Flame Spectrophotometric Determination of Copper in Ferrous Alloys JOHN A. DEAN and J. HAROLD LADY' Department of Chemistry, University of Tennessee, Knoxville, Tenn.
Organic solvent extraction can be applied to the flame spectrophotometric determination of copper in ferrous allojs. Copper, in the form of the salicylaldoxime complex, is selectively extracted from the aqueous solution of the sample with either chloroform or n-am?l acetate. Citrate was employed to buffer the solution at pH 3 and mask the iron, thereby preventing its coextraction. The organic extract is aspirated directly into an oxyacetylene flame. This procedure circumFents interferences encountered when aspirating the bulk sample containing varying amounts of diverse elements and increases the spectral emission of the copper 324.7-mp line tenfold as compared with an aqueous solution of copper. No interferences were found when the method was applied to a wide variety of ferrous alloys. Accurate measurements can be made on as little as 0.5 7 of copper per ml.
T
HE application of organic solvent extraction to flame spec-
trophotometry offers a number of advantages (e). Compared with an aqueous solution, the spectral emission of the element present in the organic extract is generally enhanced severalfold. A selective extraction avoids the introduction of high concentrations of diverse ions into the flame, thus eliminating errors that might arise due to their presence. Also minimized are variations of the physical properties of the aspirated solution, such as viscosity, droplet size, and volatility. The introduction of solutions containing high salt concentration into a flame produces considerable amounts of incandescence, apparently as a result of minute solid particle formation. Solvent estraction also affords a means for concentrating the test element. The authors have previously applied solvent extraction to the flame spectrophotonietric determination of iron (3). Copper exhibit,s tu-o rather sensitive emission lines in the ultraviolet portion of the spectrum a t 324.7 and 327.4 mp. I n addition, copper possesses a series of n.eak emission bands in the visible region (11). Lundegdrdh (9) observed that tlie radiant polver of copper was strongly dependent upon operating conditions, :ind that both lines n-ere prone to suffer strong self-absorption. Dean (1'1 has reported an extensive flame spectrophotometric study of copper and has developed methods for copper applicable t o nonferrous alloys. Hon-ever, interferences from unknoivn sources prevented the extension of the method to ferrous alloys. Prior to this, several investigators had employed the tn-o eniission lines mentioned to determine copper in biological materials ( 7 ) and other substances (4). Dean ( I ) found that the utilitjof the 324.7-mp line was hindered by self-absorption, and, n-hile not as serious in the case of the 327.4-mp line, the latter line is about one half the radiant pon.er of the former. It seemed desirable to develop a method for copper applicable to ferrous alloys and with sufficient sensitivity to facilitate the analysis of micro amounts of copper. Feigl's general discussion on tlie behavior of o-hydroxyaldosimes (6) suggested the utilization of t'his class of reagents in an extraction process. Flagg and Furman (6) developed a gravimetric method for copper using salicylaldoxime. As the work reported here was being completed, Simonsen and Burnett (1Oj 1
Pa.
Present address, \Vestinghouse Research Laboratories, East Pittsb:irgh.
reported the spectrophotometric determination of copper as the salicylaldoxime following extraction with n-amyl acetate. The copper complex of salicylaldoxime is soluble in chloroform, n-amyl acetate, and several other solvents, but most, of these latter were undesirable for flame work because of the unsteady flames that resulted. Altogether, 12 solvents were studied (8). AIost of this investigation JT-as carried out using chloroform. Copper salicylaldoxime is more readily extracted by chloroform; on the other hand, n-amyl acetate solutions provide a greater enhancement of the spectral emission. EXPERIMENTAL WORK
Apparatus. -1Beckman Model DU spectrophotometer with Model 9220 flame attachment and photomultiplier unit was used. An integral atomizer-burner, supplied with the flame attachment, vias used as the excitation source. Oxygen and acetylene lvere the gases used. The wave length knob on the spectrophotometer was replaced by a 4 to 1 gear reduction knob to facilitate positioning of the wave length dial. The copper lines are sharp, arc emission lines. One must move slowly back and forth over the peak of each line to ascertain the maximum reading. It is inadvisable to set the instrument on the apparent peak found on one particular solution and make a series of readings. The sensitivity knob on the spectrophotometer was replaced with a 10-turn dial t o permit the sensitivity settings to be reproduced. Gas flow meters were placed in the gas lines. They were calibrated using a n-et test meter t o measure the gas flow after it had passed through t,he burner. The calibration was done bjplacing a hose over the nozzle of the burner and connecting this to the wet test meter. The other burner openings were sealed off. The importance of controlling gas flows rather than gas pressures has been discussed ( 2 , 8). Reagents. A standard solution of copper (1.00 nil. equivalent to 1.00 nig. of copper) was prepared by dissolving 3.942 grams of reagent grade copper sulfate pentahydrate in 1 liter of demineralized water. Less concentrated standards, having 1.00 ml. equivalent to 20, 40, 50, and 60 -,of copper, were prepared by appropriate dilution. Solutions of salicylaldoxime in chloroform and n-amyl acetate, 1%(w./v.), were prepared by dissolving 10 grams of salicplaldoxime in the solvent and diluting to 1 liter. A standard chloroforni or n-amyl acetate solution of copper as copper salicylaldoxime, with 1.00 ml. equivalent to 25 -j of copper, was prepared by extracting 50 ml. of the standard aqueous solution of copper containing 50 -1 per ml. a t pH 3 with four 20-ml. portions of the chloroforni or n-amyl acetate solution containing salicylaldoxime. The combined extracts viere diluted to a final volume of 100 ml. Tvith additional solvent previously saturated v i t h n-ater. Less concentrated standard solutions, 1.00 ml. equivalent to 1.0, 2.0, 3.0, 4.0, 5.0, and 6.0 y of copper, were prepared by further dilution x-ith the appropriate solution of salicylaldoxime, previously equilibrated v i t h water. An ammonium citrate solution, 1.7731, was prepared by dissolving 40 grams of reagent grade animonium citrate in demineralized water and diluting to 100 nil. Flame Spectrophotometer Settings. The instrument set,tingj used to measure the copper emission \!-ere as follon-s: Sensitivity control, turns from clockxise limit Selector witch, position Phototube resistor, megohms Phototube, volts per dynode Slit, mm. Acetylene pressure, lb.,lsq, inch Oxygen pressure, lb./sq. inch Acetylene rate of flon-, cu. feet/hour Oxygen rate of flo-x, cu. feet/hour
4 0.1 22 60
0.030 5 10 4.32 7.28
Calibration Curve. A calibration curve for copper as the salic!-l:ildoxime complex in chlorofornl (Figure 1) is obtained by meas1887
ANALYTICAL CHEMISTRY
1888 uring the radiant power observed from the standard solutions prepared as described in the section on reagents. Because of self-absorption, the curve bends considerably tom-ards the concentration axis with copper concentrations exceeding 3.0 y per ml. However, if the curve is replotted as emission us. the logarithm of the copper concentration, the range of minimum relative error begins a t about 2.0 y of copper per ml., and is fairly constant to 6.0 y per ml. For n-amyl acetate solutions the radiant power is slightly larger for a given amount of copper. Both solutions are far superior to ordinary aqueous solutions.
1001
I
I
I
I
I
sAmvl
o\“
“1 / Figure 2.
/
I
Acetate
Aqueous P>cse, 25ml. (0.8 ‘Y Cu/ml.) Orcanic Phase. 5ml. I %
1
Extraction of copper as function of pH
tracts to a 25-ml. volumetric flask and bring to volume with additional solvent. Aspirate the solutions and measure the radiant power of the flame background a t 325.0 mp and of the copper line a t 324.7 mp. Bracket the unknowns with standard solutions and determine the amount of copper present by interpolation. -
0
323
RESULTS AND DISCUSSION
325 327 329 Wove Length, m p
Figure 1. Flame emission spectrum for 5 y of copper per ml.
Procedure. Weigh 0.5-gram samples into 250-ml. beakers. Dissolve in 30 ml. of 3 M hydrochloric acid, adding small amounts of nitric acid, dropwise, t o oxidize the iron. If carbon or tungsten is present, remove by filtration and n-ash the residue well with hot 1% hydrochloric acid. Dilute the sample to volume in a 100-ml. volumetric flask with demineralized m-ater. Transfer aliquots containing 15 t o I50 y of copper into 50-ml. beakers. Add 1 ml. of 1.77il.I ammonium citrate solution for each 50 mg. of sample present in the aliquot. Adjust the p H to 3.0 by the dropm-ise addition of 8.11 ammonium hydroxide. K h e n the p H exceeds 2.5 the solution assumes a green color due to the formation of the iron citrate complex. Transfer the sample quantitatively to a separatory funnel. Extract with four successive 5-ml. portions of a 1% salicylaldoxime solution in chloroform or n-amyl acetate. Shake the sample a t least 1 minute during each equilibration, Transfer the ex-
Table I summarizes the results obtained on Sational Bureau of Standards iron alloys. The results are in satisfactory agreement with the certificate values. Flame Spectra of Copper. The emission spectra of copper salicylaldoxime from a chloroform solution and the flame background for chloroform alone are shown in Figure 2. Measurements were made only on the stronger copper line a t 321.7 mp, and the background Tvas read at 325.0 mp.
0, Pressure IO0
1
7p.s.i.
Table I. Copper Analysis on Bureau of Standards Samples Sample Cast iron, 5j A.O.H. steel 35a Mo-IT-Cr-V steel 132
Manganese steel l O O a
Copper, 5 Certified Value Found I 00 1 00 , n 96 0 267 0 270, 0 268 0 149 0 153, 0 155 0 127 0 126, 0 128 0 124 . 0 122, 0 124 0 107 0 096, 0 093 0 096 0 099, 0 096 0 053 0 054, 0 053 0 0.50 0 051, 0 050 0 052, 0 052
Casting alloy 161 (64-Ki, 17-Cr, 15-Fe)
0 040
0 040, 0 037 0 036, 0 036
B.O.H. steel 14c
0 025
0 025, 0 025 0 024, 0 025
Lead-bearing steel 130 Bessemer steel 22c
0.017 0,011
0.019, 0.019 0.012,0.012 0,009, 0 . 0 0 9
C,H,
Pressure, psi
Figure 3. Emission of copper 324.7-mp line as function of fuel and oxygen pressure
Nature of Copper Extraction. Copper was extracted from an aqueous solution with a 1% solution of salicylaldoxime in either chloroform or n-amyl acetate. Copper salicylaldoxime is insoluble in water; hence, the salicylaldoxime was dissolved in the organic phase rather than in the aqueous phase in order to prevent possible precipitation of the copper complex either in the aqueous phase or at the phase interface. By this procedure the copper
1889
V O L U M E 28, NO. 12, D E C E M B E R 1 9 5 6 complex forms a t the phase interface during the extraction step and passes directly into the organic phase. The extraction of copper as a function of pH is shown in Figure 3. Copper is extracted a t a lower pH with chloroform than with n-amyl acetate. Extraction with the latter solvent has been described by Simonsen and Burnett (IO). I t was necessary to establish conditions for the extraction of copper from large quantities of iron without coest.raction of the latter. Depending upon pH, ferric iron forms either an intense reddish-purple complex or a dark purple complex with salicylaldoxime. At pH 0.5 only a slight purple tinge is visible in the aqueous phase. As the pH is raised, the purple color increases i n intensity, although remaining in the aqueous phase until p H 2 or greater, when the dark purple complex of iron salicylaldoxime passes almost entirely into the organic phase. Addition of citrate masked the formation of the iron complex. I n the pH range from 2.5 to 3.0, iron forms n light green complex with citrate which exhibits no tendency to pass into the organic phase. On the other hand, copper is readily extracted a t pH 3 in the presence of citrate. Four successive I-minute equilibrations sufficed to remove copper completely. Iron could not be detected in the extract through its flame emission lines. The pH a t which the copper complex is extracted is also a function of the salicylnldosime concentration in the extracting solvent j therefore, the procedure should be folloir.ed exactly.
enhancement effect of organic solvents on the emission of metals in the flame. .41so of importance is the aspiration rate of various solvents. The product of these two factors has been found to give hyperbolic curves when plotted against the observed emission (d, 8). Flame characteristics pertaining to the solvents employed in the study are summarized in Table 11.
Table IT. Flame Characteristics of Solvents [Fuel and oxygen flow rates adjusted to maintain a stoichiometric flame ( 8 ) . Copper concentration, 5 r/ml.] n-Amyl ChloroAcetate form Water
Flame background, scale divisions Height of inner cone, em. Flame width.o em. Flame height, em. Aspiration rate, ml./min. Net emission above background
61 0 2 0 1 0 9.5 1.09 45.0
75 0 1 5
0 7 4.5 1.18 43.8
11 0 0 5 0 5 6.0 0.77 4.7
At 2.0 c n . above the burner tip.
CONCLUSION s
The extraction method is rapid and convenient for isolating copper from the bulk sample and interfering constituents. I n addition, the use of an organic solvent gave as much as a tenfold inqease in the radiant power compared with aqueous solutions. Consequently, accurate measurements can be made a t copper concentrations as low as 0.5 y per ml. The extraction also permits concentrating the copper in the organic phase if prior sample manipulations result in an excessively large volume of solution. When used in conjunction with flame spectrophotometry, the extraction method possesses a distinct advantage over colorimetric methods in that excess complexing agents and any other colored substances in the solution have-no effect on the determination. ACKNOWLEDGMEh-T
J. H. Lady is indebted to the Tennessee Eastman Co. for a fellowship under which this work was carried out. LITERATURE CITED
Figure 4. Standard calibration curve for copper in chloroform solutions
Fuel and Oxygen Pressures. The effect of varying fuel and oxygen pressures is shown in Figure 4. The emission of the copper 324.7-mr line increases with decreasing oxygen pressure and increasing fuel pressure. Background varies similarly. Consequently, an intermediate value of fuel and oxygen pressures was chosen. One microgram of copper per milliliter gave an emission equivalent to 10 scale divisions (on the %T scale). Some burners may exhibit a tendency to blowout when the organic solvent is removed. Reducing the oxygen pressure slightly will generally overcome this difficulty. Blowout is more freauent with chloroform solutions than with n-amyl acetate solutions. Effect of Organic Solvents on Flame Emission. ~h~ heat of combustion of the organic solvent, compared with the cooling e effect of water, ie a significant factor in the consideration of the
Dean, J. -L, ANAL.CHEM.27, 1224 (1955). Dean, J . .I.,Kinth .hnual Symposium on Modern Methods of Analytical Chemistry, Lousiana State University, Baton Rouge, La., January-February 1956. Dean, J. A., Lady, J. H., ANAL.CHEM.27, 1533 (1955). Ells, V. R., J . Opt. SOC.Arne?-.31, 534 (1941). Feigl, F., “Specific, Selective, and Sensitive Reactions,” p. 196, Academic Press, Kew York, 1949. Flagg, J. F., Furman, N. H., I N D . ESG. CHEM.,ANAL.ED. 12, 529 (1940).
Griggs, 11.d.,Johnstin, R., Elledge, B. E., Ibid., 13, 99 (1941). Lady, J. H., Ph.D. dissertation, University of Tennessee, August 1955.
LundegLrdh, H., “Die Quantitative Spektralanalyse der Elemente,” Pt. I, Gustav Fischer, Jena, 1929. Simonsen, S.H., Burnett, H. &I., ANAL.C H E M27, . 1336 (1955). Singh, X. L., Proc. Indian Acad. Sei 25A, 1 (1947). RECEIVED for review April 25. 1956. Accepted September 17, 1956. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. March 1956. Taken from a portion of a dissertatton submitted by J. Harold Ladv to the Graduate School of the Universitv of Tennessee in Dartial fulfillment of the requirements for the degree of doctor of philosophi.
