contrast, the silicon-to-gallium count ratio did respond to change in aspiration rate of gallium solution but showed an insensitivity similar to the silicon-to-aluminum ratio above a critical aspiration rate of approximately 1 ml/ min. The relative standard deviation was 6.0% for flows of 1.0 to 2.48 ml/min. Matrix Effects. Matrix effects in spark analysis have been studied previously; however, matrix effects, especially interelement effects, are generally treated empirically. The recent use of atmospheres other than air (14-16) tends to reduce surface oxidation, which has been shown to account for some matrix effects (17), but the liquid layer technique offers another approach. For determination of silicon in aluminum alloys, a single analytical curve obtained by using the liquid layer technique replaced a number of analytical curves required without the liquid layer. The curves in Figure 10 are ex-
(16) C. K. Matocha, Appl. Spectrosc., 22, 562 (1968). (17) D. M . deWaal and A. Strasheim, Specfrochim. Acta, 3, 141 (1948)
amples. Preliminary studies of superalloys also have shown reduction of matrix effects when the liquid layer technique is employed (9). These alterations of matrix effects may result from the protective nature of the liquid layer, the interaction of solvent decomposition products, or the increase in current density a t the electrode surface caused by the liquid layer. These possibilities are presently under study. Received for review December 18, 1967. Resubmitted May 29, 1973. Accepted July 25, 1973. This paper is based on results from the thesis of RMB submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy, University of Illinois, 1966, and work performed a t the National Aeronautics and Space Administration’s Lewis Research Center, Cleveland, Ohio, as reported in NASA T M x-1429. Portions presented a t the 18th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1967. Work was supported in part by NSF Grants GP18910 (to HVM) and GP25909 (to RMB).
Quantitat ive Nuc lear Magnetic Resonance Analysis of Mixtures of Isomeric Arenesulfonic Acids in Sulfonation Mixtures H a n s Cerfontain, Ankie Koeberg-Telder, Cornelis Kruk, and Cornelis Ris Laboratory for Organic Chemistry, Nieuwe Achtergracht 129, University of Amsterdam, The Netherlands
Mixtures of isomeric arenemono- or disulfonic acids can be determined quantitatively by NMR spectrometric analysis in the excess of sulfuric acid reagent itself or, if another sulfonating reagent is used, in deuterium oxide as solvent. This analysis can, in most cases, be performed without requiring the constituting sulfonic acids as reference compounds. The method is based on solving a set of linear equations obtained from the area of the various absorptions of the unknown mixture. 1,2-DisuIfoarenes exhibit different PMR spectra in fuming and in concentrated aqueous sulfuric acid, as they are present as 1,2disulfonic anhydrides in the former solvent, but as the 1,2-disulfonic acids in the latter one. With mixtures of arenedisulfonic acids in which the presence of 1,2-disulfoarenes is suspected, it is therefore useful to determine the PMR spectrum in both fuming and concentrated aqueous sulfuric acid.
The quantitative separation of small amounts of sulfonic acids present in a sulfonation reaction mixture from the excess of sulfuric acid seems very unattractive, thus making impossible gas chromatographic analysis of the sulfonic acids after their conversion to sulfonyl chlorides (fluorides), or to sulfonic esters. Because of the excess of sulfuric acid, liquid-liquid chromatography of the reaction mixtures as such does not seem an appropriate method of analysis either. The quantitative analysis of mixtures of arenesulfonic acid isomers in aqueous or sulfuric acid solution can conveniently be performed by the ultraviolet spectrophoto72
ANALYTICAL CHEMISTRY, VOL. 46,
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metric multicomponent analysis ( I , 2). This method of analysis is based on a comparison of the UV absorption spectrum of the mixture with the spectra of the constituting sulfonic acids. Thus, the pure components of the sulfonic acid mixture are .required as reference compounds. This, of course, restricts the applicability of the analysis, as the synthesis of isomerically pure arenesulfonic acid is often difficult and tedious (3, 4). The infrared spectrophotometric multicomponent analysis of mixtures of nitrofluoroanthenes has been reported (5). Another spectrometric method on which a multicomponent analysis could be based is NMR. The quantitative analysis would then be based on observation of the NMR spectrum of the mixture and of the spectra of the constituting sulfonic acid components in the same solvent, and subjection of the absorptions of the mixture and of the constituents at a large number of selected chemical shifts to a least squares treatment by an electronic computer ( 1 ) . However, the chemical shifts of the various NMR absorption peaks are concentration dependent, and this would lead to a low accuracy of the analysis. Furthermore, the constituents of the mixture would again be required as reference compounds. We recently obtained a satisfactory quantitative analysis of a large number of mixtures of isomeric arenesulfonic J. M. Arends, H. Cerfontsin, I . S. Herschberg. A. J. Prinsen, and A . C. M. Wanders.Ana/. Chem.. 36, 1802 (1964). H. Cerfontain, H. G. J. Duin, and L. Vollbracht, Ana/. Chem.. 35, 1005 (1963) E. E. Gilbert, Synthesis. 1969, 1. J. E. Cooper and J. M. Paul, J. Org. Chem. 35, 2046 (1970) A. Streitwieser, Jr., and R. C. Fahey, J . Org. Chem.. 27, 2352 (1962).
acids by PMR spectrometric analysis without requiring the constituents themselves as reference compounds (612). The isomeric composition of mixtures obtained on sulfonation of disubstituted (6, 9-11) and trisubstituted ( 1 1 ) benzene derivatives was determined on the basis of the aromatic hydrogen pattern, in conjunction with the absorption pattern of the benzyl hydrogens, if present. With a sulfonation mixture of a monosubstituted benzene derivative, the relative abundance of a t least one of the three isomers can often be determined (13, 14). However, depending on the nature of the substituent, in some cases even a complete isomeric analysis can be obtained (7, 8, 1 5 ) . The aim of the present paper is to define the scope, the limitations, and the underlying principles of this quantitative method of analysis of isomeric arenesulfonic acids. Isomeric arenesulfonic acid mixtures usually result from aromatic sulfonation (16). Therefore, in addition, recommended procedures for sulfonation and the required NMR sample preparation are described. Recently Zanger has stressed the potentialities of PMR for the determination of the substitution pattern of an unknown benzene derivative ( I 7).
EXPERIMENTAL Materials. The preparation and purification of dipotassium 2,5-, 3,4-, and 3,5-toluenedisulfonate has been described (11). Apparatus. The NMR spectra were obtained with a HA 100 and an XL 100 Varian spectrometer, using the Spectro System Mu'lti Scan Averaging Technique when the arenesulfonic acid concentration was < 2 wt 70.The spectra were recorded in the sweep frequency mode under conditions far below the level of saturation. For the PMR spectra of the sulfuric acid solutions, a solution of sodium 2,2,3,3-tetradeutero-3-(trimethylsilyl)propionate (TTP) (15 wt %) in D20 or TMS (in a sealed capillary) was used as an external standard. For the PMR spectra of DzO solutions, TTP was used as an internal standard. For the fluorine NMR spectra, CC13F was used as an external standard. The chemical shift of pure T M S external (capillary) is -0.42 ppm relative to external TTP in DzO. Shielding P a r a m e t e r s . The shielding parameters of the sulfo groups on the substituents X were determined as follows. The chemical shift of the substituent X in a given compound was measured for three concentrations of that compound in between 0.05 and 1.0 moles/l. The chemical shifts varied linearly with the concentration. The chemical shift at infinite dilution was obtained by linear extrapolation to zero concentration. The chemical shift at infinite dilution was obtained by linear extrapolation to zero concentration. The chemical shift at infinite dilution of the corresponding non-sulfo group-containing compound was taken to be that of the saturated solution of that compound [the solubility of the aromatic hydrocarbons and their fluorine derivatives is very small ( I S ) ] .The shielding parameters were obtained by subtraction of the chemical shifts of the non-sulfo containing compounds from those oft he corresponding sulfo compounds. H . Cerfontain. A . Koeberg-Telder, and W . A . Zwart Voorspuy. Can. J. Chern.. 50, 1574 ( 1 9 7 2 ) . 2. R. H. Nienhuis, W . J. Spillane, and H . Cerfontain, ibid., 50, 1591 ( 1 972). A . Koeberg-Telder. Z. R. H . Nienhuis, and H. Cerfontain, ibid.. 51, 462 (1 973) H . Cerfontain, 2. R . H. Nienhuis, and W . A. Zwart Voorspuy, J. Chern. Soc.. B. 1972,2087. H . Cerfontain. A . Koeberg-Telder, and E. van Kuipers, ibid.. 1972,
2091 A . Koeberg-Teiderand H . Cerfontain, ibid.. 1973, 632. A. Koeberg-Telder and H. Cerfontain, Red. Trav. Chirn. Pays-Bas, 91, 22 ( 1 9 7 2 ) . A. Koeberg-Teider and H . Cerfontain. Red. Trav. Chim. Pays-Bas, 90, 193 (1971). M . P. van Aibada, A . Koeberg-Telder, and H. Cerfontain, ibid.. 91, 33 (1972). K . Mashimo and T . Wanai, Jap. Anai.. 21, 1079 (1972). H . Cerfontain, "Mechanistic Aspects of Aromatic Sulfonation and Desulfonation," Interscience. New York, N.Y.. 1968. M. Zanger, Org. Magn. Resonance, 4 , 1 (1972). L. M . Jackman and S. Sternhill, "Applications of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry," 2nd ed., Pergamon Press, New York, N . Y . . 1969, p 202.
Procedure. Test mixtures were made up by mixing appropriate amounts of standard solutions of the toluenedisulfonates in sulfuric acid, such that the solutions contained a total amount of 7 a t 70of toluenedisulfonic acids. Then the NMR spectra were recorded and the area of the various absorptions integrated three times, in order to minimize statistical errors. Recommended Sulfonation Procedures. To determine quantitatively the composition of an unknown isomeric sulfonic acid mixture, formed on sulfonation of an aromatic substrate, the following procedures for sulfonation, working up, and sample preparation are recommended. Sulfuric Acid Sulfonation. To an excess of sulfuric acid of the desired strength and temperature is added 2-10 wt 70of the aromatic substrate and the mixture is left until the monosulfonation is complete. Then the NMR spectrum of the sulfuric acid solution as such is recorded. Upon formation of an arenedisulfonic acid mixture in fuming sulfuric acid, in addition to the NMR spectrum of the oleum solution, another spectrum is recorded after dilution of the solvent to 95-997c HzS04. Other Types of Sulfonation (cf. 23, 14). To a given amount of substrate, either contained in a proper solvent or not, is added an appropriate amount of the desired sulfonating reagent ( e . g . , sulfur trioxide, chlorosulfuric acid, fluorosulfuric acid, disulfuric acid, or pyrosulfonic acid) as such. or contained in the same solvent. After completion of the sulfonation, the reaction mixture is quenched with and taken up in deuterium oxide. In case of turbidity, the aqueous solution is refluxed for 30 min to hydrolyze the insoluble sulfonic anhydrides. Then the NMR spectrum of the clear solution is recorded.
PRINCIPLES The quantitative analysis of a mixture of isomeric arenesulfonic acids by NMR is subject t o two basic requirements. First, the NMR spectrum of the mixture t o be analyzed consists of separated absorption regions and. second, the nuclear relaxations are such that the areas of the various absorptions are proportional to the number of nuclei involved. The second requirement renders 13C NMR unattractive for the present type of analysis. For due to the magnitude of, as well as the significant differences in, the relaxation times of the various 13C nuclei with different environments, the corresponding 13C KMR areas are in general not proportional to the number of nuclei involved. A further requisite for a quantitative analysis is that the spectra of the components of the mixture are predictable. This requirement is met for arenesulfonic acids. as the chemical shift of the substituents, such as H, F, CH3, and CF3, in a benzene derivative can be calculated by comparison with a suitable reference compound on the basis of additivity of substituent shielding parameters (6, 17). A tabulation of shielding parameters on aromatic hydrogen has recently been published (18). Data for sulfonic substituents are not available in the literature. They were determined for Dz0 and 85% H z S 0 4 as solvents by comparison of the zero concentration extrapolated chemical shifts of the PMR signals of a mixture of sodium 3.5-di- and 2,4,6-tri-deuterobenzenesulfonate(19) and the chemical shift of the PMR singlet of a saturated solution of 1.3,5trideuterobenzene (20). The dependence of the chemical shift of an aromatic hydrogen on the sulfuric acid concentration is very small for benzene, the difference for 0 and 80% H2S04 being only 0.18 ppm (21). The results are in Table I. The overall shielding effect of two sulfo substituents on aromatic hydrogen was also determined. It is strictly additive in the case of p - and n-benzenedisulfonic acid, but with the ortho isomer, deviations from additivity are observed. For concentrated sulfuric acid up t o 98% H2S04, the shielding parameters for 85% HzS04 can be applied satisfactorily (6). (19) J. K. Bosscher and H. Cerfontain.J. Chern. SOC.P . 1968, 1524. (20) C. Ris, Thesis (in English), University of Amsterdam, 1973, pp 13, 14. (21) H. Cerfontain. R e d Trav. Chim. Pays-Bas, 84, 491 (1965).
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Table I. Shielding Parameters at Infinite Dilution of Sulfonic Substituents on Aromatic Hydrogen, Methyl, Fluoro, and Trifluorornethyl Shielding parameters, A6,ppm Substituent
Solvent
2-so33-so34-so32,3-(so3-)z 2,4- (so3- ) z 2 s - (so3- ) 2 3,4-(so3-) z 2-S03H/S03-a 3-S03H/S03-a 4-S03H/S03-a 2,5-(S03H/S03-)2 2-S03H/S03-b 3-S03H/S0~-~ 4-S03H/S03-b 2,4-(S03H/S03-)? 2 3 - (S03H) z 3,4-(S03H) 2 2,3-SOzOSO23,4-SO2OSOz-
-H
Dz0
4-0.39 4-0.13 4-0.18 4-0.68
-0.95 -2.03 -4.75
-CF3
f 0.07 f 0.06 f 0.10
-1.47
f 0.05
f0.02 4-0.32 f 0.02 4-0.30 f 0.02 4-0.08 f 0.02
4-0.22 f 0.02 4-0.44 f 0.04 4-0.17 f 0.04 4-0.29 f 0.04 4-0.65 f 0.04
8 2 % HzS04
4-0.28 4-0.09 4-0.10 4-0.39
f 0.03 f 0.03 f 0.03 f 0.03
4-0.91 f 0.02 4-0.56 f 0.02 4-0.87 f 0.02 4-0.87 f 0.02
CD3N02
a The ratio C6H5S03H/C6H5S03- in 85% HzS04 is
1.4 (22). The ratio ArSOsH/ArS03- for the three toluenesulfonic acids in 82% HzS04 is 1 .O (22)
Shielding parameters of the sulfo substituents on CH3, F, and CF3 were also determined and are given in Table I.
4
4-0.26 f 0.02 4-0.06 f 0.02 $0.05 f 0.02
4-0.50 f 0.02 85% HzS04
-F
-CH3
f 0.02 f 0.02 f 0.02
I
I
I
8
I
6
For a NMR spectrum of a mixture of m isomeric arenesulfonic acids which contains n well separated regions of absorption, it follows for the area A of regionj:
/ A=
(iplxi
+ / P ? x ?+ ... : P , x , ) S i
=
j
=
(1)
1,2, ... m 1,2, ... n
x L is the mole fraction of isomer i in the mixture, ,pL the number of magnetically equivalent nuclei of isomer i which absorb in area j , and S a proportionality factor. A complete quantitative analysis can thus be obtained if Equation 1 contains a t least m independent equations. If the number of independent equations is however smaller than m, then, in general, the mole fraction of a limited number of components can still be obtained [ ( c f . Table I11 and (13, 1 4 ) ] . I
9
I
I
I
I
I
I
I
0
I
.t -b6.ppm.
/ I
,
Q
Figure 1. PMR spectra of the sulfonation mixtures of 3-bromo- and 3fluorobenzenesulfonic acid in 115% HZSOI as such (6 and D) and after dilution to 98% H2S04 ( A and C )
74
I,
II
All 9
I
8
Figure 2. PMR spectra of the test mixtures of Table I I
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1
2
Table 111. PMR Analysis of Sulfonation Mixtures of 3-Halogenobenzenesulfonic Acids in 115% H2S04
Table II. Analysis of Mixtures of Toluene-2,5-, -3,4- and -3,5-Disulfonic Acid in 104% H2S04 Mixture A, %
Isomer distribution of disulfonic acid products, %
Mixture 8.%
Isomer
Takena
Found
Takena
Found
2,53,43s-
34.5 28.8 36.7
33.6 29.3 37.1
34.6 47.1 18.3
31.3 50.4 18.3
2.5-
The total concentration of toluenedisulfonic acid in the mixtures A and B is 0.461 and 0.460 mol/l. 104% HzS04. a
The PMR aromatic hydrogen pattern of 1,2-disulfoarenes is different for fuming sulfuric acid (Le., 2100% and concentrated aqueous sulfuric acid (90-100% H2S04) as solvent, since they are present in the form of 1,2-disulfonic anhydrides in the former solvent, but as 1,2-disulfonic acids in the latter one ( 1 1 ) . This is illustrated for the disulfonic acid mixtures formed on sulfonation of 3-bromo- and of 3-fluorobenzenesulfonic acid in 115% HzS04 (Figure 1). With some substrates, the number of sulfonic acid isomers to be considered in the NMR analysis can be less than the maximum number of possible isomers because of large differences in reactivity between the various positions available for sulfonation as result of electronic or steric factors. The applicability of the present method of NMR analysis probably mainly depends on the relatively large shielding parameters of the sulfo substituents. It is expected therefore, that the method may also be applied successfully to isomeric mixtures of other aromatic compounds which contain a t least one substituent with a large shielding parameter in e . g . , arenesulfonyl chlorides and nitroarenes [cf. ( 2 3 ) ] .
3,4-
11 f 4 75 f 4 75fl 1112 11 f 4 46f3 13f2 48f2 54 f 5 9 f 2 46f3
3,5-
14f 2 14fl 43f3 39f3 46 f 5 45f4
Ref.
(6) (6)
(6)
fore thought of interest to apply the present analysis to ternary mixtures of these toluenedisulfonic acids. The results of these analysis are collected in Table 11. As expected from the assigned positions of the absorptions of the aromatic hydrogens and the methyl groups in the spectra of the test mixtures (Figure 2), the determination of the three components is very satisfactory. The applicability of the PMR analysis of a sulfonation mixture both in fuming and concentrated aqueous sulfuric acid is illustrated for the disulfonic mixtures formed on sulfonation of three m-halogenobenzenesulfonic acids in 115% H2S04. From the PMR spectrum of the to 98% H z S 0 4 diluted reaction mixture, a complete analysis in terms of the 1-halogeno-2,5-, -3,4-, and -3,5-benzenedisulfonic acids was obtained only for the fluorine and chlorine derivatives, whereas for the bromo compound only the relative amount of the 3,5-disulfonic acid was obtained (6). However, from the PMR spectra in 115% HzS04 a complete analysis was obtained for all three halogen compounds (Table 111). The agreement between the data from the analysis in 98 and 115% HzSO4 is satisfactory.
RESULTS Recently we showed that sulfonation of n-toluenesulfonic acid in fuming sulfuric acid leads to the formation of 2,5-, 3,4-, and 3,5-toluenedisulfonic acid (11). It was there(22) H. Cerfontain and B. W. Schnitger, Red. Trav. Chim. Pays-Bas, 91, 199 (1972). (23) T. Karneo, T. Hirashima, 0. Manabe, and H. Hiyarna, Kagaku To Kogyo (Osaka), 41,333 (1967): Chem. Abstr., 69, 160559 (1968).
ACKNOWLEDGMENT The authors are indebted to Mary Steeneken-Boomgaard for recording the NMR spectra, and to P. J. van der Haak for helpful comments. Received for review March 16, 1973. Accepted August 27, 1973.
Inductively Coupled Plasma-Optical Emission Analytical Spectrometry A Compact Facility for Trace Analysis of Solutions Robert H. Scott,’ Velmer A. Fassel,2 Richard N. Kniseley, and David E. Nixon Ames Laboratory-USAEC and Department of Chemistry, lowa State University, Ames, lowa 50070
This paper describes a compact inductively coupled plasma-optical emission system for the trace determination of metallic elements in solution. Theoretical considerations are presented to determine operating parameters which agree well with the empirically determined values. The aerosol desolvation system commonly used with this
type of source has been eliminated, and pneumatic nebulization is employed in place of the more elaborate ultrasonic method. Some characteristics of the plasma are reported. Detection limits are in the range 0.1 -1 0 ng/ml for most elements studied. The present facility is readily adaptable to simultaneous multielement trace analysis.
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