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(2) Hassler, J. W. “Purification with Activated Carbon”; Chemical Publishing Co.: New York, 1974; pp 23-340. (3) Cheremlsinoff, P. N.; Ellerbusch, F. “Carbon Adsorption Handbook”; Ann Arbor Science: Ann Arbor, MI, 1978; pp 1-84, 131-214, and 627-660. (4) “Calgon 1981 Adsorption Handbook”; Activated Carbon Dlvlslon: Plnsburgh, PA. (5) McLeod, H. A.; Mendoza, C.; Wales, P.; McKinley, W. P. J. Assoc. Off. Anal. Chem. 1967, 50, 1216. (6) BabJak, L. J.; Chau, A. S. Y. J. Assoc. Off. Anal. Chem. 1979, 62, 1174. (7) Colln, H.; Guichon, G. J. Chromatogr. 1977, 137, 19. (8) Stalling, D. L.; Johnson, J.; Huckins, J. N. “Environmental Quality and Safety, Supplement Vol. 111”; Coulston, F., Korte, F., Eds.; George Thlene Publishers: Stuttgart, West Germany, 1975; p 12. (9) Huckins, J. N.; Stalling, D. L.; Smith, W. A. J. Assoc. Off. Anal. Chem. 1978, 61, 32. (10) Huckins, J. N.; Stalllng, D. L.; Petty, J. D.; Smith, L. M. US. Patent
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RECEIVED for review April 6,1981. Accepted August 3, 1981.
Determination of Nitrilotriacetic Acid in Ethylenediaminetetraacetic Acid Disodium Salt by Reversed-Phase Ion Pair Liquid Chromatography D. G. Parkes,” M. G. Caruso, and J. E. Spradling I11 Ciba-Geigy Corporation, Dyestuffs & Chemicals Division, P.0. Box 1 1422, Greensboro, North Carolina 27409
The disodium salt of ethylenediaminetetraacetic acid (Na2EDTA) is added to food and pharmaceutical and highquality cosmetic products for the control of unwanted trace metal contamination (1). Trace metals in the aforementioned products are known to catalyze unwanted reactions which promote degradation of the products. Specifications regarding USP grade disodium EDTA are presented in the US Pharmacopoeia. One of the specifications concerns levels of NTA in the product (0.1% max concentration) (2). The recommended method for this analysis is a polarographic technique. The polarographic method is cumbersome, relatively nonspecific, and cannot be utilized for the analysis of NTA in other EDTA-based products. For these reasons we developed an HPLC method for this analysis. The method is simple, accurate, specific, and applicable to the analysis of NTA in other EDTA and non-EDTA-based materials. EXPERIMENTAL SECTION Apparatus. Liquid chromatographicanalysis was performed with a Perkin-Elmer Series I11 liquid chromatograph equipped with a DuPont Zorbax C-8 column, 4.6 mm i.d., 15 cm long, a Schoeffel variable-wavelength UV-visible detector, a Waters automatic injector (WISP), and an Autolab System I computing integrator. Preparation of 1% Cupric Nitrate. A 1%aqueous solution of cupric nitrate was prepared by dissolving 10 g of cupric nitrate in 990 g of distilled water. Reagents and Chemicals. Methanol (distilled in glass) was obtained from Burdick and Jackson. Tetrabutylammonium hydroxide (25% in methanol) was obtained from Matheson, Coleman and Bell. Ethylenediaminetetraacetic acid, disodium salt, dihydrate (99+%, Gold Label) and nitrilotriacetic acid (99%) were obtained from Aldrich Chemical Co., Inc. Water was obtained from a Milli Q water purification system. Phosphoric acid (reagent), ammonium hydroxide (reagent), and cupric nitrate (reagent) were obtained from Fisher Scientific. Preparations of HPLC Mobile Phase. The HPLC mobile phase consisted of 10% methanol/90% water/O.Ol M tetrabutylammonium phosphate pH 7.5. It was prepared by adding 10 mL of tetrabutylammonium hydroxide solution to -200 mL of distilled water and adjusting the pH to 7.5 with 10% phosphoric acid. The resultant mixture was transferred to a 1-L volumetric flask along with 90 mL of methanol and diluted to volume with water. The solution was then fiitered through a 0.5-pm membrane filter and degassed under vacuum. 0003-2700/81/0353-2154$01.25/0
Preparation of Standards and Samples. Standards were prepared as follows: A 10 mg/mL solution of NTA was prepared in 5% ammonium hydroxide. One gram portions of disodium EDTA (Aldrich) were added to each of six 100-mL volumetric flasks. Aliquots (0, 0.05,0.10, 0.20, 0.50, and 1.0 mL) of the 10 mg/mL NTA solution were transferred to the flasks containing the EDTA. The contents of the flasks were diluted to volume with 1% cupric nitrate solution and mixed until dissolved. An ultrasonic bath was utilized to enhance the rate of dissolution. This gave standards containing 0,0.5, 1.0, 2.0,5.0, and 10.0 mg of NTA/g of EDTA, respectively. Samples were prepared in like manner: A 1-g portion of each sample was added to a 100-mL volumetric flask. The contents of the flasks containing the samples were diluted to volume with 1% cupric nitrate solution and mixed until dissolved. Chromatographic Analysis. A %pL aliquot of each solution was analyzed by liquid chromatography. Chromatographic conditions were as follows: column = Zorbax C-8,15 cm, 4.6 mm i.d.; mobile phase = 10% methanol/90% water/O.Ol M tetrabutylammonium phosphate pH 7.5; flow rate = 2 mL/min; detector wavelength = 254 nm; detector sensitivity = 0.04 aufs. RESULTS AND DISCUSSION I t was our intent to develop a simple, yet accurate technique, for the determination of low levels of NTA in disodium EDTA. Since liquid chromatography had been used successfully in the past for related applications (3) this approach seemed viable. Initially, attempts were made to analyze NTA without prior chelation with a metal. In this instance, ion pair chromatography with low-wavelengthUV detection (Le.,
