~~
of the methylene ureas. Although MMU and DMU cannot be differentiated, the best overall resolutions were given by Solvent B and C. By these two solvent systems, each of MMU and DMU was eluted into two spots, one of which was due to urea. This is probably the result of the equilibrium reactions MMU e U F, D M U e U MMU which were accelerated by the acidic medium. The R , values decrease in the order MDU > DMTU > TMTU. It is therefore expected that methyleneureas of molecular weight greater than that of TMTIJ, possibly formed in the U F reaction, would appear at the bottom of the chromatogram. On cellulose TLC plate, each compound studied was separated clearly. It is noted that the cellulose TLC has an advantage over the silica gel TLC in that U, MMU, and DMU can be resolved without any ambiguity. In addition, MMDU can be detected on cellulose plates whereas on silica gel plate it was probably hydrolyzed by the acidic solvent medium. This suggests that MBMU is more stable than MMDU. Using the cellulose TLC technique, we have been able to detect higher homologs of methylolurea (e.g., trimethylolurea) which will be reported in a next communication. U, MMU, DMU, MDU, and DMTU are the main components often found in the U F resin. The PC technique was found useful in a low-cost semi-quantitative analysis of these compounds b 4 the standard cut-and-weigh method. The detection limit of the chromogenic reagents used with silica gel plates was about 0.1-0.5 pg for the compounds studied. p-Dimethylamino benzaldehyde (DAB) will form Schiff-type bases with primary amino group and could form a color compound with -NH- group with much less intensity. As a result it is not surprising that it could not detect compounds containing only tertiary nitrogen. The reaction of chromotropic acid (CA) with formaldehyde is well known. It may show color with compounds which could be oxidized to formaldehyde or compounds having - C H r group. Thus it can be used to detect methylol and methylene ureas. For this reason CA may be useful in detecting U F condensates which may only contain tertiary nitrogens and methylene groups. However, CA does have one poor aspect. Heating the TLC plate following spraying with CA produces an overall
+
+
Table I. Rf ( x 100) Values of Urea and Its Derivatives Substance Urea (U) 43 Monomethylolurea (MMU) 49 Dimethylolurea (DMU) 53 Bis(monomethylo1urea) methyleneether (BMME) 0 Methylene diurea (MDU) 38 Dimethylenetriurea (DMTU) 27 Trimethylenetetraurea (TMTU) ... Methylenebismethylolurea (MBMU) ... Meth ylolmethylenediurea (MMDU) . .. Whatman Chromapaper No. 1.
43
34
17
44
56
40
55
39
27 47
60 14
0
0
0
0
30
19
10
33
14
16
4
24
4
4
2
...
24
22
13
...
. ..
...
10
...
violet background, and spots of low color intensity can be obscured. Nevertheless, DAB and CA are supplementary to each other. It has been found very helpful in the TLC analysis of U F compounds that were sprayed with DAB first and then with CA to check if anything was missed by DAB. The TLC and PC techniques mentioned above have been used to advantage in our laboratory in the analysis of U F resins and moulding compounds. In these cases the methods were limited to the aqueous extractable materials only. ACKNOWLEDGMENT
The author thanks the Directors of Monsanto Australia Limited for permission to publish this paper; E. F. Jackson and D. A. Young for their interest and encouragement; and P. Mizzi for his technical assistance in parts of this work. RECEIVED for review October 15, 1971. Accepted January 6, 1972. Presented in part at the Adelaide Symposium 1971, Division of Analytical Chemistry, Royal Australian Chemical Institute.
Analyses of Potassium Terephthalate-Benzoate Mixtures by Column Chromatography M. W. Scoggins Research and Development Division, Phillips Petroleum Company, Bartlesville, Okla. 74004 THE CHEMICAL ANALYSIS of mixtures of aromatic mono- and dicarboxylic acid is a difficult problem because of the similar acidities of the materials. The difficulty is accentuated when the mixture consists of alkali salts of these acids contaminated with large amounts of alkali carbonate. Although organic acids are readily determined by GC analysis of their methyl esters or trimethyl silyl esters, the technique is not generally applicable to organic carboxylates (1-3). In addition, solu(1) G. G. Esposito and M. H. Swann, ANAL.CHEM.,34, 1048 (1962). (2) 0. S. Duron and A. Nowotny, ibid., 35, 370 (1963). (3) Alan E. Pierce, “Silylation of Organic Compounds,’’ Pierce Chemical Company, Rockford, Ill., 1968, Chap. 6.
bility problems are often encountered when the salts are converted to the free acids. Ultraviolet spectrometry has been used to resolve organic carboxylic acid or salt mixtures (4, but generally the measurement of absorbance values at different wavelengths and then solving the resulting set of simultaneous equations is not desirable for control-type analyses. We have developed a rapid method for the separation of the simple aromatic dicarboxylates from benzoates and applied it specifically to the analysis of mixtures of potassium ter(4) M. H. Swann, M. L. Adams, and D. J. Weil, ANAL.CHEM.,27,
1604 (1955).
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Table I. Analysis of Terephthalic-Benzoic Acid Blends Weight % Added Foundd TerephTerephBlend thalic Benzoic thalic Benzoic 1. 72.1 12.5 72.2 12.4 2 27.3 60.9 27.9 57.8 3b 49.2 10.6 48.6 ... 4 78.6 11.4 77.7 ... 5 30.2 5.5 30.0 ... 6 20.8 18.0 20.5 ... 7c ... 9.87 79.0 10.2 Std dev 0.54 0.78 Blends 1 and 2 contain process catalysts. b Blends 3-6 contain KOH and KzC03. c Process potassium terephthalate sample low in potassium benzoate and blended to contain 9.87 weight benzoic acid. d Average of 3 to 6 determinations. Q
Table 11. Analyses of Crude Process Potassium Terephthalate Potassium terephthalate, weight % Blended with benzoic acid to Sample Found contain Found Difference A 74.2 56.1 56.1 0.0 57.6 58.1 +0.5 B 67.5 48.4 48.6 +0.2 B 67.5 Table 111. Determination of Benzoic Acid in Terephthalic Acid Benzoic acid, weight Sample Added Foundo Difference, 0.080 97.7 1 0.082 0.13 100.0 2 0.13 0.18 100.0 3 0.18 4 0.28 0.28 100.0 0.39 106.7 5 0.36 6 0.50 0.48 96.0 0.53 100.0 7 0.53 0.95 95.0 8 1.OO Std devb 0.0071 Av recovery 99.4% a Average of 2 determinations. Includes 33 sets of duplicates from process samples. ephthalate and benzoate. Other phthalate isomers, if present, act similarly to terephthalate. In addition, a modification of the procedure allows the determination of less than 0.1 weight per cent benzoic acid in isomeric phthalic acid mixtures. The materials are separated by column adsorption chromatography and the eluted fractions are analyzed by ultraviolet spectrophotometry. EXPERIMENTAL Reagents and Apparatus. Rohm & Haas Amberlite XAD-2 resin was used as column packing (5). Absorbance measurements were obtained with a Cary Model 11 recording spectrophotometer using 1-cm quartz cells. Borosilicate glass columns, 1.2 X 25 and 1.8 X 35 cm, with a solvent reservoir at the top were used for analyzing salt mixtures and high purity terephthalic acid, respectively. The preparation of the column beds has been described previously (6). Procedure, Analysis of Mixtures. Transfer a 5-ml aliquot of sample, containing approximately 5 mg of potassium ( 5 ) Rohm and Haas Company, Philadelphia, Pa., Bulletin IE-89-65.
(6) M. W. Scoggins and J. W. Miller, ANAL. CHEM.,40, 1155 (1968). 1286
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benzoate-terephthalate mixture and saturated with NaC1, to a resin column which has been pretreated with saturated NaCl solvent. Adsorb the sample on the resin and elute with saturated NaCl solution at a flow rate of 2 to 3 ml/ minute. D o not allow air to enter the resin bed. Collect the initial 100 ml of eluate and reserve for the determination of potassium terephthalate. Elute the adsorbed potassium benzoate with water and collect the initial 150 ml in a flask containing 2 or 3 drops of concentrated HCl. Reserve this solution for the determination of potassium benzoate. Recondition the column by washing with several bed volumes of methanol and then with water. If air bubbles enter the column during the regeneration cycle, repeat the methanol wash as it is difficult to remove air bubbles during the water wash cycle. Measure the absorbance of the two collected solutions in 1-cm cells US. a reagent blank, taken from the column prior to addition of sample, by scanning the 360- to 225-nm region. Potassium terephthalate and benzoic acid have absorption maximum at 240 and 230 nm, respectively. Calculate the concentration of potassium terephthalate and benzoate using previously prepared calibration curves (calibrating solutions are not passed through the column). Both compounds adhere to Beer’s law up to concentrations of 4 and 3 mg/100 ml of solution, respectively. Benzoic acid in high purity terephthalic acid is determined in a similar manner. Approximately 0.5 gram of sample, dissolved in dilute KOH solution and saturated with NaCl, is adsorbed on the resin and the terephthalate is eluted with 500 to 600 ml of saturated NaCl solution (test for complete elution of terephthalate by scanning a portion of the eluate in the 240-nm region). Add 20 ml of water to the column and when the water just enters the resin bed, start eluting with methanol. Collect the benzoate in the initial 100 ml of aqueous methanol eluate in a volumetric flask containing a few drops of concentrated HCl. Prepare a calibration by eluting known amounts of benzoic acid, 0.05 to 3.0 mg, from the column in the same manner as for samples. RESULTS AND DISCUSSION Benzoic and terephthalic acids have characteristic, but similar, ultraviolet spectra which could be utilized to determine the individual components following separation. The separation of biphenyl mono- and dicarboxylic acid salts by elution of the latter from a non-ionic, porous polystyrene resin with 15 per cent aqueous NaCl solution (6) suggested that a more concentrated NaCl solution might elute potassium terephthalate while retaining the corresponding monoacid salt, Potassium benzoate, whose hydrophobic portion of the molecule is essentially the same fraction of the total molecule as the hydrophobic portion of biphenyldicarboxylic acid dipotassium salt, should be eluted with 15 per cent NaCl solution along with potassium terephthalate. An increase in the polar characteristic of the liquid phase, corresponding to an increase in the NaCl concentration, results in increased adsorption forces between the polymeric resin and the hydrophobic portion of the molecule. Accordingly, the benzoate molecule, containing the greater hydrophobic fraction of the molecule, should be adsorbed stronger than the terephthalate molecule at higher NaCl concentrations. Experimentally, it was found that a saturated solution of NaCl eluted the potassium terephthalate while retaining the potassium benzoate on the Amberlite XAD-2 resin. The latter salt is subsequently eluted with water. This is the basis for the present method for separating and measuring benzoate-terephthalate salt mixtures. The data in Table I indicate the accuracy and precision which can be obtained for the analyses of potassium ter-
5 .O
In
ELUATE FRACTIONS
ELUATE FRACTIONS Figure 1. Separation of potassium terephthalate-benzoate mixtures on miniature XAD-2resin columns Column, 1.2 X 18 cm i.d. Flow rate, 2.5 ml/min
Figure 2. Separation of potassium benzoate from excess potassium terephthalate Column, 1.8 X 25 cm i.d. Flow rate, 5 dimin Fraction vol,, 25 ml
Fraction vol., 10 ml ephthalate-benzoate mixtures. The blends were prepared by grinding weighed quantities of terephthalic and benzoic acids with potassium carbonate. Analyses were carried out by dissolving the sample in a slight excess of dilute potassium hydroxide. Over the 20 to 72 weight per cent range terephthalic acid, measured as potassium terephthalate, was determined with a standard deviation of 0.54 weight per cent and an average recovery of 99.6 per cent. Benzoic acid was determined in the 10 to 60 weight per cent range with a standard deviation of 0.78 weight per cent and an average recovery of 99.1 per cent. The applicability of the method to crude reaction mixtures is shown in Table 11. Two reaction products were analyzed for potassium terephthalate and then blended with benzoic acid to give the compositions shown. Analyses of the blended samples yielded results which differed from the calculated values by less than 1 per cent. The data in Table I11 indicate the accuracy and precision for the determination of low concentrations of benzoic acid in high purity terephthalic acid. These data were obtained by passage of 500-mg samples through the XAD-2 resin with 600 ml of saturated NaCl solution prior to elution of benzoate salt with methanol. The data indicate that benzoic acid can be determined in the 0.08 to 1.O per cent range with a standard deviation of 0.007 weight per cent and an average recovery of 99.4 per cent. The separation of potassium terephthalate and benzoate mixtures on the Amberlite XAD-2 resin is illustrated in Figure 1. Saturated NaCl solution rapidly elutes potassium terephthalate while retaining potassium benzoate. The adsorbed potassium benzoate is eluted with water. A column with a 1.2- x 18-cm packing operated at a flow rate of 2 to 3 ml/minute will separate a 10-mg mixture of 1 :1 potassium benzoate and terephthalate. The use of more dilute NaCl solution results in overlapping elution peaks. Figure 2 shows a similar separation for a potassium terephthalate-benzoate mixture containing in excess of 99.5 per cent potassium terephthalate. A 1.8- X 25-cm resin packing operated at a flow rate of 3 to 5 ml/minute will separate a mixture containing 650 mg of potassium terephthalate and 2.5 mg of potassium benzoate. The terephthalate salt is eluted in the initial 500 ml of saturated NaCl solution while the benzoate salt is eluted with 100 ml of methanol. Before
methanol is added to elute the potassium benzoate, a small amount of distilled water must precede the methanol, Figure 2, to prevent the precipitation of sodium chloride, which plugs the column. This elution curve indicates that potassium benzoate is retained by the resin during the passage of large volumes of the saturated NaCl eluant. Methanol can be used as the eluant only if the sample is free of strongly adsorbed, ultraviolet absorbing materials which are eluted by methanol. If these materials are present, as in the crude process samples, water is used as eluant, Potassium terephthalate in saturated NaCl solution has a characteristic ultraviolet spectrum. In this solution, the aromatic dicarboxylate has a wavelength of maximum absorbance at 240 nm with a molar absorptivity of 12,525 l./mol-cm. The presence of potassium phthalate and/or potassium isophthalate can be detected by a shift in the 240nm absorbance band to a shorter wavelength. Samples which show such a shift can still be analyzed for potassium terephthalate, with a loss of accuracy, by measuring the shoulder at 295 nm. Neither potassium phthalate nor isophthalate exhibit absorption at this wavelength (4). Potassium benzoate in aqueous solution has a wavelength of maximum absorption at 222 nm. Conversion of potassium benzoate to benzoic acid by addition of strong mineral acid results in a shift in absorbance maximum to slightly longer wavelength, 230 nm, and a 30 per cent increase in sensitivity. Benzoic acid has a molar absorptivity of 12,000 l./mol-cm at 230 nm in acid solution. Elutions with various concentrations of NaCl indicate that only a saturated solution yields quantitative separation; a 30 w/v per cent NaCl solution, approximately 90 per cent saturated, resulted in severely overlapping peaks. The flow rate of eluate had little effect on the efficiency of separation. Separations at flow rates of 1 and 3 ml/minute of saturated NaCl solution were equally efficient for the elution of potassium terephthalate from the small columns. Elution of potassium benzoate with water can be carried out efficiently at flow rates up to 5 ml/minute. The large columns permit flow rates of 5 ml/minute for both saturated NaCl solution and methanol. These flow rates allow an analysis to be carried out in a reasonable length of time.
RECEIVED for review October 4, 1971. Accepted January 11, 1972. ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
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