Factors Affecting Line Intensities in the Flame Spectrometry of Metals in Organic Solvents SIR: The use of organic solvents to enhance the intensity of atomic emission has been widely applied to flame spectrometric techniques (1). Gibson (5) investigated the increase in emission resulting from the substitution of acetone for water and indicated what percentage of the enhancement was caused by increased temperature and how much by the more efficient production of the atomic species. In this comparision of acetone and water, the feed rate of both solvents was purposely made identical. The purpose of the research presented here is to evaluate the factors leading to enhancement in the case of two different organic solvents under normal conditions of suction feed. Extraction techniques are usually employed to take advantage of the enhancement of organic solvents. An ‘‘ion exchange” extraction procedure (6) was chosen to show the applicability of this type of extraction to flame spectrometry. When an organic solution of a high molecular weight amine in its free base form is equilibrated with an aqueous acid solution such as HCl, the acid is extracted into the organic phase in the form of an alkylammonium salt. Metal ions which can form anionic species can be extracted into the organic phase according to the following equation : RSNHC1
+ FeC14- S
+ C1-
R3NHFeC14
(1)
Since various metal complexes exhibit different equilibrium constants, selectivity can be obtained by the correct choice of parameters. Such extraction procedures are ideal for flame spectrometry since preliminary separations, made in the extraction step, can eliminate interelement effects which are so troublesome in flame emission work. EXPERIMENTAL
Materials. Alamine-336, a mixture of 55% tri-n-octylamine and 45% tri-n-decylamine, supplied by General Mills, was used as the source of high molecular weight amines. Two different organic solvents, anisole and toluene, were used for the extraction and compared to aqueous solutions of ferric ion. The organic phase containing free amine was equilibrated with an equal volume of aqueous 3M hydrogen chloride. To ensure 1 : l amine-HC1 ratio, excess acid was removed by 1062
ANALYTICAL CHEMISTRY
equilibrating the separated organic phase with two equal portions of 0.3M hydrochloric acid. Potentiometric determination of the salt concentration in the organic phase was carried out in the methanol using standard sodium hydroxide. The organic phase was saturated with respect to iron by equilibrating the amine salt in the diluent with an equal volume of aqueous 0.2M ferric chloride in the presence of 9.3M lithium chloride. Standard solutions of the separated organic phase were prepared by dilution with the appropriate solvent. Apparatus. A Beckman small-bore atomizer-burner supplied with hydrogen and oxygen was used as the excitation source. A Jarrell-Ash onehalf-meter Ebert-mount grating monochrometer Model 82000 equipped with a 1 P 28 photomultiplier tube with associated electronic circuits was used for the line intensity, atomic absorption, and line reversal temperature measurements. Line intensity measurements were recorded by scanning a t 5 A. per minute with a slit width of 0.03 mm. This slit width was found to be wide enough to integrate the total 3720-A. iron line but narrow enough to resolve it from the 3737-A. line (9). For flame absorption measurements, the 2483-A. line (3) from a Hilger and Watts Q379 iron hollow cathode lamp was modulated a t 210 C.P.S. by means of a rotating chopper. A phase sensitive lock-in amplifier was used to amplify the signal from the photomultiplier tube and to isolate the modulated light from the line emitted from the flame. A slit width of 0.03 mm. was used. Flame temperature measurements were made by the line reversal technique. The continuous source was a General Electric 18 A/T 10 tungsten ribbon filament lamp. The brightness temperature of the filament was calibrated a t 6650 A. using an optical pyrometer. A solution of sodium or iron was then aspirated into the flame and the monochrometer scanned through the 3720-A. iron line or the 5890--4. sodium line. The lamp temperature was increased by gradually increasing the current until line reversal was observed on the recorder ( 6 ) . The reversal temperature for iron and sodium lines in each solvent was then calculated in the usual fashion (4). A slit width of 0.015 mm. was found to give an optimum signal to noise ratio. The optimum hydrogen flow rate was found by adjusting the oxygen pressure to 12 pounds per square inch, corresponding to a flow rate of 0.11 cubic feet per minute and aspiration rates of 0.5
ml. per minute of water, 0.5 per minute of anisole, and 0.7 ml. per minute of toluene. The intensity of the 3720-A. line was then investigated as a function of hydrogen flow rate. Optimum hydrogen to oxygen flow ratios of 2.23, 1.64, and 2.0 were observed for the anisole, toluene, and aqueous systems, respectively. Precision pressure gauges and flow meters were used to monitor gas flows. RESULTS
Calibration curves for iron in the organic solutions prepared as previously described were obtained by measuring the peak line intensity of the 3720-A. line over the range of 5-100 p.p.m. The curves were found to be linear for both the anisole and toluene. Enhancement factors of 4.9 and 10.2 were observed for anisole and toluene solutions, respectively, compared to aqueous solutions of ferric chloride. All measurements were made a t a height of 18 t o 20 mm. which gave maximum emission with a rather broad profile. A study was undertaken to find the reasons for these enhancements and the difference between the two organic solvents. The number of metal atoms that are excited a t a given temperature is given by the Maxwell-Boltzmann distribution law : N* N O e - A E / k T (2) where N * and N o represent concentrations in the excited and ground states, respectively. Relative values of the ground state populations of iron atoms in organic and aqueous solutions can be determined by atomic absorption measurements. These experimental values can now be used to calculate the relative line intensities of the element in the solvents using the following equation : Line intensity,,,. - Noolo.~-AE/kToro~ Line intensity,,. N oOP. (3) where the temperature, T,is measured by the line reversal technique previously described. The line reversal temperature of the 3720-A. iron line as well as the 5890-A. sodium line in the three solvents is shown in Table I. I n all three solvents it is noted that the reversal temperatures of the Fe 3720-A. line are higher than the Na 5890-A. line. A similar observation for
Table 1. Reversal Temperatures Measured in Various Solvents
Table 111.
Reversalotemperature K.
WaveSolvent Ele- length, To~ument A. Anisole ene Water Fe 3720 2763 2810 2700 Na 5890 2667 2667 2590 Table II.
Relative Ground State Concentrations
Relative ground state
concn. Relative (NoOrp./ solution Solvent NO,,.) feed rates 1.o Water 1.0 Anisole 3.9 1.0 1.4 Toluene 4 . 5
Relative ground state concn. per ml. 1.0
3.9 3.2
~
iron lines has been previously reported (4). There is some uncertainty in temperatures measured in unshielded flames because of the nonuniform temperature distribution in the plasma. Such temperature measurements, however, can be used to predict relative line intensities (5). Table I1 shows the increase in ground state concentrations of iron atoms for the three solvents. I n the case of anisole, which has the same feed rate as water, the increase results totally from more efficient solvent vaporization and/ or decomposition of the iron compounds. With toluene, the increase in
Solvent Water Anisole Toluene
Calculated and Experimental Intensities of the 3720 A. Line
Relative ground state concn.
Relative intensity change caused by temperature
1.00
1.00
3.90 4.50
the rate a t which the solution is aspirated into the flame accounts for a substantial fraction of the total increase in ground state concentration. However, anisole is a more efficient solvent for evaporation and/or decomposition, in spite of its higher boiling point of 155” C. compared t o 110’ C. for toluene. Using the relative ground state concentrations in Table I1 and the iron line reversal temperatures in Table I, the relative intensities can be theoretically predicted from Equation 3. A comparision of the overall observed and calculated line intensities are shown in Table 111. The experimental results agree to within 10% of the values calculated from the increase in ground state concentration and the temperature increase. The difference between the two values is probably caused by temperature inhomogeneity within the flame. The toluene solutions give peak intensities twice those of anisole even though anisole is a more efficient solvent for yielding metal atoms. However, the
1.35 2.00
Relative intensity
of the 3720-A. line
Experimental 1.00 5.30 9.00
Calculated 1.00 4.90 10 I20
increased feed rate of toluene and the higher electronic flame temperature with this solvent outweighs the higher efficiency of anisole. LITERATURE CITED
(1) Dean, J. A., “Flame Photometry,”
McGraw-Hill, New York, 1960. (2) Dean, J. A., Lady, J. H., ANAL. CHEY.27,1533 (1955). (3) Elwell, W. T., Gidley, J. A. F., “Atomic Absorption Spectrophotometry,” Pergamon Press, Oxford, 1961. (4) Gaydon, A. G., Wolfhard, H. .G., “Flames. Their Structure. Radiation. and Temperature,” Chapman and Hall; Londcin, 1960. (5) Gib!son, J. H., Grossman, W. E. L., Cookt3. W. D., ANAL. CHEM.35, 266 ( 1963j. (6) Good, M. L., Bryan, S. E., J. A m . Chem. SOC.82, 5636 (1960). (7) Moore, F. L., ANAL.CHEM.29, 1660 (1957). JACOB ELHANAN W. D. COOKE Department of Chemistry Cornel1 University Ithaca, N. Y. WORKsupported by the National Science Foundation under Grant GP-1818.
A Trifluoroacetylation-Fluorine-19 Nuclear Magnetic Resonance Technique for Characterization of Hydroxyl Groups in Poly(Propy1ene Oxides) SIR: We wish to describe significant new information in the elucidation of certain structural features of poly(propylene oxides) (PPO) which have not been directly observed before or have been in some controversy. These results were obtained by a trifluoroacetylation-lgF NMR technique (6). We prepared the ditrifluoroacetate esters of propylene glycol and the three dipropylene glycol isomers (by treatment of the alcohols with trifluoroacetic anhydride) and found that the 19F chemical shifts at 56.4 Mc. for the primary trifluoroacetates (TFA) are 15 C.P.S. to lower field than those of the secondary TFA’s. For comparison the proton chemical shift differences a t 60 Mc. between the primary and secondary acetate methyl signals of
propylene glycol and the three dipropylene glycol isomers are about 6 C.P.S. and 0 to 1 C.P.S. (solvent dependency), respectively, at 60 Mc. We have prepared the di-TFA esters of a number of racemic liquid PPO’s ranging in average molecular weights from 250 to 4000 which were prepared by the action of propylene glycol alkoxide on propylene oxide, some PPO’s partially terminated with one or more ethylene oxide units, and several ethylene glycol polymers. We have found that, in every mixture of polymer TFA esters containing both primary and secondary TFA groups, the primary TFA resonances always fall 10 to 17 C.P.S. (56.4 Mc.) to lower field than the secondary TFA resonances. There are two significant results which stem from our studies. First, in
the di-TFA esters of PPO materials (not terminated with ethylene oxide units) which we have studied thus far, we find no significant amounts of primary TFA groups to be present (less than 0.5’% of the total TFA groups). This implies that the hydroxyl end groups of PPO prepared by propylene glycol alkoxide catalyst are predominantly secondary. Other methods have in the past reported varying amounts of primary hydroxyl groups (2, 4, 6). It has been our experience that some commercial liquid PPO materials as received contain easily detectable amounts of primary hydroxyl (as measured by the technique described here) whose concentration can be reduced to an immeasurable level by a short degassing period under high vacuum. Also we have noted that VOL. 38, NO. 8, JULY 1966
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