Simple Colorimetric Determination of Phthalates in Polymers by Dye Formation

This paper describes a simple method for quantifying phthalates via the conversion of anhydrous phthalates to a dye. The phthalate was hydrolyzed with sodium hydroxide and dehydrated to form phthalic anhydride, which was converted to a marker, namely fluorescein, by reaction with resorcinol. Concentrated sulfuric acid was used as the catalyst. The presence of a phthalate was determined by absorbance spectrophotometry. The detection limit of this method for di(2-ethylhexyl) phthalate was 0.1 μmol, and the relative standard deviation with respect to reproducibility was approximately 10% (n = 3). For other phthalates, namely diisobutyl phthalate, dibutyl phthalate, bis(butylbenzyl) phthalate, di-n-octyl phthalate, diisononyl phthalate, and diisodecyl phthalate, the absorbance deviation was less than 20%, which is within the acceptable range for screening purposes. Because the phthalates were hydrolyzed to enable colorimetric determination, the identification of individual phthalates is beyond the scope of this method. However, this simple colorimetric method can easily be used to determine the total amount of phthalates in real samples at the sub-micromolar level, by converting the phthalates to dyes. The applicability of this method was investigated by analyzing actual samples containing various phthalates. The quantitative results were almost the same as those obtained by using a conventional gas chromatographic method.