I AIDS FOR ANALYTICAL CHEMISTS Rapid Dissolution Technique for Lead Alloys Harry F. Bell international Business Machines Corporation, System Products Division, Poughkeepsie, N. Y. 12602
The rapid determination of the constituents of lead alloys is of great importance to the electronics industry. Small sample sizes are frequently encountered; therefore, accurate and sensitive techniques are necessary. Atomic absorption spectrophotometry has been shown to be a valuable tool for the analysis of these alloys ( I , 2 ) . Sample dissolution, however, remains a problem because of the chemical nature of the main constituents, tin and lead. Hydrochloric acid rapidly dissolves tin, but precipitation of the partially soluble lead chloride slows down the rate of attack. Nitric acid dissolves the lead in the alloy but forms insoluble metastannic acid with the tin. The previously cited authors employ mixtures of fluoboric acid, nitric acid, and water for the dissolution of their samples. In addition, one author ( I ) uses ultrasonic agitation to speed sample dissolution. Although these procedures are satisfactory for alloys containing >50% tin, the dissolution rate for lead alloys containing >So% lead is fairly slow. Solutions of acetic acid and hydrogen peroxide are commonly used in the plating industry to strip deposits of lead from various materials (3). Experiments in this laboratory have shown that mixtures of fluoboric acid, hydrogen peroxide, and disodium ethylenediaminetetraacetate (EDTA) rapidly dissolve lead alloys a t room temperature. The addition of EDTA is necessary to prevent the formation of insoluble salts during the dissolution step. The resulting solution may be analyzed, after dilution, directly by atomic absorption spectrophotometry.
EXPERIMENTAL A Perkin-Elmer Model 403 atomic absorption spectrophotometer and a Perkin-Elmer Intensitron (Perkin-Elmer Corporation, Norwalk, Conn.) tin hollow cathode lamp were used for all samples. An air-acetylene flame and standard conditions were employed ( 4 ) . The solvent mixture was prepared by mixing equal volumes of reagent grade fluoboric acid (48%), hydrogen peroxide (30%), and EDTA (0.M). Sample sizes ranged from 20 t o 140 mg for lead alloy containing 40 and 2% tin, respectively. The samples were weighed in micro size (4 X 4 cm) disposable weighing trays on a Mettler H20T balance. The balance has a direct digital readability of 0.01 mg. The material was transferred from the trays, with the aid of water from a wash bottle, into 100-ml volumetric flasks, and 15 ml of the fresh solvent mixture was added. After the initial reaction subsided, the clear solutions were heated on a steam bath for 5 min to decompose the excess hydrogen peroxide. The samples were then cooled t o room temperature and diluted to volume. The
(1) J. Y . Hwang and L. M . Sandonato,Anal. Chern., 42,744 (1970). (2) J. U. Gouin, J. L. Holt, and R. E. Miller, Anal. Chern., 44, 1042 (1972). (3) N. L. Hall, G. B. Hogaboom, Jr., and J. B. Mohler. in "Metal Finishing Guidebook and Directory," 40th ed, N . Hall, Ed., Metals and Plastics Publications,Westwood, N.J., 1972, p 526.
(4) "Analytical Methods for Atomic Absorption Spectrophotometry," Perkin-Elmer Corp., Norwalk, Conn., March 1971. 2296
Table I . Results of Tin. Analysis Using Rapid Dissolution Method Tin, YO
Standard
Soldera Bearing metalb Soldera Soldera Solder'
Found
Certified
Re1 std dev, %
2.82
2.95 5.84 9.78 15.19 39.3
1.2 0.6 1 .o 1.5 0.7
5.87 9.73 14.97 39.6
a Spectrochemical standards (Pb/Sn), Alpha Metals. lnc.. Jersey City, N.J. OSRM 53e (84 Pb-10 Sb-6 Sn), National Bureau of Standards. Washington, D.C. S R M 1 2 7 b (40 Sn-60 P b ) . National Bureau of Standards, Washington, D.C.
standard curve was prepared from a tin stock solution (reagent grade tin metal in HC1). The final dilutions of these standards contained 15 m1/100 ml of the solvent mixture and were carried through the heating step prior to dilution. The standard curve was linear up to 150 ppm tin.
RESULTS AND DISCUSSION Identical samples of a 90/10 lead-tin alloy (0.3 g, 0.025 cm thick) were treated with 50-ml portions of the nitricfluoboric acid mixtures ( I , 2 ) and the fluoboric acid-peroxide-EDTA mixture described in this paper. All samples were allowed to stand a t room temperature until the alloy completely dissolved. The mixture containing the peroxide and the EDTA dissolved the sample in 20-30 sec, while the other mixtures required 40-90 min for complete dissolution of the alloy. The tin content of a number of different lead alloys was determined by atomic absorption spectrophotometry. The rapid dissolution technique was employed for all samples. The results of the analysis are presented in Table I. A statistical analysis of the data from Table I indicates that there is no significant difference between the results employing the rapid dissolution technique and the certified values that cannot be explained by random errors ( 5 ) . The sample dissolution rate decreases as the percentage of tin in the alloy increases. The lead goes into solution because it participates in the heterogeneous catalytic decomposition of the hydrogen peroxide and is rapidly oxidized in the process (6). Tin metal is noncatalytic with respect to hydrogen peroxide and therefore increasing amounts of tin retard the dissolution rate of the alloys. The method offers no time advantage a t alloy tin contents of greater than 50%. The diluted solvent mixture has not caused any corrosion of the atomic absorption burner head. Samples prepared by this technique have been found to be stable for a t least 24 hr. (5) K. Eckschlager, "Errors, Measurements and Results in Chemical Analysis." Van Nostrand-Reinhoid, London, 1969, p p 11 1-1 14. (6) W. C. Schumb, C. M . Satterfield, and R. L. Wentworth. "Hydrogen Peroxide." Reinhoid, New Y o r k . N . Y . , 1955, p p 479-480.
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 13, NOVEMBER 1973
The rapid dissolution technique cuts the total sample preparation time for alloys of high lead content from a minimum of 30-45 min to a maximum of 10 min. In addition, the procedure has been shown to be accurate, reproducible, and suitable for small (milligram) samples.
ACKNOWLEDGMENT The author acknowledges the valuable technical assistance of G. E. Calley. Received for review January 18, 1973. Accepted April 19, 1973.
Molecular Weight Index of the Merck Index for Mass Spectrometry l u n g Sun, D. J. Pedder, and H. M. Fales Laboratory of Chemistry, National Heart and Lung Institute, Bethesda, Md. 20074 In this laboratory, two rapid methods of identifying unit resolution mass spectra are tried before resorting to interpretation from first principles. The first method, often successful, is to use an interactive computer search of approximately 9000 mass spectra ( I ) . The second method, assuming a molecular ion can be observed or deduced, is to scan a list of organic compounds of the same molecular weight for compounds which might be expected to produce a similar cracking pattern, and to have other appropriate properties. Because the laboratory is biochemically oriented and often assists in overdose cases ( 2 ) ,the Merck Index (3) is a useful source of relevant compounds. Another index published by the Chemical Rubber Co. also contains a molecular weight index for all compounds it contains ( 4 ) . This index was compiled using atomic weights instead of multiples of the most abundant nuclides since it is for general, rather than mass spectrometric, use. This complicates its use in mass spectrometry only slightly except in the case of compounds containing bromine and chlorine. A separate table on pp C550-C557 ( 4 ) is devoted to compounds containing bromine, chlorine, sulfur, and nitrogen, especially for use in mass spectrometry. Although this tabulation is useful since it also contains the three most intense peaks, the molecular weights have still been calculated on the basis of atomic weights. For convenience, we wrote a Fortran program to create a file of molecular formulas indexed by increasing molecular weight. This list is consulted for the molecular formulas represented a t a particular molecular weight and the individual compounds are located via the molecular formula index in the Merck Index (8th edition). The molecular formulas we indexed were limited to those containing only elements from the following: C, H, N, 0, S, F, C1, Br, I, P, Si. Compounds containing other elements are expected to have distinctive isotope patterns facilitating their identification. The molecular weights were calculated as integers by the mass spectrometric convention of using the masses of the most abundant isotopes based on C = 12.0. This is normally the most. abundant peak in the isotope pattern of the molecular ion; the polychloro and polybromo compounds are the most conspicuous exceptions. A suppleS . Heller, Ana/. Chem., 44, 1950 (1972). N. C. Law, H . M. Fales, and G. W. A. Milne. Clin. Toxicol., 5 ( l ) , 17 (1972). "The Merck Index", P. G. Stecher, Ed., 8th ed., Merck 8 Go., Inc. Rahway, N. J., 1968, pp 1325-1398. "Atlas of Spectral Data and Physical Constants for Organic Compounds". J. Grasselli, Ed., Chemical Rubber Go., Press, Cleveland, Ohio, 1973, pp C18-C31.
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MOLECULAR WEIGHT
Figure 2. Maximum number of isomers found in "The Merck
Index" as a function of molecular weight based on most abundant nuclides mental listing using accurate masses for use in high resolution mass spectrometry has also been prepared. The number of compounds at each nominal molecular weight is shown in Figure 1. As expected, compounds containing an odd number of nitrogen atoms ( i e . , odd mw) are relatively scarce. From nominal molecular weight alone, as might be the case in chemical ionization or field ionization experiments, up to approximately 40 compounds might be considered in the worst case ( m / e 150). Figure 2 shows that even if the exact mass, and therefore the molecular formula, were known from high resolution mass measurements, up to approximately 10 isomeric compounds would still have to be considered at m / e 150. Obviously, consideration of even the most gross aspects of the fragmentation pattern would often allow identification in either case. This index is available from the authors in either integer or accurate mass form. The Fortran program for creating the file is also available. Received for review March 2, 1973. Accepted April 24, 1973.
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