AJ TaLar line. Three possible methods of making this determination have been suggested by Birks and Brooks (5). A recent article b y Brissey (7) also describes a technique t h a t uses an analyzing crystal of mica (2 d = 3.00 A,) that provides sufficient resolution of these lines. This laboratory has used quartz crystals (2 d = 3.04 A. and 2 d = 3.64 A,) for the past year to obtain greater resolution when necessary. Satisfactory results have been obtained by performing the analyses a t 19 kv., a voltage that is not sufficient to effectively excite the NbK spectra. With a sodium chloride analyzing Lv22vyv.y6
.."_.,
I.VO1
yy
~
ANALYTICAL CHEMISTRY
802
crystal the T a L a occurs a t 31.35" 28. The background is determined a t 33.2" 28 and the intensity value obtained as previously described. A tantalum oxide calibration curve similar to that for niobium is constructed and used to obtain the tantalum oxide content of the samples. As the total weight per cent of the two oxides obtained from the chemical separation should equal 100, this offers a check on the relative purity of the combined oxides as well as on the accuracy of the analyses. I n the oxides analyzed the impurities were too low to have any practical effect on the analyses. The weight of combined oxides received by the x-ray laboratory varies with the chemical separation procedure. With the Schoeller method larger samples can be handled and the amount of oxide separated is usually 0.2 gram or more. I n general, the M700ds method is restricted to ore samples 0.2 to 0.5 gram, and therefore the amount of oxide separated may be quite small (0.01 to 0.1 gram). . 4 modification of the usual technique is required to handle such small amounts of mixed oxides. When the weight of combined oxide received from the chemical laboratory is less than 0.2 gram, a modified procedure is used. One part of combined oxide is mixed with sufficient starch to give the required volume of sample. With such samples, standards are used that have been diluted with three parts of starch to one of combined oxides. In this procedure the niobium and tantalum lines are both excited a t 40 kv. As many small samples require a dilutiun of more than 3 to 1 with starch, a different treatment of data is also required. The intensities of the first-order N ) K a and T a L a are measured and corrected for background The calibration curve is constructed by plotting INbKalINbKa
+ ITaLa
versus weight per cent niobium oxide on a linear scale. Such a curve is shown in Figure 3. (This TaLa intensity is actually the combined intensities of a weak second-order YbKa overlapping the stronger first-order TaLa.)
B
5 ,
0 x
, 0
Figure 3.
5
IO
Ratio
15
20 25 30 35 PERCENT NIOBIUM OXIDE
ZNbKca INbKa
x
f
10
ZtsLrr
40
45
50
us. Weight $%! Niobium
Oxide in Tantalum Oxide Calibration curve for oxides considerably diluted with starch
By this procedure a calibration curve constructed using standards diluted 3 to 1 may be used to analyze very small samples that require dilution with large amounts of starch. As this method is based on the principle of comparing ratios of intensities, errors in mixing and packing the samples are minimized. Table I presents the variation in observed intensity ratios with various degrees of dilution of starch to a 50-50 mixture of niobium and tantalum oxide. It is obvious that with no correc-
tion for the difference in dilution between standards and samples the percentage error is small. Likewise, as the variation is quite uniform with change in starch concentration, a correction factor may readily be applied if necessary to use higher dilutions. A limitation on this method results from the overlapping of the second-order NbKa with the first-order TaLa line measured, as noted. Therefore, samples containing less than 1 or 2% of tantalum oxide cannot be accurately analyzed by this technique.
+
Table I. Variation of zNbKa/zNbKa ZTaLa for a Mixture of Equal.Parts of Niobium Oxide and Tantalum Oxide with Dilution by Starch G . Starch Added t o 1 G . Sample 0 1
2 3 4 5
7 10
IShlia
X lO/INbKu f I T ~ L ~
4.859 4.883 4.907 4 931 . . ~
~
4.955 4.977 5.027 5,099
Airecent paper by Nortimore and Romans ( 1 2 ) discusses the analysis of hafnium and zirconium oxides, a system analogous to niobium and tantalum oxides. The time required to complete a series of analyses with the combined n'oods and x-ray method is 4 to 5 days. The number of samples that can be handled is limited by the chemical separation procedure. The x-ray laboratory can determine the oxide ratios on 30 to 40 samples per day. The accuracy of the determination of oxide ratios depends on the amount of oxide available. The first technique described, using 0.2-gram samples, is more accurate, especially when it is necessary to determine smaller percentages of tantalum. In general, results can be expressed as 50 =k lpc, or 1 + 0.19 of either oxide by this method. The dilution technique is slightly less accurate. Ji'hen it is considered that the oxides analyzed may be only 5 to lowo of the original ores, these ratios obtained by x-ray analyses represent quite accurate anall ses of the original ore. Method B. Von Hevesy (11) describes several analytical techniques using the primary x-ray spectrograph in which the sample is made the target of a demountable x-ray tube. For several reasons this system is not suitable for routine analyses in industrial laboratories, although i t fulfills two of the major requirements necessary for accurate determinations. As the sample is made the target of an x-ray tube, very high intensities are possible, which will permit the necessary resolution to separate closely adjacent lines. Such high resolution cannot be obtained with the fluorescent x-ray spectrograph without a prohibitive loss in intensity. Consequently, the internal standard technique is limited when using the fluorescent x-rav spectrograph because many adjacent lines available for comparison lines are too weak or cannot be resolved. Among the more important requirements of elements used as reference standards are availability, absence of reference element in sample, the similarity of excitation voltage of reference element as compared to element analyzed for, and the wave length of the spectra of reference element as compared to element being determined. Table I1 lists characteristic properties of possible internal reference elements for the x-ray analyses of niobium. For the determination of niobium in ores, either molybdenum or zirconium is used as a reference element, depending on the composition of the ore. The spectral lines chosen for comparison are NbKa with either MoKa or ZrKa. These lines have suitable intensities and can be completely resolved from NbKa. Suitable comparison lines used with TaLa are HfLa, W L a , ZnKa, or CuKa. The exact technique of applying an internal standard depends
V O L U M E 2 6 , NO. 5, M A Y 1 9 5 4
803
to be of great value in the c o n c e n t r a t i o n of low-grade niobium ores. Results obBragg -4ngle (281, tained b y o p t i c a l s p e c t r o NaCl C r y s t a l Critical graphic methods and this Atomic Voltage, Kct Spectra, Kg S ectra, K Absorption Edge, Ka, KP-, Element Number Kv. A. A. degrees degrees m e t h o d a r e c o m p a r e d in M 0 42 20 0.7104 0.6323 0.6192 14.48 12.87 Table 111. Nb 41 19 0.7476 0.6529 15.24 13.56 0,6622 Zr 40 18 0.7873 0.6982 0.6888 16.05 14.30 The technique of the addiY 39 17 0.7405 0.7265 16.93 15.09 0.8307 tion of a fixed amount of a Sr 38 16.1 15.98 0.8768 0.7826 0.7716 17.89 reference element is not practical for the analysis of only a few samples because of the work involved in preparing standards. It is desirable to have on the type of sample to be analyzed. The first technique described is used for the analysis of a relatively large number of a method that does not necessitate the construction of calibrasamples of similar mineral content. tion curves but in which the internal reference element supplies -4typical ore of such a series may readily be examined mineralthe complete calibration. ogically and spectrographically to determine its approximate comFor this technique the reference standards are the same as before but added in a slightly different manner. The initial step is position. A similar base material can therefore be prepared and this is used as the matrix for dilution. For niobium oxide standto obtain an approximate value for the niobium oxide content. ards the base material would also include tantalum oxide if This can be obtained from observation of a chart record of the present in the original samples. Niobium oxide is added in varispectra of the sample. ous proportions to the matrix material to cover the anticipated To each sample is added the reference element (in this example, range of concentration. Then 0.1 gram of molybdenum or molybdenum trioxide) in a weighed amount to give approximately zirconium oxide is added to 0.9 gram of each standard and unequal concentrations of both internal standard and niobium oxknown. Thus the same concentration of the internal standard ide, The intensities, Z, of the NbKa and PIIoKa lines from these is present in every sample. samples and also from a 50-50 mixture of niobium and molybdenum oxides are determined at 50 kv. From these data the amount of niobium oxide in the sample is calculated by the following equation: Table 111. Optical and X-Ray Spectrographic Determinations of Niobium Oxide % Nb205 = % Moo3 X.(dilution factor) X NbzOa, Wt. % KbzOa, Wt. % (intensity correction factor) X zNbKa/zlloKct (unknown) Sample N o . Optical X-ray Sample No. Optical X-ray 76 1.10 1.30 in which 90 2.30 2.58 78 0.12 0.05 92 6.90 7.40 80 0.06 0.02 94 0.43 0.36 % 121003 = [weight MoOa '(weight l\IoOs Lyeight sample)] 100 82 0.21 0.17 0.23 0.20 96 Dilution factor = (weight sample weight Mo03)/weight 88 0.09 0.10 110 0.85 0.80 sample Intensity correction factor = Z x f o K a / l s b K a , obtained from the 50-50 mixture of the oxides; this term effectively calibrates The line intensities of NbKa, standard K a , and background the system are measured a t an excitation potential of 50 kv. The necessary IKbKa/zRIoKa (unknown) = intensity ratio determined from calibration curve is constructed by plotting the intensity ratios unknown sample ( N b K a divided by standard K a ) versus weight per cent niobium -4similar procedure is used for the determination of tantalum oxide. The unknowns arp determined by comparing the correoxide. Hafnium, tungsten, zinc, or copper may be used as the sponding intensity ratios to the calibration curve. internal reference element with the measulements made a t 19 kv. To analyze for tantalum the same base material is used except that now a constant amount of niobium oxide is added and the tantalum oxide is the variable. Then 0.1 gram of either hafTable IV. Niobium Oxide and Tantalum Oxide in Samples nium dioxide, tungsten trioxide, zinc oxide, or copper oxide is Containing Variable Cassiterite added to 0.9 gram of each sample. The samples are excited a t KbnOa, W t . % Trt?Os, K t . % 19 kv., 50 ma., and the calculations performed like those for Sample N o . Sn, % A B A €3 niobium. 17.4 21.0 34.1 21.0 21.7 182 61.6 60.5 303 7.2 3.1 4.2 T h e best choice of matrix composition, while not critical, is 65.6 62.7 31 1 0.5 3.1 3.9 that approximating the average composition of the ore samples 17.6 17.7 3.8 3.9 31 3 51.5 9 . 5 9.9 315 6 0 . 1 2 . 7 2 . 5 as regards their x-ray absorption characteristics. By this technique 15 to 20 samples per day can be analyzed for both niobium and tantalum. The accuracy of the determinations depends on the concentration of oxide in the sample. The The characteristics of the IVLa and HfLa are similar to those lower limit of detection is about O . O l ~ oniobium oxide and 0.0370 of TaLa, but there may be some restriction on the use of either tantalum oxide. By several modifications of the equipment, of these elements for a tantalum analysis. If tungsten is used, such as the use of a lithium fiuoride crystal, a Sollar slit collimathe L spectral lines from the primary x-ray beam that are scattor, and a krypton-filled Geiger tube, the lower limit of detection tered by the sample would be added to the fluorescent tungsten for niobium may be extended to 0.001%. spectrum. *4s the scattered radiation is relatively weak, it is not On samples containing about 0.1% niobium oxide the results difficult to correct for this added intensity when the concentrahave a spread of ~ k 0 . 0 3 7 ~With . higher concentrations the tion of tungsten in the sample is above about 5%. Hafnium possible variation was 5 to 10% of the amount present, with the oxide is an excellent standard but it may be difficult to obtain least variation for samples of high concentration. The accuracy a sufficient quantity. of the tantalum analyses ie slightly lower than for niobium in the The K a spectra of copper and zinc fall close to the La spectrum low concentration range I n the higher concentrations the reof tantalum and hence either may be employed as an internal sults are comparable. standard. By this type of analysis results were obtained which proved Results by this method of analysis for niobium and tantalum
Table 11.
Spectral Characteristics of Those Elements of Possible Use as Internal Standards for Niobium
1.
++
ANALYTICAL CHEMISTRY
804
oxides are compared in Table IV with those obtained by method A on samples in which a tin concentration varied from 0.5 to 60%. The other elements present in significant amounts were titanium, iron, silicon, and manganese. The equation used in the second modification of method B was critically examined to determine the possible accuracy of the method. The first two terms on the right side of the equation involve only weighing and mixing. The weight of standard added is usuallymore than0.5 gram so that weighing errors are less than 1%. The complete mixing of standard and unknown is facilitated by using only -325-mesh material and both mechanical tumbling in a bottle and brushing through a 100-mesh screen. As a large area of sample is analyzed, possible errors caused by segregation are negligible. The third term involves only intensity measurements on the 50-50 mixture of reference and niobium or tantalum oxide. The intensity determinations can be made to a high degree of accuracy by recordingasufficientnumberof counts. Therefore, if the fourth term can be accurately determined, satisfactory results may be obtained by this method. It is necessary to make a preliminary chart record of the spectra of the sample in the region of the strong niobium and tantalum lines to detect the possible presence of interfering elements and overlapping lines. If overlapping lines are present, other comparison lines may be used. Recently, other analyzing crystals have been obtained that provide greater resolution than 30dium chloride without a sacrifice of intensity. It has been found important to maintain absolute line intensities below about 400 counts per second by suitable adjustment of the x-ray tube current. I n this region it is not necessary to correct for the dead time of the Geiger tube. The use of internal standards is completely valid only if the various elements in the matrix of the sample affect the reference line and niobium or tantalum equally. I n his book, von Hevesy (11) states ". , .Only if we have a reference line that entirely shares the fate of the line of the element to be determined, so far as absorption and excitation is concerned, can the intengity ratio of the two lines furnish a trustworthy method of quantitative analysis." However, it is usually possible to use lines sufficiently close together to satisfy this condition for practical analyses. By this technique 5 to 10 samples per day may be analyzed for niobium and tantalum. The accuracy of the analysis of uncomplicated ores is about that obtained for Method A. For very complex systems similar methods are employed, but more precautions as to interfering elements must be taken. This lnboratory has analyzed pyrochlores, rutiles, columbite-tantalites. monazites, and other systems by this method with an accurpy equal to the chemical methods. The resulting saving in time and espense is very important. Method C. This method is based on the assumption that, over a limited range, there is a linear relationship hetween the concentration of a n element and the intensity of its spectral lines. It is possible to measure the intensity of a spectral line before and after addition of some of the element analyzed for and correlate the data to obtain the original concentration in the samplc. The following equations are used to perform the necessary calculations:
where
z
= concentration of Iib205or TasOr in unknown before
X' y
=
addition
XD
= concentration of l\jb205or T a 2 0 6added to sample
= intensity of NbKa or TaLa before addition
IZ
ZZ + 2/ = intensity of NbKa or TaLa after addition D = dilution factor = weight of sample/weight of sample 1
plus weight of oxide added
K and K' = constants whose value depend on the experimental limit k" conditions; in the limiting case -= K
Y+O
If the assumption is made that k" = K and if Equation 1 is divided by Equation 2 the following relation is derived:
This method has been most successful for the analyses of lowgrade ores. I n the low Concentration range (0 to 5%) the linear relationship is maintained so that no corrections are necessary. For higher concentrations the linear relationship is limited to narrow ranges, so that the amount of oxide required to be added is critical. If too little oxide is added, experimental errors may be large. If the amount added is too large, the approximation of a linear relationship is poor, The best results have been obtained when the oxide added is of such a quantity that I Z / I Z ~ +=v 0.7 to 0.8. If high accuracy is required, the sample may be analyzed by a series of successive dilutions. The results are then estrapolated back to zero addition to obtain the best answer. From analyses performed, results are reported as 0.5 f.0.05% or 10 zk 0.5y0. For higher concentrations Method B has been employrd. hlethod C has been most useful for the analyses of samples containing elements that interfere with hlethod €3. I t is necessary to consider only overlapping spectra for method C. -4 recent paper by Despujols (8) describes an application similar to Method C for the analyses of zinc ores. Table V presents a comparison of optical and fluorescent x-ray spectrographic analyses of ores for niobium oside by Method C.
Table V. Optical and X-Ray Spectrographic Determinations of Niobium Oxide Nb205, W t . "0 Sample No.
Optical
X-ray
CONCLUSIONS
All three methods have been employed for routine analyses of niobium-tantalum ores. Method A is accurate if the chemical separation is satisfactory but it is very time-consuming and espensive. Method B, for most samples, is as trustworthy as A and requires very little time per sample. hfethod C has been used for specilized samples, especially low-grade niobium pyrochlore. The methods described are applicable to the analyses of many other ores, for example, hafnium and zirconium, uranium and thorium, iron and manganese, selenium, tungsten, molybdenum. By the use of Methods B and C it is possible to analyze a mineral quantitatively for all major constituents above atomic number 22. For example, recently this laboratory determined niobium, tantalum, iron, manganese, tin, and titanium in a columbite-tantalite in less than one day. The considerable savings in time and effort in the analysis of ores for both niobium and tantalum by this fluorescent x-ray spectrographic technique may have an important economic consequence. At the present time, the high cost of chemical analysis of these ores often prohibits sampling and possible subsequent
V O L U M E 26, NO. 5, M A Y 1 9 5 4 development of comparatively small deposits or concentrations of niobium-tantalum ores. It has been estimated that a single accurate chemical analysis of this type costs about $100 to perform. Unless this cost is a relatively small fraction of the total value of a deposit, such deposits have not been considered economical to exploit. T h e n many complete analyses can be made rapidly and inexpensively, the cost of an initial analysis should 110 longer be a detrimental factor to the development of small deposits of niobium-tantalum ores. ACKNOW LEDGMElrr T
The cooperation of Allan H. hfacmillan and Thomas E. Green, who performed the chemical analyses, and of Maurice J. Peterson, who made the spectrographic analyses, is appreciated. George E. .4shby, now with the J. J. Maguire Co., did much of the preliminary work in developing the equipment and the general technique of quantitative mineral analysis. This investigation was performed under the direct supervision of John E. Conley, with the general supervision of Paul M. iimbrose. Their continued interest in this problem encouraged the authors to prepare this paper for presentation and for final publication.
805 LITERATCRE CITED
(1) Adler, L., and Axelrod, J. AI., J . Opt. SOC.Amer., 43, 769 (1953). ( 2 ) ;Itkinson. R. H.. Steipman. J.. and Hiskes. C. F.. ANAL.CHEY., 24,477 (1952). (3) Behr. F. -4.. and Zinparo. P. W., Xorelco Reporter, 1, 3 (1953). (4) Birks, L. S., Rev. Sci.>nstr., 22, 891 (1951). (5) Birks, L. S., and Brooks, E. J., . ~ N A L . CHEY.,22, 1017 (1950). ( 6 ) Birks, L. S., Brooks, E. J., and Friedman, H., IM., 25, 692 (1953). (7) Briswy, R. M.,Ibid., 24, 1034 (1952). (8) Despujols. J., J . phys. radium, 13, Suppl. to No. 2, 31A (1952). (9) Friedman, H., and Birks, L. S.,Rea. Sci.Instr., 19, 323 (194s). (10) Gillam, E., and Heal. H. T., Brit. J . A p p l . Phys., 3, 353 (1952). (11) Hevesy, G. von, “Chemical Analysis b y X-Rays and Its rlpplications,” Kew Tork, RIcGraw-Hill Book Co., 1932. (12) Rlortimore, D. hl., and Romans, P. A,, J. Opt. Soc. Amer., 42, 673 (1952). (13) Schoeller. W. R.. and Powell. -4.R., Analust, 50, 485 (1925); 53, 264 (1928). (14) Woods, G. A., Atomic Energy Research Establishment, Harwell, England, CRLiAE-62 (1950). RECEIVED for review September 24, 1953. Accepted January 22, 1954. Presented in p a r t a t the Pittsburgh Conference on Analytical Chemistry and -4pplied Spectroscopy, 1953.
Some Interferences in Flame Photometry ROY D. CATON, JR.,
and
RAYMOND W. BREMNER
Department o f Chemistry, Fresno State College, Fresno, Calif.
Interference considerations have played a very prominent role in the measurement of metal ions in solution by some workers, but certain aspects of interference effects caused by the so-called inert materials have been given little or no consideration by others. Viscosity obviously is an important factor in the flame spectrophotometric determination of metal constituents in solution. Because the rate of flow of the solution through an aspirator or orifice into the flame is a function of its viscosity, several authors have added various substances to standard solutions to give viscosities approximating those of the solutions being analyzed. The present study w-as undertaken in the hope of finding viscosity correction factors or otherwise correlating the effects of viscosities. This was accomplished in part, but other effects, dependent upon the particular viscosity-regulating additive used, play an important role in changing the flame intensity. Particle size, which is one of these effects, is studied by means of photomicrographs.
T
HERE are many analytical techniques for determining
small quantities or low concentrations of most of the elements which exhibit flame spectra, but the flame method offers the analyst two considerable advantages: It provides a precision which cannot be matched by any other spectrochemical method and a complete analysis can be made in a few minutes. Water, biological fluids, and liquid food products are easily analyzed for the alkali and alkaline earth metals with high specificity. Minerals, ceraniicF, glass, soils, alloys, metals, biological tissues, and food products are easily analyzed after an appropriate solution of the substance is made. However, the presence of foreign ions and molecules in the unknown substance is often the most troublesome factor in flame photometry and causes the majority of errors and a loss of ana-
lytical accuracy. Berry et a / . ( 1 ) and Parks et al. (10) denionstrated the depressing effects of inorganic acids and inorganic salts upon the flame intensities of sodium and potassium. Other workers have shown similar results (3, 8). Several workers have also shown how the presence of certain organic solutes may either enhance or depress flame intensity, depending on the substance used (1-3, 6, 9, 10). Several workers have briefly discussed the effects of viscosity of the solution and attributed a portion of the interference of foreign substances to their alteration of the rate of atomization of the sample into the flame (1,2,4, 5,9). Berry et al. ( 1 ) used sucrose ap an example of a compound which caused large errorq because of viscosity. Bills et al. ( 2 ) showed that the presence of 5000 p p.m. of glycerol with 5 p p.m. of sodium depressed the flame intensity 24% while it depressed the rate of atomization into the flame by only 6%. Mosher et al. (9) showed the depressing effects of varying amounts of gelatin upon the flame intensity of sodium and potassium and attributed these effects to viscosity. Conrad and Johnson (4)also attributed errors in the analysis of petroleum oils to viscosity. Some authors (1, 2 ) advocate correcting for interferences by dilution of unknowns to minimize them. Empirical correction cui vee have also been wed with success (4,‘7,8,11),and a method eliminating the interfering effects of diverse ions in water analyG i G by the addition of “radiation buffers” has been employed by Keet et al. ( I f ) I n many cases, standards are compounded to approximate the composition of the sample being analyzed (9,10). The references mentioned are not an exhaustive list, but serve to indicate the present status of the problem of interference. The present study was made to determine the magnitude of interference attributable to viscosity only, in some solutions containing interfering substances. The question of whether or not the particle size of the spray might have some bearing upon the interference effects of foreign substances also presented itself and a study of this aspect v.-as made. It was decided to ex-
