ANALYTICAL SCIENCES DECEMBER 1998, VOL. 14 1157 1998 © The Japan Society for Analytical Chemistry Notes Spectrophotometric Determination of Ethylenediaminetetraacetic Acid and Its Related Compounds with p-Carboxyphenylfluorone, Titanium(IV) and Hydrogen Peroxide1 Yoshikazu FUJITA, Itsuo MORI and Takako MATSUO Osaka University of Pharmaceutical Sciences, Nasahara, Takatsuki, Osaka 569–1094, Japan Keywords Ethylenediaminetetraacetic acid and its related compounds, spectrophotometry, p-carboxyphenylfluorone- titanium(IV) complex, hydrogen peroxide Ethylenediaminetetraacetic acid (EDTA), an (polyethylene glycol-p-nonylphenylether, Nakarai aminopolycarboxylic acid, is widely used in industry, Tesque) and 0.8% Amphitol 24B (betaine lauryldimethyl- detergents and foods. Recently, environmental pollu- aminoacetate, Kao Chem.) in the final concentration. tion by EDTA is getting more and more serious due to A buffer solution of pH 5.5 was prepared by mixing a its strong affinity with highly toxic heavy metals and its 0.2 M disodium hydrogenphosphate solution and a 0.1 resistance to biodegradation. Thus, it is imperative that M citric acid solution. Reagent-grade chemicals were sensitive and selective means of analysis are available. used throughout. Pure water was prepared by purifying We have reported some sensitive spectrophotometric deionized water with a Milli-Q Labo system just before methods2,3 for the determination of hydrogen peroxide use. (H2O2) based on fading of a dye-titanium(IV) complex A Shimadzu spectrophotometer (Model UV-160) with in the presence of EDTA. We speculated that a method 1.0-cm matched silica cells was used for an absorbance which would utilize fading of a dye-titanium(IV) com- measurement. The pH measurements were made with a plex in the presence of H2O2 would serve as a sensitive Horiba (F-11) pH meter in combination with a calomel determination for EDTA. Here, we report on a spec- glass electrode. trophotometric method for the determination of EDTA and its related compounds based on fading of a p-car- Standard procedure boxyphenylfluorone (PCPF)-titanium(IV) complex in The following components were mixed in a 10-cm3 –6 the presence of H2O2. volumetric flask: a solution containing 1.0×10 – 6.0×10–6 M of EDTA and its related compounds, 0.5 cm3 of a 2.0×10–4 M titanium(IV) solution, 0.2 cm3 of a –2 3 Experimental 1.5×10 % H2O2 solution, 0.5 cm of the surfactant solution, 2.5 cm3 of the buffer solution and 0.4 cm3 of a Reagents and apparatus 1.0×10–3 M PCPF solution. The mixture was diluted to A stock solution (1.0×10–3 M, 1 M=1 mol dm–3) of 10 cm3 with water, transferred into a test tube, mixed EDTA was prepared by dissolving disodium ethylene- well and kept at 60˚C for 25 min. After the solution diaminetetraacetate (Dojindo Lab.) with water. had been cooled in water to room temperature, the dif- Working solutions were prepared by the dilution of the ference in the absorbance (∆A) between the resultant stock solution. Other EDTA-related compounds, solution and a reagent blank solution prepared under EDTA-metal chelates and EDTA derivatives, were also the same conditions was measured at 615 nm against obtained from Dojindo Lab. A titanium(IV) solution water. (2.0×10–4 M) was prepared from a stock solution (Wako Pure Chem. Co. Ltd., 1000 µg/cm3) by dilution with 0.1 M hydrochloric acid, as required. Solutions of all dyes, Results and Discussion which had been synthesized according to the method described in the literature4,5, were prepared in 1.0×10–3 Optimization of reaction variables M methanol solution containing one drop of hydrochlo- In preliminary experiments, the effect of dyes was –2 ric acid. A H2O2 solution (1.5×10 %) was prepared by studied by measuring the difference of absorbance (∆A) diluting a 30% hydrogen peroxide (Mitsubishi Gas between a dye-titanium(IV) and a dye-titanium(IV)- Chem. Co. Ltd.) with water. A surfactant solution was EDTA solutions in the presence of H2O2. The dyes obtained as a mixed solution of 0.4% Triton N-101 used were PCPF, o-sulfophenylfluorone (SPF), o- 1158 ANALYTICAL SCIENCES DECEMBER 1998, VOL. 14 Fig. 1 Structures of dyes studied hydroxyhydroquinonephthalein (QP), vanillylfluorone (VF), salicylfluorone (SF), phenylfluorone (PF) and m- fluorophenylfluorone (MFPF). These structures are shown in Fig. 1. These dyes could be arranged in the Fig. 2 Absorption spectra of PCPF-Ti(IV) and PCPF solu- –5 –5 following order with respect to sensitivity: PCPF>QP, tions. [Ti(IV)]=1.0×10 M; [PCPF]=4.0×10 M; [H2O2] SPF>VF, SF>> PF, MFPF. =3.0×10–4%; [surfactant]=0.5 ml of the surfactant solution/10 The best pH range was from 5.2 to 5.9 by using 2.5 ml; pH=5.5. Curve 1: PCPF-Ti(IV) or PCPF-Ti(IV)-H2O2 cm3 of 0.2 M disodium hydrogenphosphate/0.1 M citric solutions. Curves 2, 3 and 4: PCPF-Ti(IV)-H2O2-EDTA solu- acid buffer solution. tions (EDTA concentration(M): curve 2=2.0×10–6; curve 3= × –6 × –6 The amount of PCPF was examined while maintain- 4.0 10 ; curve 4=6.0 10 ). Curve 5: PCPF or PCPF-H2O2 ing a fixed final concentration of titanium(IV)(1.0×10–5 solution. Reference: water. M). The use of 4.0×10–5 M PCPF in the final concen- tration was the best with regard to the reaction rate and stability of color development and in consideration of Calibration curves absorbance of a PCPF-titanium(IV) solution (a reagent A calibration curve for EDTA was constructed by the blank). standard procedure. A good linear relationship was In order to stabilize the color development and to observed over the 1.0×10–6 M – 6.0×10–6 M range of increase the sensitivity, the effects of different surfac- EDTA in the final volume of 10 cm3. This procedure is tants were examined: nonionic (Triton N-101, Tween about 3 – 20 times more sensitive than the other spec- 20, Brij 35, Triton X-100), anionic (sodium dodecylsul- trophotometric methods, such as bathocuproine-copper6, fate, perfluorooctanoic acid, sodium di(2-ethylhexyl)- Pyrocatechol Violet-bismuth7, phosphomolybdic acid8, sulfosuccinate), cationic (stearyltrimethylammonium 1,2-naphthoquinone-4-sulfonic acid phenylthiosemicar- chloride, cetylpyridinium chloride, cetyltrimethylam- bazone-bismuth9 and 4,7-diphenyl-1,10-phenanthro- monium chloride) and amphoteric (Amphitol 24B, line-disulfonic acid-iron10 methods, and is as sensitive Swanol AM-103, sodium N-lauroylsarcosine). The as HPLC methods.11 The relative standard deviation simultaneous use of Amphitol 24B with a nonionic sur- (RSD) for five runs of 2.0×10–6 M EDTA was 1.8%. factant, particularly Triton N-101, was the most effec- The calibration curves for EDTA related compounds tive. (EDTA-metal chelates and other aminopolycarboxylic 3 –2 When 0.1 – 2.0 cm of 1.5×10 % H2O2 was used, ∆A acids) were also constructed under the optimum condi- was found to remain constant. Thus, 0.2 cm3 of tions. EDTA-metal chelates and aminopolycarboxylic –2 1.5×10 % H2O2 solution was chosen as described in acids include EDTA-calcium(II), EDTA-zinc(II), the standard procedure. ∆A was considerably influ- EDTA-manganese(II), EDTA-cobalt(II), EDTA-Cu(II), enced by the mixing order. The best mixing order was EDTA-iron(III), trans-1,2-diaminocyclohexane- EDTA, titanium(IV), H2O2, surfactant, buffer solution N,N,N′,N′-tetraacetic acid (CyDTA), diethylenetri- and PCPF. amine-N,N,N′,N″,N″-pentaacetic acid (DTPA), N-(2- The color formation in this reaction system did not hydroxyethyl)ethylene-N,N′,N′-triacetic acid (EDTA- occur instantaneously at room temperature. Thus, the OH), nitrilotriacetic acid (NTA), 1,3-diamino-2- effects of incubation temperature and time were exam- hydroxypropane-N,N,N′,N′-tetraacetic acid (DPTA- ined at 40, 50, 60, 70 and 80˚C. A maximum and con- OH), ethylenediaminedi(o-hydroxyphenylacetic acid) stant ∆A value was obtained on heating at 60˚C for 25 (EDDHA) and glycolethylenediaminetetraacetic acid min, following by cooling to room temperature. The (GEDTA). The sensitivities of EDTA-copper(II) and ∆A value remained constant for at least 5 h after the GEDTA were remarkably low. EDTA-copper(II) was solution had been cooled to room temperature. Figure could be determined in the presence of thiosulfate ion 2 shows the absorption spectra of PCPF-titanium(IV) (5.0×10–4 M). EDTA-iron(III) could not be determined and PCPF solutions under the standard procedure. at all. The reason why EDTA-iron(III), even in the presence of a reductant, gives no response is probably ANALYTICAL SCIENCES DECEMBER 1998, VOL. 14 1159 that free iron(II) ion released from EDTA-iron(III) Table 1 Determination of EDTA in foods (µg/g) forms a deeply colored complex with PCPF. In conclu- Founda sion, though the present method is not free from the a a, b Sample RSD , Recovery , interference of coexisting aminopolycarboxylic acids, Proposed Other % % EDTA-metal chelates except EDTA-iron can be deter- method methodc mined as total amounts of EDTA. A 20.2 18.9 1.8 101.9 B 32.8 31.6 2.3 97.5 Effects of foreign substances C 54.3 —d 2.2 100.3 The effects of many foreign substances on the deter- D 132.3 —d 2.0 98.0 mination of EDTA were examined. Inorganic ions such E 32.0 32.8 2.2 101.2 as sodium, potassium, calcium, magnesium, cobalt, A, soft drink; B, salada dressing (emulsion type); C, canned zinc, tin, aluminum, chloride, sulfate, nitrate, iodide, mushroom; D, canned crab meat; E, canned green peas. thiosulfate, thiocyanate, cyanide and bromate ions did a.