blanks were determined by adding a knoivn amount of n-heptadecanoic acid to the fat and analyzing for fatty acids as described. I n this particular sample, the amounts of lauric, linoleic, and linolenic acids actually chromatographed \?-ere approximately 2, 10, and 2 y, respectively. The peak areas for these small amounts of acid are difficult to measure, and the results are erratic using a packed column and a thermal conductivity detector. For samples of 25 to 200 y of free fatty acids the recoveries as shown are excellent. -4bsolute recoveries generally run from 60 t o 95%; quantitative results without the use of an internal standard would be unobtainable. The recrystallized C17 acid used as the internal standard, when carried through the esterification procedure and chroniatographed, yielded only a trace of impurity a t a position corresponding to the CI3 saturated acid. Kone of
the fat samples analyzed in this study has shown a peak at this position. Figure 1 shows the chromatogram for the separation of the C17 standard and the CISfatty acids. The retention time for methyl palmitate is 12 minutes and its separation from the C17acid is also complete. The poly(viny1 acetate) (PVA) used as the partitioning medium is preferred to the polyestertype materials that have been reported (2, 6, 7 ) . PVA is readily available commercially and does not show excessive “bleeding.” The products of the thermal degradation of PVA are acetic acid and polyvinylacetylene (S), and in preliminary stability studies a t 205” C. only a slight initial loss of acetic acid (0.5 mg. per hour) was found. Thus, if the separated fatty acid methyl esters are collected for spectrophotometric study, one can readily compensate for traces of acetic acid collected with the sample.
LITERATURE CITED
(1) Alford, J. A . , Elliott, L. E., Food
380 (1952).
’
(4) Hornstein, I., Elliott,, L. E., Crowe, 1’. F., S a t u r e 184, 1710 (1959). ( 5 ) Keulemane, 4.I. AI., Verver, C. G., “Gas Chromatography,” p. 32, Rein-
hold, Kew York, 1957.
( 6 ) Lipsky, S. R., Landowne, R. A., Godet, SI.R., Biochim. et Biophys. Acta 31,336(1959). (7) Orr, C. H., Callen, J. E., J . Ani. Chem. Soc. 80, 219 (1958).
RECEIVEDfor revieiv October 7 , 1959. Accepted December 18, 1959. Taken from a thesis submitted by Irwin Hornstein in partial fulfillment of the requirements for the Ph.D. degree, Georgetown University. Illention of specific commercial materials or equipment does not constitute recommendation for their use above similar materials and equipment of equal value.
Colorimetric Determination of Traces of Acids in Dimethyl Terephthalate A. L. HENSLEY Research and Development Department, Standard Oil Co. (Indiana), Whiting, Ind.
b Traces of acids in dimethyl terephthalate can be determined by reaction of the acid with the potassium salt of bromothymol blue in tolueneN,N-dimethylformamide solvent. Diminution of the blue color of the bromothymol blue ion i s directly related to the acid number of dimethyl terephthalate. Water and carbon dioxide must be excluded.
P
of synthetic fibers have set a very lom acid-number specification on esters of phthalic acids they purchase as raw materials. An accurate routine method is, therefore, required for determining very low acid numbers, usually below 0.1 mg. of potassium hydroxide per gram. Of the esters, dimethyl terephthalate is the most difficult to analyze. Because it is sparingly soluble in suitable solvents, conventional indicator titrations give diffuse end points with the dilute titrant required. Conductometric titrations give better end points but are too timeconsuming for routine use. I n a method developed for measuring the acidity of fatty oils ( I ) , the acid in the oil reacts with the sodium salt of a colored indicator acid, and the color RODUCERS
542
ANALYTICAL CHEMISTRY
change produced is compared visually with standards to determine the amount of acid present. This reaction has been made the basis of a rapid spectrophotometric method for determining the acid number of dimethyl terephthalate. The ester is dissolved in a selected solvent, and the potassium salt of bromothymol blue is added. Acid in the ester reacts nith the bromothymol blue anion to form the bromothymol blue acid; the decrease in intensity of the blue color of the anion is proportional to the original acid concentration of the sample. INDICATOR A N D SOLVENT
I n developing the method, a satisfactory combination of indicator and solvent had to be determined. The relative strengths of organic acids are not always constant when measured in various solvents; therefore, it was necessary to select an indicator acid that is weaker in the solvent chosen than the acids present in dimethyl terephthalate. The dissociation constants of the aromatic acids most likely to be present in dimethyl terephthalate require that the color of the indicator change in the p H range of 8 to 10 in water. Of the
indicators tested, bromothymol blue (dibromothymolsulfonphthalein) was the most satisfactory. The ionized, or basic, form of this indicator absorbs strongly a t a wave length where the un-ionized acid form absorbs only slightly. Because the ionized form is a relatively weak base, errors from carbon dioxide absorption and ester hydrolysis are small. A suitable solvent should dissolve the ester readily without decomposing it, be polar enough to dissolve the indicator salt, be stable, and be neutral or slightly basic. N,K-Dimethylformamide (DMF) satisfies all these requirements; however, in this solvent, the phthalic acids behave as monobasic acids with respect to bromothymol blue. Doubling the acid number thus obtained for an ester would not yield the correct value because the acidity is present as unknown proportions of phthalic acids and their monoesters. This difficulty Tvas avoided by the use of 25% toluene-75% DIIF, in which solvent bromothymol blue is a weaker acid than the second carboxyl of the phthalic acids. Both forms of the indicator are soluble and stable in the solvent when protected from strong light.
To prepare the indicator solution,
dissolve 0.40 gram of bromothymol blue in 400 ml. of reagent grade DlIF in a 500-ml. volumetric flask, and slowly add 0.5~1-potassium hydroxide in ethyl alcohol until a perinanent dark blue solution is obtained. Dilute the solution to volume v i t h DLIF and transfer it to a 5-nil. reservoir-type buret fitted with a drying tube containing Ascarite and Drierite. Khen diluted t o 100 ml. with 1 : 3 toluene-DlIF, 2 ml. of solution should have an absorbance of 0.5 t o 0.8 when measurcd against a solvent blank a t 610 mp in 1-em. cells in a Beckman B spectrophotometer or cquivalent. Although reagent grade toluene and D M F contain practically no carbon dioside, both absorb it when exposed to the atmosphere. Thus, to prepare the solvent, mix 3 parts of D M F and 1 part of toluene from newly opened reagent bottles. Store the solvent in a reservoir protected by a drying tube filled with -4scarite and Drierite. CALIBRATI0N
Dissolve enough terephthalic acid of known acid number in the solvent (1 :3 toluene-DNF) to make a solution containing acid equivalent t o 0.100 mg. of potassium hydroxide per ml. Dilute 10 ml. of this solution to 100 ml. with solvent. T o prepare a calibration curve covering the range from 0 to 0.2 mg. of potassium hydroxide, add 2.00 ml. of the indicator solution from a microburet to appropriate amounts of the dilute standard acid solution, dilute to 100 ml. with solvent, read absorbance a t 610 nip against a solvent blank in 1-em. cells in the spectrophotometer, and plot absorbance against milligrams of potassium hydroside. The calibration curve does not follow Beer’s law, but gives a curved line of decreasing negative slope with increasing acid concentration. PROCEDURE
To determine the acid number of dimethyl terephthalate, weigh out a sample containing acid equivalent to 0.05 t o 0.20 mg. of potassium hydroxide, and transfer it to a 100-ml. volumetric flask. Add about 75 ml. of solvent (1: 3 toluene-DMF), stopper, and shake the flask to dissolve the sample. Add 2.00 ml. of indicator solution, dilute to volume with solvent, and mix. Kithin 1 hour, read the absorbance as when calibrating. Obtain milligrams of potassium hydroxide from the calibration curve and, from the weight of the sample, calculate acid number (milligrams of potassium hydroxide per gram). DISCUSSION
To establish the precision, one analyst performed six successive daily analyses on a sample and a, second analyst performed four, both using the same calibration. Averages found were 0.061 and 0.065 mg. of potassium hydroxide per gram, with a standard deviation of
Table
1.
Determinations
of
0.040
0.027 0.020
Typical Dimethyl Terephthalates
Conductometric Titration
... 0.25, 0.30, 0.32
0.29 0.10, 0.10, 0.09
0.10
0.03, 0.05, 0.04 0.04 0.02, 0.03, 0.03
0,027
0.02, 0.02, 0.02
0.02 0.01, 0.02, 0.02 0.017 17
0.004 in both cases. This work was repeated on a second sample of dimethyl terephthalate, with each analyst using his own calibration curve. Average acid numbers found were 0.042 and 0.041, with standard deviations of 0.0014 and 0.0019, respectively. The difference when analysts use common and separate calibration curves is not significant, and the standard deviations show the method has high precision. I n the first of two tests of accuracy, known amounts of terephthalic acid were added to dimethyl terephthalate containing no detectable acid, and the acid numbers were determined. Acid numbers from duplicate analyses of three samples were : Knowm
of
Acid Number
Colorimetry (This Method) 0.96, 0.87, 0.91 Av. 0.91 0.283, 0.283, 0.288 Av. 0.285 0.129, 0.127, 0.129 Av. 0.128 0.033, 0.036, 0.031 Av. 0.033 0.029, 0.028, 0.028 Av. 0,028 0.026, 0.026, 0.024 Av. 0.025 0.022, 0.016, 0.017 Av. 0.018 Coefficient of variation, % 4.7
Found 0 042, 0.042 0.028, 0.027 0.022, 0.021
Average error is 0.001 acid number. The positive bias indicates that the ester probably contained an undetected acid impurity. I n the second test of accuracy, typical samples of dimethyl terephthalate were analyzed by this method by conductometric titration, and by conventional indicator titration. I n the conductometric titrations, the sample was dissolved in 1:1 DMF-toluene and titrated with alcoholic potassium hydroxide. I n the indicator titrations, phenolphthalein was the indicator, alcoholic potassium hydroxide was the titrant, and 1:4 ethyl alcohol-toluene was the solvent. Results are shown in Table I. I n all instances, results agree as indicated by the coefficients of variation. However, the two titrimetric methods have such poor precision compared with the colorimetric method, that these results show the excellent precision of the latter. Although the colorimetric method is accurate, rapid, and sensitive, a few
Indicator Titration 0 . 7 6 , 0.91, 0.90 0.86 0.26, 0.33, 0.41 0.33 0 08, 0.15, 0.14 0.12 0.04, 0.04, 0.07 0.05 0.02, 0.04, 0.05 0.037
0.02, 0.04, 0.03 0.03 0.01, 0.03, 0.03 0.023 35
precautions are necessary. Even when protected from air, the indicator solution fades slowly, so that the calibration curve should be checked every day. h’ew batches of solvent or indicator solution usually change the calibration curve; hence, a new curve should be prepared each time either is changed. The amount of indicator solution must be accurately measured because the decrease in the color added is measured. A simple calculation shows that, if a 1gram sample of an ester with an acid number of 0.01 is taken, an error of 5y0 in the volume of indicator solution can produce an error of 100%. Once the flask containing the sample is opened, measurements should be made promptly because the solvent absorbs carbon dioxide from the air, and the color fades. CONCLUSION
The method has an acid number detection limit of 0.003 mg. of potassium hydroxide per gram, is more accurate than existing methods for low acid number materials, and is rvell suited for routine use where many acid numbers are to be determined. Under these conditions, the time for one analysis can be reduced to 15 minutes. The technique can be readily extended to the determination of acid numbers of other light-colored nonacid substances, such as other esters, edible oils, turbine oils, transformer oils, alcohols, and most organic solvents. Selection of solvent and indicator for each system will be determined primarily by solubility of the sample and the dissociation constant of the acid to be measured. LITERATURE CITED
(1) Illarionon,, W. W., Kogan, I. S., Mikrochemie 21, 11 (1936).
RECEIVEDfor review October 9, 1959.
Accepted December 29, 1959.
VOL. 32, NO. 4, APRIL 1960
.I543
