pellets, not commercially available concentrate, and should not be stored in polyethylene. The ruthenium chloride solutions were standardized by ignition of the chloride to dryness, reduction in hydrogen, and weighing as the metal. PROCEDURE
Fume aliquots containing from 100 to 300 y of ruthenium with 3 ml. of sulfuric acid to remove chloride and nitrate. If nitrate is present, add a fourfold excess of 12M hydrochloric acid to prevent the premature volatilization of the tetroxide. Transfer the sample with water to a still of the type shown in Figure 2 and add a slurry of 0.5 gram of sodium bismuthate in 6N sulfuric acid with the aid of several 6N acid washes. Add water if necessary to keep the acid normality a t 6 or less. Adjust the air supply to about 2 bubbles per second, and distill the ruthenium by heating the mixture to boiling for about 5 minutes. Catch the condensate in 15 ml. of iced 1N sodium hydroxide to which 1 ml. of 5% sodium hypochlorite has been added. Warm the catch solution to room temperature and dilute to volume with 1N sodium hydroxide. Measure the absorbance in 1-em. cells at 385 mp against a reagent blank. If 5-cm. cells are used, 40 y of ruthenium can be determined in the initial aliquot. Calculate the concentrations by comparing the absorbance measurements for the samples with the values obtained for a pair of standards on the same day. There are small day to day variations in the standard values as a result of unavoidable reduction of the tetroxide to dioxide by mater betn-een the still pot and the receiver. The suspended dioxide can be removed by prolonged centrifugation but it is unnecessary. For any set of standards determined on the same day by the
same analyst, there is no observable deviation from Beer’s law.
Dissolution of Uranium-Ruthenium Alloys. Uranium alloys containing u p to 5% ruthenium have been dissolved in a 4 t o 1 hydrochloric t o nitric aqua regia. The dissolution is carried out in a n Erlenmeyer flask by covering the sample with water and adding the acid mixture as rapidly as the vigor of the reaction will safely permit. When hydrogen evolution has ceased, a fourfold excess of acid is added, and the sample boiled to solubilize the ruthenium residues. If the dissolution is not pushed, or if the aqua regia is added sparingly, the ruthenium will not dissolve completely. When this occurs, it is necessary to separate the finely divided ruthenium by a tedious centrifugation, dissolve it in a sodium hypochlorite-sodium hydroxide mixture, acidify with hydrochloric acid, and combine this with the rest of the solution. DISCUSSION
The precision obtained in the analysis of uranium alloys for ruthenium using this method has been very good. In 12 duplicate determinations selected a t random from the files, the precision a t the 95% confidence level was =t1.5% and the maximum deviation was 2.0%. Although no exacting confirmation of the accuracy of the method has been attempted, the completeness of the distillation as checked by the use of tagged ruthenium, the simple nature of the colored ruthenium species, and the precision also indicate that the accuracy is good. Negative bias may occur from carelessness in the distillation steps, but this is readily detected by obvious de-
creases in the precision. One independent accuracy check mas made against a ruthenium metal powder sample with a reported purity of 99 %. A weighed amount was dissolved in sodium hypochlorite, acidified with hydrochloric acid, and diluted to volume. By analysis the metal was 98.5% ruthenium. The molar absorbance index obtained for perruthenate at 385 mp is 2150 10, for ruthenate at 465 mp, 1730 It 10. The values reported by Connick and Hurley ( 2 ) at the same absorbance maxima appear about 47, high. Their value for ruthenium tetroxide a t 315 mp is also 4% higher than the values reported by TT’ehner and Hindman (7) and Silverman and Levy (6).
+
*
ACKNOWLEDGMENT
The authors thank Luke DeGraff and Donald AI. RIacDonnell for technical aid in preparing this paper. LITERATURE CITED
(1) Banks, C. V., O’Laughlin, J. W,, AKAL.CHEM. 29. 1412 (19571.
Kansas 1948. (4) Marshall, E. D., Richard, R. R., ANAL. CHEM.22. 795 (19,501. (5) Silverman, M.’ D., Leiy, H. A,, Oak Ridge Natl. Lab., “Studies of Upper Valence States of Ruthenium in Aqueous Solutions,” ORNL 746 (August 1950). ( 6 ) Stoner. G. A.. ASAL. C H E h f . 27. ‘ 1186 (1955). (7) Wehner, Philip, Hindmnn. J. C., J . Am. Chem. SOC.72, 3911 (1950). (8) Yaffe, R. P., Voigt, A. F., Ibid., 74, 5043 (1952). RECEIVEDfor reviem June 30, 1958. Accepted September 26, 1958.
High-Sensitivity, Recording, Scanning Flame Spectrophotometer M.
T. KELLEY,
D. J. FISHER, and H. C. JONES
Analytical Chemistry Division, Oak Ridge National laboratory, Union Carbide Nuclear Co., Oak Ridge, Tenn.
b The design and performance of a high-sensitivity, recording, wave length-scanning, flame spectrophotometer are described. Considerations are discussed that govern choices of recorder, monochromator, multiplier phototube, and filter components. Special advantages of this instrument include gains in precision and in dependability resulting from the use of a chart recorder, the reduction of hazards arising from the analysis of 178 *
ANALYTICAL CHEMISTRY
radioactive samples, exceptional performance in the red spectral region, great stability and sensitivity, unusual operational versatility, minimizing of anion and salt interferences by the high sample dilution possible, and reduction of spectral and flame background interferences by use of a good monochromator. Generally, the relative standard deviation for analyses is less than 1%, and linear calibration curves are obtained.
T
of the high-sensitivity, recording, scanning flame spectrophotometer described has evolved from that of single-beam instruments designed and built at the Oak Ridge National Laboratory in February 1953 (6) and from the design of a doublebeam flame spectrophotometer built in June 1952 (4). Two instruments of the latest design, described herein, have been built and are in use: ORNL Models Q-1457A and Q-1887. ComHE DESIGN
Figure 2. Top view of wove length drive, monochromator, multiplier phototube, and flame source
Figure
1.
Front view of flame spectrophotometer
plete and detailed mechanical and electrical draivings (Q-1887 series) of the new instrument ar? available from the authors. For analyses a t a f i x d wave length the use of a strip chart recorder has several important advantages. The greater precision of measurement possible by taking the average of the chart reading during the entire aspiration of a sample rather than by using the instantaneous, fluctuating value or highly damped value of an indicating meter is of great benefit. The average of the chart reading taken during the aspiration of solvent after each sample is subtracted from the average reading for the sample. Instrumental drifts are thus compensated for by means of the chart record. Periodic scheduled aspirations of standard solutions are made, with readjustment of multiplier phototube gain, if required, to prevent errors due to change in flow rates or multiplier phototube gain. However, the principal reason for the alternate aspiration of sample and of solvent is to prevent the gradual accumulation of solids a t the burner orifice. An experienced analyst can recognize incipient trouble, such as thc burner's becoming plugged or the multiplier phototube's becoming noisy or starting to drift, from the appearance of the chart record during the aspiration of a sample or standard. These advantages \\.ere likewise realized in the recording instrument designed by King and Priestley (7). The need to analyze radioactive, as well as nonradioactive, samples and to analyze for elements that have flamc emission lines in the red spectral region -for example, potassium, rubidium, and cesiumdemanded a highly sensitive and stable flame spectrophotometer that operates well in the red region of the spectrum. The use of
sensitive multiplier phototubes and of a monochromator having a wide aperture (f 4.4) and good dispersion in the red spectral region provides high sensitivity. The hazards arising from the analysis of radioactive samples can therefore he minimized with this sensitive instrument by making a relatively high dilution of the sample. In addition, a filter may be used which removes radioactive particles from the exhaust gases ( 1 ) . The chamberless, lowmemory Beckman burner requires but a minimum volume of sample and works well with both aqueous and organic solvents; these arc also advantages in performing analyses at high sensitivity or in analyzing radioactive samples. With careful control of gas flow, this burner is an unusually stable flame source. The Farnsn-orth developmental-Type 16PMI/6836 multiplier phototube is srnsitive and is particularly useful in the wave length region from 0.7 to 1.2 microns (5'). Developments in flame photometric instrumentation have been summarized recently by Meloche (to),who discusses the exceedingly important advantages for general purpose or research analyscs that result from the use of a good monochromator. Spectral and flame background interferences are greatly reduced. The high sensitivity of this instrument usually allows a sample to he considerably diluted for flame analysis. This minimizes anion or salt interfcrences because it results in the formation in the flame of relatively small solid particles which can be more efficiently excited. The wave length-scanning device provided in this instrument can be conveniently used for the determination of the spectral background adjacent to the analytical wave length. This method of eliminating cation inter-
Figure 3.
Functional block diagram
of flame spectrophotometer
ferences has bcen described by Margoshes and Vallce (9). DESCRIPTION OF INSTRUMEN1
The Model 91887 flame spectrophotometer is shown in Figure 1. Not included are the Beckman pas control panel, the wave length d r b e mechanism, and the optional exhaust gas-filter assembly. The control unit is mounted below the strip chart recorder, and contains the power supply for the multiplier phototube and the multiplier phototube current-measuring circuit and controls. A top view photograph (Figure 2) shows the wave length drive mechs, nism, grating monochromator, the housing (with dark slide removed) for the multiplier phototube, and the Beckman flame source. This assembly can he mounted either on a bench top under an exhaust duct or in a hood. The exhaust duct may contain a filter, if needed (1). A functional block diagram of the flame spectrophotometer is shown in Figure 3. Flame Source. The flame source consists of a Beckman (Catalog No. 9215) flame source, hydrogen-oxygen burner. and auxiliarv control Dane1 for gas regulation. ' Monochromator. The monochromator chosen for the two single-beam instruments of most recent construction is a Bausch & Lomb (Catalog No. VOL 31, NO. 2, FEBRUARY 1959
179
33-86-40-01) grating monochromator, which is ordered without a light source. It is attached to a mounting base, as are the flame housing and the wave length drive mechanism. Mechanical drawings of this base, and of mounting adapters for these components (Q-1887-7) are available. Provision is made for placing a 2 X 2 inch filter in the optical path in order to eliminate trouble from overlapping spectral orders. The exit lens of the monochromator is removed. A light-diffusing disk is substituted for it for multiplier phototubes such as the Types 1P21 and 6217, where it results in an enhancement of stability with no loss of sensitivity. The housing for the multiplier phototube (Drawing Q-1887-6) bolts over the exit slit so that the exit light impinges directly onto the cathode of the multiplier phototube. I n order to render the monochromator lighttight, either sealing compound is used around the seams in the monochromator or suitable gaskets are fabricated and installed. The characteristics of the monochromator are as follows: focal length, 250 mm.; linear dispersion, 66 A. per mm. in the first order; range, 2000 to 14,000 A. in the first order; and equivalent aperture ratio, f/4.4. The grating is blazed for the ultraviolet of the first order unless, when ordering the monochromator, one requests blazing to favor another spectral region. Obviously, other commercial monochromators could be substituted, and the best choice depends upon the nature of the analyses to be made. Wave Length Drive Mechanism. This is fabricated from standard hIinneapolis-Honeywell Brown recorder chart drive components. (Mechanical details are shown on drawings &-1887-5 and -8.) It is possible to use interchangeably any combination of the standard Brown chart drive motors and chart drive gear trains. Several gear ratios are available with a given gear train. A typical combination is motor No. 77311-1 and gear train KO. 78663-1 with a recorder chart speed of 4 inches per minute. The relationship between wave length drive and displacement of chart paper depends upon the selection of these wave length drive parts and upon the speed of the recorder chart. The above combination gives 1.15 inches of chart travel per 10 mp of first-order wave length drive. The instrument may be used for flame spectrophotometric determinations a t a fixed wave length. The wave length drive may be used to obtain corrections conveniently for background adjacent to the analytical wave length. The instrument may also be used to scan and record the flame spectrum within a selected wave length regionfor example, to check the purity of reagents that are to be used in the preparation of standard solutions or to examine samples qualitatively. Power Supply. The negative, highvoltage power supply is conventional (Figure 4). It is stabilized against power line fluctuations by means of a 180
ANALYTICAL CHEMISTRY
Sola constant-voltage transformer. The cathode of the multiplier phototube is connected to negative high voltage through a fine-gain adjustment rheostat and a coarse-gain adjustment tap switch. These controls are used to adjust the gain of the multiplier phototube. The upper limit of useful voltage for each multiplier phototube is determined by the relative amounts of its over-all gain, noise, and dark current a t various voltages. The tap switch selects along a series string of Type 8582 voltage regulator tubes, which provides increments of 85 *2 volts. The output of the power supply is very stable. In the low position of the Voltage Hi-Lo switch, the output ranges from 600 to 1275 volts and is used with Types 1P21, 1P22, 1P28, 5819, and 6217 multiplier phototubes. I n the high-switch position, the range is from 1360 to 2040 volts; this position is used with Farnsworth Type 16PM1 tubes. Background Control. The positive output of the power supply is off ground by 85 volts. A multiturn potentiometer, designated flame background compensator, is connected across the first two Type 85A2 tubes, and an adjustable, positive (bucking) or negative (additive) constant current, derived therefrom, is delivered to the load resistor and is algebraically summed with the output current of the multiplier phototube. This control compensates for the output of the multiplier phototube that is due to flame background and to its own dark current. The background control is used, while solvent is being aspirated through the burner, to set the recorder pen to about 2% of full scale. This fraction of the full-scale value is subtracted from the recorder readings obtained while a sample or a standard is being aspirated. Multiplier Phototube. The multiplier phototube t o be used is chosen on the basis of the spectral sensitivity needed for the analyses t o be made. For example, a Type 1P21 is suitable for determinations of magnesium, sodium, or lithium; a Type 6217 for d e terminations of sodium, lithium, or potassium; a Type 16PM1 for determinations of potassium, rubidium, or cesium. Types 1P22 and 1P28 are interchangeable with the Type 1P21, and Type 5819 is interchangeable with Type 6217; these may also be used whenever their spectral sensitivity is suitable. The Type 1P28 is preferred for use a t short wave lengths. The Type 6217 is more satisfactory than the 1P22 for work in the red spectral region. It should be emphasized that the sensitivity, spectral range, noise, optimum applied voltage, and signal-tonoise ratio of individual multiplier phototubes of a given type all vary within wide limits; therefore, a selected multiplier phototube of a certain type may far outperform the “typical” one. Types 1P22 and 6217 multiplier phototubes will have a high noise level for some time after exposure to room light. In general, for all multiplier
phototubes, significant reductions in the amounts of noise and dark current are obtained after several days of operation during which voltage is applied constantly to the multiplier phototube. It is recommended that voltage be applied to the multiplier phototube a t all times except during maintenance operations or the changing of the multiplier phototube. The glass envelopes of the 1P21 and 1P22 multiplier phototubes have been provided, except for the signal window, with a coating of Aquadag that is connected electrically to the cathodes. This improves the time stability of the tube. The housing of the Type 6217 multiplier phototube is provided with a Mu metal magnetic shield. The output current of the multiplier phototube for full-scale deflection of the 10-mv. recorder is only 0.002 pa. This unusually low operating level eliminates signal drifting that is due to fatigue effects, and results in excellent low-drift characteristics. In order to improve the stability of the multiplier phototube, care is taken to avoid concentrating the light beam from the monochromator upon a small portion of the multiplier photocathode. The diffusing disk is not used with the Type 16PM1 or in wave length regions where ipaque. A Tvne 7102 and the DuMont
that they could be used instead of the FarnswGth Type 16PM1. Load Resistor and Sensitivity Control. A 5-megohm, current-measuring load resistor is used. The signal developed across this resistor is directly proportional t o the intensity of light being emitted in the flame, as seen through the spectral passband of the monochromator, and to the spectral sensitivity of the multiplier phototube. The signal developed across one tenth of the resistor may be connected to the recorder by turning the sensitivity switch t o the low position. The latter position is used for determining the additional sample dilution needed when a reading obtained with the sample is higher than the scale covered by a calibration curve. The load resistor is shunted by a 0.047-pfd. damping capacitor (C4). The time constant of the current-measuring circuit is thus 0.25 second. It is a compromise chosen when one wishes to use reasonably fast wave length scanning even though the output of the flame source is inherently fluctuating, particularly as seen under these conditions of high sensitivity. More damping-e.g., by increasing C4 from 0.047 to 0.47 pfd.may be used to smooth out the response if one measures a t a fixed wave length or if one scans wave lengths at a slower rate. The use of a resistor of lorn value, as compared with values required for nonmultiplying phototubes, together with the use, as shown on the circuit diagram, of Fluorothene insulation for the current-measuring circuit elements and of Teflon-insulated hookup wire eliminates need for desiccation and
I
i~
VOL. 31,
NO. 2,
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FEBRUARY 1959
181
permits stable operation in the presence of chemical fumes or high humidity. Brown Recorder. A IO-mv., Brown strip chart recorder having a 2-second pen balance motor is used. I n a nonscanning instrument a slower speed motor would be satisfactory. The polarity of the recorder is reversed by reversing the polarity of the standard cell and the working cell. A modified Rrown-type 40X amplifier is installed in the recorder. The modification consists of replacing the original, lowimpedance, transfornier input circuit by a high-impedance, RC-coupling, input circuit so that the recorder may be used to record the current through the 5-megohm load resistor. Details of all modifications t o the recorder are fully described in the circuit diagram (Figure 4) and in drawings Q-1887-11 and -12. In some of the flame spectrophotometers built prior to these, a 50-mv. Brown recorder was used. By appropriate choice of the magnitude of the load resistor and of the sensitivity of the recorder, one may arrive a t the over-all instrument sensitivity required for the analyses t o be performed. Other design parameters, particularly the spectral sensitivity of the multiplier phototube and of the monochromator, must also be considered. PERFORMANCE SPECIFICATIONS
The precision of analyses made with the flame spectrophotometer described herein is dependent upon the magnitude of sample interferences and upon the qualities of the specific monochromator and multiplier phototube that are used. Generally speaking, for established analytical procedures such as those for about 5 y of lithium or potassium per ml., the relative standard deviation is less than 1%. Other authors (8) have stated that it is necessary to use the double-beam principle in flame photometry in order to obtain precision of better than =k5%, but the studies described here have not indicated that this is necessary. The drift, including multiplier phototube drift, is typically less than 1% of full scale per hour when a 50-mv. recorder is used. Consecutive readings taken on the same sample or standard should check each other to within better than =t0.5% of full scale. The sensitivity of analysis depends on several variables, such as identity of the solvent, sensitivity of the multiplier phototube, span of the recorder, characteristics of the monochromator, and contamination of the solvent and container by the element being determined. For example, a t 6708 A. with a 50-mv. recorder, the Bausch & Lomb grating monochromator with a standard blaze grating, a Type 6217 multiplier phototube, and an ethanolor methanol-water solvent, less than 0.001 p.p.m. of lithium can be easily 182
ANALYTICAL CHEMISTRY
d
c 4 ‘k x 1 10 % of
Full Scale
6 p p rn.
I
c1
Figure 5. Typical data used to plot calibration curve Flame photometric analysis for cesium; 16PM1 multiplier phototube; 852.1 rnp
measured. With a grating blazed to favor the visible red region, a 10-mv. recorder, and water as solvent, 0.0005 p.p.m. of lithium gives a reading that is 20y0 of full scale, and the pen fluctuations are but 2% of full scale. For comparison, Whisman and Eccleston, who used a modified Beckman Model DU monochromator a t G708 A. with a type 1P28 multiplier phototube, report a detection limit for lithium of about 0.2 p.p.m. (12). Linear calibration curves are obtained over reasonable concentration ranges. The most usual concentration range of calibration curves used in established analytical methods is 0 to 10 p.p.m. A manual has been written that describes checkout and field maintenance procedures and that contains a troubleshooting guide for this instrument ( 5 ) . Many hours of reliable operation of this instrument have been obtained with a minimum amount of maintenance. METHOD OF ANALYSIS
Calibration curves are prepared by the use of standard solutions. The spectral purity of the standards should be checked by means of the flame spectrophotometer. Detailed operating procedures have been prepared (4, 6). The monochromator slit width and wave length settings and the type of multiplier phototube to be used are specified on the calibration curve for the element to be determined. Beakers of solvent are
aspirated betn-een beakers of simple or standard. The approximate settings of the multiplier phototube voltage controls and the background control are also given on the calibrat’ion curves. These controls are exactly adjusted as follows: Khile a standard solution and then the solvent are being aspirated, the voltage controls and the background control are respectively adjusted to obtain recorder values corresponding to those on the previously prepared calibration curve. Usually a standard solution is used in n hich the concentration of the element is 80% of the fullscale mlue on the calibration curve and the respective recorder values are 82 and 2%. The instrument may then be operated for long periods of time without readjustment. Periodically, a standard solution is aspirated to prove that conformity with the calibration curve is being maintained. Data typical of those us2d t o plot a calibration curve are shown in Figure 5, which also illustrates the appearance of the chart records obtained with this instrument a t a fixed wal e length. A 50-mv. B r o m recorder was used. These data were used to prepare a calibration curve for the flame spectrophotometric analysis of 1 to 10 p.p.m. of cesium in water solution. DISCUSSION
Results obtained in the determination of aluminum and lanthanum have been described ( 2 , If). ACKNOWLEDGMENT
The authors acknowledge significant contributions by others a t the Oak Ridge National Laboratory: L. C. Bate, P. R. Bell, C. C. Courtney, R. A. Dandl, G. A, Holt, G. E. Tipton, D. D. Walker, and A. D. Williams. The Q-1887 series drawings were prepared by 11. M. Bowelle and J. E. Perciful. The sensitivity data for lithium are the work of W. R. Laing, Cyrus Feldman, C. A. Pritchard, and R. L. Walker. The precision data were obtained by K. R. Laing. The wave length-drive mechanism was originally designed by W. A. Blevins. The data of Figure 5 are the work of Cyrus Feldman and W. B. Estill. The ORNL master analytical manual methods mere prepared under the editorship of H. P. Raaen. LITERATURE CITED
(I) Edgerton, J. H., Davis, H. G., Henlev, L. C., KelleJ-, M. T.,AN.4L. CHEW 28,557 (1956).
( 2 ) Eshelman, H. C., Dean, J. A., Menis, O., Rains, T. C., Zbid., 31, 183 (1959). (3) Farnsworth Electronics Co., Fort Kayne 1, Ind., “Multiplier Phototube TJ pe 6836/16PL11 Tentative
Information (Developmental Type)” (June 15, 1956). (4) Fisher, D. J., Oak Ridge National Laboratory Master Analytical Manual, TID-7015 (Section l), Method Nos. 1 003051 and 9 003051 (6-15-56), Office of Technical Services, De t of Commerce, Washington 25, D. (5) Fisher, D. J., Specification S o . SI162 18-12-57). available from Instrument’Department, Instrumentation and Controls Division, Oak Ridge National
8..
Laboratory, Oak Ridge, Tennessee. (6) Fisher, D. J., Laing, W. R., Ibid., Method Nos. 1 003050 and 9 003050, pp. 1-22 (Aug. 14, 1953). (7) King, W. H., Jr., Priestley, W.,Jr., Am. SOC.Testing Materials, Spec. Tech. P u b l . 116, 97 (1951). (8) Lewin, S. Z., ANAL. CHEM.30, 17A (July 1958). (9) Margoshes, M., Vallee, B. L., “Flame Photometry and Spectrometr . Principles and .4pplications,” in d l . I11 of
“Methods of Biochemical Analysis ” Glick, D., ed., Interscience, New Yo&, 1956. (10) Meloche, V. W.. ANAL. CHEX 28, . 1844 (1956): (11) Menis, O., Rains, T. C., Dean, J. A., Ibid., 31, 187 (1959). (12) Whisman, M., Eccleston, B. H., Ibid., 27, 1861 (1955). RECEIVED for review February 15, 1958. Accepted August 28, 1958.
Extraction and Flame Spectrophotometric Determination of Aluminum H. C. ESHELMAN’ and JOHN A. DEAN Department o f Chemistry, University of Tennessee, Knoxville, lenn. OSCAR MENIS and T. C. RAINS Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, lenn.
b Aluminum can b e selectively extracted from an acetate-buffered solution adjusted to p H 5.5 to 6.0 with a 0.1 M solution of 2-thenoyltrifluoroacetone in 4-methyl-2-pentanone; or from an acetate-buffered solution adjusted between p H 2.5 and 4.5, and containing N-nitrosophenylhydroxylamine (cupferron), with 4-methyl-2-pentanone. The organic phase is aspirated directly into an oxyacetylene or an oxyhydrogen flame and the emissivity of aluminum is measured a t either the atomic line a t 396.2 rnp or the sharp oxide band head a t 484 mp. When a 4-methyl-2-pentanone rather than an aqueous solution of aluminum is aspirated into the flame, the emissivity is increased 100-fold. At a slit width of 0.030 mm. the sensitivity is 0.5 y of aluminum per ml. per scale T). The calibration curve division is linear from 5 to 40 y of aluminum per ml. Tolerance limits for a wide diversity of ions have been determined. When necessary, preliminary separation methods have been devised to remove interfering elements.
(70
A
flame spectrophotometric method has been developed that is sufficiently sensitive to determine aluminum in low concentrations in a variety of materials. Except for a brief reference to the emission of aluminum in an oxycyanogen flame ( I S ) , only indirect methods (S, 8, 10, 12) for the determination of aluminum have been reported, based on the depressant DIRECT
1 Present address, Chemistry Department, Southwest Louisiana Institute, Lafayette, La.
effect of aluminum upon the flame emission of calcium. All other ions that might depress the calcium emission must be absent, as well as any ions that might enhance or otherwise affect this emission. Direct flame photometric methods for aluminum in aqueous solutions lack sufficient sensitivity and selectivity. However, when a 4-methyl-2-pentanone rather than an aqueous solution of aluminum is aspirated into the flame, the emissivity is increased approximately 100-fold and the interference of large numbers of elements is obviated. Sufficient sensitivity and freedom from spectral interference can be attained by use of either the atomic line a t 396.2 mp or the oxide band head a t 484 mp for making the emissivity measurement. The sensitivity is 0.5 y of aluminum per ml. per scale division (% T scale) EXPERIMENTAL WORK Apparatus. The flame spectrophotometers used have been described
(2,C).
Reagents. 4-Methyl-2-pentanone, practical grade. Bluminum, standard solution, 100 y per ml. Dissolve 0.100 gram of pure aluminum mire (Fisher, Certified ACS grade) in hydrochloric acid, then dilute to 1 liter with demineralized water, ’Y-Nitrosophenylhydroxylamine(cupferron), 0.1M. Dissolve 1.6 grams of reagent in 100 ml. of water. If the aqueous solution is strongly colored, make it slightly ammoniacal and purify the solution by equilibrating it with 100 ml. of 4-methyl-2-pentanone for 15 minutes. Allow the phases to separate and remove the aqueous layer containing the S-nitrosophenylhydroxylamine. Store the reagent in a dark place.
ZThenoyltriBuoroacetone (TTA), 0.1M. Dissolve 5.5 grams of the technical grade reagent in 4methyl-2-pentanone, then dilute to 250 ml. with additional solvent. Store the solution in cool place away from light. Instrumental Settings. The instrument settings for the ORNL and Beckman Model DU flame spectrophotometers are as follows: Beckman ORNL Sensitivity control, yo adjust 50 High Selector switch, position 0.1 High Phototube resistor, megohms Blue-eensitive, RC.4 1P28 22 5 Phototube, volts per dynode 60 75 0.030 0.25 Slit. mm. Spectral slit width, mp At 396 mp 0.6 1.6 At 484 mp 1.2 1.6 Slit Width. \i7Tith the Beckman Model D U flame spectrophotometer, the slit width used was 0.030 mm.; this corresponds t o a spectral slit width of 0.6 mp a t 396.2 mp. The atomic lines of aluminum a t 394.4 and 396.2 mp are incompletely resolved from each other a t slit widths exceeding 0.050 mm. A similar range of slit widths is recommended for the oxide band a t 484 mp. F u e l and Oxygen Flow Rates. Optimum flow rates of oxygen and acetylene are different when a combustible solvent is aspirated. Also, a larger volume of organic solvent is aspirated per minute (2.0 ml.) as compared with an aqueous solution (1.0 ml.). Figure 1 shows a plot of the emission intensity of aluminum as chart divisions above the flame background us. the acetylene pressure for various oxygen pressures; Figure 3 shows the flow rates which corVOL. 31, NO. 2, FEBRUARY 1959
* 183
