Instrumental Reduction of Background Signal of Fuel-Rich Oxyacetylene Flames Used in Atomic Absorption Spectrometry SIR: The degree of dissociation of very stable metal monoxides in the flame can be increased by increasing the fuel-to-oxygen ratio to shift the chemical equilibrium between the metal monoxide and metal toward the latter (5, 7 ) . This is especially true when using acetylene fuel and 01-ganic solvents for the samples. One problem associated with these flames is their intense background emission. In this investigation, vanadium, tin, and aluminum are extracted from aqueous solutions into 4-methyl-2-pentanone (hexone) and aspirated into highly fuel-rich oxyacetylene flames. By using an atomic absorption spectrophotometer constructed by simple modification of a Beckman Model DU, the large signal due to the intense flame background emission can be easily and effectively removed. INSTRUMf NTAL
A Beckman Model DU Spectrophotometer with the phototube housing and cell compartment replaced by a side window type housing is used as the monochromator. h 1P28 photomultiplier tube is used as detector. An ax. amplifier, photomultiplier high voltage power supply and a.c power supply for the hollow cathode discharge tubes (H.C.D.T.) were built according to Box and Walsh (1). The light pipe (d), aspirator-burner (Be(kman No. 4030), quartz lenses (10-crn. focal length), and H.C.D.T. are mounted and aligned as in Figure 1 on a 50-cm. optical bench (Ealing Corp, Cambridge 40, Mass.). EXPERIMENTAL
Vanadium and tin are completely extracted into hexone from 1 M HC1 solutions containing ammonium Nnitrosophenylhydroxylamine (cupferron). One ml. of 3% cupferron per mg. of metal is more' than enough for complete extraction. Eshelman and coworkers (4) have investigated the
Table 1.
A-C POWER SUPPLY
Figure 1. Experimental setup used to make atomic absorption measurements in fuel-rich O&Hz flames
extraction of aluminum in detail. In this investigation, the aqueous aluminum sample containing a suitable amount of cupferron is adjusted to a p H between 2.5 and 4.5 with acetic acid and extracted with hexone. Other metals extracting cupferron have been tabulated (6). The optimum conditions for the analysis of vanadium, tin, and aluminum were experimentally determined and are given in Table I. The spectral lines used were predicted to be the most sensitive from their gii values (3). This was verified experimentally. RESULTS AND DISCUSSION
Vanadium, tin, and aluminum analytical curves are linear up to 200, 500, and 50 p.p.m., respectively. Absorbances are, respectively, 0.10, 0.08, and 0.16 a t 50 p.p.m. Minimum detectable concentrations (1% change in source signal) in hexone are 1.5, 2.0, and 0.5 p.p.m., respectively. Other workers (5, 7 ) using similar flames and more expensive equipment have obtained slightly poorer sensitivities for vanadium and aluminum. The relative standard deviation for 11 consecutive readings is 2% a t an absorbance of 0.07. Metals that occur in appreciable
Optimum Experimental Conditions for the Atomic Absorption Spectrophotometric Analysis of Vanadium, Tin, and Aluminum
Aspiration rate of Burner Line, Flow r a k cc./min. hexone, ht., Metal mp 0 2 CsH2 ml./min. mm." V 318.4b 4800 5800 1.16 3.0 Sn 286.3 6900 7700 3.00 3.8 A1 309.3 6200 7300 2.72 3.5 Vertical distance from burner tip t o axis of light pipe. b Unresolved triplet: 318.3, 318.4, 318.5 mp. 5
quantities with vanadium and tin, and metals that interfere seriously with aluminum emission (4) have no effect on the absorbance, even in concentrations several times that of the vanadium, tin, or aluminum. Chloride, nitrate, and sulfate, likewise have no effect. The excellent results obtained with the modified DU are primarily due to the great reduction in the signal due to the intense flame background emission. This is best shown by the flame background spectra reproduced in Figure 2. All spectra shown were obtained with the flame aspirating hexone and ad-
Slit width, mm. 0.030 0.030 0.015
H.C.D.T. current, ma., a.c 6 14
D
4
Figure 2. Flame background spectra at several experimental conditions (see text) VOL 36, NO. 4, APRIL 1964
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943
justed to the conditions described for aluminum (Table I). Curve A was run in this manner a t a slit width of 0.015 mm. Curve B was run in the same manner as A , but with the light pipe removed. This illustrates the improvement that can be obtained by the use of a light pipe. Curve C was run in the same manner as A , but with the flame emission chopped mechanically by a chopper between the flame and light pipe. This illustrates the flame background which would be observed in making emission measurements under these flame conditions when a light pipe is used. Because the copper used blocked out effectively half of the flame emission, curve C was run a t a slit width of 0.021 m1.-Le., 0.015 X .\/2. Curve D was run in the same manner as C, but with the light pipe removed. This illustrates the flame background which would normally be observed in emission measurements under these
flame conditions. Comparison of curves C and D also illustrates the improvement attainable by the use of a light pipe. Comparison of curves B and D illustrates the improvement attainable by the use of an a x . amplifier in atomic absorption measurements in these flames. In curve A , one notes the absence of prominent OH band spectra. It is also interesting to note that the aluminum line used (309.3 mp) occurs on the peak of one of the most intense OH bands. The method and apparatus described should lend themselves well to the determination of many elements having a strong tendency to form compounds in the flame. One disadvantage of these fuel-rich flames aspirating organic solvents is the high acoustical noise and intense white radiation. For prolonged operation it is recommended that the burner be encased in some type of housing.
LITERATURE CITED
(1) Box, G. F., Walsh, A., Spectrochim. Acta 16,255 (1960). W. J.. Dean. J. A.. Anulvst 12) . ,87,Carnes. 748 (1962). ’ (3) Corliss, C. H., Bosman, W. R., Natl. Bur. Std. (U. S.),Monograph 53, July 20, 1962. (4) Eshelman, H. C., Dean, J. A., Menis, 0..Rains. T. C.. ANAL.CHEM.31. 183 (1959). ’ (5) Fassel, V. A., Mossotti, V. G., Ibid., 35,252 (1963). (6) Freiser, H., Chemist-Analyst 51, 62 (1962). (7) Slavin, W., Manning, D. C., ANAL. CHEM. 35,253 (1963).
J. D. WINEFORDNER CLAUDE VEILLON
Department of Chemistry University of Florida Gainesville, Fla. TAKEN in part from the M.S. thesis of Claude Veillon, submitted t o the Graduate School of the University of Florida in November 1963.
Spectrophotometric Determination of Vanadium in EthylenePropylene Rubber Solutions with 3,3’-Diaminobenzidine SIR:Our interest in the determination of traces of vanadium in solutions of experimental ethylene-propylene rubbers (EPR) in organic solvents prompted us to search for a rapid and simple analytical procedure. Until recently, two methods were in general use in our laboratory. A colorimetric method dependent on formation of the phosphotungstate complex required the elimination of interfering ions through the use of a mercury cathode cell (4). This method while precise and accurate is time-consuming. Because of the nature of the samples, a rapid emission spectrographic method (9) did not always yield results of suitable precision and accuracy. Analysis by neutron activation while fast and accurate requires expensive equipment that is not always available. The present method is based on a procedure proposed by Cheng (1) for the determination of traces of vanadium through the use of diaminobenzidine (DAB). The reaction is specific for vanadium, and interfering substances such as strong oxidizing agents, highly colored substances, and certain anions (1) can easily be avoided. For readily ashed material, the rapid finkh enables the analysis of about ten samples a day. Properties of the reagent and conditions of the reaction have been thoroughly explored by Cheng. Since this method is relatively new, a comparison of results obtained by this method with 944
ANALYTICAL CHEMISTRY
those obtained by neutron activation analysis should be of interest. Our first attempt to analyze vanadium consisted of wet ashing the elastomer solution with nitric and perchloric acids followed by reacting the resulting solution with DAB. Further investigation indicated that the hydrochloric acid and hydrogen peroxide that are formed during the wet oxidation step reduce vanadiumw) as suggested by McCoy (2). Dry ashing the sample in a platinum crucible, however, proved quite satisfactory. To increase sensitivity, cells of a 5-cm. path length were used for some analyses. The use of these cells permitted the preparation of a calibration curve ranging from 5 to 50 pg. vanadium per 25 ml. The calibration curve similar to that described by Cheng was used for samples of higher vanadium content. EXPERIMENTAL
Reagents and Apparatus. Beckman
Spectrophotometer DU with 1- and 5-cm. cells. Standard vanadium solution. Prepare 5 pg. per ml. of 50 pg. per ml. from ammonium metavanadate. 3,3’-Diaminobenzidine tetrachloride (DAB) solution. J. T. Baker reagent grade. Dissolve 0.1 gram of reagent in 20 ml. of water and store in refrigerator under an inert atmosphere. Procedure. Ash a 10-gram sample of elastomer solution in a platinum
crucible by charring on a hot plate followed by heating over a Meker burner. Add 10 ml. of water and 2 ml. of nitric acid to the crucible, then wash the contents into a 100-ml. beaker. To ensure complete removal of vanadium, melt a gram of potassium pyrosulfate in the crucible, cool, then wash the salt into the beaker with hot water. Evaporate the contents of the beaker to about 10 ml. and transfer the contents to a 25-ml. volumetric flask. Before diluting to the mark, add 1 ml. of 85% Dhosphoric acid and 1 ml. of DAB solution. Measure the absorbance a t 470 nib in a suitable cell, using a reagent blank, after standing 15 minutes in the dark. PreDare a calibration curve by adding suitable aliquots of standard vanadium solution to 100-ml. beakers that contain about 50 ml. of water, 2 ml. of nitric acid, and 1 gram of potassium pyrosulfate. Reduce the volume to about 10 ml. by boiling and transfer the contents to a 25-ml. volumetric flask. React with phosphoric acid and DAB solution as described for sample analysis. For vanadium contents from 5 to 50 pg.. measure the absorbance in a 5-cm. cell using a reagent blank. Cse a 1-cm. cell for 50 to 250 pg. of vanadium. DISCUSSION OF RESULTS
Table I presents data showing adequate recovery of vanadium that had been added to vanadium-free E P R solutions as ammonium vanadate.
