using standards prepared from the additive used in blending the oil. These determinations were compared to direct determination using standards prepared from additive, 417, as previously discussed. Zinc content of additives used to prepare standards was determined by atomic absorption standard additions. The degree of bias that can exist in direct atomic absorption determinations is demonstrated by the data presented in Table V. Direct determinations using standards prepared from a single additive, 417, are consistently lower than direct determinations using standards prepared from the additive used in blending. The bias was confirmed by comparison of the direct determinations to a determination by X-ray fluorescence for selected samples in Table V.
for possible errors due to variations in the chemical structure of the zinc containing compound (3-6). Standards prepared in this manner should also be made to contain base stock oil a t roughly the same concentration as samples diluted for measurement. Even if the additive used in blending is unknown to the analyst, the addition of base stock oil to the standards used will eliminate a determinate error and improve accuracy of the method.
ACKNOWLEDGMENT The authors thank M. W. Bell, J. K. Fogo, and R. N. Wheatley for providing X-ray fluorescence and wet chemical data respectively. Helpful discussions with F. W. Stechmeyer are also gratefully acknowledged. LITERATURE CITED
CONCLUSIONS From the data presented here, it is apparent that bulk effects of various additive components in the atomic absorption determination of zinc in lubricating oils cannot always be eliminated by simple dilution. A bias which can be as much as 10% relative may be obtained if a direct method is used which utilizes the same standards for the determination of all types of lubricating oils. Bulk effects can be removed by a standard additions approach or by matching standards and samples closely. Matching standards with samples by preparing the standards from the additives used in blending the oils is the most convenient approach. This procedure will also correct
B. Osborne Ed.. lnd. Lubr. Tribol., 25, 63 (1973). G. E. Peterson and H. L. Kahn, At. Absorpt. News/.,9, 71 (1970). M. Kashiki and S.Oshima, Anal. Chim. Acta, 55, 436 (1971). J. Guttenberaer and M. Marold. Fresenius' 2.Anal. Chem.. 262(2). 102 (1972). S.T. Holding and P. H. D. Matthews. Analyst (London), 97, 189 (1972). M. Kashiki and S. Oshima, BunsekiKagaku, 20, 1398 (1971). R. Herrmann in "Analytical Flame Spectroscopy", R. Mavrodineanu, Ed., Springer-Verlag New York Inc., New York, N.Y., 1970, p 468. G. D. Christian and F. J. Feldman, "Atomic Absorption Spectroscopy", Wiley-lnterscience, New York, N.Y., 1970, p 68. (9) J. E. Allan. Spectrochim. Acta, 17, 467 (1961). (10) A. J. Lemonds and 6. E. McClellan, Anal. Chem., 45, 1455 (1973). (11) E. Schmall, Exxon Chemical Company, personal communication, 1974.
(1) (2) (3) (4)
RECEIVEDfor review August 16, 1974. Accepted May 5, 1975.
Extraction of Copper(l1) from Aqueous Thiocyanate Solutions into Propylene Carbonate and Subsequent Atomic Absorption Spectrophotometric Determination B. G. Stephens and H. L. Felkel, Jr.' Department of Chemistry, Wofford College, Spartanburg, S.C. 2930 7
Mulford ( I ) has pointed out that atomic absorption spectrophotometric (AAS) analysis usually requires very little in the way of preliminary sample preparation because of the inherent sensitivity and specificity of the technique. However, for some applications, a preliminary separation and concentration of the element of interest is necessary. This paper is concerned with the applicability of propylene carbonate (4-methyl-1,3-dioxolane-2-one) as an extractant for AAS methods. T o evaluate the behavior of propylene carbonate as an agent for solvent extraction coupled with AAS, an investigation of the extraction of copper(I1) from thiocyanate solutions was initiated. Ideally, a solvent used to extract a species from an aqueous phase should be colorless, nontoxic, immiscible, and have little tendency to form emulsions. This work employing propylene carbonate for the first time as a solvent for extractions with AAS, demonstrates that the solvent is entirely satisfactory for use in conjunction with AAS. Propylene carbonate, even though appreciably water-soluble (21.2 ml per 100 ml of water a t 24 "C) (2),extracts the copper(I1)-thiocyanate complex rapidly, completely, and neatly. Propylene carbonate, when aspirated into an airPresent address, Department of Chemistry, Purdue University, Lafayette, Ind. 47907. 1676
ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
acetylene flame, produces a clear, steady flame, with complete combustion; there is no absorption due to the solvent. Propylene carbonate is colorless, nonhydroscopic, noncorrosive, chemically stable, and practically odorless. I t is essentially nontoxic in oral doses or by skin absorption. I t is more dense than water and has a high dielectric constant (69 esu at 23 "C). Propylene carbonate has been used as a solvent for polymers and plasticizers and a number of inorganic salts (3). It has been proposed as a solvent for electrochemistry and electron paramagnetic resonance spectrometry ( 4 ) . The solvent has already been shown to be an effective extractant for ferroin-type iron(I1) complexes (2) and the pentan-2,4-dione complex of iron(II1) ( 5 ) .Propylene carbonate has been used to extract molybdophosphoric acid (6) and molybdenum(V1) from aqueous solutions ( 7 ) . I t has also been used to extract simultaneously copper and iron subsequent to spectrophotometric determination (8). Allan (9) has reported on the use of ammonium pyrrolidine dithiocarbamate to complex copper prior to its extraction into methyl isobutyl ketone. It was reported that copper could be extracted from a t least 6N HC1 and 9N H2S04. Also mentioned in this work is the fact that nitric acid decomposes the complexing agent and ideally should not be present. Munro ( 1 0 ) has shown that the copper concentration in the methyl isobutyl ketone phase varied with
0 6
0 5
0
I
i
I
I
i
I
I
I
I
2
3
4 PH
5
6
7
L
I
e
Figure 1. Effect of pH on the extraction of copper(l1) from aqueous thiocyanate solutions into propylene carbonate
the extraction ratio and the acidity of the solution from which the extraction was performed, making it necessary to check the extraction efficiency from standards and samples t o ascertain that it is constant. Methyl isobutyl ketone has also been widely used as an extractant prior to AAS determination of many other metals ( I ) . The solvent seems to have many characteristics as a solvent that recommends its use in conjunction with AAS. Those who work with the solvent, however, should be aware that it may be irritating to eyes, mucous membranes, and in high concentrations, narcotic ( 1 1 ) .
EXPERIMENTAL Apparatus and Operating Conditions. Separatory funnels with Teflon stopcocks and plastic stoppers were used for the extractions. A Perkin-Elmer Model 290-B atomic absorption spectrophotometer with a copper hollow-cathode source was employed to obtain quantification measurements. A Corning Model 10 p H meter with a combination electrode was used for measurement of pH. T h e operating conditions that were used are given in Table I. Reagents. All reagents were analytical grade. A stock solution (6.07 X 10-3M) was prepared by dissolving 0.3855 g of copper wire in 10 ml of 1:l nitric acid over low heat. After evaporating to approximately 2 ml, 5 ml of sulfuric acid were added, and the solution was heated until dense white fumes were evolved. Two milliliters of hydrochloric acid were added and the solution made t o a final volume of one liter; a 3.64 X lO-*M copper solution was prepared fresh daily from the stock solution and contained 5 ml per liter of 12M hydrochloric acid. A solution of 3M ammonium thiocyanate was prepared in deionized water. Distilled water was passed through a monobed ion exchange column composed of Dowex 50W-X2 resin in the hydrogen form and Dowex 1-X1 resin in the hydroxide form. Technical grade propylene carbonate was vacuum-distilled in all-glass apparatus. General Procedure. Place an aqueous solution of the sample in a n appropriate separatory funnel. Add 4 ml of 3M ammonium thiocyanate and 5 ml of saturated sodium chloride for each 50 ml of solution. Adjust the p H of the solution to about 1 with 6M hydrochloric acid or 3M aqueous ammonia. Add enough propylene carbonate to give about 4 ml of extractate. This can be accomplished by adding 10 ml of propylene carbonate for each 50 ml of aqueous phase, shaking the funnel for a few seconds, adding 4-ml portions of the solvent followed by shaking until a second phase appears, and adjusting the final volume of the extractate to about 4 ml. Shake the funnel vigorously for about 10 sec and allow the phases to separate while swirling intermittently. Drain the lower phase into a 25-ml volumetric flask and rinse the stopcock bore and funnel stem with about 1 ml of propylene carbonate. Extract three more times employing 4 ml of solvent followed by a 1-ml washing each time. Make the combined extracts and washings to volume with propylene carbonate. Aspirate the extractate according t o the instrumental conditions given in Table I. The amount of copper in the sample may be obtained by referring to calibration
oL 30
10
MOLES
0
S C N - 20 I1 x 1 0 ~ )
Figure 2. Effect of thiocyanate concentration on the extraction of copper(l1) from aqueous solutions into propylene carbonate
Table 1. Experimental Conditions Perkin-Elmer Model 290-B spectroptotometei coupled !\ith a Perkin-Elmer Model 303 burner regulator
Wavelength, 3247 A Range, ultraviolet Slit. 7 A Lamp current. 5 mA Air Supply. 30 psi Flowmeter, 2.8 Acetylene Supply. 10 psi Flowmeter, 3.2 Sample uptake rate, 2.5 ml/min Scale expansion, 1 Sensitivity, 0.15 pg/ml c'u for 1%absorption Flame characteristics Height, 3 . 5 m m
Width. 5.15 cm graphs. T h e extractate should contain between 2 and 20 gg/ml of copper for optimum precision.
RESULTS AND DISCUSSIONS Effect of pH. The conditions of the general procedure were followed and the p H was varied using dilute HC1 and NaOH solutions. The 50 ml of aqueous phase contained 193 fig of copper. The results depicted in Figure 1 demonstrate that the copper(I1)-thiocyanate complex ion is effectively extracted in the p H range 0.0-5.0. The pH recommended for routine analysis is 1.0. Reagent Concentration. The amount of ammonium thiocyanate present in the aqueous phase was optimized by using the general procedure for an aqueous volume of 50 ml containing 211 wg of copper. The results are shown in Figure 2. For aqueous volumes of 200 and 300 ml, containing 4 ml of 3M ammonium thiocyanate and 5 ml of saturated sodium chloride per 50 ml, the expected absorbance due to copper is approximately 12.6% low for the 200-ml sample and 7.9% low for the 300-ml sample. Stability of Extract. Even though the yellow-brown color of the copper(I1)-thiocyanate complex ion fades with time, the absorption of the aspirated extractate remains constant for a t least two days. Distribution Coefficient. The distribution coefficient for the distribution of the copper(I1)-thiocyanate complex ion between aqueous and propylene carbonate phases was ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
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Table 11. Effect of Diverse Species I
o c
Species
:
2L
08[ 0 6
0 m v)
determined by using the conditions of the general procedure. An aqueous volume of 50 ml that contained 422 pg of copper and 4 ml of 3M ammonium thiocyanate was extracted with 4 ml of propylene carbonate and allowed to stand for 30 min. The distribution coefficient was determined by comparing the absorbance due to the copper in the propylene carbonate phase to that due to the copper remaining in the aqueous phase. The copper in the aqueous phase was transferred to a propylene carbonate phase by multiple extraction with the solvent prior to determination of the absorbance. This was accomplished to ensure that possible effects on absorbance due to solvent dissimilarities would be minimized. The distribution ratio was found to be 78 a t 28 “C. When the aqueous layer contained 1.5 grams of NaC1, the distribution ratio was 93 a t 25 “C. Effect of Water in the Extract on AAS Operating Characteristics. Using the conditions of the general procedure, 422 pg of copper was extracted with two 10-ml portions of propylene carbonate. Five-milliliter portions of the extractate were made to 25-ml volumes by adding the following: 1) 20 ml of “anhydrous” propylene carbonate, 2) 13.3 mi of “anhydrous” propylene carbonate and 6.7 ml of propylene carbonate that had been saturated with water, 3) 6.7 ml of “anhydrous” propylene carbonate and 13.3 ml of propylene carbonate saturated with water, and 4) 20 ml of propylene carbonate saturated with water. The absorbances of the four solutions were within f 2 % of the mean absorbance. Therefore, the amount of water in the extractate is not critical and calibration curves prepared a t one extraction temperature can be used for analyses run a t another temperature. This is fortunate because a t 20 “ C the propylene carbonate phase contains 6.2% water and increases to 8.4% water a t 30 “C (12).In another experiment, the absorbances due to equal concentrations of copper in water and in propylene carbonate extracts were virtually identical. Unfortunately, this demonstrates that the presence of propylene carbonate p e r se does not enhance the absorbance due to copper-or probably any other metal for that matter. Enhancement results only from the increase in concentration of the metal by virtue of its extraction from a larger to a smaller volume. Calibration Graph. When various amounts of copper were extracted according to the general procedure, the calibration curve shown in Figure 3 was obtained. Effect of Diverse Species. The general procedure was used incorporating 106 pg of copper. Cations were added as solutions of their nitrate, chloride, or sulfate salts, and anions as solutions of their sodium, potassium, or ammonium salts. The volume of the aqueous phase was -50 ml. The 1678
ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
Aluminum (III) Antimony (111) Arsenic (111) Barium (11) Bismuth(II1) Cadmium(T1) Calcium(I1) Cerium (Iv) Chromium (111) Chromium (VI) Cobalt (11) Cobalt (11) Cobalt (11) Gold(1II) Iron(III) Lead(I1) Lithium (I) Magnesium (11) Manganese (IT) Mercury(I1) Mercury(I1) Mercury (11) Molybdenum (VI) Nickel (11) Strontium (11) Thorium (IV) Tin (11) Titanium (IV) Tungsten(V1) Uranium (VI) Vanadium (V) Zinc(11) Zinc(I1) Zinc(I1) Bromide Fluoride Iodide Nitrate Phosphate Phosphate Sulfate Tet raborat e
pg present
“I interference
27,100 122,000 209,000 137,000 209,000 112,000 40,100 140,000 52,000 2 16,000 58,900 29,500 14,500 169,000 55,800 207,000 6,940 120,000 54,900 201,000 100,500 50,000 20,000 58,700 87,600 232,000 119,000 47,900 91,500 270,000 98,900 65,400 32,700 16,300
2.10 -1.12 0.82 1.06 0 .o 0 .o 2.10 1.30 1.40 4.20 -7.63 -9.78 -1.01 -2.32 -0.04 -1.40 1.03 0.40 -0.40 -13.7 -5.59 -0.34 -0.50 1.20 1.06 -0.50 -1.10 2.80 0.24 -2.70 -3.20 -18.7 -7 -80 -1.01
79,900 18,000 764,000 4,060,000 6,880,000 1,390,000 8,640,000 155,000
1.80 2.20 -0.90 0 .o 1.20 0 .o 1.70 0.30
results shown in Table I1 are grouped according to cations and anions in alphabetical order. Any error less than 3.0% can reasonably be considered to be within the precision of the method itself. Analyses. The copper in National Bureau of Standards Sample 85B wrought aluminum alloy and Sample 19G open hearth steel was determined. Approximately 0.2-g portions of the aluminum and 1.5-g portions of the steel were each weighed into 250-ml volumetric flasks, dissolved in 10 ml of aqua regia, boiled to remove the volatile nitrates, and made to volume. Twenty-five-milliliter aliquots were transferred to separatory funnels, diluted to -50 ml, and the copper extracted according to the general procedure. Because of the high iron content of the steel sample, approximately 4 g of sodium pyrophosphate were added to each 25-ml aliquot prior to the addition of the ammonium thiocyanate to hinder the formation of the dark red iron (111)-thiocyanate complex ion. In the absence of the pyrophosphate, it is very difficult to distinguish between the phases because of their opaqueness. When pyrophosphate
carbonate is readily accomplished. Since propylene carbonate is more dense than water, the tedium of extractions using solvents lighter than water is avoided and multiple extractions tend to cancel factors affecting the distribution coefficient in single extraction systems. The interferences due to mercury, zinc, and cobalt occur because these metals form extractable species with thiocyanate and hinder the extraction of the copper(I1)-thiocyanate complex ion. However, more than 14.5 mg per 50 ml of sample of these metals can be accommodated with little effect on the accuracy of the method.
Table 111. Determination of Copper in NBS Standard Samples S B S Sample 19G S t e e l
h B S Sample Y5B Aluminum Allo) Sariple
\liquor
-.Cua
Sample
Aliquot
1
1
1 1 1 2 2 2 2
2 3 4 1 2 3 4
3 -97 3.98 4.08 4.17 3.93 3.99 4.05 4.09
1 1 1 1
1 2 3 4 1 2 3 4
2 2 2 2
AV = 4.03
0:
cub
0.090 0.092 0.092 0.094 0.094 0,090 0.093 0.093
LITERATURE C I T E D (1) C. E. Mulford, At. Absorp. News/., 5, 88 (1966). (2) B. G. Stephens and H. A. Suddeth, Anal. Chem., 39, 1478 (1967). (3) "Propylene Carbonate Technical Bulletin," Jefferson Chemical Co., Houston, Texas, 1960. (4) R . F. Nelson and R. N. Adams, J. Necfroanal. Chem., 13, 184 (1967). (5) 6. G. Stephens, J. C. Loftin, W. C. Looney, and K. A. Williams, Analyst (London). 96, 230 (1971). (6) R . J. Jakubiec and D.F. Boltz, Mlkrochim. Acta. 1199 (1970). (7) K. Murata and S.Ikeda. J. lnoro. Nucl. Chem.. 32. 267 (1970) (8) B. G. Stephens, H. L. Felkel. :r., and W. M. S p h , Anal 'Chem., 48, 692 (1974). (9) J. E. Allan, Spectrochlm. Acta, 17, 459 (1961). (IO) D.C. Munro, Appl. Specfrosc., 22, 199 (1966). (11) P. G. Stecher, Ed., "The Merck Index," 8th ed., Merck 8 Co., Inc., Rahway, N.J., 1968, p 590. (12) James C. Loftin, Wofford College, Spartanburg, S.C., personal communication, 1973.
AV = 0.092
a NBS analyses: reported, :?1.99%;average, 3.9970; range, 3.97%4.0370. NBS analyses: reported, 0.093%; average, 0.093%; range, 0.089%-0.100%.
is used as a masking agent, a t least 112 mg of iron per 50 ml of aqueous phase can be tolerated. The results are shown in Table 111. CONCLUSION
RECEIVEDfor review December 9, 1974. Accepted April 24,
The determination of copper by atomic absorption after its extraction as the thiocyanate complex into propylene
1975.
New Zeeman Method for Atomic Absorption Spectrophotometry Hideaki Koizumi and Kazuo Yasuda Naka Works Hifachi Ltd. Katsufa, Ibaraki, Japan
Because of the importance in environmental research, it is highly desirable to develop a simple instrument which enables one to determine accurately trace elements in foods, living materials, and atmosphere in a short time. Recently, T. Hadeishi et al. have proposed a new technique for the detection of mercury in which the hyperfine structure of Ig9Hg has been used as the light source of atomic absorption spectrophotometry ( I ) . Furthermore, Hadeishi has introduced a significant improvement of the technique by using the isotope effect ( 2 ) . The magnetic field of about 7 kgauss was applied to the light source in the direction of the propagation of the light beam, and circularly polarized Zeeman components u+ and u -of lgSHg were used as reference and absorbing light, respectively. In the latest work, Hadeishi et al. used H and u* components of *04Hg as the absorbing and the reference light respectively ( 3 ) .These methods developed by them are far superior in background correction to the conventional methods. However, it is difficult to apply their techniques to the determination of various elements because the special isotope should be used for the light source. In the present article, we report the Zeeman method for atomic absorption spectrophotometry in which the magnetic field is applied to the light source of natural mercury in the direction perpendicular to the propagation of the light beam, and the Zeeman components are used as a reference and an absorbing light. By studying profiles of the absorption and the emission line of natural mercury, the following results were obtained; The magnetic field of 15
kgauss is sufficient to shift the u* components of the natural mercury lamp from the absorption line because the width of the absorption line is smaller than 23 GHz and the hyperfine splittings of natural mercury light source at this field are smaller than the absorption line width. Therefore, it is possible to use the H and u* components a t 15 kgauss as an absorbing and a reference light, respectively. A very simple atomic absorption spectrophotometer was constructed by using the linearly polarized Zeeman components of a natural mercury lamp as the light source. The detection of trace mercury of about 70 picograms could be achieved in 1 minute. Because it is unnecessary to use the special isotope, the present method is widely applicable for the detection of various elements such as Cd, Pb, Cu, Zn, Cr, and Al, as will be reported elsewhere. I t should be emphasized that, according to our method, chemical pretreatment is unnecessary for a quantitative analysis for mercury. When we observed emission spectra of mercury in a magnetic field transverse to the direction of the light source, the spectral line splits into three components which correspond to transitions of AM = 0, AM = fl,Le., H and u* components. Whereas the H line is linearly polarized and parallel to the magnetic field, the uc and u- lines are linearly polarized and perpendicular to the magnetic field. The intensity of the H line is equal to that of u* lines, if self-absorption of each line does not take place in the light source. When the magnetic field of larger than 15 kgauss is applied to the light source, only the wavelength of H com-
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