Ind. Eng. Chem. Fundam.
356
Wellstead, P. E.; Zanker, P. Znt. J . Control 1979, 30, 27. Ydstie, B. E.; Liu, L. K. I n Proceedings of the American Control Conference, San Diego, CA , 1984. Ydstie. E. E.; Sargent, R. W, H. I n Proceedings of the 6th International Federation of Automatic Control Symposium on Estimation and System Parameter Identification, Washington, DC , June 1982.
1986,25, 356-359 Ydstie, B. E.: Kershenbaum, L. S.: Sargent, R. W. H. AIChE J . 1985, 3 1 , 1771.
Received for review June 11, 1984 Accepted September 17, 1985
Effect of Citrate Buffer Additive on the Absorption of NO by Solutions of Ferrous Chelates Elizabeth A. Grlfflths and Shlh-Ger Chang" Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720
Citrate buffer in solutions can act as a ligand and compete with other ligands for complexation with ferrous ion. As a result, more than one type of ferrous chelate and/or ferrous mixed chelates may be formed. Subsequently, the absorption capacity of NO in the system is affected. We have studied this effect on Fe"(IDA), Fe"(NTA), and Fe"(EDTA and determined the equilibrium constants, enthalpies, and entropies for the coordination of NO to Fe"(Cit) and to Fe' (IDA) as a function of pH and temperature in aqueous solutions. NO absorption studies and polarographic results indicate a previously unrec nized mixed-ligand complex, Fe"(CiXIDA), which has a larger stability constant for NO than either Fe"(Cit) or Feqs(1DA)alone at 25 O C .
1
Introduction In a system for removing nitrogen and sulfw oxides from wet flue gas, nitrosyl ferrous chelates are formed (Littlejohn and Chang, 1982; Chang et al., 1982, 1983; Lin et al., 1982) when ferrous chelates are added in the scrubbing liquor to promote the solubility of NO. The reaction between the complexed NO and HSO5.2. In addition, it was employed to show that only Fe"(Cit) formed a t pH 6.5; for Fe"(IDA)NO, AH" = -9.13 kcal mol-', AS" = -6.33 eu, and K = 2.05 X lo5 M-l at 25 "C, pH 6-7; and for Fe"(Cit)(IDA)NO, K 23.3 X lo5 M-' at 25 "C and K 15.5 X lo4 M-' at 55 "C. Our previous studies of the equilibrium constant of Fe"(Cit)NO at pH 5.1 found it to be much smaller (Littlejohn and Chang, 1982) than that determined here. This is because the stability constant of ferrous citrate is only lo3,' (Hamm et al., 19541, and the one-to-one molar ratio of citrate ion to ferrous ion used in our previous studies was insufficient for all the ferrous ion to be in the form of Fe"( Cit). 0
-0.6
-0.3 Edr,
v8.
-0.9
S.C.E., Volts
Figure 5. Polarograms of Fe"(IDA)(Cit)NO and Fen(Cit)NO a t pH 6 a n d 25
OC.
Fe"(Cit)NO are recorded at pH 6. The half-wave potentials of their characteristic cathodic waves (Uchiyama et al., 1977; Kaneko and Nozaki, 1977) are -0.76 and -0.785 V, respectively. The sudden abruptness of the waves indicates a multielectron process. In analogy to Fer'(EDTA)NO, a similar system investigated by linear scanning voltammetry (Uchiyama and Muto, 1981), the electrode reaction can be described by [Fe"(Cit)NO]-
[Fe"(Cit)HNO-I2[Fe"(Cit)NHOH]-
--
+ 2e + H+
+ H+
+ e + H+
[Fe"(Cit)HNO-I2- (7)
[Fe"(Cit)NHOH][Fe"(Cit)]-
(8)
+ NH20H (9)
Conclusion Ferrous chelates can be used to augment the solubility of nitric oxide substantially in a wet flue gas simultaneous denitrification and desulfurization system. The Fe"(Cit) and Fe"(1DA) complexes are not as effective in absorbing NO as either Fe"(EDTA) or Fe"(NTA). The advantage of citrate is that it can act as both a ligand and a buffer and it is inexpensive. The effect of the citrate buffer on the binding of NO by ferrous chelates was calculated and examined experimentally. Consequently, the mixed-chelate complex Fe"(Cit)(IDA) was found. (The theoretical calculations applied only when there were no new species formed.) The equilibrium constants, enthalpies, and entropies for the coordination of NO to Fe"(Cit), Fe"(Cit)(OH-), Fe"-
Acknowledgment We appreciate the support and encouragement of Earl Evans, Joseph Strakey, and John Williams. We thank Dave Littlejohn for assistance in running polarographic experiments. This work was supported by the Assistant Secretary for Fossil Energy, Office of Coal Research, US. Department of Energy, under Contract No. DE-AC0376SF00098 through the Pittsburgh Energy Technology Center, Pittsburgh, PA. Registry No. NO, 10102-43-9. Literature Cited Chang, S.G.; Littlejohn, D.; Lin, N. H. In Flue Gas Desulfurization; American Chemical Society: Washington, DC, 1982; ACS Symp. Ser. No. 188. Chang. S. G.; Littiejohn, D.; Lynn, S. Environ. Sci. Techno/. 1983, 17, 649. Ha", Randall E.; Schull, C. M.; Grant, D. M. J . Am. Chem. SOC.1954, 7 6 , 21 11. Hishinuma, Y.; Kaji, R.; Akimoto, H.; Nakajima, F.; Mori, T.; Kamo, T.; Arikawa, Y.; Nozawa, S. Bull. Chem. SOC.Jpn. 1979, 52, 2863. Intorre, B. J.; Martell, A. E. J . Am. Chem. SOC. 1961, 8 3 , 3618. Kaneko, H.; Nozaki, K. Rev. Polarogr. 1977, 2 3 , 68. Lin, N. H.;Llttlejohn, D.; Chang, S. G. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 725. Lingane, J. L. J . Am. Chem. SOC. 1946, 6 8 , 2448. Llttlejohn, D.; Chang, S. G. J . Phys. Chem. 1882, 8 6 , 4, 537. Meites. L. J . Am. Chem. SOC. 1951, 73, 3727. Ottmers, D. M. "Evaluation of Regenerable Flue Gas Desulfurization Processes"; revised report; Radian Corporation: Austin, TX, July 1976; VOl. EPRI RP535-1, EPRI FP-272. Ogura, K.; Watanabe. M. Electrochim. Acta 1982, 27, 1, 111-4. Ras, M. Janardhan; Sethuram, B.; Navaneeth Rao, T. Bull. SOC.Chim. Belg. 1982, 91(2), 111-6. Ringbom, A. Complexation of Analytical Chemistry; Interscience: New York, 1963; pp 37, 332, 356. Tokmadzhyan, M. A.; Dobrynina, N. A.; Martynenko, L. I.; Alchudzhan, A. A. I z v . Akad. Nauk SSSR, Ser. Khim. 1975a. 460-2. Tokmadzhyan, M. A.; Dobrynina, N. A.; Martynenko. L. I.; Alchudzhan, A. A. Zh. Neorg. Khim. 1975b, 20(10), 2842-3. Uchiyama, S.;Nozaki, K.; Muto, G. Bunseki Kagaku 1977, 2 6 , 219-244. Uchiyama, S.;Muto, G. J . Elecfroanal. Chem. 1981, 127, 275-9.
Received f o r review June 25, 1984 Accepted August 9, 1985
