Determination of Major Constituents by Atomic Absorption Spectrometry Fredric J. Feldman, Jane A. Blasi, and Stanley B. Smith, Jr. Instrumentation Laboratory, Inc., I I3 Hartwell Ave., Lexington, Mass. 02173 ONLYRECENTLY has it become possible to apply the internal standard technique to atomic absorption analysis. The authors have reported the determination of both major and minor constituents of zinc die-cast by atomic absorption spectroscopy utilizing the internal standard and found increases in both precision and accuracy (I). The present paper describes the use of the aforementioned techniques for the determination of major constituents of cement, steel, aluminum alloys, copper alloys, animal feeds, etc. Zinc, copper, chromium, aluminum, ,manganese, calcium, iron, nickel, and silicon are among many of the metals determined. In the majority of cases, the metal determined was present at a concentration of greater than 5% in the original material. Simultaneous dual-channel analysis of zinc and copper in brass and nickel and chromium in stainless steel was also performed. The precision of the analyses and a comparison of the direct determinations and internal standardization with respect to the precision and accuracy of the measurement are made. Analytical results using the method of standard additions us. external calibration standard are also compared. (1) S. B. Smith, Jr., J. A. Blasi, and F. J. Feldman, ANAL.CHEM.,
40, 1525 (1968).
EXPERIMENTAL Reagents. All chemicals used were of reagent grade. Distilled, deionized water was used throughout. Atomic absorption measurements were made with an Instrumentation Laboratory dual-double beam atomic absorption spectrophotometer (Model 153) as described previously ( I ) . Internal Standard Operation. Described previously (I). Simultaneous Dual Channel Operation. The two hollow cathodes are placed in their respective channels. The A channel grating monochromator is set for the wavelength of the A hollow cathode. An interference filter is placed in the B channel. Each channel is operated with completely independent controls for zeroing, standardization, and readout. Procedures. IRONAND COPPERALLOYS(Low SILICON). The metals are dissolved in a minimum amount of nitric acid and taken up to volume with dilute hydrochloric acid. SILICONALLOYS. The sample is dissolved in a minimum amount of hydrochloric acid. Nitric acid is added dropwise to complete solution. The silica precipitate is filtered and ignited in platinum or nickel crucibles. A 1 :1 mixture of potassium carbonate and sodium carbonate is added to the crucible and the silicon is fused. The fusion is dissolved in a minimum amount of dilute hydrochloric acid and added to the original filtrate. The filtrate is taken up to volume with water.
Table I. Instrumental Conditions A Channel
Concn. Sample Al-bronze
Element
cu Ni Fe AI
Cement
Si-bronze Mn- steel Stainless steel AI-allo y
Feed, animal Brass a
Ca Mg Fe A1 Si
cu
Si Mn Fe Cr Ni Mg AI
Ca Mg cu Zn
X (nm) 249.1 247.6 358.1 396.1 422.7 285.2 372.0 396.1 251.6 249.1 251.6 403.0 358.1 425.4 300.2 285.2 309.2 422.7 285.2 249.1 307.6
(ppm)= 800 500 800 200 20 1 .o 30 100
B Channel
H.C., mA
10 15 1
4 8 9 8 8 5 8 8 10 4 9 6 8 6 8 5 7 8 5
1000
4
600
5
40 800 60 40 1600 120 100 0.5
Slit, p 80 80 40 80 80 80 40 80 80 80 80 80 80 80 80 80 80 160 80 80 80
Internal standard Cd
X(nm)
H.C., mA
228.8
9
Mn Cr
403.0 357.9
8 8
Cd
228.8
9
Cr
357.9
8
Cd
228.8
Cd Zn Cr
228.8 307.6 357.9
9 5 8
Flame, (CzHr) Air-lean Air-stoich Air-stoich NrO-lean Air-lean Air-stoich Air-stoich NnO-lean NnO-rich Air-lean NrO-rich Air-lean Air-stoich NZO-rich Air-stoich Air-stoich NrO-lean
Cd Mn Mn
228.8 403 .O 403.0
9 8 8
Air-stoich Air-lean Air-lean
Approximate working concentration. ~
~~
VOL. 41, NO. 8,JULY 1969
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Table 11. Precision Data Relative standard deviation % Sample Cartridge brass Feed Cement Al-alloy Al-bronze Al-alloy A1 alloy Stainless steel A1 alloy Mn steel O.H. steel
Element Zn
Direct 0.73 0.83 1.29 0.83 0.48
Mg A1 Mn
cu Fe
1.55
0.83
Mg Ni
0.51
cu
0.63 0.70 0.94
Mn Mn
Internal standard 0.21 0.26 0.93 0.58 0.44 0.77 0.58 0.18 0.53 0.49 0.36
Table 111. Per Cent Composition of Samples Analyied Element A.A. % Sample NBS Z 82.25 82.00 cu Al-bronze 164a 3.72 3.75 Ni 4.05 4.00 Fe 9.59 9.60 A1 69.20 69.50 cu Cartridge brass 1101 30.30 30.34 Zn 61.48 61.50 Ca Cement 4.25 4.25 Mg 3.27 3.25 Fe 5.04 A1 5.00 90.93 90.93 cu Silicon bronze 158a 3.03 3.00 Si 65.26 65.40 Ca Cement 0.415 0.42 Mg 3.71 3.70 Fe 4.97 4.93 A1 21 .OO 21.05 Si 3.99 3.99 cu Al-Alloy 85-B 0.61 0.62 Mn 0.24 0.24 Fe 1.49 1.47 Mg 90.0 A1 90.1 Mn 0.412 0.418 Steel open hearth 1.89 1.88 Mn Mn steel l00b Fe 97 97 15 Cr 15 Stainless steel 348 Ni 26 26 13 Cr 13 Stainless steel 73-C 0.920 0.916 Mn Steel open hearth 18.2 18-20" Cr Stainless steel 10.8 8-12' Ni Allegheny Ludlum 59 56-65' cu Machinable brass 35 33-36" Zn 4.90 4-7" Ca Feed 0.288 Mg
CEMENT. The cement sample is dissolved in a minimum amount of hydrochloric acid. The silica is filtered and treated as above. ANIMAL FEED. The feed samples were treated in two ways. Initially the sample was wet ashed with a mixture of nitric, sulfuric, and perchloric acid in the ratio of 3 :1 :1. After the solution cleared, it was taken up in dilute hydrochloric acid and analyzed for calcium and magnesium. An alternate method was also investigated. This method involved adding a I-gram sample to 100 ml of 10% HCl. The volume was reduced 50 ml of a hotplate by gentle boiling. The solution was filtered and taken to volume with water. The results of the Ca and Mg determination agreed for both methods and therefore the latter method was used.
RESULTS AND DISCUSSION The optimum instrumental conditions used for the analyses are summarized in Table I. In some cases, instrumental curve correction was made to linearize the working curve. The choice of spectral lines and flames was made to give maximum precision at the concentration determined without regard to sensitivity. The object here was to increase the precision of the method by determining the metal at the highest concentration commensurate with precision and possibly eliminating further dilution. Calibration curves for the direct and internal standard methods were linear in most cases. Table I1 summarizes the precision data. In all cases, the addition of an internal standard increased the precision of the analysis and in some cases actually straightened the working curve. The magnitude of the correction is dependent on the metal, but averages to a factor of 2 better for an internal standard determination. For example, copper which can be determined precisely because it is relatively flame insensitive is not affected to any great extent by the addition of an internal standard whereas the precision of an iron determination in an aluminum alloy is increased two-fold with the use of an internal standard. A thorough discussion of this effect of internal standardization on atomic absorption measurements and the mechanism of its action has been made (2). The samples analyzed and the per cent compositions determined are summarized in Table 111. In most cases good agreement is observed, illustrating the accuracy of the determination. The accuracy of any determination will of course be limited by the precision if run over an extended period of time and assuming no bias, instrumental or otherwise. Simultaneous Determinations. In most of the above analyses, internal standard techniques are used. However, be~~~~~
a
Table IV. Instrumental Conditions for Simultaneous Analysis A Channel
M Sample
Element
Xa (nm)
Cement Mn-steel Stainless steel Al-alloy Al-bronze Brass
Mg Fe Cr cu
0
~
(2) F. J. Feldman, 15th Spectroscopy Symposium of Canada, Toronto, October 1968.
Estimated values.
cu cu
285.2 358.1
Concn. ppm 1-2 1000-1800
Slit, p 80 80
Element Ca Mn
425.4 324.7 249.1 249.1
100-200 4-8 400-800 600-1000
80 80 80 80
Mg Ni
Ni Zn
B Channel
(nm) 422.7 403.0
Concn. ppm 15-30 20-40
341.4 285.2 341.4 307.6
40-80 1.5-2.5 40-80 300-600
Xa
Wavelengths used were chosen because curve was linear or could be curve corrected for the concentration range.
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ANALYTICAL CHEMISTRY
cause the Instrumentation Laboratory Model 153, is a dualchannel instrument, with completely independent electronics for each channel, simultaneous analysis can be performed when internal standardization is not being run. Actually internal standardization is simultaneous analysis with the resultant ratio of the signals. Table IV lists the optimum conditions for the simultaneous analyses. The precision of a simultaneous analysis was not as good a s an analysis utilizing internal standardization. However, the precision of most of the analyses was better than 1%. The main convenience of a simultaneous determination is the time saving feature which was greater than a factor of 2, because dual standards could be used. This saving alone could make simultaneous analyses worthwhile in many laboratories where single element atomic absorption procedures were considered too time-consuming. Table V summarizes the accuracy of some of the simultaneous determinations. In all cases, good agreement was obtained.
Table V. Simultaneous A.A. Determinations Sample
Element
Stainless steel 304
Cr
Mn steel lOOb AI-alloy 85b AI-bronze 164a Stainless steel 348
Ni Mn Fe cu Mg cu Ni
Chem.
A.A.
18-20 8-12 1.89 97.0 3.99 1.49 82.25 3.72
Cr
15.0
Ni
26.0
18.2 10.8 1.90 96.0 4.01 1.50
82.0 3.75 15.2 26.3
RECEIVED for review January 23, 1969. Accepted April 23, 1969.
Solvation of Aluminum(lll) Ion in Dimethylsulfoxide-Water Solutions by Proton Magnetic Resonance Spectrometry D. P. Olander, R. S. Marianelli, and R. C . Larson Department of Chemistry, Uniuersity of Nebraska, Lincoln, Neb. 68508
DURINGTHE PAST SEVERAL years there has been an increasing interest in the use of nuclear magnetic resonance spectrometry (NMR) to determine coordination numbers and solvent exchange rates of metal ions in nonaqueous solvents (1-5). Because it is difficult to obtain strictly anhydrous conditions in most nonaqueous solvent systems, studies of the effect of residual water on the ion-solvent interactions need to be made. Separate NMR signals can be obtained for coordinated and free solvent molecules when the exchange of the molecules of the primary solvation sphere of the cation with the bulk solvent molecule is sufficiently slow (6). From the work of Thomas and Reynolds with aluminum(II1) in dimethylsulfoxide (DMSO) (3) and that of Fratiello et al. with aluminum(II1)chloride-water-DMSO (7-9), it seemed reasonable that we could determine the relative solvation of aluminum (111) ions by water and DMSO in a mixed solvent system by proton magnetic resonance spectrometry. An attempt has been made, therefore, to obtain quantitative information, in (1) N. A. Matwiyoff and H. Taube, J. Amer. Chem. SOC.,90, 2796
(1968). (2) S. Nakamura and S. Meiboom, ibid., 89, 1765 (1967). (3) S. Thomas and W. L. Reynolds, J. Chem. Phys., 44,3148 (1966). (4) W. G . Movius and N. A. Matwiyoff, Inorg. Chem., 6, 847 (1967). (5) N. A. Matwiyoff and W. G. Movius, J. Amer. Chem. SOC.,89, 6077 (1967). (6) J. A. Pople, W. G. Schneider, and H. J. Bernstein, “HighResolution Nuclear Magnetic Resonance,” McGraw-Hill Book Co., Inc., New York, 1959, p 221. (7) A. Fratiello and D. P. Miller, Mol. Phys., 11, 37 (1966). (8) A. Fratiello, R. E. Lee, V. M. Nishida, and R. E. Schuster, J. Chem. Phys., 47, 4951 (1967). (9) A. Fratiello and R. E. Schuster, Tetrahedron Letters, 46, 4641 (1967).
the form of equilibrium constants, on the interaction of water with aluminum(II1) ions in DMSO, with the hope that such information will be of assistance in interpreting the results of studies of inorganic cations in nonaqueous media. EXPERIMENTAL
Samples were prepared by adding measured amounts of water with a microburet to aliquots of anhydrous DMSOaluminum(II1) solutions of known aluminum(II1) concentration, in a 5-ml volumetric flask, and diluting to volume with anhydrous DMSO. Density measurements showed that the volumes of DMSO and water were additive over the composition range used and that the density of the Al(C10&DMSO solutions did not vary significantly from that of pure DMSO. The anhydrous solutions were obtained by adding twice recrystallized, hydrated City Chemical Corporation aluminum(II1) perchlorate to Matheson, Coleman and Bell distilled DMSO, and removing the water with Matheson, Coleman and Bell “Linde” type 3A l/le-inch molecular sieves in a modified Soxhlet extractor (10) at 55 “C. The perchlorate salt was used to minimize the possible formation of anion complexes (11). The sieves were dried for two hours in vacuo at 300 “C before being used. The water content of a typical solution was reduced from 0.3 to 0.01M in ca. 5 hours drying time. The drying procedure was carried out for approximately 24 hours to assure an essentially anhydrous solution for sample preparation. The anhydrous DMSO was distilled from molecular sieves and stored in a glass stoppered bottle over freshly dried sieves. Aluminum(II1) concentrations were determined gravi(10) P. Arthur, W. M. Haynes, and L. P. Varga, ANAL.CHEM., 38, 1630 (1966). (11) T. C. Hoering, F. T. Ishimori, and H. 0. McDonald, J . Amer. Chem. SOC.,80, 3876 (1958). VOL. 41, NO. 8,JULY 1969
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