Nuclear Magnetic Resonance Study of the Thermal Decomposition of Ethylenedinitrilotetraacetic Acid and Its Salts in Aqueous Solutions David L. Venezky and William B. Moniz
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 14, 2015 | http://pubs.acs.org Publication Date: January 1, 1969 | doi: 10.1021/ac60270a005
Chemistry Division, Naval Research Laboratory, Washington, D . C. 20390
Aqueous and deuterium oxide solutions of ethylenedinitrilotetraacetic acid (H4EDTA), disodium ethylenedinitrilotetraacetate (Na2H2EDTA), and tetralithium ethylenedinitrilotetraacetate (Li4EDTA) were degassed to remove dissolved oxygen and then heated at 200 OC in sealed NMR tubes for periods ranging from one hour to over 300 hours. Periodic examination of the contents using proton NMR showed that decomposition occurred rapidly in the order H4EDTA > Na2H2EDTA> Li4EDTA. Approximate rates for the degradation processes have been determined. The longest halflife, that for 0.1M Li4EDTA solution, is approximately 13 hours at 200 OC. The NMR results suggest a primary, pH dependent, stepwise loss of (-CH2C02-) groups from EDTA4- to form (-02CCH2)NHCH2CH2N(CHzC02-)2, A NH,C H,CH zN(CH$02-)2, a nd (CH 2NHCH*C02-)2. slower, secondary decomposition produced products not yet identified.
MOSTanalytical procedures employing ethylenedinitrilotetraacetate (EDTA4-) solutions are carried out at temperatures of 100 "C or less; however, other uses of the chelating agent require higher temperatures. For example, in pressurized utility boilers that operate at temperatures up to about 320 "C, HrEDTA and its salts have been used as cleaners, scale preventers, and corrosion inhibitors (1-6). The thermal stability of ethylenedinitrilotetraacetic acid (H4EDTA) and its salts in solution and in the solid state were reviewed recently (7, 8). Although laboratory tests have been conducted to determine the temperature at which EDTA4- solutions can be utilized without thermal decomposition, complete experimental details and results are not available (7). Edwards and Merriman (7) report that in the absence of oxygen EDTA4appears to be stable to at least 204 "C, and solutions containing 100-200 ppm EDTA4- give indications of complete stability of the free chelate (metal chelate) up to 250-60 "C for periods of two hours. EDTA 4- in aqueous solution containing dissolved oxygen appears to act as an oxygen scavenger at temperatures above 180 "C. Inactivation of the chelating ability of EDTA4- in solution at temperatures of 149 "C and above has been attributed to dissolved oxygen ( I , 5, 6). The corrosion rate of metals (2, 6) and scale prevention in boilers ( 5 , 7) are drastically affected by dissolved oxygen and EDTA4- at high temperatures. Edwards and Merriman (7) report that one part of oxygen equivalent causes 50 to 100 parts EDTA4- to be J. K. Rice, Proc. Amer. Power Conf.,26,814 (1964). W. R . Merriman, ibid., 26, 824 (1964). J. J. Roosen and J. V. Levergood, ibid.,27,790 (1965). L. G. Friedle, ibid., 27, 801 (1965). C. Jacklin, ibid., 27, 807 (1965). J. A, Lux, ibid., p 817. J. C. Edwards and W. R. Merriman, Proc. bzt. Water Conf., Eng. SOC.of W . Pa., 24, 35 (1963). (8) D. L. Venezky, NRL Report 6674, Naval Research Laboratory, Washington, D. C., Dec. 1967.
(1) (2) (3) (4) (5) (6) (7)
nearly worthless in preventing deposits in water boilers. Although other oxidizing agents (9), such as peroxide, hypochlorite, permanganate, thiosulfate, ceric ions, and vanadate ions, have been found to oxidatively degrade EDTAd-, the organic products of the degradations have not been thoroughly studied. The analytical methods for the estimation of residual EDTA4- in solutions heated to high temperatures fail to take into account the possible formation of new nonvolatile chelating agents through the degradation of EDTA4-. Employing NMR techniques which recorded the proton spectra of the organic chelating agent, we studied and report herein the rate of thermal decomposition of oxygen-free aqueous and deuterium oxide solutions containing ethylenedinitrilotetraacetic acid (HIEDTA), disodium ethylenedinitrilotetraacetate (NazH2EDTA) and tetralithium ethylenedinitrilotetraacetate (Li,EDTA). Some of the decomposition products have been tentatively identified on the basis of the NMR data. EXPERIMENTAL Apparatus. NMR spectra were recorded using a Varian HA-100 spectrometer operating in the field sweep mode. The water peak was used as the locking signal. Our choice of water as the reference was dictated by the desire to maintain a pure system; furthermore, it is not known if the commonly used internal reference for aqueous solutions, sodium 3-(trimethylsilyl)-l-propanesulfonate (TSS), is stable at 200 "C. Because our measurements showed that the chemical shift of 4.775 ppm between the water peak and TSS changes by less than 1 Hz (0.01 ppm) over the pH range 2 to 13, we feel that sufficient accuracy is maintained using a water reference. As far as practicable, identical instrument settings were used for a given sample; the control sample was used to reset any variations in peak intensities. This facilitated quantitative measurement of spectral changes. Electronic integration could not be employed because of the close spacing between peaks, and the proximity to the lock signal. Consequently, signal heights were measured from the recordings. Peak positions were measured to 1 0 . 1 Hz with a frequency counter. Reagents. H4EDTA (Fisher Certified) and Na2H2EDTA. 2H20(Baker Analyzed) were used without further purification. Li4EDTA was prepared by reacting H4EDTA with an excess of lithium carbonate, filtering excess lithium carbonate from the boiling mixture, and evaporating the filtrate to concentrate the product. After freeze drying, the hygroscopic powder was analyzed for EDTA4- content by the method of Piibil and VeselT (IO). The analytical results, infrared spectrum, and DTA were consistent with the anhydrous salt. Distilled water and deuterium oxide (Bio-Rad Laboratories, 99.84 DzO)were used as solvents to prepare the appropriate mol solutions.
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(9) P. N. Palei and N. I. Udal'tsova, Zh. Anal. Khim., 15, 668 (1960). (10) R. Piibil and V. VeselL, Chemist-Analyst, 56, 83 (1967). VOL. 41, NO. 1, JANUARY 1969
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Procedure. A 0.25M stock solution of NazH2EDTA in H 2 0 was prepared and the required amounts transferred to a standard thin-wall NMR tube. All other H 2 0 and DzO solutions were prepared by dissolving in an NMR tube the appropriate quantity of solid in 1 ml of solvent. The acid was not soluble in HzO or DzO and only 0.5 ml of the mixture was prepared. For solutions, one half of the liquid was transferred to a second NMR tube, freeze-thaw degassed four times under reduced pressure to Torr) and finally the solution or mixture was sealed under reduced pressure. About 15 p1 of HzO was added to the DzO solutions to serve as a locking signal for the NMR spectrometer. The samples in the unsealed NMR tubes were retained as standard solutions. The sealed NMR tube was placed in an 8” length of l/a” steel pipe capped at each end. The tubes and protective jacket were heated in an oven set at 200 “C 5 ” ; length of heating period designates the time the tube was in the oven. The tube was allowed to cool 45 minutes before it was removed from the protective jacket. The NMR spectrum of a sample was determined initially and following each period of heating. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 14, 2015 | http://pubs.acs.org Publication Date: January 1, 1969 | doi: 10.1021/ac60270a005
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RESULTS AND DISCUSSION
Because oxygen affects the stability of EDTA4- in aqueous solutions (2, 5-7), special care was taken to conduct our experiments in the absence of dissolved oxygen. Four cycles of the freeze-pump-thaw degassing were used to remove traces of dissolved oxygen from the solutions sealed in the NMR tubes. By simulating the degassing on 25 ml of distilled water, we found approximately 0.08 ppm of residual oxygen. However, this quantity of oxygen is attributed to gas absorbed during sample handling in the gas chromatographic method used to determine dissolved oxygen (11). Consequently, the thermal decompositions may be considered to be taking place in the absence of dissolved oxygen (12). The proton NMR spectrum of MaEDTA (M = H and/or monovalent metal ion) consists of two lines, in a 2 :1 intensity ratio, corresponding to the two kinds of protons in the molecule-those situated on the carbon atom a to the carbonyl group (8 per molecule) and those situated on the carbon atoms between the two nitrogens (4 per molecule) (Figure la): MOzCCHz
\ / MOzCCHz
I
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/O
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I 90
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l I l l I t I f f 110 120 130 140 150
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Figure 1. NMR spectra of 0.18M aqueous solution Of NazHzEDTA 2 H z 0 under reduced pressure. Heated at 200 “C
CHzCOzM NXHzCH2-N
/
\ CHzCOzM
If M is H, the rapid exchange of this acidic proton averages its signal with that of the bulk water solvent. Because the spectrometer is operated in such a way that the locking signal is derived from the protons in the bulk water peak, information on the carboxylic acid protons is not obtainable. The symmetry and configuration of the MIEDTA molecule are such that there is no observable spin coupling between protons. The nitrogen atoms are presumably relaxing (changing spin states) rapidly due to quadrupolar effects, thus washing out nitrogen-proton spin-coupling. NazH2EDTA(0.18M) in HzO was the first substance for which solution thermal decomposition was attempted ; consequently, mild temperatures and short heating periods were employed initially. After 16 hours at 120 “C and 16 hours at 150 “C, no observable changes in the NMR spectrum oc-
e
a. Unheated b. 1.5 hours c. 3.5 hours
d. 9.5 hours
12
ANALYTICAL CHEMISTRY
(11) J. W. Swinnerton, V. J. Linnenbom, and C. H. Cheek, ANAL. CHEM., 34, 1509 (1962). (12) C. P. Poole, Jr., “Electron Spin Resonance,” Interscience, New York, N. Y.,1967, p 629.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 14, 2015 | http://pubs.acs.org Publication Date: January 1, 1969 | doi: 10.1021/ac60270a005
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nz
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Figure 2. NMR spectra of 0.17M deuterium oxide solution of Na2H2EDTA.2H20sealed under reduced pressure. Heated at 200 "C
a. Unheated b. 1 hour c. 5 hours
1
I
d. 10 hours e. 20 hours
I
I curred compared to the spectrum of an unheated sample (Figure la). It appears that 200 "C is about the lowest temperature at which decomposition proceeds at any significant rate, After 1.5 hours at 200 "C,gross changes in the spectrum were apparent (Figure lb). The signal at 89 Hz had decreased in relative intensity, and a new signal appeared slightly upfield from it, at 92 Hz. In addition, a pair of multiplets centered at 88 and 137 Hz appeared. As heating proceeded, the signal at 89 Hz decreased further in intensity, while the signals centered at 92, 88, and 137 Hz continued to grow. After 3.5 hours at 200 "C (Figure IC) a signal appeared at 115 Hz, slightly upfield from the 113 Hz peak. The significant changes in the spectrum upon additional heating to 9.5 hours involve the 89 and 113 Hz signals, which decreased in intensity and moved to 112 and 144 Hz, respectively, before disappearing (Figure Id). After much longer heating times (up to 320 hours), additional signals to higher field (ca. 190 and 207 Hz) were noticeable. A sample of Na2H2EDTAwas run in DzO under the same conditions as the above aqueous solution. The NMR spectrum of the unheated solution is shown in Figure 2a. After 1 hour at 200 "C,the signal at 87 Hz had completely disappeared, while the signal at 109 Hz had decreased in amplitude significantly (Figure 2b). The multiplets centered at 88 and 137 Hz were comparable in size and appearance to those of
Figure 3. NMR 0.26M aqueous LtEDTA sealed duced pressure. 210 "C
spectra of solution of under reHeated at
a. Unheated b. 5 hours c. 45 hours d. 137.5 hours e. 209.5 hours
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NazH2EDTA > H4EDTA may be a result of resonance stabilization of the free carboxylate ions, which is reduced in the protonated group. It is also possible that the (13) L. G. SillCn and A. E. Martell, "Stability Constants of MetalIon Complexes," Special Publication No. 17, The Chemical Society, London, 1964.
acidic proton plays an active part in the degradation process, perhaps through a hydrogen-bonding mechanism. Proton association with nitrogens in EDTA4- has been reviewed recently by Fujiwara and Reilley (14); a possible backside displacement of the (-CHzCO2-) group by the acidic proton is suggested. Effects of pH and Lewis acids on the rates of decomposition and on the nature of the decomposition products are currently under investigation. Overriding the questions of mechanism and relative stability is the evidence that none of the compounds studied is indefinitely stable in oxygen-free aqueous solution at temperatures of 200 "C and higher. Because a doubling in rate is usually observed for every 10 OC increase in temperature, at temperatures substantially greater than 200 "C the lifetime of the initial compounds would be short indeed. The structural assignments reported herein are tentative. We are currently preparing the postulated intermediates and will report the results in a future paper. RECEIVED for review July 1, 1968. Accepted October 7, 1968. (14) Y . Fujiwara and C. N. Reilley, ANAL.CHEM., 40, 890 (1968).
Computation of Channels Ratio in Liquid Scintillation Counting. A Theoretical Consideration C . T . Peng Radioactivity Research Center and Department of Pharmaceutics! Chemistry, School of Pharmacy, University of California, San Francisco Medical Center, San Francisco, Calif. 94122
A semi-empirical method is given for computing channels ratio in quench correction in liquid scintillation counting from two parameters k and y, which are ratios of integral count rates obtained from quenched and unquenched homogeneous samples using the discriminator levels defining the channel width as base lines. I n addition, a procedure for adjusting instrument settings to given channels ratio is outlined. The use of parameters k and y is advantageous in allowing sensitive adjustment in operating conditions to be made to achieve optimal correlation of counting efficiency with channels ratio and also in permitting interrelation of correlation curves obtained on different spectrometers.
AMONGTHE MANY methods reported for quench correction in liquid scintillation counting of homogenous samples, the channels ratio method has the advantage and convenience of affording information on quench correction simultaneously with the counting of the sample. 'This method, as described originally by Baillie (1) and later elaborated by Bush (2), 16
ANALYTICAL CHEMISTRY
involves a comparison of the count rates registered in the two channels of a spectrometer which are so selected that one channel monitors and the other counts different portions of the beta energy spectrum of a radionuclide such as aH, IC,or ssS. In the absence of quenching, the ratio of the count rates in the monitor and the counting channels remains constant. As quenching occurs, a decrease in the intensity of the scintillation from the sample takes place, resulting in a diminution of pulse height with a downward shift of the photon intensity spectrum. Because a slight shift of the photon intensity spectrum invariably causes a change in the channels count rate ratio, it is practical, based on this principle, to utilize the deviation of the channels ratio of a quenched sample from that observed of an unquenched sample to determine the degree of quenching.
(1) L. A. Baillie, Int. J. Appl. Radiat. Isotopes, 8, 1 (1960). 35, 1024 (1963). (2) E. Bush, ANAL.CHEM.,
