Determination of Organic Acids in Solutions of Chromic Acid HERBERT H. BARBER, Jr., and WILLIAM
H. CLINGMAN,
Jr.
American Oil Co., Texas City, Tex.
,An improved method for determining monobasic and dibasic acids in hydrocarbon oxidation products provides for a separation of the carboxylic acids from objectionable quantities of inorganic materials and determinations of both total and dibasic acids by liquid chromatography. A technique is provided for identifying the isomeric monobasic acids by mass spectrometry of individual chromatography fractions. Unidentified carboxylic acids are readily classified as monobasic or dibasic acids. The method has been applied to the oxidation products of tert-butylcyclohexane.
m E
k: , .
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-SILICIC ACID
.. ,' .
COARSE GLASS FRIT
1 ,
A
analytical problem arising in the oxidation of organic compounds, such as hydrocarbons or alcohols, is the separation of the products obtained. I n particular, oxidations with chromic acid can give a complex mixture of monobasic and dibasic acids. For example, such mixtures have been obtained from the ozonization of tert-butylcyclohexane in the presence of chromic acid ( 2 ) . Prior to identification and analysis, these acids must be separated not only from the chromic acid but also from each other. In recent years, developments in liquid-liquid partition chromatography have greatly facilitated the analysis of carboxylic acids ( I , 3 ) . I n these methods, 1-butanol-chloroform mixtures are used to elute both monobasic and dibasic acids from a silicic acid column. Chromic acid, however, is also eluted as a broad peak, which interferes with the determination of carboxylic acids. Thus, these procedures are not applicable in the presence of chromic acid. I n addition, several monobasic and dibasic acids, such as glutaric and formic acids, have the same retention volumes, and cannot be individually determined. T o avoid these difficulties, the procedure of Bulen et al. ( I ) has been modified. Chromic acid and other inorganic materials are first removed from the sample. . Two chromatograms are then obtained, one of the total carboxylic acids and one of the dibasic acids alone, the monohasic acids being
cm.1 D
GENERAL
/STOPCOCK
PRESSURE ARM
T NO 2
,VOLUMETRIC
SIPHON
COLLECTOR
RECEIVER
Figure 1.
Chromatographic column
determined by difference. Isomeric monobasic acids in any one fraction are determined by mass spectrometry. The technique makes possible the separation of complex mixtures of carboxylic acids in the presence of chromic acid. APPARATUS A N D REAGENTS
The chromatographic apparatus is shown in Figure 1. All reagents were of analytical reagent grade. Freshly boiled (to remove carbon dioxide) distilled water was used throughout the procedure. The solvent indicator solution was prepared by pipetting 25 ml. of 0.02% phenol red in aqueous solution into 475 ml. of absolute methanol and neutralizing to a pink end point. The silicic acid was sized slightly coarser than that described by Bulen et al. ( 1 ) . Sedimentation of Mallinckrodt's chromatographic grade silicic acid mas repeated about 15 times with the fines being discarded each time. EXPERIMENTAL
Estimate of Total Dibasic Acid. T h e dibasic acid content was first
estimated on a n aliquant portioii of the sample by precipitation of t h e silver salts according t o t h e method of Hoot ( 4 ) . Before addition of the silver were reniol-cd nitrate, Cr04--2and SO*-* as their barium salts and C r + 3 \ins removed by precipitating the hydroxitl(~. The silver salts were dissolved in I.\. nitric acid and Ag+ was volumetricnlly precipitated. Removal of Chromate. Based o n this estimate, a n aliquant sample containing approximately 1.5 nieq. of carboxylic acid was analyzed. Cr201-2 v a s reduced to Cr+3by slowly bubbliiig sulfur dixode into the ice-cold solution. -4fter neutralization with sodium h ydroxide, chromium hydroxide was filtered off and digested twice with two 150-ml. portions of boiling 2Yo sodium sulfate to remove adsorbed organic salts. The combined filtrates were then concentrated by evaporation, transferred to a small Erlenmeyer flask, and evaporated to dryness. The dry sodium salts of the carboxylic acids were free of the undesired chromate, but now \yere contaminated with about 6 grams of sodium sulfate. Removal of Sodium Sulfate. T h e dry salts were pulverized in the Erlenmeyer flask and mixed ivith 24 grams of d r y silicic acid and 12 nil. of 22Y sulfuric acid. A slurry transfer of this mixture in 35 volume Yo of 1butanol in chloroform was made t o t h e t o p of a slurry-packed silicic acid column containing 24 grams of silicic acid buffered with 16.5 ml. of 0.5A. sulfuric acid. Approximately 300 nil. of 30 volume yo 1-butanol in chloroform were required to elute the total carboxylic acids from the column. The carboxylic acids were neutralized with sodium hydroxide and the solution was evaporated to dryness. Chromatography of Total Carboxylic Acids. T h e dry sodium salts of t h e carboxylic acids were crushed and mixed with 2 grams of silicic acid and 1 ml. of 21V sulfuric acid. This mixt u r e was quantitatively transferred t o t h e t o p of a chromatographic column, which had been previously packed with 48 grams of silicic acid and 33 ml. of 0.5N sulfuric acid using t h e method of Bulen et al. ( I ) . Precautions were taken to prevent trapping air bubbles in the top of the column and a glass wool plug was inserted above the sample. Elution was clone by stepwise increases in the concentration of 1butanol. The following solvent schedule VOL. 31, NO. 12, DECEMBER 1959
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Table 1.
Retention Volumes of Saturated Acids
SPECTROMETER
Retention Vol., M1. Mobile Phase
Acid Monobasic Saturated CSisomers Saturated Csisomers Saturated CCisomers Propionic Acetic Formic Dibasic tert-Butyladipic Adipic Glutaric Succinic Malonic Oxalic
GLASS PLUG
78 95
118
200 350 660
ACID
190 470 685 850 940 1200
Figure 2. Mass spectrometer sample container for acid analysis
6'o
r
DIBASIC ACIDS
Table II. Effect of Chromium(ll1) Removal on Organic Acid Chromatography
Organic A4cid, Mea. Anal- ?'4, ReTheory ysis covery
Acids 6-tertButyladipic Acetic Adiuic Foimic
0.597 0.511
85.6
0.277 0.260 93.9 0.299 0.260 87.0 0.279 0 298 106.8 0.395 0.377 95.4 Succinic j Total 1.847 1.706 92.4 4 Listed in order of increasing retention volume. b These acids not separated.
a TOTAL ACIDS
--
Table 111. Effect of Hydrochloric Acid Treatment on -Dibasic Acid Recovery
Acid tert-Butyladipic Succinic Adipic Glutaric
Organic Acid, Meq Before After treat- Treatment ment 0.989 1.018 1.798 1.788 1.408 1.430 1.536 1.550
EVALUATION
% Recovery 103.0 99.4 101.7 101.0
Table IV. Carboxylic Acids from Ozone-Ch romic Acid 0 xi d a tion of fert-Butylcyclohexane
Mole
7-J
Acid Yield 15.6 tert-Butyladipic 14.0 Adipic 9.1 Glutaric 9.5 Succinic 48.2 Total Dibasic acid by silver ion method 46 Propionic Nil Pivalic 3.4 Acetic 8.4 Formic 4.7
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ANALYTICAL CHEMISTRY
solution was added from a dispensing buret to each fraction. Identification of Monobasic Acids. T h e isomeric pentanoic acids were identified b y mass spectrometry of t h e chromatography fraction. T h e sample entry system shown in Figure 2 provided a suitable means of introducing t h e acids into the mass spectrometer. D r y sodium salts of t h e acids were obtained b y titrating the pentanoic acid fraction with sodium hydroxide and evaporating away the solvents. These salts were transferred to the sample entry system and attached to the mass spectrometer. Air was evacuated from the tube prior to adding two drops of 60 to 80% sulfuric acid to the salts. The free acids along with some water vapor were introduced into the mass spectrometer for qualitative identification. The use of 80% sulfuric acid rather than 60% acid did not appreciably reduce water vapor. Concentrated (98y0) sulfuric acid could not be used because it decomposed the salts of the monobasic acids to produce carbon monoxide, carbon dioxide, and sulfur dioxide. Chromatography of Dibasic Acids. T h e dibasic acids were determined by t h e same method of chromatography used for total acids. All t h e fractions except t h e peaks for the pentanoic and butyric acids were composited and evaporated t o dryness. Concentrated hydrochloric acid was added t o t h e d r y sodium salts. Evapoiation in vacuo for 24 to 48 hours a t room temperature completed the removal of the monobasic acids and excess hydrochloric acid. The dry acids were pulverized and quantitatively transferred to the top of the chromatographic column. The same solvent schedule was employed as for total carboxylic acids. Positive identification of dibasic acids was made by infrared analysis of sodium salt mulls ( 5 ) .
FRACTION NUMBER
Figure 3. Analysis of tert-butylcyclohexane oxidation product
was used: 100 ml. of chloroform, 450 ml. of 5 volume yo 1-butanol in chloroform, 200 ml. of 15 volume % 1-butanol in chloroform, 300 ml. of 25 volume yo 1-butanol in chloroform, and 300 ml. of 35 volume % ' 1-butanol in chloroform. About 5 hours were required to make the run. The initial flow of 350 t o 400 ml. per hour dropped somewhat as the concentration of 1-butanol in the mobile phase was increased. Titration of 5.6-ml. fractions was made with 0.01N and 0.05N sodium hydroxide in 90 volume yo methanol. No more than 1.5 ml. of 0.01N sodium hydroxide was added to a fraction before switching to the higher normality solution. Use of two strengths of standard alkali prevented excessive dilution of the indicator. Exactly 2 ml. of neutralized solvenbindicator
OF
PROCEDURE
T o facilitate identification of the carboxylic acids, peak effluent volumes were measured for the individual acids (Table I). These retention volumes are independent of the presence or absence of other acids. The procedure was further evaluated by analyzing a synthetic blend of monobasic and dibasic acids in aqueous solution (Table 11). In addition to the organic acids (O.OlM), the solution also contained chromic sulfate (0.2V) in order to resemble a hydrocarbon oxidation product (2). The chromium was first removed according to the procedure given above, and then the mixture was chromatogra; hed. The recovery of each individual acid was good. The losses were probably due to adsorption on the gelatinous hydroxide precipitate. As indicated in Table I, propionic and tert-butyladipic acids were eluted together, while formic and glutaric acids also had similar elution volumes. Thus, to obtain a complete
,
analysis for each of the acids, it would be necessary to remove the monobasic acids and chromatograph the dibasic acids alone. After obtaining the first chromatograph, the monobasic acids are removed b y treatment of their sodium salts with concentrated hydrochloric acid and subsequent evaporation in vacuo. Independent experiments established that there was no loss of dibasic acids during this part of the over-all procedure. Known weights of tert-butyladipic, succinic, adipic, and glutaric acids were treated separately with 13 ml. of concentrated hydrochloric acid (Table 111). After evaporation, each of the acids was recovered quantitatively. ANALYSIS OF NAPHTHENE OXIDATION PRODUCTS
The procedure described has been applied to the analysis of tert-butylcyclohexane oxidation products where a combination of ozone and chromic acid was the oxidant ( 2 ) . All of the expected dibasic acids were completely separated by the analytical method, and the resolution obtainable is illustrat,ed in Figure 3, which contains chromatograms for a typical analysis. Because monobasic and dibasic acids were not completely resolved, interpretation of the total carboxylic acid
chromatogram (Figure 3) was greatly simplified once the dibasic acid chromatogram had been obtained. Propionic and formic acids could be readily measured by backing out the amounts of the interfering dibasic acids as determined in the second chromatogram. T h e peak corresponding to acetic acid is absent from the dibasic acid chromatogram, indicating complete removal of the monobasic acids using the hydrochloric acid procedure. It was also established, using known blends, that there was no separation of the isomeric C, and Ca monobasic acids. The Ca and Ca fractions can be separated during the first chromatogram, however, and the isomers determined b y mass spectroscopy. The monobasic Csfraction shown in Figure 3 was identified as pivalic acid in this manner. The results of the analysis illustrated are given in Table IV. The silver ion method (4) for obtaining total dibasic acids was in agreement with the chromatography results. Although dibasic acids predominated in the naphthene oxidation product, a complete acid analysis was possible only with the technique described above. CONCLUSION
A liquid-liquid chromatography procedure has been developed for the
analysis of monobasic and dibasic acids in the presence of interfering inorganic substances such as chromic acid. The latter can be removed as chromium hydroxide, with good recovery of the carboxylic acids. I n addition, interfering monobasic acids can be separated from the dibasic acids by evaporating a concentrated hydrochloric acid solution of their sodium salts. A mass spectrometer technique was developed for the positive identification of pentanoic acid isomers. This technique is suitable for the identification of butanoic acids; however, the latter were not present in the products from the oxidation of tert-butylcyclohexane. The total analytical procedure should be generally applicable to reaction systems in which chromic acid or other oxidants are used to oxidize organic: molecules to carboxylic acids. LITERATURE CITED
W.A,, Varner, J. E., Burrell, R. C.. ANAL. CHEJI. 24, 187 (19512). (2) Clingman, W.H., K a d s n o r t h , F. T., Znd. Eng. Chem. 5 0 , 1257 (1958). ( 3 ) Corcoran, G. B., ANAL. CIIEX. 28, 168 (1956). ( 4 ) Hoot, K. F., Kobe, I
