Determination of Tin and Antimony in Type Metal Using Atomic Absorption Spectrophotometry Joseph U. Gouin, Janet L. Holt, and Ralph E. Miller Tests and Technical Control Sercice, US.Gocernment Printing Ofice, Washington, D. C.
TYPEMETAL is a lead-base alloy containing 3 to 7 per cent tin, 3 t o 17 per cent antimony, and impurities of copper, bismuth, and arsenic. An atomic absorption procedure for the determination of tin and antimony, the major components in type metal, was sought which would be faster and simpler than present standard wet chemical methods of analysis. Mostyn and Cunningham ( I ) described a procedure for t h e determination of antimony as a major constituent in lead but n o mention for the determination of tin was made. A further search of the chemical literature revealed n o detailed procedure for the determination of tin as a major constituent in lead. With this background, a procedure for the determination of these two elements was developed utilizing a mixture of fluoboric and nitric acid which introduced two time saving features. First, by simply changing flame conditions, tin and antimony can be analyzed in the same solution rather than necessitating two separate samples as in the standard wet chemical methods. Second, as seen in Table I, n o close scrutiny of the acidity is required after a certain minimum amount of acid has been used as is required in the procedure of Mostyn and Cunningham. In addition, this dilute acid mixture causes n o visible corrosion of the nebulizer and burner head as was reported when similar but more concentrated acids were used (2).
Table I. Effect of Acid Concentration on Sn and Sb Volume of acid mixture, ml Tin results, % Antimony results, 33 4.63 12.21 65 4.58 12.00 130 4.51 12.02 190 4.56 12.00 Table 11. Operating Conditions Antimony Copper Current of hollow cathode lamp, mA 15 18-20 Spectral line, A 2175.8 3247 Slit opening, mm 1 . 0 o r O. 3 1.0 Pressure reading on acetylene tank, psi 12-13 12-13 Acetylene pressure reading on burner regulator con8-10 8-10 trol box, psi Acetylene flow rate reading, 8 8 l./min Air pressure of plant air 40 60 supply line, psi Air pressure reading on burner regulator control 26-30 26-30 box, psi 23 23 Air flow rate reading, I./min
Tin 30
2354. a 1.o
12-13 8 9 60
26-30 23
EXPERIMENTAL
Apparatus and Operating Conditions. All tests were carried out o n a Perkin-Elmer Model 403 atomic absorption spectrophotometer using a three-slot burner head. Because of the lack of sensitivity for tin and antimony in the absorption mode, the concentration mode which amplifies the absorption readings was used for the analysis. Table I1 lists the spectral lines in operating conditions used for the fuel and oxidant. The lower air pressure of the plant air supply line for antimony listed in Table I1 was required because a signature press connected on t h e same air line as used by this atomic absorption instrument caused fluctuating air flows which affected the antimony readings when the press was operating. To correct the condition, a pressure reducing valve was installed t o reduce the plant air pressure from 80 psi t o 40 psi. Two other preventive precautions were taken. A check valve was installed and a small empty cylinder of several liters volume was placed before the burner regulator control box as a compensating reservoir. A less sensitive spectral line, 2311.5 A, a t a slit opening of 1.0 mm. can also be used for the determination of antimony. Reagents. All reagents used were of analytical grade. The acid mixture for dissolving samples consisted of 3 parts fluoboric acid (48% aqueous), 6 parts nitric acid (70% aqueous), and 30 parts distilled o r deionized water. The tartaric acid was dissolved in water to give a 1.2% solution. The standard stock solution of type metal constituents used (1) R. A. Mostyn and A. F. Cunningham, ANAL. CHEM.,39,
433 (1967). (2) J. Y . Hwang and L. M. Sandonato, ibid., 42, 744 (1970). 1042
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for adjusting type metal standard solutions was made from metals which were 99.9999 pure. Interference Effects. Pure metal (99.9999 %) of antimony, tin, and lead were dissolved in equal amounts of fluoboric and nitric acids t o test the interference effects of these elements on each other. Tables I11 and IV give the results of this experiment. The concentration mode readings listed in Table 111 are about three times the absorption readings and in Table IV about ten times t h e absorption readings. After a study of the readings, it was decided that t h e best and most practical procedure for background correction due t o major constituents and trace impurities in making the type metal standards was t o use the actual type metals rather than synthetically preparing them. Standard Preparation. Low and high standards to be used in a bracketing method of analysis were prepared for each formulation of type metal (See Table V for type metal formulations), using actual samples whose tin and antimony content had been determined previously by titration. For example, linotype standards were made with linotype type metal, and so forth for the other formulations. Each standard was prepared t o contain the correct amount of both tin and antimony, thus avoiding t h e preparation of a separate set of standards for each element. To adjust the amounts of tin and antimony when needed, additional increments from prepared stock solutions were added. The spread betheen low and high standards was restricted t o a range of 0.5 t o 1.O % for improved accuracy. Operating Concentration Range. Curves plotting the concentration of tin and antimony cs. the concentration mode readings of the instrument showed a linear range for the tin
Table 111. Effects of Tin and Lead on Antimony Absorption Readings concn mode Sb, g/l. Sn, g/l. Pb, g/l. 378 ... ... 0.06 366 0.40 ... 0.06 365 0.06 ... 0.41 366 0.42 0.06 ... 366 ... 0.06 0.02 367 ... 0.06 0.03 ... 380 0.06 0.04 371 0.06 0.02 0.40 0.03 358 0.06 0.41 361 0.06 0.42 0.04 Table IV. Effects of Antimony and Lead on Tin Absorption Readings concn mode Sb, g/l. Sn, g/l. Pb, g/l. ... 177 0.02 ... 178 0.02 0.40 ... 175 0.02 0.41 ... 186 0.02 ... 0.42 169 ... 0.05 0.02 ... 0.02 0.06 168 ... 0.08 168 0.02 173 0.40 0.05 0.02 182 0.06 0.41 0.02 186 0.08 0.42 0.02
determination from 0 pg per ml t o 50 pg per m l a n d for antimony f r o m 0 pg per ml t o 70 pg per ml. To operate in these ranges a n d t o determine both tin and antimony o n t h e same sample, the following sample sizes were used. For monotype a n d stereotype metals, 0.3750 gram was used, but for linotype metal 0.5000 gram was needed and for electrotype metal 1.OOOO gram was needed. Procedure of Analysis. Dissolve the accurately weighed samples in 65 ml of the acid mixture, add 20 m l of tartaric acid solution, and then dilute t o 1 liter in a volumetric flask. At the operating conditions given previously, zero the atomic absorption spectrophotometer with a fluoboric acid solution equal in concentration to the samples; then run the samples between the high and low standards interpolating to calculate the sample concentrations. Procedure of Analysis by Wet Chemistry. When determining tin and antimony in type metal, the samples (1.0000 gram) are dissolved in 15 ml of concentrated sulfuric acid with approximately 3.0 grams of potassium sulfate. The antimony sample is titrated with potassium permanganate according t o a procedure by McNabb and Wagner (3). The tin sample is titrated with iodine according to a procedure by McDow, Furbee, and Clardy ( 4 ) . I t is recommended when standardizing t h e permanganate a n d iodine solutions to use as a standard a lead-base bearing metal which closely resembles type metal in composition. Standard sample 53e obtained from t h e National Bureau of Standards is a satisfactory example. DISCUSSION AND RESULTS The atomic absorption results compared satisfactorily with the titration results. Of a total of 40 different samples, all t h e atomic absorption results for tin were within 0.20% of the results obtained by titration. Of a total of 55 different samples, 91 (50 samples) of the atomic absorption results
z
(3) W. M. McNabb and E. C. Wagner, Ind. Eng. Chem., 2. 254 (1930). (4) I. B. McDow, K. D. Furbee, and F. B. Clardy, ibid., 16, 555 (1944).
Table V. Formulation of Type Metals Sb content, Sn content, % Type metal 3.5 3.5 Electrotype 11.5-12.0 4.5 Linotype 16.5-16.8 7.0 Monotype 13.0 7.0 Stereotype Table VI. Copper Determination in Type Metal Colorimetric method (neo-cuproine extraction), % Atomic absorption method, % 0.025 0.026 0.025 0.026 0.031 0.030 0.030 0.030 0.030 0.025 0.031 0.031 0.031 0,025 Table VII. Effects of Tin, Antimony, and Lead on Copper Absorption Readings cu, Sn, concn Pb Sb, mode g/500 ml g/500 ml g/SOO ml g/500ml 0.0010 105 0.0010 1.0000 104 0.0010 1.0000 0.1500 104 0.0010 1.OoOo 0,0600 104 0.0010 1.0000 0.1500 0.0600 105
for antimony were within 0.20% of the results by titration, 96% (53 samples) were within 0.30%, and none were over 0.40%. The fact that 9 % of the type metal tested for antimony by atomic absorption did not agree closer than 0.20 % with the titration results was thought t o be due, in part, t o the tendency of antimony t o segregate in type metal, especially in the higher ranges of 14 t o 17%. The standard deviation for the determination of tin by atomic absorption and titration is 10.05 %. The standard deviation for antimony determination by atomic absorption is *0.08% and by titration 1.0.06z. The only problem experienced with some of the standards was that the antimony concentration tended t o decrease over a n extended period of time. The suspected cause was hydrolysis of antimony which, though gradual, was precipitating the element out of solution. Tartaric acid was added to maintain the antimony in solution for the life of t h e standards. Lead can also be determined in a fluoboric acid solution in the presence of tin and antimony, but it is unnecessary for control work in type metal. However, lead analysis is important in lead fluoboric plating solutions. Determination of lead in fluoboric acid-tin plating solutions was investigated and very good agreement was obtained between analysis by atomic absorption and the lead sulfate precipitation method ( 5 ) . F o r example, results of two samples taken a t different times gave by t h e precipitation method 64.0 and 63.1 grams per liter as compared to 64.3 and 63.1 grams per liter, respectively, by t h e atomic absorption method. Analysis of impurities in type metal was also investigated using samples dissolved in fluoroboric acid. For example, (5) “Electroplating Engineering Handbook,” 2nd ed., A. K. Graham, Ed., Reinhold, New York, N.Y., 1962, p 293. ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
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copper analysis in type metal by atomic absorption gave very good agreement with results obtained by the neo-cuproine extraction method (6) as seen in Table VI. It is recommended that, when analyzing for impurities, a 2.5-gram sample be dissolved and diluted t o 250 ml t o make a more concentrated sample, and then use conditions listed in Table 11. Table VI1 shows the effects of tin, antimony, and lead o n copper in fluoboric acid. I n summary, analysis of the major constituents and trace impurities of type metal can be done by atomic absorDtion with n o special preparation or treatment except for preparation of standards when samples are dissolved in the fluoboric(6) A. R. Gahler, ANAL. CHEM., 26, 577 (1954).
nitric acid mixture. The atomic absorption results compare very well with standard wet chemistry results and in most cases the procedure is quicker and simpler.
ACKNOWLEDGMENT This paper has been approved for publication by A. N. Spence, Public Printer of the United States. The work was done in the Tests and Technical Control Service under the direction of George G. Groome, Technical Director. RECEIVED for review September 20, 1971. Accepted December 13, 1971. Mention of commercial products does not imply endorsement by the U.S. Government Printing Ofice.
Phototautomerism in the Lowest Excited Singlet State of 4-Methylumbellife~rone G . J . Yakatan,' R. J . Juneau, and S. G . Schulman College of Pharmacy, UniGersity of Florida, Gainesville, Flu. 32601 COUMARIN IS WIDELY distributed in nature and some of its derivatives are of great importance in chemistry and medicine. Many coumarins are naturally fluorescent. Goodwin and Kavanagh (1, 2 ) measured the fluorescence of several coumarin derivatives as a function of pH. Fluorescent indicators have been used for acid-base titrations. F o r example, Chen (3) indicated that a coumarin derivative, 4-methylumbelliferone, was a useful indicator to follow the p H change induced in the carbonic anhydrase catalyzed hydration of Cor. Several workers have utilized fluorogenic coumarin substrates in enzyme determinations. Robinson ( 4 ) measured p-glucosidase activity by the fluorescence of 4-methylumbelliferone released by action of the enzyme o n 7-(/3-~-glucopyranoxyloxy)-4-methylcoumarin. Mead et al. ( 5 ) measured the activity of the same enzyme o n the galactoside substrate. Similarly, sulfatase has been measured by its action o n the sulfate ester of 4-methylumbelliferone (6, 7) and alkaline phosphatase by its activity o n the phosphate ester (8). The numerous applications of the fluorescence properties of the 7-hydroxycoumarins (umbelliferones) has led to much recent interest in the description of the fluorescence of these molecules in solution (3, 9-11). Creaven and coworkers (9) reported that 7-hydroxycoumarin showed excited state ioniza-
tion from p H 1 to 9. Below p H 9, the excitation shift to shorter wavelength was attributed to the change in the ground state, from the anion to the non-ionized molecule, but the fluorescence emission remained that of the ionized species. These same authors found it difficult to explain the fluorescence shift to longer wavelength at p H 1 and postulated that the fluorescing species at p H 0-1 could be a dimer hydrogenbonded through the free 7-hydroxyl group (9). Fink and Koehler (10) studied the fluorescence of 7hydroxycoumarin in the p H range 1.2 to 11.2. They found a n intense blue fluorescence band which was p H independent for all pH's greater than 2.2. At p H values between 1.2 and 2 . 2 , the latter emission red shifted and decreased in intensity. These authors assigned this phenomenon to a n excited-state acid-base equilibrium since the ground state pK, of 7-hydroxycoumarin was approximately 8 (10). Here, the postulate that the lower energy emission was due to the unionized molecule while the higher energy blue fluorescence was due to the 7-hydroxycoumarin anion, indicated that the nature of the fluorescence of coumarin and its derivatives was still in question. This, coupled with our interest in excited state equilibria led us to reinvestigate the nature of the fluorescence of COUmarin and some of its derivatives.
Present address, College of Pharmacy, University of Texas, Austin, Texas 78712.
EXPERIMENTAL
( I ) R. H. Goodwin and F. Kavanagh, Arch. Biochern. Biopliys., 27, 152 (1950). (2) Zbid.,36, 451 (1952). (3) R . F. Chen, A m / . Lett., 1,423 (1968). (4) D. Robinson, Biocliem. J . , 63, 39 (1956). (5) J. A. R . Mead, J. N. Smith, and R. T. Williams, ibki., p 39. (6) W. R . Sherman and E. F. Stanfield, ibid.. 102, 905 (1967). (7) G. G . Guilbault and J. Hieserman, ANAL. CHEM.,41, 2006 (1969). (8) G. G. Guilbault et a/., A t i d . Lett., 1, 333 (1968). (9) P. J. Creaven, D. V. Parke, and R. T. Williams, Blochem. J., 96, 390 (1965). (10) D. W. Fink and W. R. Koehler, ANAL.CHEM., 42, 990 (1970). (11) W. R. Sherman and E. Robins, ibrd., 40, 803 (1968). 1044
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Apparatus. Absorption spectra were obtained using a Beckman DB-GT spectrophotometer. Fluorescence measurements were performed o n a Perkin-Elmer MPF-2A fluorescence spectrophotometer whose monochromators were calibrated against the xenon line emission spectrum and whose output was corrected for instrumental response by means of a rhodamine-B quantum counter. Reagents. Coumarin was purchased from Aldrich Chemical Co., Cedar Knolls, N. J., and 4-methyl-7-hydroxycoumarin was obtained from K and K Laboratories, Plainview, N.Y. Both compounds were recrystallized from 95 ethanol. The 4-methyl-7-methoxy derivative was prepared by methylating 4-methyl-7-hydroxycoumarin with dimethyl sulfate and potassium carbonate in acetone. The methylated product was
