315
Anal. Chern. 1984, 56,315-317
However, we have outlined the limitations of the standard eluting sequence when dealing with asphaltenes. To overcome the interference effect of the low solubility of asphaltenes in cyclohexane, we have suggested using a cyclohexane-benzene (60/40) mixture as the first eluent. But we must be aware that such a change obviously modified the meaning of the neutral, acid, and basic fractions recovered which no longer match the corresponding API 60 fractions.
t
Q Q INJECTOR
LITERATURE CITED
I m e we/ c /$
I
FRACTION RECOVERING
I
Flowfsgddr? / c / / r
Figure 1. The experimental chromatography assembly.
It can be noted on Table I11 that a difference appears in the proportion of weak acids and weak bases related to moderate acids and moderate bases which is modified, with the distribution of strong acids and strong bases remaining basically the same. In spite of these differences, the balance and reproducibility remain acceptable. So the separation scheme described here can be used as a preparative method which is well suited for crude oils, heavy ends, maltenes, and resins.
Jewell, D. M.; Albaugh, E. W.; Davis, E. W.; Ruberto, R. G. Ind. Eng. Chem. Fundam. 1974, 13, 1974. Jewell, D. M.; Weber, J. H.; Bunger, J. W.; Plancher, H.; Latham, D. R. Anal. Chem. 1972, 4 4 , 1391. Amat, M.; Arpino, P.; Orrlt, J.; Lattes, A,; Guiochon, G. Analusis 1980, 8(5), 179. Schmltter, J. M.; Ignatiadis, I.; Arpino, P.; Guiochon, G. Anal. Chem. 1983, 55, 1685-1688. Algelt, K. H.; Latham, D. R.; Jewell, D. M.; Selucky, M. L. "Chromatography In Petroleum Analysis, Chromatographic Sclence Series"; Cazes, J., Ed.; Marcel Dekker: New York, 1979; Vol. 11, p 197. Galya, L. G.; Suatoni, J. C. J. Li9. Chromatogr. 1080, 3 (2), 229. Jewell, D. M. "Chromatography in Petroleum Analysis, Chromatographic Sclence Serles"; Cazes, J., Ed.; Marcel Dekker: New York, 1979; Vol. 11, p 275. Mc Kay, J. F.; Amend, P. J.; Cogswell, T. E.; Harnsberger, P. M.; Erickson, R. B.; Latham, D. R. Prepr. Div. Pet. Chem., Am. Chem. SOC. 1977, 22, 708. Desbene, P. L.; Lambert, D.; Maquet, J.; Lemerle, J.; Huc, A. Y., unpublished results.
Received for review March 7,1983.Accepted October 3,1983.
Determination of Ethylenediaminetetraacetic Acid in Boiler Water by Liquid Chromatography David L. Venezky* and Walter E. Rudzinski'
Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375 Tetrasodium ethylenediaminetetraacetate (Na,EDTA) is an active ingredient in boiler treatment formulations, and it has been used for metal oxide solubilization and for maintenance of metal ions in a sequestered state in solution (1,2). At high temperatures (200"C), EDTA may decompose forming iminodiacetate (IDA) and other degradation products (1-3). It is therefore imperative that a procedure be devised that would allow a determination of total EDTA in boiler water. The ASTM method for determination of EDTA is based on a spectrophotometric determination of the reduction of color intensity in a solution containing zirconium and xylenol orange indicator (4).The method is susceptible to interference from other chelating agents and phosphate which can be present in boiler water up to the range 10-25 mg/L (2). A variety of chromatographic methods have been developed for the analysis of EDTA. Gas chromatography has been used for the simultaneous determination of EDTA, IDA, and N(2-hydroxyethyl)iminodiacetate,HEIDA (5). The method however involves a preliminary derivitization step w h k l & time-consuming. An ion chromatography method is available, but the method may be susceptible to interference from phosphate ion, and it has a limited linear range (6). Two methods utilizing ion pair high-performance liquid chroma-
* Permanent address, Department of Chemistry, Southwest Texas State University, San Marcos, TX 78666.
tography (HPLC) have been reported; however, the methods complex are based on the formation of a stable CU(EDTA)~(7, 8). These methods are not applicable to boiler water samples which contain large amounts of iron and nickel which compete effectively with copper for the EDTA. In order to circumvent most of the limitations in the current procedures, we have devised an HPLC procedure which can determine total EDTA in the presence of Fe3+, Cuz+,Ni2+, phosphate, Ca2+,and Mg2+. The method is based on the chromatographic determination of Fe(EDTA)- using octadecylsilane (ODS) bonded phase as the stationary phase and tetrabutylammonium bromide (TBABr) in acetate buffer as the mobile phase.
EXPERIMENTAL SECTION Apparatus. The liquid chromatography was performed with a Varian 8500 pump equipped with an Aerograph high-pressure septumless injector. A Whatman guard column adapted with a Varian inlet fitting and filled with C0:Pell ODS 30-38p packing material was prepoeitioned prior to the analytical column (Ul4.6 mm i.d., 25 cm long). An Aerograph 254-nm trasphere ODS-~W, UV detector and a Heath Schlumberger dual pen chart recorder were also used. Helium was used as the solvent pressurizing gas. An SMI IIIA direct current argon plasma emission spectrometer (Spectrametrics, Inc., Andover, MA) was used for the element analyses. Reagents and Chemicals. Water was obtained from a Millipore Water Purification System. Methanol was obtained from
This article not subject to U.S.Copyrlght. Published 1984 by the American Chemical Society
316
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
Fisher. Tetrabutylammonium bromide (TBABr) was obtained from Eastman; EDTA, tetrasodium salt was obtained from Baker. The iron (FeC13)was purchased as a DILUT-IT Ionic Standard Concentrate (Baker) and diluted to 1L with HC1. The resulting solution was 1ppt (part per thousand). All other reagents were ACS reagent grade. Preparation of Mobile Phase. TBABr was dissolved in 0.05 M sodium acetate which had been adjusted to a pH of 4.5 with acetic acid. The final concentration of TBABr was 0.0162 M. This aqueous solution was then mixed with methanol resulting in a final water:methanol ratio of 964. The mobile phase was filtered and degassed with Type LS 5.0-pm filters. Sufficient mobile phase was prepared so that all standards and samples could be run identically. Preparation of Standards and Samples. The Na4EDTA stock solution was prepared by dissolving 2.279 g of analytical grade Na4EDTAin 500 mL of 0.05 M sodium acetate which had been adjusted to a pH of 4.5 with acetic acid. All standard solutions were then prepared by diluting the stock solution with acetate buffer. CU(EDTA)~was prepared by adding 1.00 mL of a 0.064 M CuS04solution in acetate buffer to 1.00 mL of Na4EDTAstandard andthen diluting to 25.00 mL with mobile phase. Fe(EDTA)- was prepared by adding 2.00 mL of a 1 ppt FeCl, solution to 1.00 mL of Na4EDTA standard and then diluting to 25.00 mL with mobile phase. Test solutions were prepared to simulate boiler water, which was treated with EDTA. They were prepared by adding 2.00 mL of a 1 ppt FeC13 solution to 1.00 mL of a 2% EDTA solution heated in an autoclave (190-210 "C) and then diluting to 25.00 mL with mobile phase. The boiler water samples were prepared by adding 1 mL of 1 ppt FeC13 to 5 mL of boiler water and then diluting to 10 mL with mobile phase. The samples were "spiked" by adding between 1 and 20 pL aliquots of the Na4EDTA stock solution, to 10 mL of sample. Chromatographic Conditions. Flow rate was 60 mL/h, chart speed was 0.5 in./min, sample size was 10 pL, unless otherwise specified, dead volume was 2.50 mL, and detector setting range was 0.64-0.02 aufs.
RESULTS AND DISCUSSION It was our intention to develop a simple, yet accurate method for the determination of total EDTA in boiler water. One approach which had proven effective (7,8) involved formation of the chromophore, CU(EDTA)~-.The complex is stable with a conditional stability constant, k j = 1011at pH 4.5 (9). The k j takes into account the formation of hydroxide complexes and the protonation of EDTA. The chromatographic peak corresponding to the complex was symmetric with a capacity factor, k' = 5.0. A plot of response vs. micrograms injected yielded a linear response between 690 ppm and 1 ppm Na4EDTA, a sensitivity of 0.0265 absorbance units/microgram injected, and a limit of detection of 1 ppm. Unfortunately, CU(EDTA)~is not stable in the presence of Fe3+. If Fe3+is added to CU(EDTA)~-, then an exchange equilibrium occurs between Fe3+and CU(EDTA)~with the resulting formation of Fe(EDTA)- (conditional stability cona t p H 4.5) (9, IO). Once all the available Fe3+ stant = has reacted with CU(EDTA)~then the relative peak heights corresponding to Fe(EDTA)- and excess CU(EDTA)~remain constant. It is therefore necessary to assure that all the Fe3+ has reacted. The CU(EDTA)~completely reacts with Fe3+ in approximately 2 h. Figure 1illustrates the chromatogram obtained on injecting a mixture of Fe(EDTA)- and Cu(EDTA)*- wherein all the Fe3+ is chelated and the excess EDTA is then complexed with Cu2+. Since iron is a major interference in the analytical method for the determination of EDTA, we attempted to analyze EDTA as the Fe(EDTA)- complex. The chromatographic peak corresponding to the complex was symmetric with a capacity factor, k' = 0.88. Injecting Fe(EDTA)- as a sample and plotting peak height as a function of micrograms of
2 I
"R Figure 1. Chromatogram obtained with a sample containing: (1) excess CuSO ; (2) 200 ppm Na,EDTA complexed with an equimolar amount of Fe3'; and (3) 72 ppm Na,EDTA as Cu(EDTA)*-. The sample
was analyzed as described in the Experimental Section with the absorbance setting at 0.32 aufs. V , is retention volume. Table I. Effect of the Addition of an Interferent on Relative Peak Height (RPH)' RPH interferent concentration cu2+ 2.6 X M (165 ppm) 1.00 Ni2+ 6.8 x M (40 ppm) 1.00 H,PO; 1.5 x M (145 ppm) 0,93 CaZ 1.25 x M (50 ppm) 1.00 Mg2+ 2.06 x l o v 3M (50 ppm) 1.00 a Sample consisted of 4.2 X lo-* M Na,EDTA (182 ppm) in a 2-fold molar excess of FeCl, prepared in the mobile phase. +
Na4EDTA injected yielded a linear response between 1013 ppm and 0.5 ppm Na4EDTA with a sensitivity of 0.0706 au/pg, and a limit of detection of 0.3 ppm. The minimum detectable quantity of EDTA was 35 ng (determined as Na4EDTA). The relative standard deviation of the chromatographic method was 1.5%. In order to understand the role of interfering ions, a number of potential interferents were added to a prepared sample (see Table I). Copper, nickel, calcium, and magnesium did not interfere; phosphate at a concentration of 145 ppm caused a 7% reduction in the height of the chromatographic peak. In order to evaluate the method, a number of solutions were prepared to simulate boiler water treated with EDTA. A 2% EDTA treatment solution (sample 1)was prepared by mixing Versene 220 (technical grade Na,EDTA, Dow Chemical Co.) with tap water. The solution was heated rapidly in an autoclave to a temperature of 190 OC and a pressure of 11.7 bar (170 psig). Analyzing the sample for trace metal ions by using a direct current plasma (DCP) emission spectrometer, high levels of iron (250 ppm), calcium (20 ppm), and magnesium (6 ppm) were found. These levels are all higher than the levels found in boiler water (2). Analyzing the sample resulted in a total Na,EDTA concentration of 1.88 f 0.05%. The low value can be attributed to decomposition of some of the EDTA at the elevated temperature. A second sample of 2% EDTA was heated to a temperature of 210 OC and maintained at a pressure of 17.2 bar (250 psig) for 1.75 h. The analysis indicated 1.15 f 0.07% Na4EDTA. Approximately 50% of the EDTA had decomposed. These results are corroborated by nuclear magnetic resonance (NMR) data which indicate that Na4EDTA has a half-life at 210 OC on the order of 1-3 h (3). Finally, a boiler water sample obtained from a steam propulsion boiler (oil-fired natural circulation Babcock and Wilcox D-type steam generator on a "chelant treatment") containing
317
Anal. Chem. 1904, 56,317-319
ppm added Na,EDTA, the sample was found to contain 1.2 ppm Fe(EDTA)-.
i
ACKNOWLEDGMENT We thank Ramanathan Panayappan for performing the elemental analyses of the simulated boiler water sample and John Liebermann for many helpful discussions. Registry No. EDTA, 60-00-4; water, 7732-18-5. LITERATURE CITED
Figure 2. Chromatograms obtained with a sample containing boller water "spiked" with Na,EDTA to a final added concentration of (1) 0 ppm, (2) 0.5 ppm, (3) 1.3 ppm, (4) 2.3 ppm, (5) 4.6 ppm, and (6) 9.1 ppm. Retention volume ( V , ) of Fe(EDTA)- is 4.20 mL. The detector
(1) Motekaitis, Ramunas J., Cox, X. B., 111; Taylor, Patrick; Martell, A. E.; Miles, Brad; Tvedt, Tory J., Jr. Can. J . Chem. 1982, 60, 1207-1213. (2) Venezky, David L.; Dausuel, Robert L., Jr. "EDTA as a Boiler Water Treatment"; Proceedings of the 40th International Water Conference, Pittsburgh, PA, 1979; Paper No. 1WC-79-6. (3) Venezky, David L.; Moniz, William B. Anal. Chem. 1969, 41, 11-16. (4) Annu. Book ASTM Stand. 1982, ASTM 031 13-80 Part 31 (Water), 727-731. (5) Sniegoski, Paul J.; Venezky, David L. J. Chromatogr. Sci. 1974, 12, 359-361. (6) Chen, S. G.; Cheng, K. L.; Vogt, Corazon, R. Mikrochim. Acta 1983, 473-481. (7) . . Perfetti. Gracia A.; Warner, Charles R. J . Assoc. Off. Anal. Chem. 1979, 62, 1092-1095. (8) Parkes, D. G.; Caruso, M. G.; Spradling, J. E., I11 Anal. Chem. 1981, 53, 2154-2156. (9) "Keys to Chelation"; Dow Chemical Co.: Midland, MI, 1969. (10) Martell, Arthur; Sillen, Lars G. Chem. SOC., Spec. Pub/. 1964, No. 17,Section 11.
setting is 0.04 aufs.
2 ppm phosphate ion was analyzed for total EDTA. By use of the method of standard additions, the sample was spiked with N%EDTA. See Figure 2. After the data were linearized from five different spiked samples, and extrapolated to 0.0
RECEIVED for review August 18, 1983. Accepted October 4, 1983. W. Rudzinski wishes to thank the American Society of Engineering Education (ASEE) for a Summer Fellowship at the Naval Research Laboratory.
Iridium/Iridium Oxide Electrode for Potentiometric Determination of Proton Activity in Hydroorganic Solutions at Sub-Zero Temperatures Silvano Bordi,* Marcello Carll, and Giorgio Papeschi Department of Chemistry, University of Florence, Via Gin0 Capponi 9, 50121 Florence, Italy
Sergio Pinzauti Department of Pharmaceutical Sciences, University of Florence, Via Gino Capponi 9, 50121 Florence, Italy At present spectrophotometry is the most widespread method for paH+determination in cryobiochemistry work. However, this technique is rather complex, cumbersome, and time-consuming for routine work (1). In an attempt to setup a simpler and quicker method for measuring proton activity a t sub-zero temperatures, some researchers have modified (2) the glass electrode, replacing the internal solution with a hydroorganic mixture. The modified glass electrode however exhibits several drawbacks including increase in electrode impedance and decrease in response speed as the temperature decreases. The aim of this work is to verify the feasibility and performance of the Ir/Ir02 electrode for paH+potentiometric measurements in hydroorganic solutions in the -20 to +20 "C temperature range. The use of iridium-thick iridium oxide electrodes with a view to producing practical pH sensors has so far been limited to few investigations in aqueous solutions ( 3 , 4 )or in biological
fluids (5,6) but they are particularly attractive candidates for a future generation of pH sensors because of their stability, their low impendance, and their fast response.
EXPERIMENTAL SECTION Iridium wire of 99.9% purity, 0.5 mm in diameter, was supplied by Engelhard Industries, Newark, NJ. Platinum wire of 99.9% purity, 0.2 mm in diameter, was supplied by Metalli Preziosi, Milan, Italy. All reagents used throughout were of analytical grade. Bidistilled water was used for preparing the solutions. A combined Radiometer GK 232C glass electrode was employed for measuring the pH of aqueous solutions, in conjunction with a pH meter with resolution of 0.01 pH (SEAC Model 106,Florence, Italy). The iridium/iridium oxide electrode was prepared by welding 1 cm of Ir wire t o 10 cm long Pt wire, and it was oxidized by heating in an electric oven at 600 "C, the iridium wire being wetted with saturated NaHC03 solution. The oxidation process was
0003-2700/84/0356-0317$01.50/00 1984 American Chemical Society
