Pola rogra p hic Determi nati0 n of Te repht ha Iic Acid in Mixtures of Phthalic Acid Isomers MAYNARD E. HALL and RONALD C. McNUTT' The Chemstrancl C o p , Decatur, Ala.
A polarographic procedure has been developed for the determination of terephthalic acid a t concentration levels as low as 0.1% in mixtures of phthalic acid isomers. The supporting electrolyte is 1 .OM aqueous lithium hydroxide. The first reduction wave of terephthalic acid is used for quantitative measurements, and the repeatability of the method a t the 2% concentration level for triplicate determinations is 10.1 %. A single analysis requires approximately 25 minutes. Only the terephthalic isomer produces two reduction waves in strong aqueous lithium hydroxide solutions.
T
HE GROWING industrial importance of phthalic acid isomers in the manufacture of alkyd resins, plasticizers, and synthetic fibers has emphasized the need for the development of analytical methods for these isomeric acids. A literature survey reveals that the analysis of phthalic acid isomers is mainly by ultraviolet light absorption, polarographic, and precipitation techniques. Also, most of the analytical work reported has been for phthalic acid, with very little work done in isophthalic and terephthalic acids. Swann and Adams ( 5 ) recently reported the use of infrared spectroscopy for the detection of phthalic acid isomers which could materially assist in quantitative analyses by subsequent methods such as ultraviolet absorption spectroscopy. A method based on ultraviolet spectroscopy was reported by Swann, hdams. and Weil (6) for mixtures of the phthalic acid isomers. The concentrations of the acids are calculated by simultaneous equations based on the absorptivities of the acids a t certain mave lengths. Polarographic investigations have been limited almost entirely to ophthalic acid. Furman and Bricker (3) were the first to investigate the polarography of phthalic acid. Their investigations were a t p H levels below 5. At a p H of 4.7 they found no wave due to phthalic acid in a sodium acetateacetic acid buffer system, and below
Present address, Vanderbilt Univer-
sity, Nashville, Tenn.
a p H of 4.7 the reduction waves obtained were believed to be due to the undissociated acid. Recently Hanus and Ryvolova (4) made a polarographic study of phthalic acid and some of its esters, and they reported results very similar to those of Furman and Bricker. Whitnack and Gantz ( 7 ) also made polarographic studies of alkyl esters of phthalic acid. Known precipitation methods are the ASThI procedure (1) for phthalic acid and the Bryce-Smith (2) procedure for the determination of terephthalic acid in admixture with isophthalic acid, which is based on difference in solubility of thallous salts of the acids. This method was adequate for concentrations of terephthalic acid a t the 20% level, but it is doubtful if it would apply to concentrations of 1to 2% or less. I n investigating the polarography of phthalic acid isomers generally, a polarographic procedure for the determination of small amounts of terephthalic acid in mixtures of phthalic acid isomers was developed in strong
lithium hydroxide solution, and this report describes such a procedure. APPARATUS AND REAGENTS
Polarographic measurements were made with a Leeds & Northrup Type E Electrochemograph. Both an Htype and a Sargent KO. S-29398 electrolysis cell with a mercury pool anode were employed. The capillary flow rate was 1.67 nig. per second. The capillary Constant, m2/3t1/6, was 1.88 mg.213 sec.-l/* determined with open circuit. The mercury column, h, was 64 cm. All experimental measurements were made in an air-conditioned room with the temoerature held constant to 25" i 0.5" C: Tetra-n-butylammonium hydroxide, 1.OM. ourchased from Southwestern hnalhical Chemicals, Austin, Tex. Lithium hydroxide, purified grade, Fisher Scientific Co. Phthalic acid, Eastman Khite Label. Isophthalic acid, a highly purified grade made in this laboratory by hydrolysis of the dimethyl ester. Terephthalic acid, Eastman White Label, further purified in this laboratory
Figure 1 . Polarograms of phthalic acid isomers Supporting electrolyte 1 .OM tetra-n-butylamrnonium hydroxide
IO -E 1/2 vs.MERCWPl POOL ANODE VOL. 32, NO. 9, AUGUST 1960
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The acid had an acid titration equivalent to 99.5+% purity. METHOD
For terephthalic acid concentrations of less than 1%, weigh approximately 0.5 gram of sample of phthalic acid isomers into a 50-ml. beaker. Add 10 to 15 ml. of 1.OM lithium hydroxide and stir with a magnetic stirrer at room temperature until the sample dissolves (ca. 5 minutes). Transfer the sample solution quantitatively to a 25-ml. volumetric flask and dilute to the mark with 1.OM lithium hydroxide solution. Transfer a portion of the sample solution to the electrolysis cell and bubble an inert gas through the solution to remove the dissolved oxygen. Record the polarogram between -1.7 and -2.1 volts (US. S.C.E.). Measure the diffusion current and calculate the amount of terephthalic acid present from a calibration curve.
TEREPHTHALIC ACID
PHTHALIC AND ISOPHTHALIC ACIDS
1.60
1.70 180 1.90 2.00 2.10 - E 1/2 VS SCE
Figure 2. Polarograms of phthalic acid isomers in 1 .OM lithium hydroxide
CALIBRATION
Prepare a standard solution of terephthalic acid in 1.OM lithium hydroxide to contain approximately 1.0 mg. of acid per ml. Pipet aliquots of the standard solution into separate 25-ml. volumetric flasks and dilute to the mark with 1.OM lithium hydroxide solution. Obtain polarograms as described above. Measure diffusion currents for concentrations ranging from 0 to 5 mg. of terephthalic acid per 25 ml. of electrolyte solution.
Table 1. Comparison of Theoretical and Experimental Values for Terephthalic Acid
Added, M g . oTerephthalic ture phthalic Phthalic Acid, h k No. acid acid Added Found ~
i
1 2 3 4
~
20.8 61.2 84.7 98.3
78.0 40.0 10.7 0.0
2.00 1.00 5.00 0.50
2.00 0.95 5.10 0.45
DISCUSSION OF EXPERIMENTAL WORK
I n developing a polarographic procedure for the terephthalic acid in phthalic acid isomers i t was necessary first to investigate the behavior of all three acids a t the dropping mercury electrode. For such investigations tetran-butylammonium hydroxide was used as the supporting electrolyte because this electrolyte does not interfere, as does lithium hydroxide, with any of the reduction waves of the phthalic acids. Figure 1 shows polarograms of all three acids in 1.OM tetra-n-butylammonium hydroxide, using a mercury pool anode. Phthalic and isophthalic acids, produce only one reduction wave, while terephthalic acid produces two waves. The reduction waves overlap so much, however, that no quantitative procedure could be developed for any one acid in the presence of the other two isomers. Also in limited studies with tetra-n-
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ANALYTICAL CHEMISTRY
butylammonium hydroxide, it was noted that the height of the first wave was proportional to the concentration of terephthalic acid but not the height of the second wave. However, by using 1. O M lithium hydroxide as the electrolyte, the first wave only of terephthalic acid is separated sufficiently from the other two isomers to permit quantitative measurements in mixtures (Figure 2). Also the sensitivity for terephthalic acid is greater in 1. O M lithium hydroxide electrolyte solution. The diffusion current produced by the first terephthalic acid reduction is dependent on the lithium hydroxide concentration up to l . O M , above which i t remains constant. I n 0.1-44 lithium hydroxide electrolyte the amount of diffusion current obtained for unit
concentration of terephthalic acid is only one fourth the current obtained in 1.0X concentration of base. From a study of electrolyte concentration, the recommended lithium hydroxide concentration for quantitative 11-ork is from 1.0 to 1.55f. The diffusion current was linear with concentrations of terephthalic acid up to 30 mg. per 25 ml. of electrolyte. However, a t these high concentrations (20 to 30 mg.) of terephthalic acid, a maximum was obtained that could not be eliminated with alkyl aryl polyether suppressor. Data in Table I indicate the accuracy that can be obtained for terephthalic acid on analyzing synthetic mixtures of the three phthalic acid isomers. The polarographic method can determine terephthalic acid in concentrations as low as 0.1% in the presence of other phthalic acid isomers. The halfwave potential for the first wave of terephthalic acid in 1.0III lithium hydroxide is approximately - 1.93 volts us. S.C.E. The plateau separating the first and second waves of terephthalic acid as seen in Figure 2 has a fairly steep slope, making the measurement of the total wave height not as precise as for a flat plateau; however, the repeatability a t the 2% level for triplicate determinations was =kO.l%. ACKNOWLEDGMENT
The authors express their appreciation to C. W. Tate for supplying the purified terephthalic and isopthalic acids. LITERATURE CITED
(1) Am. Soc. Testing Materials, Phil-
adelphia, Pa., “ASTM Standards,’’ D 1307 and D 1306,1954. (2) Bryce-Smith, D., Chem. &. I n d . ( L o n d o n ) 1953,244. (3) Furman, N. H., Bricker, C. E., J. A m . Chem. SOC.64,660 (1942). (4) Hanus, R., Ryvolova, A., Chem. listy 50,46 (1956). (5) Swann, M. H., Adams, M. L., AXAL. CHEW30,1322 (1958). (6) Swann, M. H., Adams, M. L., Weil, D. J., Ibid., 27, 1604 (1955). (7) Whitnack, G. C., Gantz, E. St. C., Ibid., 25, 553 (1953).
RECEIVED for review December 9, 1959. Accepted April 18, 1960. Analytical Section, Southeastern Regional Meeting ACS, Richmond, Va., Sovember 5-7, 1959. Contribution N o . 56 from The Research Center of the Chemistrand Corp.
