Despite the apparent fit of second order kinetics attributed to Equation 1, a similar pH and concentration dependence could conceivably arise for Equation 2 as well. Similarities between the IDA system and the previously studied systems encompass those features associated almost entirely with the Mo-bonded oxygen atoms. Such a correlation is not unreasonable because these atoms will be relatively unaffected by slight structural variations in the Mo-bonded ligand. Dimerization (polymerization in the Mo2EDTA system) is observed for all three systems. The observed stoichiometry and infrared data support a structure with a single 0
Mo/ \Mo bridge. From the pH dependence of the ligand proton resonances, MOOJDA-~, M O O ~ M I D A - and ~ , (MoO&EDTA-4 are concluded to be protonated. The absence of infrared absorption bands around 1750 cm-l indicates that the carboxylate groups are not protonated. This fact, in conjunction with the magnitude of the chemical shifts upon protonation, supports the scheme in which the doubly-bonded oxygens are protonated (2). Finally, the magnitude of the JAX coupling and the appearance of the A proton resonances in HzO and DzO suggest
the possibility of using the M O O J D A - ~complex as a probe for analyzing the composition of H20-Dz0 mixtures. In such a mixture the NMR spectrum of the A protons is an AB pattern (from N-D) superimposed on an ABX pattern (from N-H). The areas of the AB and ABX signals are directly proportional to the DzO and HzOconcentrations, respectively, which can then be determined by electronic or mechanical integration. A similar technique for H20-D20 analysis using N,N-dimethylbenzylamine (MBA) as a probe has already been reported (9). The use of Mo031DA-* as a probe may be more useful in certain cases because of the larger doublet separation (7.3 cps compared t o 5.1 cps for MBA), the MOOSIDA-2 may be used in neutral solutions (MBA solutions require prior acidification), and the M O O ~ I D A -may ~ be added as a solid to the solutions being analyzed (NazMo03IDA with 4 to 6 waters of hydration may be precipitated from a 1:1 Na2Mo04, HJDA solution at pH 5 to 6 by adding ethanol).
RECEIVED for review January 19, 1967. Accepted May 22, 1967. Investigation supported by the National Science Foundation (GP-4423). (9) D. E. Leyden and C. N. Reilley, ANAL.CHEM., 37, 1333 (1965).
Sensitivity of Manganese Determination by Atomic Absorption Spectrometry Using Four Solvents Fredric
J. Feldman, Robert E. Bosshart, and Gary D. Christian
Dicision of Biochemistry, Walter Reed Army Institute of Research, Walter Reed Army Medical Center, Washington, D.C.
ORGANIC SOLVENTS are often used to increase the sensitivity in atomic absorption analysis ( I ) . Manganese has been extracted, for example, into methyl isobutyl ketone using ammonium pyrrolidine dithiocarbamate as the chelating agent (2). This procedure suffers from instability of the complex as well as certain interferences. This paper describes a study of the atomic absorption spectrometry of manganese in four solvents: acetone, methyl isobutyl ketone (MIBK), ethanol, and water. Optimum conditions for the determination of trace amounts of manganese are reported. Possible relationships between the physical properties of the four solvents and the absorbance of the manganese are presented and discussed. With acetone as the solvent as little as 2 ppb of manganese can be determined directly.
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EXPERIMENTAL
Reagents. Reagent grade solvents and distilled, deionized water were used without further purification. Stock solutions of manganese containing 100 pg per ml were prepared by dissolving 0.0308 gram of manganous sulfate (MnS04. H20) in 1.00 ml of hydrochloric acid and diluting to 100.0 ml with the appropriate solvent. Working solutions of 0.0020.100 pg per ml were prepared by dilution of this stock solution just before use. Apparatus. All atomic absorption measurements were made with a Jarrell-Ash atomic absorption spectrophotometer (Model 83-000) as previously described (3). The fuel and supporting gases were hydrogen and air, respectively. A
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Figure 1. Relationship between per cent absorption and hydrogen and air pressure for 100 ppb manganese in acetone (1) J. E. Allan, Spectrochim. Acta, 17, 467 (1961). (2) R. F. Mansell, Atomic Absorptiorz Newsletter, 4, 276 (1965). (3) F. J. Feldman and W . C. Purdy, Anal. Chim. Acta, 33, 273 (1965). VOL. 39, NO. 10, AUGUST 1967
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Burner Height (mm) Figure 2. Relationship between per cent absorption and burner height for 100 ppb manganese A. Water Curve I. Manganese in solvent B. Ethanol Curve 11. Pure solvent Curve III. Difference between Curves I and I1 C. Acetone D. Methyl isobutyl ketone
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ANALYTICAL CHEMISTRY
Curve I Curve P Curve JE ,I
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Westinghouse manganese hollow-cathode lamp (neon-filled) was used as the light source. All absorption measurements were recorded on the Bristol Dycamaster recorder supplied with the instrument. The 2795-A reasonance line of mangaqese was the most sensitive, and was used for the measurements unless otherwise specified. Scale expansion techniques were used t o increase the signal. RESULTS AND DISCUSSION
To confirrn that all the manganese was dissolved in each of the solvents., 1 ml of each of the four stock solutions (containing 100 kg,/ml of manganese) was evaporated and the residue transferred to a 25-ml volumetric flask by washing with 2 drop!; of hydrochloric acid, followed by water. These solutions were then aspirated into the flame. In each case, the per cent absorption was identical, indicating that a n equal concentration of manganese was present in each solvent, including water. Fuel Effects. The effects of hydrogen and air pressures on the absorption of manganese were studied. Figure 1 shows representative curves for acetone solutions. For all solvents a corresponding increase in the absorption was observed as the air pressure was increased. This increase in absorption continued until the air pressure reached 18-22 psi, after which any further increase in the air pressure caused only a small or negligible change in the absorption of manganese. Although the optimum conditions in the four solvents deviated slightly, no significant difference in absorption was noticed when the air pressure varied from 19 t o 21 psi and the hydrogen pressure varied from 6 to 10 psi. Although the relative semitivities of manganese in the solvents were different, the optimum fuel-air ratios were similar. Therefore, 20 psi air and 8 psi hydrogen were used as the optimum fuelair ratios for manganese for all further studies. Burner Effects. The series of curves illustrated in Figure 2 was obtained with a manganese concentration of 100 ppb. The burner heights (listed in mm in the figures and discussion) are inverse to the distance of the burner tip from the light path. Forty-five millimeters is at the tip of the burner (base of the flame) and 0 mm is near the tip of the flame. The corresponding pure solvents were run as a blank. Subtracting the absorption due to the blank from the sample absorption gave net absorption curves for manganese, which increased linearly with increasing burner heights; the usual gaussian type curve was not found. The ground state atoms of manganese are greatest a t the base of the flame and decrease with increasing flame height due to a variety of causes, among them, oxide and/or other compound formation, increasing flame volume and flame temperature. It would seem from these curves that a maximum burner height (base of the flame) would give optimum results for the manganese determination. However, a t this burner height the absorption due to the blank is also a t a maximum, and when scale expansion techniques are used, the blank values are quite significant. Thus, the resulting value is a difference of two large numbers. Therefore, the optimum burner height chosen was where the absorption due t o the blank was zero (water always gave a blank). For acetone, MIBK, and ethanol the burner heights were 20, 28, and 25 mm, re-
spectively, Undoubtedly, some sensitivity is lost by not using the maximum burner height, but the elimination of a n extremely high blank (at maximum scale expansion) justifies this manipulation. For some of the solvents, a negative blank absorption was observed which was detected by the instrument used. This was due to a slight emission of the solvents a t this wavelength. The sensitivity of manganese in the various solvents was determined. The concentration of manganese giving rise t o a 5 relative absorption above the blank was 25, 1, 3, and 15 ppb for water, acetone, MIBK, and ethanol solution, respectively. Five per cent was chosen as the lower limit of detection, rather than 1 recommended by most workers, because of the higher noise level at maximum scale expansion. Acetone appears to be the best solvent. As little as 1 ng/ ml of manganese can be determined in acetone. Solutions of manganese in acetone a t 2, 4, 6, 8, 10, 12, and 16 ppb were run using the optimum conditions listed; the corresponding relative per cent absorption was 12, 19, 25, 35, 42, 48, and 62, respectively. The base line drifted during the time required to run several samples and a blank was run after every two or three samples; a line was drawn from blank to blank and measurements were made from this line at each sample signal. There was no blank signal under the conditions used. The reading for 16 ppb corresponds to about 6.2 % absorption (scale expansion was approximately l o x ) which gives a n absorbance reading of 0.028. This compares with a n absorbance of 0.0017 for 17 ppb manganese in MIBK found by Joyner and Finley ( 4 ) using a premix burner; this was the limit of sensitivity for their measurements, a factor of 10 less than in the present work. Solvent Effects. A plot of the absorbance of 100 ppb manganese solutions cs. the viscosity (in each of the solvents) showed no linear relationship. A linear relationship, however, was observed between the absorbance (log % relative absorption) of the above solutions and the product of density and viscosity (“Handbook of Chemistry & Physics,” 24.0” C). This relationship was used to predict the relative absorption in ethyl acetate of 100 ppb manganese as 41. The experimental value was 40. The slope or shape of a plot of this type will vary with the concentration of manganese in the different solvents due t o curvature of the calibration curves over wide concentration ranges. However, the relative orders of sensitivities obtainable in the different solvents can be predicted from these data. This relationship has not been investigated with other metals. The plot using 100 ppb manganese can be used to predict the absorbance in other solvents; conversely, a n approximation of the viscosity of a solvent can be made if the other parameters are known. The product of the density and viscosity is related to the flow rate although a plot of the flow rate L‘S. the absorbance is not linear.
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RECEIVED for review December 19, 1966. Accepted May 16, 1967. (4) T. Joyner and J. S. Finley, Atomic Absorptiotz Newsletter, 5 , 4 (1966).
VOL. 39, NO. 10, AUGUST 1967
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