than 0.5%. Therefore, t h e compensator need be calibrated only monthly. Repeatability. A number of Samples were run in duplicate t o determine the repeatability of the method. Results are listed in Table VI. ~~~
Table VI. Repeatability of Hydrogenation of Unsaturates (Palladium Catalyst)
Sample Meq./g. Butene polymer A 4.31 4.24 Butene polymer B 9.31 8.96 Butene polymer C 6.23 6.28 Butene polymer D 5.17 5.05 Butene polymer E 19.96 19.94 Butene polymer F 2.31 2.35 Hexadecene-la 8.91 8.96 Decene polymer 1.01 0.97 Mixed-olefin polymer 1.61 1.65 42% Octene-2 in xylene 7.01 7.02 Hydrog. cat. poly gasoline 0.05 0.06 Hydrog. cat. poly ~gasoiine 0.24 0.22 Diisobutylene 17.95 18.00 Decenel 13.90 13.90 1-Ethynylcyclohexanol 30.50 30.75 sec-Butyl benzene“ 47.07 46.92 Naphthalene” 79.52 80.00 Standard deviation = 0.12 Chromatographed. Rhodium catalyst used. 0
Time Requirements. The elapaed time per determination will vary with the sample. Pure olefins can be reduced in about 20 minutes, gasoline fractions require 2 to 4 hours, and lowmolecular-weight polymers (about 800) may require 8 to 10 hours. The man-hours per determination will vary with the time required for the hydrogenation and with the number of units operated concurrently. With a dual unit about 1man-hour is required per sample. This could probably be improved by the use of more than two units. ACKNOWLEDGMENT
The author thanks J. R. Glass for valuable assistance in designing the apparatus and for helpful suggestions in the development of the method. LITERATURE CITED
(1) Am. SOC.Testing Materials, D115957T, p. 589, 1957. (2) Balandin, A. A., Khidekel, M. L., Dokl. Akad. Nauk SSSR 123,83 (1958). (3) Bateman, L., Shipley, F. W., J. Chem. SOC.1958, 2888. (4) Braude, A., et al., Zbid., 1954, p. “C“0
%.
0010.
( 5 ) Colson, A. F., Analyst 79, 298 (1954).
(6) Davia, H, S., et al., J . Am, Chem. floc. 54,2340 (1932). (7) Eisch, J., Gilman, H,, C h . Reu. 57 525 (1957). (8) kachinaai, H. E., Bergmann, E. D., J. Am. C h . SOC.72 5651 (1950). (9) Farmer, E. H., Gahey, R. A. E., J. Chem. SOC.1933 (IO) Farmer, E. iTiley, R. A. E., Nature 131, 60 (1953). (11) Gilman, G., Cohn, G., Aduan. Calalysia 9, 733 (1957). (12) Joshell, L. M., IND.ENQ.CHEM., ANAL.ED. 15, 590 (1943). (13) Judd, S. H., Nicksic, S. W., Division of Petroleum Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959. (14) Knight, H. B., et al., J. Am. Chem. Soc. 75, 6212 (1953). (15) Lebedev, S. V., et al., J. Chem. SOC. 1925, p. 417. (16) Zbid., 1930, p. 321. (17) Ogg, C. L., Cooper, F. J., ANAL. CHEM.21, 1400 (1949). (18) Savacool, R. V., Ullyot, G. E., Zbid., 24, 714 (1952). (19) Swan Tiong, S., Waterman, H. I., Chzm. Znd. (Paria) 81 (2), 204 (1959). (20) Zbid., 81 (31, 357 (1959). (21) Unger, E. H., ANAL. CHEM. 30, 375 (1958). (22) Vandenheuvel, F. A., Ibid., 24, 847 (1952). (23) Waterman. H. I.. et al.. J . Znat. Petrol. 42,349 (1956). ’ RECEIVEDfor review April 28, 1966. Accepted July 25,1966. Presented at the First Middle Atlantic Regional Meeting, ACS, Philadelphia, Pa., February 1966.
e
~
Behavior of 2,4-Dinitrobenzenesulfonic Acid as an Acid Catalyst in Acetylation Reactions DONALD J. PIETRZYK and JON BELISLE Deparfment o f Chemistry, University of Iowa, Iowa City, Iowa
b The catalytic behavior of 2,4dinitrobenzenesulfonic acid (DNBS) i s examined in acid-catalyzed acetylation reactions and compared to HCI04 and p-toluenesulfonic acid (PTS). It is observed that DNBS and HC104 are superior catalysts to PTS. Perchloric acid appears to be better than DNBS only when the acetic anhydride concentration or when the acid catalyst concentration is very low. This trend follows the acidic strength exhibited by the three acids. Quantitative results for the acetylation of a variety of amines, alcohols, sugars, and polymers using DNBS as catalyst are reported. In addition to being strongly acidic, DNBS has other advantages which suggest its use over HC104. DNBS is a solid, readily available and purified, it appears to be safe and stable when heated in organic mixtures, and does not result in a highly colored acetylating mixture. Its notable disadvantage is a side reaction that occurs when aniline derivatives 1508
ANALYTICAL CHEMISTRY
52240
are acetylated in the presence of DNBS is superior to PTS heat. (HCIO4 cannot be used) for analysis of polymers of certain structures.
T
HE ACETYLATION and related re-
actions have been used for a long time for the analysis of hydroxyl and amine functional groups. A major advance in this area in recent years has been the use of acid catalysis with emphasis on HC1O4 (4). This method which proceeds more rapidly than the previous ones and is carried out at room temperature has been used for alcohols (4), phenols, thiols, and amines (a, ketoximes and vic-dioximes (Q),alkoxysilanes (?), mercaptosilanes (I), and micro analysis of hydroxyl group (IO). The principal disadvantages of the HClO4-acetic anhydride method are its inability to be used at elevated temperatures, potential hazard if carelessly used, and color formation with time. Experiments in our laboratory (8) on
the acidity of aromatic sulfonic acids in nonaqueous media indicated that several of the nitro substituted acids are close in acid strength to HClO,. It was then of interest to examine 2,4dinitrobenzene sulfonic acid (DNBS) as an acid catalyst in the acetylation reaction. The sulfonic acid previously used, p-toluenesulfonic acid (PTS) (4, IS), is a weaker acid than HC104. This resulted in a milder acetylating reagent which required a longer reaction time or heat to keep the reaction time short. I n addition to being strongly acidic, DNBS has the advantages of being a solid, readily available, readily purified if needed, and appears stable to heat. The latter property along with its strongly acidic nature suggests that DNBS would be very suitable &s a general acid catalyst. I n this report, data are presented which show that DNBS can be used LU a replacement for HC1O4 in acidcatalyzed acetylations. The experiments also serve as further evidence for
the highly acidic character of DNBS and its usefulness as a general strong acid catalyst. EXPERIMENTAL
Reagents, Inorganic reagents and organic solvents were obtained in good grades from readily available sources. Purification of p-toluenesulfonic acid (PTS), obtained from Eastman Organic Chemicals and 2,4-dinitrobenzenesulfonic acid (DNBS), obtained from Eastman and Pfister Chemical was previously described (8). If the DNBS is highly colored, it should be treated with decolorizing charcoal so that the DNBS-acetylating reagent would have only a slight yellow color. White label acetic anhydride was obtained from Eastman and the ethyl acetate from Fisher Scientific Co. and distilled from PzOs. Alcohols and amines were obtained from readily available sources and were purified in certain instances. The sources of the polymers are listed in the tables. Solutions. For the time and varying acid and acetic anhydride concentration studies, stock solutions were made in ethyl acetate at concentrations such t h a t when a 5-ml. aliquot was pipetted into a 3-ml. aliquot of an amine or alcohol solution, the concentration for the mixture as listed in the tables was obtained. Stock solutions of the amine or alcohol in ethyl acetate were prepared so that a 3-ml. aliquot delivered 4 mmoles of the hydroxyl or amine group. Sodium hydroxide of about 0.6JI was prepared by mixing 125 ml. of saturated, carbonate free, aqueous NaOH, 285 ml. of water, and 3600 ml. of methyl alcohol. This was standardized against potassium acid phthalate. The indicator was an aqueous mixture of 1 part of 0.1% neutralized cresol red and 3 parts of 0.1% neutralized thymol blue. For quantitative measurements the acetylating reagent contained 3.8 ml. of 72% HCI04, 6.8 grams of DNBS, or 4.6 grams of PTS, 33 ml. of acetic anhydride, diluted to 100 ml. with ethyl acetate. The sulfonic acids were each first added to a small amount of ethyl acetate, the anhydride was added and the solutions were allowed to stand for 15 to 30 minutes. The rest of the ethyl acetate was added, and the final mixture was allowed to stand for an additional 30 minutes. Caution. The addition of reagents for the HC104 acetylating reagent is that of Fritz and Schenk (4), and it is important to follow their recommended procedure and handling of the solutions to avoid hazardous conditions. When 5 ml. of the acetylating solution is mixed with 3 ml. of the ethyl acetate containing the sample, the final concentration of the acid and anhydride are 0.14M and 2.211i, respectively. Procedure. Weigh accurately a sample containing approximately 4 mmoles of hydroxyl or amine into a 125-ml. glass-stoppered flask and pipet in 3 ml. of ethyl acetate t o dissolve Or, pipet accurately the sample.
Table 1.
Per Cent Reaction as Function of Time and Acid Catalyst
Reaction, %
Reaction time, min. 5
Compound Methyl alcohol
5 15 5 15
Cy clohexanol
into the flask a 3-ml. aliquot of a n ethyl acetate solution which contains 4 mmoles of t h e functional group. Pipet into the mixture exactly 5 ml. of the acetylating reagent and allow the reaction to proceed for the prescribed amount of time. Add 1 to 2 ml. of water, shake the mixture, and add 10 ml. of 3 to 1 pyridine-water solution. The hydrolysis of the excess anhydride should be complete within 5 minutes. Titrate the mixture with NaOH titrant using the indicator color change of yellow to violet to mark the end point. A reagent blank containing identical aliquots is determined in the same fashion, and the difference between the blank and the sample is used for the calculation. If the reaction requires heat, use an iodine flask and place the flask in a water bath maintained a t 65" C. The reservoir of the flask should be moistened with the pyridine-water solution or ethyl acetate to prevent loss of the volatile anhydride and acetic acid. The recommended procedures and reagent mixtures described are similar to those of Fritz and Schenk (4). These should be consulted if a more detailed description is required. DISCUSSION
The reaction path for HC104-catalyzed acetylation of an alcohol is portrayed in the following steps (2, 3). AcnO
+ H+
AcZOH+ Ac+
+ ROH
-+AczOH+
Ac+
(1) (2)
+
(3)
+ HOAC ROAc + H +
DNBS
PTS
81.6
81.1
16.1 25,5 8.0 13.8 6.1 11.1
84.8 87.2 87.2 72.8 72.8
15
%Propyl alcohol
HCIO.
The importance of the acidity of the acid catalyst which supplies the proton to the anhydride molecule and eventual production of the acetylium ion is clearly demonstrated. Perchloric acid as the source of the hydrogen ion in acid-catalyzed acetylations has been extensively studied (4-6). The effect of HC104 concentration, type of solvent, reaction time, anhydride concentration, and the hydrolysis of the acetylating reagent has been established. Studies on the catalytic ability of several monoprotic acids which are all weaker acids than HC1O4 in nonaqueous media were also reported. Initial experiments were designed to show the catalytic ability of DNBS and compare it to HC1O4, an acid of similar strength, and to PTS, a weaker acid.
82.4 87.2
87.4 73.2
73.2
The per cent reaction for the acetylation of several common alcohols in ethyl acetate solvent as a function of time for the three acids is reported in Table I. The acetylating reagent when added to an aliquot of the alcohol sample solution resulted in a mixture containing 0.44M acetic anhydride and 0.037M acid in ethyl acetate solvent. This mixture in relation to the mmoles of alcohol is not sufficient in acetic anhydride concentration to cause 100% reaction (about a 1.04 alcohol to anhydride molar ratio). Therefore, the extent of catalysis is easily discernible. The results show that HC104 and DNBS are very similar in their behavior and are superior to PTS. When the sample size of the alcohol was reduced by one third, quantitative results were obtained for the HC104 nd DNBS reagents in 5 minutes, wh e for PTS the per cent reaction increased to 25.9, 12.9, and 10.5% for methyl alcohol, 2-propyl alcohol, and cyclohexanol, respectively. The catalytic ability of the three acids is further illustrated by considering the effect of acid concentration on acetylation and is reported in Table 11. In these experiments the scetylating reagent when added to an aliquot of the sample solution resulted in a mixture containing the acid concentration listed in Table I1 and 2.2M acetic anhydride in ethyl acetate. The reaction time was 5 minutes. At 0.0013M or higher acid concentration quantitative results were obtained for the four compounds for DNBS and HC104 as the catalyst. On the other hand, when using PTS, quantibtive results were obtained a t only high acid concentration and only for three of the compounds which again illustrates the superior catalytic activity of DNBS and HC104. Although not shown, similar results were obtained for 0isopropyl phenol which, like phenol, was only partially acetylated by PTS catalyst. At 0.0013M acid there is an indication of a difference in the catalytic activity of HCIOa and DNBS. For example, in the case of cyclohexanol, although not quantitative in 5 minutes, there is a greater per cent reaction for HClOd than for DNBS catalyst. It is also interesting to note that in the absence of an acid catalyst extensive reaction still takes place in the case of
a
VOL 38, NO. 1 1, OCTOBER 1966
1509
Table
II.
Per Cent Reaction as Function of Acid Catalyst Concentration
Reaction, % Molarity
Acid PTS DNBS HC104 PTS DNBS HClOi PTS DNBS HClOi DNBS HC104
0.14 0.05 0.01 0.0013
...
0
Ethyl alcohol
Phenol
Cyclohexanol
+Ethylaniline
99.3 97.4 98.8 66.7 101.5 98.1 16.2 99.3 97.8
88.9 102.7 97.8 71.8 101.4 98.7 68.2 103.4 97.3
98.8 98.7 101.7
99.1 100.5 102.1
98.1 101.3 5.0 101.0 100.3 52.7 85.2
4.6
68.2
97.3 102.5 98.7 98.7 99.4 101.1 99.6 98.6
phenol and o-ethylaniline, the latter being quantitative. The phenol either serves in part as its own acid catalyst and reacts via the phenoxide ion while apparently bme catalysis due to the amine accounts for the latter reaction (11).
Table 111. Per Cent Reaction as Function of Acetic Anhydride Concentration at Constant Acid Catalyst Concentration
Acetic
2.2 1.8 1 .o 0.64
Anhydride to
3.8 3.1 1.8 1.1
98.3 99.4 98.5 99.0
101.9 99.8 99.4 33.5
Table IV. Acetylation of Hydroxyl and Amine Compounds Using Different Acid Catalysts"
Compound Methyl alcohol Ethyl alcohol n-Propyl alcohol 2-Propyl alcohol n-Butyl alcohol %Oct 1 alcohol ~yc~o%exanol %Methyl cyclohexan01 Phenol o-isc-Propyl phenol' S,&diis+Propyl phenolb %Methyl-&-tertbutyl phenolb Galactosec *Ethyl anilineb N-Methyl-ptoluidine" pphenetidine"
Recovery, % HClO4 DNBS 98.6 98.9 99.2 98.3 101.3 98.8 101.7
97.1 96.8 98.9 99.1 98.7 99.2 98.7
99.2 98.7 105.6
99.3 98.7 100.6
101.0
98.6
106.6 96.2 102.1
100.7 96.5 100.5
91.7 99.8
93.2 100.0
a Reaction time of 10 minutes except where noted. b Samples obtained from Ethyl Corp. =Reaction time of 30 minutes for the sugar and 20 minutes for the amines.
1510
e
ANALYTICAL CHEMISTRY
Differences in catalytic activity of HClO4 and DNBS can be seen by comparing the per cent reaction found for different concentrations of acetic anhydride. Data for ethyl alcohol are reported in Table I11 for a bminute reaction time. I n these experiments the acetylating reagent when added to an aliquot of the alcohol sample solution resulted in a mixture containing 0.005M acid and the acetic anhydride concentration listed in Table 111. In the case of HClO4, quantitative results were obtained for all concentrations of acetic anhydride. If the anhydride concentr% tion is 0.64M, or an anhydride to alcohol ratio of 1.1, quantitative results are not obtained for the alcohol when using DNBS. Similar results were found for the acetylation of cyclohexanol. As a result of these data, it is concluded that HC104 and DNBS are superior acid catalysts to PTS. On the other hand, differences between HCIOl and DNBS are seen only near the breaking point of the reaction; that is, at very low acid concentration or a t very low anhydride concentration. The observed catalytic activity follows the acidic strength exhibited by the acids in nonaqueous media (8) where HCIOa and DNBS were found to be very strong acids with the former slightly stronger and both considerably stronger than PTS. One of the limitations of the perchloric acid acetylating reagent is its color development with time ( 4 ) . This characteristic was not a problem with the DNBS reagent as color developed only slightly with time. The acetylating power of the reagent did not appear to be affected over a 2-week period. Aliquots were taken of concentrated alcohol or amine solutions for the previous data in order to maintain a constant sample weight. T o further demonstrate that DNBS is capable of catalyzing the acetylation reaction in the same manner as HC1O4, quantitative results, averages of two or more analyses on directly weighted samples, are re-
ported in Table IV for primary and secondary alcohols, phenols, a sugar, and anilines, some of which are sterically hindered. The sampIes were dissolved in 3 ml. of ethyl acetate, and the acetylating reagent was added. The final concentrations were 0.14Macid and 2.2M acetic anhydride. No attempt was made to purify most of the samples. The fact that the results for DNBS are similar to HC104, the stoichiometry of which has been well established, demonstrates its quantitative behavior. Several of the compounds were also acetylated in the presence of PTS and as before only partial acetylation occurred for the phenols which again substantiates the superior catalytic activity of HC104and DNBS. A major limitation in the use of HClO4 as an acid catalyst in acetylation as well as other acid-catalyzed reactions is that it can't be heated in organic mixtures. Therefore, a catalytic combinstion of a strong acid and heat can never be realized with HClO4. DNBS, which has been shown to be near in acid strength to HC104 (8) and to be almost equivalent to HC104 in catalytic ability a t least in acetylation, does not suffer from this problem. We have in purification procedures heated concentrated solutions of the acid to near dryness a t steam bath temperatures and to a few waters of hydration in a heated rotary evaporator. In addition to the heat experiments described here, we have refluxed DNBS, acetic anhydride, ethyl acetate and anisole mixtures for up to 1 hour. Consequently, DNBS appears to be safe and stable to heat a t least under the described conditions. Experiments on the effect of heat on the rate of acetylation using DNBS as catalyst are reported in Table V. It was found that many of the aniline derivatives precipitate as sulfonate salts (8) prior to acetylation when using the DNBS reagent. Therefore, the extent of reaction for the nitroanilines is very low for the unheated DNBS reagent, quantitative for the unheated HCI04 reagent where no precipitation occurs, and increases significantly for heated DNBS reagent. The heating tends to overcome precipitation, and quantitative results can be obtained if the reaction time is controlled. For example if 30 and 10 minute reaction times at 65' C. are exceeded for p - and onitroaniline, a side reaction occurs which consumes acid (or anhydride) and high results are obtained (Table V). This side reaction occurred only in the case of the anilines. For example, phenol and ethyl alcohol were heated a t 65' C. for 4 hours without any effect on the stoichiometry of the reaction. The sugars were found to have low solubility in ethyl acetate. Again heating, in this case gentle warming on a hot plate, shortens the reaction time
by increasing the solubility. There WM no evidence of charring. Glycerol dissolves very slowly in ethyl acetate, and if continuous stirring is not employed when using the HC104 reagent, low results are obtained. Gentle warming of the sample in the presence of DNBS again shortens the reaction time. I n all of the previous examples heat is used to overcome precipitation or low solubility in ethyl acetate. Consequently, the true effect on rate of reaction is not easily discernible. 2,6Diethylaniline was found to be soluble in ethyl acetate and slow reacting which is probably due to steric hindrance. The reaction of this compound with unheated acetylating reagent and heated (65' C.) DNBS reagent is reported in Table V as a function of time. Unheated, there is little difference between HC104 and DNBS as catalyst. However, when heated in the presence of DNBS the increase in the rate of reaction is clearly discernible. A side reaction is also observed as in the case of the other anilines. It is interesting to note that PTS as catalyst caused a similar side reaction when heated. Experiments with other solvents were not performed. Fritz and Schenk (4) report several other solvents as alternates to ethyl acetate. Our experience (8) has shown that the anilines studied formed sulfonate precipitates when titrated with DNBS in all the alternate solvents except triethyl- or tributylphosphate. The acetylation reaction has been applied to polymer analysis. Stetzler and Smullin ( I S ) used p-toluenesulfonic acid as a catalyst for the analysis of hydroxyl content in polyoxyalkylene ethers. Complete acetylation was obtained in 15 minutes a t 50' C. They confirmed the fact that the strong acid catalyst, HC1O4,would not work for this type of structure due to cleavage of the ether linkage. Siggia ( l a ) suggests oxidation of the ether linkage as the source of high results for hydroxyl content. DNBS was then examined as an acid catalyst for the acetylation of the hydroxyl group in macromolecules, summarized in Table VI, since it is a strong acid like HC104 but does not possess the oxidizing power of HClO4. Concentration of the DNBS reagent was similar to that used in Table IV. The stoichiometry of the reaction was checked by measuring yo OH for a fixed sample weight as a function of time (up to 1 hour) and then as a function of sample weight for a fixed reaction time. Per cent hydroxyl for comparison was also measured by another acetylating procedure (la). The Span, Tween, and Carbowax samples which were similar to those examined by Stetzler and Smullin ( I 3) were quantitatively acety-
lated when using DNBS as catalyst at the first reaction time (10 minutes) studied. Additional time or change in sample weight did not affect the stoichiometry. Span 60 and 65 are monc- and tristearates of sorbitan, respectively, while Tween 20 and 40 are polyoxyalkylene derivatives of sorbitan monolaurate and monopalmitate, respectively. Carbowax 1000 is a polyethylene glycol of approximate molecular weight 1OOO. No breakdown in the ether linkage by using the strong acid catalyst DNBS was observed. The Pluronic samples of approximate molecular weight 2250 are polyethers like Carbowax. They differ, however, in that they are block polymers with polyoxyethylene groups on both ends of a polyoxypropylene chain. Acetylation as a function of time (only part of the data is reported in Table VI) revealed a side reaction. The increased yoOH, which was accompanied by color formation, is probably due to reaction
Table V.
Compound pNitroaniline +Nitroaniline Ethyl alcohol Phenol Glycerol Lactose Galactose
a t the oxygen-secondary carbon bond.
It is interesting to note the fast rate of acetylation and that Pluronic L-62 which has 10% more polyoxyethylene has less of an interfering side reaction for a given time than L61 which tends to support the suggested side reaction. Acetylation of polyvinyl alcohol (Elvanol 52-22 and 71-30 obtained from D u Pont) was attempted with DNBS as catalyst. Unfortunately, the data are difficult to evaluate because of insolubility of polyvinyl alcohol in ethyl acetate. If the acetylation is attempted a t room temperature, no reaction is evident, while at elevated temperature (55' C.) acetylation did occur. However, it was accompanied by a change in the sample to a large solid mass. This caused poor and fleeting end points in the titration which WM probably due to trapping of the solution in the solid mass. The longer the reaction time, the worse the titration became. At the lower reaction times (10
Effect of Heat on Acetylation
Reaction time, min. 10 30
HClO4 95.3
10
101.5
30 240
Reaction, % DNBS 30.8 51.4 4.1 10.1
240
15 15 15 30
88.15
77.1
97.9
81.9
96.2
96.5
95.1b
15
30
DNBS (65' C.) 76.1 104.2 101.7 119.7 97.8 100.2 94.95 93 * 45 97.35 97.P 97.4"
2,6-Diethylanilined
Solution stirred initially when acetylating reagent added. Continuous stirring by magnetic stir bar. Gentle warming on hot plate. d Sample obtained from Ethyl Cor . ' 133.9% reaction for PTS under ties, conditions. a
Table VI.
Compound Span 60' Span 65b Tween 20b Tween 40b Carbowax 1000~ Pluronic L61d
10
10 10
10
10 2
5
Pluronic G62d
Acetylation of Polymers
Reaction time, min.
DNBS 7.32 2.06
Hydroxyl, % Py-( Ac0)~Oa 7.40
3.18 2.94 3.45 2.07
10 20 2
5
2.01
3.11 2.92 3.39 1.42
1.26
10 2.09 2.49 20 Pyridine-acetic anhydride (3: 1); reaction time 75 minutes at 55" C. Atlas Chemical Industries. 0 Union Carbide Corp. * Wyandotte Chemicals.
V O L 38, NO. 1 1 , OCTOBER 1966
151 1
and 20 minutes) approximate % ’ OH could be calculated but is not reported since the true yo OH is unknown, nor could reproducible results be obtained with the pyridine-acetic anhydride reagen t . ACKNOWLEDGMENT
The authors thank Ethyl Corporation, Atlas Chemical Industries, E. I. du Pont de Nemours and Co., Union Carbide Corp., and Wyandotte Chemicals Corp. for data sheets and generous samples of several of the compounds, and Alan D. Wilks for help with some of the analyses.
LITERATURE CITED
(1) Berger, A., Magnuson, J. A., ANAL. CHEW36, 1156 (1964). (2) Burton, H., Praill, P. F. G., J. Chem. SOC.1950. 1203. (3) Ibid., 1951,-522. (4) Fritz, J. S., Schenk, G. H., ANAL. CHEM.31, 1808 (1959). (5) Ibid., 32, 987 (1960). (6) Gutnikov, G., Schenk, G. H., ANAL. CHEM.34, 1316 (1962). (7) Magnuson, J. A., ANAL. CHEM.35, 1487 (1963). (8) Pietrzyk, D. J., Belisle, J., Ibid., 38, 969 (1966). (9) Schenk, G. H., Ibid., 33, 299 (1961). (10) Schenk, G. H., Santiago, M., Microchem. J. 6, 77 (1962).
(11) Schenk, G. H., Wines, P., Mojzis, C., ANAL. CHEM.36, 914 (1964). (12) Siggia, S., “Quantitative Organic
Analysis via Functional Groups,” Third
Ed., Wiley, New York, 1963. (13) Stetsler, R. S., Smullin, C. F., ANAL.CHEM.34, 194 (1962). RECEIVEDfor review Ma Accepted July 20, 1966. & n % i J g z sistance came from a grant .(GM 123106-01) from the National Institutes of Health and from a Du Pont Fellowship (1965-1966) for one of the authors (JB). Presented in part at the First Midwest Regional American Chemical Society Meeb ing, Kansss City, November 4-5, 1965.
Models for the Analytical Applications of Dialysis Using Differential Kinetics R. F. BROMAN‘ and R. C. BOWERS2 Department of Chemistry, Northwestern University, Evansfon, 111. Mathematical expressions describing the dialysis of two-component mixtures of nonelectrolytes and of three-component mixtures of univalent ions are presented for both finite and infinite bath techniques. Experimental verification of these equations for single electrolyte dialysis has been carried out. Model calculations made using the expressions cast doubt on the general analytical utility of the method of dialysis based on differential kinetics for the determination of components in a mixture. Optimum values of the concentration ratio of components in the mixture and differences between diffusion coefficients of the components are discussed. The dialysis method of analysis appears to be most useful when the components are present in nearly equal concentrations and their diffusion coefficients differ by a factor greater than two. Brief experimental work with potassium chloride-hydrochloric acid mixtures supports these conclusions.
S
Hanna, and Serencha (2f,88) have applied the technique of dialysis using differential kinetics to the determination of components in binary and ternary mixtures. They reported good results for the analysis of such mixtures as nitric acid-acetic acid, potassium chloride-potassium bromide, and nbutylamine-tert-butylamine. The dialysis method was applied in a manner analogous to previous work with differential chemical reaction rates as analytical tools (16), and the differential physical reaction rate of dialysis was
6020 1
described using the mathematics of chemical reaction rates. The mathematical treatment of parallel first-order chemical reaction rates (9) is identical with that for the rate of dialysis of mixtures of nonelectrolytes, but it has long been recognized that ionic mixtures do not diffuse in the same manner ( f a ) . McBain and coworkers (18, 23) have studied in some detail the coupled diffusion of ions within a concentration gradient in a sintered glass disk. Earlier workers have noted the process of coupled diffusion of ions in solution (19, 20) and in membranes (4, 7 , 14, f7). If the dialysis technique is to be analytically useful, the theories must account for this coupled ionic diffusion. An appraisal of the theoretical models has been made here in an attempt to predict the conditions under which the technique may be satisfactorily utilized. A brief experimental verification of the theory and experiments of the type reported by Siggia, Hanna, and Serencha (21), designed to test the general analytical usefulness of their technique, is also presented.
IOGIA,
1512
ANALYTICAL CHEMISTRY
THEORETICAL
Solution of Fick’s second law for the appropriate boundary conditions leads to the general mathematical equation for diffusion through a membrane. Barnes (1) and Dvorkin (8) have solved the problem for dialysis of a single nonelectrolyte (which is also applicable to single electrolyte dialysis). These solutions include expressions for the initial establishment of a finite concentration gradient across the membrane. Steady-state
flux equations for coupled ionic diffusion have been derived by Gose (fl), who considered the diffusion of three ionic species. A steady-state solution to the dialysis problem is most convenient for our purposes. Initial establishment of the concentration gradient occurs rapidly, especially for thin, wide-pored membranes and for solutes with relatively large diffusion coefficients. The time required for the attainment of steady state is negligibly small compared to the time over which the entire dialysis experiment is conducted. The following assumptions have been made in the derivations: The membrane does not possess any ion exchange properties; activity coefficients are taken as being equal to unity; diffusion coefficients are independent of concentration; the thickness of the membrane is small enough so that the membrane volume is negligible compared to the solution volume; diffusion is restricted to the membrane; and for the electrolyte case, no electric current flows through the dialysis cell. It has also been assumed that specific solute-solute and solute-membrane interactions as well as osmotic effects would be accounted for in the magnitude of the membrane diffusion coefficient, an integral diffusion coefficient. Solutions have been obtained by solving the general flux equation 1 Present address, De artment of Chemistry, University of gebraska, Lincoln, Neb. 68508 2Present address, College of Liberal Arts and Sciences, Northern Illinois University, DeKalb, Ill. 60115
