Microcolorimetric Method for o-Phthalates M. H. SWANN Coating and Chemical laboratory, Aberdeen Proving Ground, Md.
b Color-forming reactions of o-phthalates and succinates are presented for their direct determination or rapid detection in all types of coating.
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in the presence of o-phthalic acid, which interferes and thus limits the usefulness of the succinate method. Dihydronaphthazarin is formed from succinates by the reaction:
CONDENSATION ereaction between o-phthalic acid (o-benzenedicarboxylic acid) or anhydride and hydroquinone to form the highly colored compound quinizarin was investigated
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for its analytical value both as a rapid means of detecting o-phthalates in coatings and as a specific method for quantitative analysis in all types of coating materials. The increasing number of synthetic resins blended with alkyds accounts for the usefulness of a method of general applicability. Although yields of 100% cannot he obtained. rigid control of the conditions of reaction yields accurate and reproducible results. The method is applicable to dried coating films whether clear or pigmented. Less than one drop of enamel vehicle is needed for a single determination and small flakes of coating material can be chipped or scraped from finished surfaces and analyzed.
All of the modifying resins and other dicarboxylic acids including iso- and terephthalic acids have been tested for interference but none affect the determination except the adduct of maleic anhydride with isoprene. This product is indistinguishable from o-phthalates by this test, producing the same color and approximately the same yield. The adduct with butadiene does not develop color by this reaction. Phthalate ester plasticizers such as dibutyl phthalate will respond to the test and cannot be distinguished from polymeric phthalates unless separated by charcoal adsorption as previously described (2). PROCEDURES
Qualitative Identification of oPhthalates. A drop of resin solution
The determination is made in a small 25-ml. , glass-stoppered flask, and the sample made to react with 1 gram of hydroquinone and 2 ml. of concentrated sulfuric acid in an oven a t 1-15" C. for 2 hours. After dilution with water, the yellow color is extracted with benzene and ita intensity is measured photometrically a t its absorbance maximuni of 480 mp. For more positive identification, the benzene can be shaken with aqueous alkali, which becomes intensely violet with an absorbance mayimum of 575 mp.
As anticipated from the reaction, both maleic acid and succinic acid will condense to form colored products with hydroquinone, but neither interferes in the phthalic determination; the color formed by maleic acid is not extractable with benzene and the color formed by succinic acid is decomposed after 2 hours a t 145" C. However. succinic acid can be measured quantitatively by a modification of this procedure, except 1352
ANALYTICAL CHEMISTRY
CONCENTRATION mgJ100 mi
Figure 1 . Color developed by phthalic anhydride, measured at 480 mp
is placed in the bottom of a borosilicate glass test tube and approximately 1 gram of hydroquinone is added. Two milliliters of concentrated sulfuric acid are added and a thermometer is inserted. The tube is inclined and heated very slowly over a small Bunsen flame with gentle agitation or stirring until the temperature reaches 190" C. It is then cooled, slowly diluted with approximately 25 ml. of water, transferred to a separatory funnel, and shaken with 50 to 75 ml. of benzene. A yellow color in the benzene layer indicates o-phthalates. For confirmation, the aqueous layer is discarded and the benzene is washed first with water and then with dilute (0.1 to 0.2Ar) aqueous alkali. A violet color results in the aqueous layer. Qualitative Identification of Succinates. T h e above procedure is used for succinate identification except t h a t a larger sample, 3 t o 4 drops of resin solution, is needed because of the lower sensitivity. A red color is extracted by the benzene, which turns blue if washed with alkali. Both of these color formations are obscured by the more intense colors formed by phthalates if present. Quantitative Procedure for Phthalates. A small sample of resin vehicle representing 0.1 t o 0.3 gram of nonvolatile matter is weighed into a 100ml. volumetric flask, dissolved in benzene or acetone, and diluted t o volume. An aliquot of 5 ml. or less, which represents not more than 4 mg. of phthalic anhydride, is transferred to a glass-stoppered, 25-ml., Erlenmeyer flask and dried in an oven a t 105' C. Exactly 1.000 gram (+0.001 gram) of hydroquinone is added and exactly 2.0 ml. of concentrated sulfuric acid. The stopper is vented and the flask is placed in an oven a t 105 to 110' C. for hour; then it is transferred to a preheated oven a t 145' C. for 2 hours. Gentle agitation is required two or three times in this period. It is important that the 145' C. oven has good temperature control with quick recovery after closing of the oven door and that it does not exceedothe maximum temperature more than 1 . On cooling, the sample is slowly diluted with small portions of water, transferred to a 250-ml. separatory funnel with water, and diluted to a volume of 25 to 30 ml. Approximately 75 ml. of benzene are added with thorough agitation. On separation, the aqueous layer is drawn off, shaken with 20 ml. of benzene, and discarded. Both benzene portions are washed separately with water before being combined. Xhen acid-free, the benzene is poured
from the top opening of the funnel. filtered through paper which has been dampened with benzene into a 100-ml. volumetric flask, and diluted to volume. The absorbance is measured spectrophotometrically a t 480 mH and the phthalate content of the aliquot taken is determined from a graph prepared with potassium biphthalate as standard. using a water solution of the standard and drying aliquots equivalent t o 0.5 to 4.0 mg. of phthalic anhydride. Quantitative Procedure for Succinates. The procedure for succinates is identical to t h a t outlined for phthalates, except t h a t larger samples estimated to contain from 4 t o 25 mg. of succinic acid are used and the oven temperature is reduced to 135’ C. The red color is extracted with about 40 ml. of benzene, so t h a t the final dilution can be reduced to 50 ml., and the absorbance of these solutions determined a t 520 mp. A working curve can be prepared with succinic acid as standard, drying the aliquots a t temperatures below 100” C.
Table I.
Resin
Analysis of Typical Alkyd Resins
Phthalic Anhydride, % Present (ultraFound violet) (colorimetric)
1 2 3 4 (phenol modified)
11.8 37.5 43.0 40,6
11.4,11.8 38.8,38.8 42.6,43.5 41.4,40.5
DISCUSSION
Accuracy of the method is not so good as some of the older methods for determining phthalic anhydride ( I ) , but its broader application and lack of interference from other acids and resins are unique and valuable. Duplicate determinations should always be made and all analyses and standardizations should utilize the same equipment. The yield
of colored condensation products varies with time and temperature of reaction. The 135” C. temperature established for succinates gives maximum yield of color. Highest yields in the phthalate method are attained a t temperatures above the prescribed 145” C., but the higher temperatures cause foaming of the reagent which begins to decompose under the attack of concentrated acid. This explains the need for good oven temperature control. The variations in points used to plot the working curve for phthalates (Figure 1) illustrate the anticipated precision. Some analytical results are shown in Table I. LITERATURE CITED
.4m. SOC. Testing Materials, “hSTM Standards,” Designation D 1306 and D 1307, 1954. ( 2 ) Swann, M. H., Adams, M. L , Espoposito, G. G., ANAL.CHEW27, 1426
(1)
(1955).
RECEIVEDfor review December 18, 1956. .4ccepted A4pril27, 1957.
Spectro-Visual Method for Determining End Points Application to Titration of Soh ble Sulfate HARLEY H. BOVEE and REX J. ROBINSON Chemistry Department, University of Washington, Seattle 5, Wash.
F A new technique is described for detecting the end point in the titration of sulfate with standard barium chloride using the indicator tetrahydroxyquinone. The sample is illuminated with light transmitted by Wratten filter No. 45 and the end point is observed as a change from green to blue. The method is simple and the end point is sharp and reproducible. This principle of filter selection should b e applicable to other visual titrations.
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volumetric determination of sulfate ion with barium chloride using the disodium salt of tetrahydroxyquinone as an internal indicator has been described by several investigators (4, 6-9, 11). The color change of the indicator is from yellow to orange-red but is not sufficiently sharp for accurate work, particularly by the inexperienced operator. I n an effort to improve and standardize the end point determination, Hallett and Kuipers (4) suggested comparison with light transmitted by two thicknesses of Wratten No. 21 red filter to indicate the near approach of HE
the end point. Ogg, Willits, and Cooper ( 7 ) carried this procedure one step further by taking as the end point the exact match of the sample solution with a standard polished glass filter having 37% spectral transmittance a t 550 mp, Both the sample and filter were lighted from below to assist the color matching. I n 1953, Storlazzi and Ransom ( I O ) noted that a sulfate titration viewed by light transmittance through a R r a t t e n S o . 45 blue filter gave a green to blue color shift a t the end point. The present work was directed towards investigation of this color change and establishment of an accurate and reproducible procedure for determining the end point. EXPERIMENTAL
Equipment and Reagents. To provide suitable underlighting, a light table was constructed from a metal box: two l b w a t t fluorescent lamps, and a translucent glass cover. The cover was masked with opaque paper except for two openings, to furnish light beams viewing the sample and reference solution simultaneously. No. 45 Wratten filters were mounted between glass
plates and secured over the light openings. 4 Fisher Electrophotometer was used for the photometric titrations. The spectral transmittance curves for the indicator were run on the Beckman DU spectrophotometer. The barium chloride solutions were approximately 0.004, 0.02, and 0.1N. These solutions were standardized against hydrazine sulfate, using the titration procedure described. The indicator reagent was tetrahydroxyquinone, supplied by W.D. & L. D. Betz Co.; it was used in the dry state. Selection of Filter. The visual color change in the titration solution may be understood from the spectral transmittance curves for the tetrahydroxyquinone indicator both before and after the end point shown in Figure 1. Similar spectral curves hal-e been obtained by Lee and others ( 5 ) and Walter (12). From curve 1 it is apparent that the indicator before the end point has a high rate of transmittance in the red portion of the spectrum extending well into the yellow before dropping to a minimum in the blue-green range a t about VOL. 29, NO. 9, SEPTEMBER 1957
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