the site energy distribution curves ( I I ) showed to be the most uniform with respect to adsorption energies. The variation in the column efficiency with surface coverage of 2-propanol on the wide-pore gel is shown in Figure 3. This curve shows that column efficiency increases as the organic layer becomes more dense. Clearly, surface uniformity increases as the coverage increases, and column efficiency and tailing improve as a consequence. As has already been pointed out, covering the suiface also tends to destroy selectivity. Adsorption site energy distribution curves can be directly related to the gas-solid chromatography behavior of the adsorbent. The chemically modified gels having the most uniform site energy distribution profiles (see Figure 6 of Reference 11) also yielded the best gas-solid chromatography efficiency values and the least peak tailing. The best GSC performance was obtained with the wide-pore gel modified with I-decanol, which was shown by the adsorption isotherm studies to have experienced the most marked reduction in high-energy adsorption sites. This work is not extensive enough to provide a clear-cut answer as to why this preparation had the most desirable properties. It had the highest surface coverage, and other results seemed to indicate that degree of coverage is the most important factor, at least in the case of small molecules as treating agents. However, it is also reasonable to believe that the longer and more bulky the hydrocarbon “tail” attached to the surface, especially if it is
long enough to bend over, the more drastic will be the change in surface characteristics. The heat of adsorption for hexane on the unmodified grade 12 gel (Table I) was about 10.7 kcal/mol and for 1-hexene about 12.2 kcalimole. Clearly, the value for 1-hexene was higher because of the specific interaction between the 1-hexene n-bonds and the surface hydroxyls of the adsorbent. Table I1 shows that the heats of adsorption on the grade 62, widepore gels varied from about 7.4 to 8.8 kcaljmol for hexane while the 1-hexene heats varied from about 7.6 to 9.9 kcal/ mole. In general, modification of the wide-pore gel decreased the heat of adsorption of l-hexene; however, there were exceptions. The lower heats for I-hexene on the modified wide-pore gels are probably due to the blocking or elimination of some of the specific interaction sites. Rogers (20) also measured heats of adsorption on various modified silica gels. Some of his modified gels had higher heats of adsorption for benzene than the control gel and a few had lower values. The present work seems to be in agreement with the values reported by Rogers. RECEIVED for review December 29, 1969. Accepted August 24, 1970. This work has been supported by Grants No. GP-4206 and GP-8361 from the National Science Foundation. (20) E. K. Hurley, M. F. Burke, J. E. Heveran, and L. B. Rogers, Separ. Sci., 2,275 (1967).
s Chroma Sidney Siggia and Lee R. Whitlock University of Massachusetts, Amherst, Mass. 01002 Application of pyrolysis gas chromatography is demonstrated for the determination of arylsulfonic acids and their salts. The principal pyrolysis products of sulfonic acids are sulfur dioxide and the parent hydrocarbon. Sulfur dioxide is recovered quantitatively; the parent hydrocarbon is recovered at the 50% level or less. For favorable cases, addition to the pyrolysis sample of carbohydrazide, which readily produces hydrogen atoms at the pyrolysis temperature, significantly increases the parent hydrocarbon recovery. For benzenesulfonic acid, the recovery of benzene i s increased to 98%. A linear response for both sulfur dioxide and the parent hydrocarbon is obtained for a range of sample weights from 0.2 to 1000 pg. Because of the sensitivity of gas chromatography, this method is useful for the direct analysis of aqueous solutions down to the parts-per-million concentration of sulfonate. The presence of sodium sulfate does not interfere. y making sample solutions basic, the interference from sulfuric acid in the sulfur dioxide measurement is eliminated. Because of the multiple-sample capacity of the microreactor-type pyrolysis unit, up to six samples may be analyzed per hour. The optimum pyrolysis temperature is between 710 and 790 “C.
SULFONICACIDS have resisted analysis because of their limited reactivity and lack of volatility. Furthermore, their analysis is often complicated by the presence of sulfuric acid used in their manufacture. Several workers have reported procedures which extend the range of gas chromatography to analysis of
these compounds. Kirkland ( I ) has converted mixtures of alkyl- and arylsulfonic acids to the corresponding volatile sulfonyl chloride and methyl ester derivatives with subsequent analysis by gas chromatography. Reporting a qualitative study, Parsons (2) prepared the sulfonyl fluoride derivatives which he separated by gas chromatography. Siggia, Whitlock, and Tao (3) quantitatively converted mixtures of arylsulfonic acids and their salts to sulfite and phenols by fusion with potassium hydroxide. The resultant sulfite was measured volumetrically, and the phenols were determined by gas chromatography. Ito and Hara ( 4 ) determined together the sulfate and the sulfonate group by reducing the dried sample with a tin(1I)-strong phosphoric acid reagent heated at 300 ‘C for I5 minutes in a closed system. The hydrogen sulfide evolved was determined by gas chromatography. However, no distinction between sulfonate and sulfate could be made. Pyrolysis gas chromatography (PGC) has become an indispensable method for studying nonvolatile compounds. The applications have principally been to identification and structural analysis of synthetic polymers, but studies of other (1) J. J. Kirkland, ANAL.CHEM., 32, 1388 (1960). ( 2 ) J. S. Parsons, J. Gus Chromutogr., 5, 254 (1967).
(3) S,Siggia, L. R. Whitlock, and J. C . Tao, ANAL.CHEM., 41, 1387
(1969). (4) S. Ito and T. Ham, Nippon Kugakii Zussi, 90, 1027 (1969).
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types of nonvolatile materials have also been made. Liddicoet and Smithson (5) have pyrolyzed detergent-range alkylbenzenesulfonates (ABS). The “fingerprint” pyrograms obtained were for the qualitative identification of these complex compounds. Burke (6) applied PGC to aqueous solutions of ABS compounds as a method to differentiate and identify the various kinds of alkyl groups present. Combining pyrolysis and desulfonation by phosphoric acid, Lew (7) mixed samples S and linear alkylbenzenesulfonates (LAS) with either P2Q6or H3POd. Pyrolysis of this mixture at 400 O C resulted in instantaneous desulfonation producing the hydrocarbon intact, with up to 60 % conversion. The method was quantitative for carbon number and isomer distribution. Despite the use of pyrolysis techniques for establishing important structural information, the quantitative measurement of sulfonic acid concentrations using PGC has not been reported. Utilizing a different chromatographic approach, Scoggins and Miller (8) employed a non-ionic porous polystyrene resin for the separation of mono- and disulfonated aromatic compounds. Similarly, Skelly (9) analyzed biphenyl sulfonates by an electrophoresis separation followed by ultraviolet measurements of the separated sulfonates. However, with each approach the discrimination for isomeric or homologous species was insuflicient to permit analysis of these mixtures. Overcoming this problem, Stehl(10) used ion exchange high pressure column chromatography for the fast separation of a number of aromatic sulfonates including some isomeric and homologous compounds. This paper describes a method for the quantitative determination of sulfonic acids and their salts based on pyrolysis gas chromatography and measurement of either sulfur dioxide or the parent hydrocarbon. The sulfur dioxide recovery is quantitative for a large number of sulfonic acids including polysulfonated compounds. The parent hydrocarbon recovery is not quantitative but is reproducible. Analysis based a n measurement of the parent hydrocarbon is possible by comparison to known standards. In favorable cases, the parent hydrocarbon recovery can be increased significantly by adding carbohydrazide to the sample before pyrolysis. For example, the benzene recovery from benzenesulfonic acid is increased from 51 to 98 %. The purpose of the carbohydrazide is to serve as a hydrogen atom donor to the parent hydrocarbon free radical initially formed in the pyrolysis reaction. As a result of the pyrolyzing procedure developed, sample sizes can vary from 1 rng of sulfonic acid down to parts-permillion concentration in aqueous solutions. EXPERIMENTAL
eagents. All sulfonic acids were obtained from Eastman Organic Chemicals in the purest grade available. Benzenesulfonic acid and p-toluenesulfonic acid were used as received. m-Benzenedisulfonic acid (Eastman, Technical grade) was freed from sulfuric acid by conversion to the calcium salt and filtering to remove CaS04. After addition of KzCQ3and filtering to remove CaCQ,, the potassium salt was converted to the acid by ion exchange using Dowex 5OW-8X. All other sulfonic acids were purified by recrystallization of the sodium ( 5 ) T. W. Liddicoet and L. H. Smithson, J. Amer. Oil Chem. SOC., 42, 1097 (1964).
(6) M. F. Burke, P h D . Thesis, Virginia Polytechnic Institute, Blacksburg, Va., 1966. (7) H. E. Lew, J . Amer. Oil Chem. SOC.,44, 359 (1967). (8) M. W. Scoggins and J. W. Miller, ANAL.CHEM.,40, 1155 (1969). (9) N. E. Skelly, ibid., 37, 1526 (1965). (10) R.H. Stehl, ibid., 42, 1802 (1970). 1728
salts from alcohol-water or water alone. The sodium salts were checked for purity by elemental analysis for sulfur. The salts were then converted to the acids by ion exchange. Dodecylbenzenesulfonate was obtained from K 8r K Laboratories, Inc. (Plainview, N. U.)and was used as received, Stock solutions of sulfonic acids were prepared by dissolving the acid in the appropriate volume of water to give solutions containing 0*1- to 0.5-mole/l. Aliquots of these solutions were analyzed by titration with standard sodium hydroxide to a Bromthymol Blue end point. Stock solutions of the sulfonate salts were prepared by dissolving weighed samples of the compounds in appropriate volumes of water to give solutions containing 0.1-moleil. All solutions for pyrolysis were prepared by dilution of the stock solutions. Sulfur dioxide (99.98%) was obtained from J. T. Baker, Specialty Gas Division, commodity No. 6-6481. Silicic acid (325 mesh) was obtained from Fisher Scientific Co. Carbohydrazide was obtained from Olin Mathieson Chemical Corp., New Haven, Conn. All materials for the preparation of analytical columns were obtained from Analabs, Wamden, Conn. Apparatus. PYROLYSIS UNIT. The pyrolyzer was purchased from the Perkin-Elmer Corp., Norwalk, Conn. (Pyrolysis Accessary 154-0825). The construction of this unit is based on a design described by Ettre and Varadi (11). A detailed diagram of the tube-type microreactor pyrolysis unit used in this investigation including all modifications which were made on the commercial unit is given by Siggia, Whitlock, and Tao (3). To obtain an accurate knowledge of the pyrolysis oven temperature, the oven temperature control dial was calibrated at the start of this investigation. This was done by placing a chromel-alumel thermocouple inside the quartz tube at a point coincident with the normal sample boat position. Carrier gas was flowing through the unit when the calibration measurements were made to ensure a true reading. The samples were contained in micro-size platinum boats (11-mm long) supplied by Fisher Scientific Co, The boats were cleaned after each analysis by brisk brushing followed by ignition in an open flame for a few seconds. Gas CHROMATOGRAPH. The pyrolysis unit was connected to a Perkin-Elmer Model 900 gas chromatograph equipped with a thermal conductivity detector. To connect the pyrolysis unit to the Model 900, a special injection port assembly is required (Perkin-Elmer Part No. 009-0276). The chromatographic measurements were recorded with a 1-millivolt Speedomax Model-W potentiometric recorder (Leeds and Northrup, Philadelphia, Pa.). All peak area measurements were made by the method of cut and weigh. Helium flowing at a rate oi‘ 60 ml/min was used as the carrier gas. Procedure. SAMPLE PREPARATION. All samples were prepared for pyrolysis from aqueous solutions. If solid samples are to be analyzed, they should first be dissolved with a known volume of water. A final concentration of no greater than 0.5-1M sulfonate (about 0.1 wt %) should be used. The lower limit of concentration depends only on the sensitivity of the gas chromatographic detector being used. As many as six samples may be prepared and loaded into the pyrolysis unit at one time. To each sample boat was added approximately 3 mg of silicic acid for each 10 pl of aqueous solution that was to be pyrolyzed. For extremely dilute solutions as much as 50 p1 of solution can be used. The sample solution was added using either a 10- or 50-pul Hamilton syringe. The water was evaporated from the samples by placing the boats in a small bell-jar heated to 80 ‘efor about 20 minutes. Vacuum was applied by a water aspirator. Finally the sample boats were placed in the storage area side-arm of the pyrolysis tube. Before the first sample was pyrolyzed, the system was flushed (11) K. Ettre and P. F. Varadi, ANAL.CHEM., 35569 (1963).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
with carrier gas for 4-5 minutes. Manipulation of the boats inside the pyrolysis unit is described elsewhere (3, 11). GASCHROMATOGRAPHIC CONDITIONS.Analytical columns were fabricated from I/,-inch stainless steel tubing. When measuring sulfur dioxide and the parent hydrocarbons of benzene- and toluenesulfonic acid, a 7-foot Porapak Q (SO/ 100 mesh) column was used. The column temperature was programmed from 70 to 220 "C at a rate of 26 "C/min. Pyrolysis products from all other sulfonic acids were measured using a 10-foot column packed with 10% Carbowax 20M on Anakrom ABS (90,400 mesh). The column temperature was programmed from 100 to 240 "C at 26 "C/min. When only the sulfur dioxide pyrolysis product was measured, the Porapak Q column was used for all compounds. The column temperature was programmed from 70 to 140 "C at 26 "C/ niin. The detector temperature was normally held at 200 "C, and the bridge current was 225 mA except when noted otherwise. Quantitation of Pyrolysis Products. PROCEDURE A. To determine the mole per cent recovery of sulfur dioxide from the pyrolysis of sulfonic acids, a calibration curve of moles cs. peak area for sulfur dioxide was prepared. A 50-pl Hamilton gas-tight syringe was used to inject known volumes of sulfur dioxide for the peak-area measurements. The volume used was adjusted to standards conditions of temperature and pressure (0 "C, 760 mm Hg). To convert the adjusted volumes to moles sulfur dioxide, the syringe was calibrated in molesper-$ sulfur dioxide by analyzing the gas volumes delivered from the syringe for total oxygen according to the method of Meade et al. (12) as modified for gas analysis. Mole per cent recoveries of the parent hydrocarbon were determined by peak area comparisons to known hydrocarbon standards run under similar gas chromatographic conditions. PROCEDURE B. A second much simpler procedure used to obtain quantitative data from the pyrolysis products was to pyrolyze known amounts of the sulfonic acid compound being analyzed. Calibration curves were plotted for either moles or weight sulfonate taken US. peak areas for either sulfur dioxide or the parent hydrocarbon (depending on which pyrolysis product was used for the analysis), Samples to be analyzed were then pyrolyzed under similar conditions, and results were obtained by comparison to the calibration curve. This procedure is especially useful for routine analysis of samples where knowledge of the absolute per cent recovery of the pyrolysis products is not necessary. RESULTS AND DISCUSSIOK
The type of pyrolysis unit and the method of sample preparation contributed greatly to the success of this method. A microreactor (tube and boat)-type pyrolysis unit was used in this investigation rather than a filament type unit, because of the following features : Quantitative analyses are easier since the sample size can be accurately measured and varied over any desired range. A change in characteristics of the system due to aging is less likely. The exact temperature of the pyrolysis oven can accurately be measured and maintained constant over a wide range of temperatures. The large ratio of pyrolysis oven mass to sample mass permits considerable latitude in the sample handling procedure. The multiplesample capacity of the pyrolysis unit results in considerably faster analysis times. The unit is commercially available. Microreactor-type pyrolysis units packed with stable materials such as quartz wool, glass beads, and Chromosorb P have been reported (13), but these were used exclusively for (12) C. F. Meade, D. A. Keyworth, V. T. Brand, and J. R. Deering, ANAL.CHEW,39, 512 (1967). (13) M. Verzele, K. Van Cauwenberghe, and J. Bouche, J. Gas Chromatogr., 5, 114 (1967).
direct injection of liquid samples. In these filled reactors, intimate contact of the sample with a large hot surface area is obtained. The same principle was used here by placing a layer of several milligrams of a powdered, inert material (diluent) over the bottom of the sample boat. The sulfonate sample, dissolved in water, was introduced into the boat as a solution and then dried. Water gives no pyrolytic trace and can be used as a solvent without fear of any remaining water present with the sample entering into the pyrolysis reaction. Many organic solvents are not stable and so cannot be used (14).
The effect of the diluent is to give a very high surface area for heat transfer and a thinner film cf sample. These factors tend to decrease secondary reactions and reduce the amount of pyrolysis residue. Also, any dependence of sample size on the course of the pyrolysis reaction is effectively eliminated by the diluent. The choice of diluent was important. Several materials were investigated to ascertain which would give the optimum results. Identical amounts of p-toluenesulfonic acid were pyrolyzed while varying only the type of diluent. The lowest recovery of sulfur dioxide and toluene was obtained when no diluent was used. Diatomaceous earth (Anakrom AB, 100/110 mesh), pumice stone powder, and ground quartz wool increased recovery by 15 to 25%. Alumina (30/200 mesh) and beach sand (- 100 mesh) increased the hydrocarbon recovery, but the sulfur dioxide recovery decreased. The greatest increase in recovery of both pyrolysis products, as much as 50%, was obtained using silicic acid (325 mesh). This diluent was used throughout the investigation. Pyrolysis Conditions. Significant pyrolysis begins to occur to 350 "C for sulfonic acids and at 450 to 550 "C for their salts. The temperature of the pyrolysis oven was raised stepwise in increments of 50" from 400 to 950 "C in order to determine the temperature a t which the greatest recovery of sulfur dioxide and the parent hydrocarbon occurred. No noticeable effect on the pyrolysis product peak shapes nor on the number of peaks produced was observed. Only their magnitude changed as the temperature was increased. The optimum temperature for sulfur dioxide recovery was between 710 and 750 "C while that for the parent hydrocarbon was between 740 and 790 "C. The relatively high pyrolysis temperature required for optimum results is probably due to heat transfer rather than to any correlation between this temperature and bond fission for sulfonic acids. The actual temperature at which pyrolysis occurs is probably well below the equilibrium temperature of the furnace and cannot be measured (15). However, the broad temperature range through which the pyrolysis products do not change in number or character points to the fact that the sulfur-carbon bond in sulfonic acids is significantly weaker than the rest. A typical pyrogram is shown in Figure 1, Only two significant peaks, sulfur dioxide and the parent hydrocarbon, result from the pyrolysis of most sulfonic acids. A third major peak in the pyrogram is water released from the silicic acid diluent. The small unidentified peaks eluting before water are also from silicic acid. Measurement of Sulfur Dioxide. Identification of sulfur dioxide as a primary pyrolysis product was made by trapping the component as it eluted from the gas chromatograph in an aqueous sodium tetrachloromercurate solution. Positive identification of the sulfur dioxide was made according to the (14) T. Wolf and D. M. Rosie, ANAL.CHEM.,39, 725 (1967). (15) F. Farre-Rius and G. Guiochon, ibid.,40, 998 (1968).
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Table II. Analysis of Benzenesulfonic Acid Found, m Taken,
Based on
PLg
so2
985 354 241 106 53 13
991 345 237 104 53 13
Based on benzene 980 348 248 103 52 12
Table 111. Effect of HzSOe and NaOH on the Pyrolysis of Benzenesulfonic Acid. TIME [minl
Figure 1. Pyrogram of ,u-toluenesulfonic acid Peak 1. water; 2. SOz; 3. benzene; 4. toluene
Table I. Analysis of Sulfonic Acids and Salts Based on Measurement of Sulfur Dioxide Standard Analysis* deviaCompound mole, % tion Benzenesulfonic acid 98.5 1.5 m-Benzenedisulfonicacid 99.2b 1.8 1,3,5-Benzenetrisulfonicacid 97.6~ 2.8 p-Toluenesulfonic acid 99.4 1.4 p-Aminobenzenesulfonic acid 98.8 1.7 98.0 2.2 5-Amino-2-chlorobenzenesulfonicacid 99.5 1.2 2-Naphthalenesulfonic acid 97.9 1.9 Na benzenesulfonate Na 2-naphthalenesulfonate 98.4 2.0 Na dodecylbenzenesulfonate 94.Sd 2.2 a Based on 5 determinations. * Based on 2 moles SOz per mole sulfonic acid. c Based on 3 moles SOz per mole sulfonic acid. d True purity of compound was not known.
procedure of West and Gaeke (16). To determine if the pyrolysis residue contained sulfur in any form, six 1.5-mg samples were pyrolyzed and the combined residues were analyzed by elemental analysis for sulfur. None was detected down to 0.01 %. To demonstrate the determination of sulfonic acids by measuring sulfur dioxide, a wide range of compounds was studied. The results of the analyses are given in Table I. Quantitation was done according to Procedure A for sulfur dioxide. Complete recovery of sulfur dioxide was obtained in each case. No effect on the recovery could be observed due to other functional groups present on the molecule. No change in pyrolysis conditions or temperature was necessary to obtain complete recovery from the polysulfonated compounds. Monosulfonic acids yield one mole while di- and trisulfonic acids yield two and three moles of sulfur dioxide, respectively, per mole of sulfonic acid pyrolyzed. The sodium salts, however, required a temperature between 75 and 100 "C higher than that used for the acids. Additionally, when pyrolyzing the sodium salts, approximately double the amount of the silicic acid diluent was used. The standard deviation for five trials was less than 3.0% in each case. The analysis of a (16) P. W. West and 6.C. Gaeke, ANAL.CHEM.,28, 1816 (1956). 1722
Compound HzS04 NaSO4
Moles ( X 106) Taken Found 1.73 1.56 10.5 None 0.535 0.529
Found, % based on
so2
90.2 0 98.8
Na benzenesulfonate NaOHb Benzenesulfonic acid 0.894 0.905 101.4 H2S0dC NaOHd a Pyrolysis temperature, 720 "C. * 2.9 X mole NaOH; sample solution pH, 11.2. 1.4 X lo+ mole H2S04. 1.1 X mole NaOH; sample solution pH, 8.9.
+ +
+
dodecylbenzenesulfonate sample, material commonly used in the manufacture of detergents, is included in Table I although no further assessment of its actual purity was made. The lack of dependence on sample size is demonstrated by the data given in Table 11. Samples of benzenesulfonic acid were pyrolyzed at 720 OC. Calibration of the pyrolysis products according to Procedure B was used to obtain the results. A linear response for amount of sulfonic acid taken us. both the sulfur dioxide and the benzene peak areas was obtained for sample sizes ranging from approximately 10 to 1000 pg. Interferences in the Sulfur Dioxide Measurement. The presence of sulfate, either as sulfuric acid or as the alkali metal salt, is a common interference with many types of sulfonate analyses procedures. To investigate the possibility of sulfuric acid interference, samples of sulfuric acid were prepared and pyrolyzed according to the procedure used for sulfonic acids. The pyrogram showed a large peak which was identified as sulfur dioxide. Further investigation showed that SOzis produced in yields up to 90% from sulfuric acid. Samples of sodium sulfate were pyrolyzed in an identical manner, but no SO2 was produced. These experiments indicated that the sulfuric ac:d interference can be eliminated by neutralizing the sulfonic-sulfuric acid sample with NaOH and pyrolysis of the sodium salts. To determine if the presence of NaOH had an effect on the recovery of Son, samples of benzenesulfonic acid sodium salt were prepared and varying amounts of NaOH were added. The results showed that samples up to p H 11.5 can be analyzed with no decrease in the recovery of SO*. Table 111 shows the data obtained in this investigation. Analysis of PPM Concentrations of Aqueous Solutions of Sulfonic Acids. Few methods exist for the analysis of sulfonic acids at the parts-per-million level in aqueous solutions. Since this method can be used for aqueous solutions and since samples are taken directly without chemical pretreatment or separations steps, analysis of very dilute solutions is possible.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
n
n I
I
I
1
2
3
I
B
U 4 5
TIME \ m i d
Figure 2. Pyrogram from PPM sulfonic acid analysis Pyrogram A. 5.4 ppm benzenesulfonic acid on silicic acid diluent Pyrogram B. Silicic acid diluent alone Conditions: column, 6-ft; 10% Carbowax 20 M; oven temperature, 80 T; detector temperature, 100 "C; detector current, 350 mA; He flow rate, 60 cc/rnin; attenuation, 1X ; pyrolysis temperature 720 "C
T I M E Irntn)
Figure 3. Pyrograms of benzenesulfonic acid with and without carbohydrazide added to the sample Table IV. Analysis of PPM Concentrations of Aqueous Solutions of Benzenesulfonic Acid. Taken, Found, PPm Ppm Found, % 128 124 97.0 72.0 73.8 102.5 37.8 36.0 95.4 5.4 4.8 89.0 a Based on SOn. b Average of 3 determinations. Table V.
Parent Hydrocarbon Recovery of Sulfonic Acids CarboNormal hydraInpyrolyzide crease, Compound sis, % added, Z % 51 98 92 Benzenesulfonic acid m-Benzenedisulfonicacid 21 68 224 1,3,5-Benzenetrisulfonicacid 4 17 325 p-Toluenesulfonic acid 50 56 12 2-Naphthalenesulfonic acid 49 65 33 54 28 - 48 p-Aminobenzenesulfonic acid
Several different standard solutions of benzenesulfonic acid were prepared from a stock solution by successive dilutions. The final concentrations ranged from 5 to 130 pg/ml sulfonic acid. Aliquots of 30 to 50 p1 of each solution were used for analysis. Quantitation was done according to Procedure B. The results of the analyses are given in Table IV. Figure 2 depicts a typical pyrogram obtained in these analyses along with a pyrogram of the diluent alone, under identical conditions. To measure sulfur dioxide a t these levels, it was necessary to set the thermal conductivity detector
Peak 1-water,2-SO2,3-benzene Pyrogram A. Benzenesulfonicacid on silicic acid diluent Pyrogram B. Benzenesulfonic acid with carbohydrazide added to the sample Pyrogram C. Carbohydrazidealone Conditions: column, 7-ft, Porapak Q ; oven temperature, programmed from 70 to 220 "C at 26 'Cimin; detector temperature, 200 "C; detector current, 225 mA; pyrolysis temperature, 790 "C
for maximum sensitivity, The cell temperature was 100 "C and the bridge current was 350 mA. The best results were obtained with the Carbowax 20M column temperature held isothermally a t 80 "C. Substitution of a microcoulometric sulfur detector (17-19) for the thermal conductivity detector should permit the analysis of even more dilute solutions than reported here. Measurement of the Parent Hydrocarbon. Besides sulfur dioxide, the parent hydrocarbon can be used as a measure of the starting sulfonate. The mole per cent recovery for the parent hydrocarbon of several sulfonic acids is given in Table V. For most monosulfonic acids, the recovery was near 50%. For di- and trisulfonic acids, the recovery decreases drastically. No recoveries were quantitative. However, for each, the per cent recovery is reproducible and remains the same over a wide range of sample sizes as was demonstrated by the data given in Table I1 for the analysis of benzenesulfonic acid when measuring benzene. The parent hydrocarbon recovery from ABS materials was considerably lower than 50% because of partial pyrolysis of the alkyl-side chain. (17) R. L. Martin and J. A. Grant, ANAL.CHEM., 37, 649 (1965). (18) E.M.Fredericks and G. A. Harlow, ibid., 36, 263 (1964). (19) P. J. Klaas, ibid., 33, 1851 (1961).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
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A study of the effect of certain modifications on the pyrolysis reaction for the purpose of increasing the parent hydrocarbon recovery was made. The free radical pyrolysis reaction mechanism initially produces, in the case of benzenesulfonic acid, the phenyl radical which then abstracts a hydrogen atom from an unpyrolyzed molecule (or possibly a pyrolysis fragment) giving a benzene molecule. (The SOzOH radical, also initially formed, readily decomposes further to sulfur dioxide and a hydroxyl radical.) The pyrolysis residue remaining is probably a thermally stable polymer formed by the condensation of phenyl radicals. In an attempt to increase the recovery of the parent hydrocarbon, the helium carrier gas was changed to hydrogen in the hopes that the hydrogen molecule could serve as a source of hydrogen atoms for the parent hydrocarbon free radical initially formed. However, no change was observed in the hydrocarbon recovery upon pyrolysis of either benzene- or p-toluenesulfonic acid. This was probably a result of the high bond dissociation energy of the hydrogen molecule compared to the limited reactivity of the aromatic parent hydrocarbon free radical. A second approach investigated was to mix with the sulfonate sample a compound that readily produces hydrogen atoms at the pyrolysis temperature and then forms either a stable free radical or rearranges to a stable compound which would not in any way enter into the main pyrolysis reaction. Compounds of this type include tetralin and cyclohexadiene which aromatize at high temperatures giving naphthalene and four hydrogen atoms or benzene and two hydrogen atoms. Other compounds such as butylated hydroxytoluene and its dimer [2,2'-methylenebis(6-tert-buty1-4-methylphenol)]produce one and two hydrogen atoms, respectively, and form stable free radicals. These materials increased the hydrocarbon recovery, but their use was severely restricted because of their partial pyrolysis producing fragments which tend to interfere with the pyrogram peak area measurements for the parent hydrocarbon.
1724
Finally, hydrazides (known reducing agents) were investigated. Carbohydrazide was found to give two hydrogen atoms per molecule at the pyrolysis temperature (790 "C) but nothing was produced which could interfere with the hydrocarbon measurements since all of its pyrolysis products elute before benzene. Several grams of the material was ground lo a fine powder and 2 to 3 mg was added to each sample boat in place of the diluent. The recoveries of the parent hydrocarbons when carbohydrazide was included with the sample are given in column 2 of Table V along with the per cent increase. Typical pyrograms of benzenesulfonic acid with and without carbohydrazide and of carbohydrazide alone are shown in Figure 3. The benzene recovery from benzenesulfonic acid was quantitative and the usual pyrolysis residue completely disappeared. The benzene recovery from benzenedi- and trisulfonic acid was not quantitative, but the recoveries increased more than 200 and 300%, respectively. Others showed smaller increases, except for p-aminobenzenesulfonic acid which showed a decrease in the aniline recovery. This may be due to reduction of the amino group by the hydrazide. As is evident from Figure 3, pyrogram B, the sulfur dioxide recovery is reduced and cannot be measured when carbohydrazide is used. Best results were obtained when the hydrazide was used alone without added diluent. Hydrazide mixed with various amounts of diluent always resulted in lower recoveries of hydrocarbon. Carbohydrazide is partially soluble in water. Added sample solution adheres to the undissolved particles much in the same way as with the silicic acid diluent. The optimum pyrolysis temperature was not changed by the presence of the hydrazide.
RECEIVED for review July 20, 1970. Accepted September 23, 1970. This work was supported by National Science Foundation grant GP 12171.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
