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1 University of Groningen Dissocition constnts nd thermodynmic properties of mino cids used in CO2 bsorption from (293 to 353) K Hmborg, E. S.; Niederer, J. P. M.; Versteeg, Geert Published in: Journl of Chemicl nd Engineering Dt DOI: /je700275v IMPORTANT NOTE: You re dvised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Plese check the document version below. Document Version Publisher's PDF, lso known s Version of record Publiction dte: 2007 Link to publiction in University of Groningen/UMCG reserch dtbse Cittion for published version (APA): Hmborg, E. S., Niederer, J. P. M., & Versteeg, G. F. (2007). Dissocition constnts nd thermodynmic properties of mino cids used in CO2 bsorption from (293 to 353) K. Journl of Chemicl nd Engineering Dt, 52(6), DOI: /je700275v Copyright Other thn for strictly personl use, it is not permitted to downlod or to forwrd/distribute the text or prt of it without the consent of the uthor(s) nd/or copyright holder(s), unless the work is under n open content license (like Cretive Commons). Tke-down policy If you believe tht this document breches copyright plese contct us providing detils, nd we will remove ccess to the work immeditely nd investigte your clim. Downloded from the University of Groningen/UMCG reserch dtbse (Pure): For technicl resons the number of uthors shown on this cover pge is limited to 10 mximum. Downlod dte:

2 J. Chem. Eng. Dt 2007, 52, Dissocition Constnts nd Thermodynmic Properties of Amino Acids Used in CO 2 Absorption from (293 to 353) K Espen S. Hmborg, John P. M. Niederer, nd Geert F. Versteeg*, Deprtment of Science nd Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlnds, nd Procede Group BV, P.O. Box 328, 7500 AH Enschede, The Netherlnds The second dissocition constnts of the mino cids β-lnine, turine, srcosine, 6-minohexnoic cid, DLmethionine, glycine, L-phenyllnine, nd L-proline nd the third dissocition constnts of L-glutmic cid nd L-sprtic cid hve been determined from electromotive force mesurements t tempertures from (293 to 353) K. Experimentl results re reported nd compred to literture vlues. Vlues of the stndrd stte thermodynmic properties re derived from the experimentl results nd compred to the vlues of commercilly vilble mines used s bsorbents for CO 2 cpture. 1. Introduction Aqueous solutions of mines re frequently used for the removl of cid gses, such s CO 2 nd H 2 S, from vriety of gs strems. In prticulr, queous solutions of lknolmines nd blends of lknolmines re widely pplied in gs treting. 1 It hs been reported tht lknolmines undergo degrdtion in n oxygen tmosphere, usully encountered in the tretment of flue gses. Aqueous mino cid slt solutions might be n ttrctive lterntive to lknolmines. They hve been found to hve better resistnce to degrdtion, nd their rectivity with CO 2 is comprble to queous lknolmines of relted clsses. Due to their ionic nture, queous solutions of slts of mino cids hve negligible voltility nd, mong others, lso hve higher surfce tension thn queous solutions of lknolmines. 2-4 The second dissocition constnts of β-lnine, turine, srcosine, 6-minohexnoic cid, DL-methionine, glycine, L- phenyllnine, nd L-proline nd the third dissocition constnts of L-glutmic cid nd L-sprtic cid hve been determined from (293 to 353) K in this work by the use of electromotive force mesurements. Prts of the results extend the temperture rnge of vilble literture dt for the compounds under investigtion. 2. Procedure 2.1. Amino Acids. β-alnine, turine, srcosine, 6-minohexnoic cid, DL-methionine, glycine, L-phenyllnine, nd L-proline dissocite in queous solutions ccording to - O 2 C-R-NH H 2 O h - O 2 C-R-NH 2 + H 3 O + (1) where the presence of the protonted mino cid zwitterion is neglected. The equilibrium constnt cn be determined by electromotive force (EMF) mesurements using combined glss ph electrode. A combined glss ph electrode hs two well-known unplesnt properties. * Corresponding uthor. E-mil: geert.versteeg@procede.nl. University of Twente. Procede Group BV. Figure 1. Influence of the dimensionless overll mollity of β-lnine on ln(k 2,exptl): b, experimentl vlues; - -, liner regression. (1) Filure to obey the Nernst eqution perfectly. This error rises due to the lkline error nd the symmetry potentil, which cn vry with the chnge of the ctivity of H 3 O + in the solution under investigtion. In strong lkline solutions, errors my pper due to the contribution of lkline ions (e.g., N +, K +, etc.), nd the mesured ctivity of H 3 O + ppers to be higher thn the ctul ctivity. (2) A tendency to drift in potentil by millivolts over periods of minutes to dys. The stndrd potentil of the electrode is not constnt in time. To overcome the unplesnt properties described bove, combined glss ph electrode cn be trnsferred bck nd forth between two solutions of different compositions while continuously recording the electrochemicl potentil. Mesurements comprble to the ccurcy of system including stndrd hydrogen electrode cn be chieved if used with noise-free voltmeter, proper shielding, nd surrounding temperture mtching of the solutions. The sme electrode hs to be used during both mesurements, nd the time period between the two mesurements should be s short s possible. 5 A combined ph /je700275v CCC: $ Americn Chemicl Society Published on Web 11/15/2007

3 2492 Journl of Chemicl nd Engineering Dt, Vol. 52, No. 6, 2007 Tble 1. Comprison of Correlted Vlues of ln(k) nd Thermodynmic Properties with Literture Dt for Protonted MDEA T/K this work Kmps 21 Oscrson 10 Kim 22 Littel 11 Schwbe 12 ln(k) r G m /kj mol r H m /kj mol electrode nd two-cell system were used to determine the dissocition constnts nd Extrpolted vlue. Ag(s), AgCl(s) 3 M KCl(q) HCl(q,mj HCl ) Glss Electrode (I) Ag(s), AgCl(s) 3 M KCl(q) NOH(q, mj NOH ) - O 2 C-R- NH + 3 (q, mj -O2 C-R-NH 3 +) Glss Electrode (II) HCl ws chosen to be used in cell I s reference solution, since combined ph electrode cn determine the ctivity of H 3 O + with higher ccurcy in n cidic solution thn in bsic solution. As the ctivity of pure solid is set to unity, the Nernst eqution for cell I results in nd for cell II results in E I ) E (T I ) - RT I F ln( H 3 O+ Cl -) I (2) E II ) E (T II ) - RT II F ln( H 3 O+ Cl -) II (3) As both cells re t the sme temperture, T I ) T II, the stndrd potentil of the combined ph electrode is the sme during both mesurements, E (T I ) ) E (T II ) T II ln( H3 O+ Cl -) II ) F(E I - E II ) + T R I ln( H3 O+ Cl -) I (4) In the cse of exct temperture mtching of the two cells, T I ) T II ) T, the ctivity of the inner reference electrolyte of the combined ph electrode is identicl during both mesurements, ( Cl- ) I ) ( Cl- ) II ln( H3 O+) II ) F(E I - E II ) + ln( RT H3 O+) I (5) As HCl is completely dissocited, the dissocition of wter cn be neglected in cell I. The mss blnces for the mino cid, NOH, nd wter in cell II re nj -O2 C-R-NH 3 +) n -O2 C-R-NH 3 ++ n -O2 C-R-NH 2 (6) Electroneutrlity results in nj NOH ) n N + (7) nj H2 O ) n H 2 O + n H 3 O+ + n OH - (8) n N + + n H3 O+ ) n -O2 C-R-NH 2 + n OH - (9) The chemicl equilibrium conditions for both rections present re H3 O+ OH - K w (T) ) 2 H2 O -O2 C-R-NH 2 H3 O+ K(T) ) -O2 C-R-NH 3 + H2 O (10) (11) For given temperture nd composition, the electromotive forces E I nd E II nd the temperture in ech cell re mesured. Activities of HCl nd KCl re estimted using the excess energy model of Pitzer from Holmes 6 nd Pbln, 7 respectively. The ctivity of wter follows from the Gibbs-Duhem eqution. A very brief outline of the excess energy model of Pitzer used in this work is given in Appendix A. Activities of the compounds present in cell II re pproximted using the modified Debye-Hückel term in Pitzer s eqution, i.e., neglecting binry nd ternry prmeters. The ctivity coefficient of the zwitterion structure of the mino cid is set to unity for ll mollities nd tempertures. The influence of pressure on the chemicl rections is neglected, nd it is set to 1 br during the described clcultions. The chnges in the compositions of the electrolyte cells due to the outflow of the KCl electrolyte from the electrode re neglected. The wter dissocition constnt, K w (T), is tken from Fisher. 8 With the given informtion, eq 4 to eq 11 re solved itertively to yield the true number of moles of ech species present in cell II, s well s preliminry vlue for the dissocition constnt of the mino cid. This dissocition constnt is clled preliminry becuse it is clculted out of set of equtions in which for cell II the ctivities re not exctly known. The experiments re performed t

4 Journl of Chemicl nd Engineering Dt, Vol. 52, No. 6, Tble 2. Comprison of Correlted Vlues of nd Thermodynmic Properties with Literture Vlues for β-alnine T/K this work My 24 Gillespie 13 Dey 14 Mjumdr 15 Boyd 23 Christensen r G m /kj mol r H m /kj mol Extrpolted vlue. Tble 3. Comprison of Correlted Vlues of nd Thermodynmic Properties with Literture Vlues for Turine T/K this work King r G m /kj mol r H m /kj mol Extrpolted vlue. different overll mollities of the mino cid, nd the true equilibrium constnt of the dissocition of the mino cid is determined in two-step liner extrpoltion procedure lim mj NOH,II )const. mj - O2 C-R-NH 3 +f0 [K 2,exptl (T, mj -O2 C-R-NH 3 +, mj NOH,II )] ) K 2,exptl (T, mj NOH,II ) (12) lim K 2,exptl (T, mj NOH,II ) ) K 2 (T) (13) mj NOH,II f0 However, if mj NOH,II is smll enough (e.g., mj NOH,II e 0.01 mol kg -1 ), the second extrpoltion is not necessry Dicrboxylic Amino Acids. L-Glutmic cid nd L- sprtic cid dissocite ccording to - O 2 C - HO 2 R-NH + C- 3 + H 2 O h - O 2 C - -O2 R-NH + C- 3 + H 3 O + (14) - O 2 C - -O 2 R-NH + C- 3 + H 2 O h - O 2 C - -O2 R-NH C- 2 + H 3 O + (15) Tble 4. Comprison of Correlted Vlues of nd Thermodynmic Properties with Literture Vlues for Srcosine T/K this work Dtt 28, r G m /kj mol r H m /kj mol Extrpolted vlue. Tble 5. Comprison of Correlted Vlues of nd Thermodynmic Properties with Literture Vlues for 6-Aminohexnoic Acid T/K this work Smith 30 Gillespie 13 Brndriz r G m /kj mol r H m /kj mol Extrpolted vlue. Electroneutrlity results in nj H2 O ) n H 2 O + n H 3 O+ + n OH - (18) n N + + n H3 ) n - O2 C O+ - -O2 C - R-NH n - O2 C - -O2 C - R-NH 2 + n OH - (19) The chemicl equilibrium conditions for both rections present re where the presence of the protonted mino cid zwitterion is neglected. Mesurements of the third dissocition constnts of L-glutmic cid nd L-sprtic cid were performed in mnner similr to tht described bove. However, eq 6 to eq 13 hd to be dpted to describe the composition in cell II nj - O2 C - HO 2 C - R-NH + 3 ) n - O2 C - -O2 C - R-NH n - O2 C - -O2 C - R-NH 2 (16) H3 O+ OH H2 K w (T) ) 2 O K(T) ) - O2 C - -O2 C - R-NH 2 H3 O+ - O2 C - -O2 C - R-NH + 3 H2 O (20) (21) nj NOH ) n N + (17) Becuse between one nd two moles of NOH hve to be dded to ech initil mole of mino cid in this cse, the

5 2494 Journl of Chemicl nd Engineering Dt, Vol. 52, No. 6, 2007 Tble 6. Comprison of Correlted Vlues of nd Thermodynmic Properties with Literture Vlues for DL-Methionine T/K this work Pelletier r G m /kj mol r H m /kj mol Extrpolted vlue. extrpoltion to zero ionic strength is performed ccording to lim mj NOH,II f0 [K 3,exptl (T, mj mj - O 2 C - R-NH + f0 3 m HO 2 C Alknolmines. Mesurements of the dissocition constnt of protonted methyldiethnolmine (MDEA) were performed to vlidte the experimentl setup nd technique. Protonted MDEA dissocites ccording to HCl ws used in cell II insted of NOH, nd consequently eq 6 to eq 13 were dpted to describe the composition of the cell Electroneutrlity results in The chemicl equilibrium conditions for both rections present re The liner extrpoltion to zero mollities ws done s in eqs 12 nd 13 lim mj HCl,II )const. mj MDEA f0 - O2 C - HO 2 C - R-NH + 3, mj NOH,II )] ) K 3 (T) (22) MDEAH + + H 2 O h MDEA + H 3 O + (23) nj MDEA ) n MDEA + n MDEAH + (24) nj HCl ) n Cl - (25) nj H2 O ) n H 2 O + n H 3 O+ + n OH - (26) n MDEAH + + n H3 O+ ) n Cl - + n OH - (27) H3 O+ OH H2 K w (T) ) 2 O MDEA H3 O+ K(T) ) MDEAH + H2 O (28) (29) [K exptl (T, mj MDEA, mj HCl,II )] ) K exptl (T, mj HCl,II ) (30) lim K exptl (T, mj HCl,II ) ) K(T) (31) mj HCl,II f0 3. Thermodynmic Reltions To the experimentlly determined dissocition constnts, the well-known thermodynmic reltions re pplied r G m )-RT ln K (32) r H m )-R d ln K d(1/t) (33) nd the chnge of stndrd stte properties (T ) T ) K) for the dissocition of n mino cid in wter re clculted from eq 32 to eq 33 by use of 9 where i ) 0 for protonted MDEA, i ) 1 for β-lnine, turine, srcosine, 6-minohexnoic cid, DL-methionine, glycine, L- phenyllnine, nd L-proline, nd i ) 2 for L-sprtic cid nd L-glutmic cid. The further thermodynmic reltions cn be clculted by the reder 4. Experimentl For the EMF mesurements, ph voltmeter (Metrohm 780) with resolution of 0.1 mv nd 0.1 K ws used, together with combined ph glss electrode (Metrohm ) with 3 M KCl (Metrohm ) inner reference electrolyte nd n integrted Pt1000 temperture sensor. When not in use, the electrode ws stored in storge solution (Metrohm ). Before ech mesurement, the electrode ws crefully rinsed with distilled wter nd dried with pper tissue. The cells were completely filled with the electrolyte solutions, plced in temperture controlled wter bth, nd seled between ech mesurement. The experiments were performed under nitrogen tmosphere inside glovebox to prevent CO 2 from the ir from bsorbing into the electrolyte solutions. During ech mesurement the electromotive force (E/mV) nd the temperture in the cell (T/K) were recorded. Mesurements were performed from (293 to 353) K t 10 K intervls nd t K. The overll mollity of HCl in cell I nd in cell II ws held constnt (mj HCl,I mj HCl,II 0.01 mol kg -1 ) during mesurements of the dissocition constnts of protonted MDEA. The overll mollity of MDEA ws between ( nd ) mol kg -1. During mesurements of β-lnine, turine, srcosine, 6-minohexnoic cid, DLmethionine, glycine, L-phenyllnine, nd L-proline, the overll mollity of HCl in cell I nd NOH in cell II ws held constnt in ll solutions (mj HCl,I mj NOH,II 0.01 mol kg -1 ). The overll mollity of the mino cids ws between ( nd ) mol kg -1. During mesurements of L-glutmic cid nd L- sprtic cid, the mollity of the mino cids ws between mol kg -1 nd mol kg -1, nd the mollity of sodium hydroxide ws pproximtely 1.5 times tht of the mino cid. The overll mollity of HCl in cell I ws held constnt (mj HCl,I 0.01 mol kg -1 ). Mesurements where the temperture in cell I nd in cell II devited by more thn ( 0.1 K were not considered in the described clcultions. 5. Chemicls ln K i ) A + B + C ln(t) (34) T r S m ) ( r H m - r G m ) T r C P,m ) d r H m dt (35) (36) MDEA [ ], β-lnine [ ], turine [ ], srcosine [ ], 6-minohexnoic cid [ ], DLmethionine [ ], glycine [ ], L-phenyllnine [ ], L-proline [ ], L-glutmic cid [ ], L-sprtic cid [ ] (Sigm-Aldrich), nd NOH [ ] nd HCl [ ] (Merck) were used s supplied. NOH nd HCl were provided s 0.1 M stndrd solutions, diluted to the desired

6 Tble 7. Comprison of Correlted Vlues of nd Thermodynmic Properties with Literture Vlues for Glycine T/K this work Dtt 28,29 King 33 Owen 34 Gillespie 13 Clrke 35 Iztt r G m /kj mol r H m /kj mol Extrpolted vlue. Journl of Chemicl nd Engineering Dt, Vol. 52, No. 6, Tble 8. Comprison of Correlted Vlues of nd Thermodynmic Properties with Literture Vlues for L-Phenyllnine T/K this work Iztt r G m /kj mol r H m /kj mol Extrpolted vlue. mollities, nd checked by mens of titrtion. Wter ws deminerlized nd further purified by vcuum distilltion. 6. Results nd Discussion Experimentl results t verged tempertures for the dissocition constnts of protonted MDEA, the second dissocition constnts of β-lnine, turine, srcosine, 6-minohexnoic cid, DL-methionine, glycine, L-phenyllnine, nd L-proline, nd the third dissocition constnts of L-glutmic cid nd L-sprtic cid re given in Appendix B with experimentl uncertinties. The experimentl uncertinties re due to inccurcies in E I nd E II of ( 0.5 mv. The results re lso given with the verge nd mximum reltive devition between the experimentl dt nd the vlues from the liner fit. These numbers provide insight into the ccurcy of the liner extrpoltion. The experimentl equilibrium dt re given s Supporting Informtion with Run no. corresponding to Appendix B. In Figure 1, ln(k 2,exptl ) is plotted s function of the dimensionless overll mollity of β-lnine ccording to eq 12 nd eq 13. The extrpoltions to zero mino cid mollity were done by liner regression for MDEA, β-lnine, turine, srcosine, 6-minohexnoic cid, DL-methionine, glycine, L-phenyllnine, nd L-proline. The intercept t zero mino cid mollity is ln- (K 2,exptl ) nd ln(k exptl ) in the cse of MDEA. For L-glutmic cid nd L-sprtic cid, liner extrpoltion yields ln(k 3 )t the intercept of zero ionic strength ccording to eq 22. In Tble 1 to Tble 11, correlted experimentl results of the dissocition constnts nd the vlues of the stndrd stte thermodynmic properties re given nd compred to vilble literture vlues. Literture dissocition constnts tht re bsed on the molrity scle were converted to the mollity scle by K ) 1 F w ( m ) c K c (37) Tble 9. Comprison of Correlted Vlues of nd Thermodynmic Properties with Literture Vlues for L-Proline T/K this work Smith 37 Azb r G m /kj mol r H m /kj mol Extrpolted vlue. Tble 10. Comprison of Correlted Vlues of ln(k 3) nd Thermodynmic Properties with Literture Vlues for L-Glutmic Acid T/K this work Albert 18 Wilson 19 ln(k 3) r G m /kj mol r H m /kj mol where F w is the mss density of pure wter tken from Sul 20 nd K c is the dissocition constnt bsed on the molrity scle. Experimentl results of the dissocition constnts of protonted MDEA hve been reported by Kmps, 21 Oscrson, 10 Kim, 22 Littel, 11 nd Schwbe. 12 The results re given in Tble 1 nd Figure 2. The correlted vlues from this work gree well with the results by Kmps 21 nd Oscrson, 10 with n verge reltive devition (in ln(k)) of 0.01 % nd 0.03 %, respectively. The vlues extrpolted to higher tempertures re within 0.08 % nd 0.21 % of the results by Kmps 21 nd Oscrson, 10 respectively. The results by Kim, 22 Littel, 11 nd Schwbe 12 re within 0.46 %, 1.76 %, nd 0.58 %, respectively. The vlues of r G m nd r H m gree well with the literture vlues. The lrgest reltive devitions in r G m re from those of Schwbe 12 with 0.58 % nd in r H m re from those of Kmps 21 with 2.65 %. The correlted vlues of the second dissocition constnts of β-lnine re in the best greement with the results by Boyd, 23 with n verge reltive devition (in ln(k 2 )) of 0.31 %. The results by My, 24 Gillespie, 13 Dey, 14 nd Mjumdr 15 devite by 0.87 %, 1.24 %, 1.49 %, nd 1.10 %, respectively. The differences between the correlted vlues of this work nd the litterture vlues for β-lnine re lrger thn those of protonted MDEA. My 24 did not tke ctivity coefficients of the compounds into considertion when determining the second dissocition constnts from experimentl dt; hence, the ctivity coefficients were set to unity for ll species. If the ctivity

7 2496 Journl of Chemicl nd Engineering Dt, Vol. 52, No. 6, 2007 Figure 2. Dissocition constnts of protonted MDEA: b, exptl results this work; - -, fit this work; 3, Kmps; 21 4, Oscrson; 10 open tringle pointing left, Kim; 22 open tringle pointing right, Littel; 11 ], Schwbe. 12 Tble 11. Comprison of Correlted Vlues of ln(k 3) nd Thermodynmic Properties with Literture Vlues for L-Asprtic Acid T/K this work Smith 30 Btchelder 38 ln(k 3) r G m /kj mol r H m /kj mol coefficients re set to unity in the present work, the correlted vlues from this work hve n verge reltive devition of 0.04 % compred to My. 24 The results by Gillespie 13 differ with n verge reltive devition of 1.24 % from the correlted vlues of this work. Gillespie 13 determined r H m experimentlly by flow clorimetry nd used temperture specific vlue of ln- (K 2 ) to derive temperture-dependent vlues of ln(k 2 ). By this, the ccurcy of the results from the clorimetric dt will follow the ccurcy of the temperture specific vlue of ln(k 2 ) used. Gillespie 13 chose vlue from Christensen. 25 Christensen 25 described the estimtion of the ctivity coefficients indequtely, nd consequently, it is difficult to explin the differences between the results of Gillespie, 13 Christensen, 25 nd this work. In the work of Dey 14 nd Mjumdr, 15 the ctivity coefficients were set to unity for ll compounds. By pplying this to the present work, the second dissocition constnts differ by 0.55 % nd 0.17 %, respectively. Boyd 23 estimted the ctivity coefficients of the compounds using the Dvies eqution, 26 form of the Debye-Hückel eqution. By pplying the Dvies eqution to this work, the correlted results hve reltive verge devition of 0.19 % compred to Boyd. 23 Vlues of the thermodynmic properties of β-lnine re given by My, 24 Gillespie, 13 nd Boyd. 23 The results by Boyd 23 re in best greement with the results from this work, with devition of 0.30 % nd 0.21 % in r G m nd r H m, respectively. Experimentl vlues of the second dissocition constnts of turine re reported by King. 27 Correlted vlues from this work re in greement with the results by King 27 with n verge reltive devition (in ln(k 2 )) of 0.02 %. The thermodynmic vlues re lso in greement, with reltive devition of 0.10 % nd 0.72 % in r G m nd r H m, respectively. Vlues of the second dissocition constnts of srcosine re reported by Dtt. 28 The correlted vlues hve n verge reltive devition of 0.06 % from the results by Dtt. 28 Vlues of r G m nd r H m given by Dtt 29 hve reltive devition of 0.09 % nd 2.02 % from the vlues of this work. Smith, 30 Gillespie, 13 nd Brndriz 31 reported vlues for the second dissocition constnts of 6-minohexnoic cid, nd the vlues devite by 1.29 %, 1.32 %, nd 0.36 % (in ln(k 2 )) from this work, respectively. In the work of Smith, 30 the ctivity coefficients of the mino cid re set to unity. By pplying this to the present work, the verge reltive devition is 0.43 %. Gillespie 13 mesured r H m by flow clorimetry nd used temperture specific vlue of the dissocition constnt from Smith. 30 With ctivity coefficients set to unity in this work, the reltive difference is 0.12 %. The reltive difference between this work nd the work of Brndriz 31 is 0.36 %. Smith 30 reported vlues for r G m nd r H m, nd the vlues hve reltive devition of 1.24 % nd 0.53 % from this work, respectively. Vlues of the second dissocition constnts of DL-methionine re reported by Pelletier. 32 The results hve n verge reltive devition of 0.15 % (in ln(k 2 )) from the correlted vlues of this work. The vlues of r G m nd r H m hve reltive devition of 0.64 % nd 0.23 %, respectively. Vlues of the second dissocition constnts of glycine hve been reported by Dtt, 28 King, 33 Owen, 34 Gillespie, 13 Clrke, 35 nd Iztt. 16 The results by the uthors hve reltive devition (in ln(k 2 )) of 0.04 %, 0.05 %, 0.04 %, 0.09 %, 3.40 %, nd 0.08 % from this work, respectively. Dtt 29 nd King 33 reported vlues with reltive devition of 0.05 % nd 0.13 % in r G m nd of 0.45 % nd 0.45 % in r H m, respectively. Owen 34 nd Iztt 16 lso reported vlues for r H m with reltive devitions of 2.95 % nd 0.11 %. Iztt 36 reported vlues of the second dissocition constnts of L-phenyllnine, nd the results devite (in ln(k 2 )) with n verge of 0.33 % from the correlted vlues of this work. r H m from Iztt 36 devites by 2.6 % from this work. Vlues of the second dissocition constnts of L-proline re reported by Smith 37 nd Azb, 17 nd the vlues hve n verge reltive devition of 1.04 % nd 1.29 % (in ln(k 2 )) from the correlted vlues of this work. In the work of Smith, 37 the ctivity coefficients of ech compound were set to unity. By pplying so to this work, the vlues by Smith 37 hve n verge reltive devition of 0.15 %. The vlues of r G m nd r H m from Smith 37 hve reltive devition of 1.07 % nd 3.85 %. Vlues of the third dissocition constnts of L-glutmic cid hve been reported by Albert 18 nd Wilson 19 t ( nd ) K, nd the results devite by 1.59 % nd 0.31 % (in ln(k 3 )) from the correlted results of this work. Albert 18 used titrtion technique to determine the third dissocition constnts without tking ctivity coefficients into considertion. However, the lrge reltive devition from this work cnnot be explined. In ddition to the potentiometriclly determined result t K, Wilson 19 lso crried out mesurements of the third dissocition constnts t elevted tempertures using kinetic pproch. Those results contin lrge uncertinties nd re not compred to the results from this work. Smith 30 nd Btchelder 38 hve reported vlues of the third dissocition constnts of L-sprtic cid. The results by these uthors hve reltive devition of 1.49 % nd 3.16 % (in ln- (K 3 )), respectively, from the correlted results of this work. Smith 30 used the Debye-Hückel term, log (γ (m) ) )-A φ z i 2 I m, to estimte the ctivity coefficients of the compounds. By pplying the term to the present work, the results by Smith 30

8 Journl of Chemicl nd Engineering Dt, Vol. 52, No. 6, Tble 12. Comprison of Vlues of the Stndrd Stte Thermodynmic Properties for Commercilly Avilble bsorbents to this work compound ref r G m /kj mol -1 pk r H m /kj mol -1 6-minohexnoic cid this work L-proline this work β-lnine this work srcosine this work L-sprtic cid this work L-glutmic cid this work mino-1-propnol (MPA) glycine this work piperzine (PZ) mino-2-methyl-1-propnol (AMP) monoethnolmine (MEA) 2-(2-minoethoxy)ethnol (DGA) DL-methionine this work L-phenyllnine this work turine this work diisopropnolmine (DIPA) diethnolmine (DEA) methyldiethnolmine (MDEA) this work Tble 13. Prmeters for Eqution 44 (0) β KCl (1) β KCl C KCl prmeter kg mol -1 kg mol -1 kg 2 mol -2 u e e u e e-4 u e e u e e e-8 u e e e-11 u e e u e e e-3 f L(T r, 1 br) e e e-5 f L(T r, P r) e e e-5 f G(T r, 1 br) e e e-4 f G(T r, P r) e e e-4 K K Tble 14. Prmeters for Eqution 46 prmeter B V KCl /kg (mol br) -1 q q q e-8 q e-10 q e-3 q e-7 q q e-9 q e-12 q e-5 q e-10 q q e-12 q e e-8 q 15 devite by 1.08 %. Btchelder 38 lso estimted the ctivity coefficients by Debye-Hückel term. Even though the estimtion of the ctivity coefficients differs from the term used in this work, the lrge devition cnnot be explined. Vlues of the thermodynmic properties of the third dissocition re reported by Smith, 30 nd the reltive devitions from this work in r G m nd r H m re 1.48 % nd 8.25 %. The results of the dissocition constnts presented in this work re in greement with the bove listed literture vlues. As shown, the vlues of the estimted ctivity coefficients of the compounds hve n influence on the finl results. Accurte Tble 15. Prmeters for Eqution 48 (0) β HCl (1) β HCl C HCl prmeter kg mol -1 kg mol -1 kg 2 mol -2 q e-3 q q e q e e-5 q e e estimtions of the ctivity coefficients re importnt to chieve ccurte results of the dissocition constnts. During the mesurements described, NOH is used to deprotonte the mino cids nd form n mino cid slt. KOH nd LiOH hve lso been used to form mino cid slts to see the effect on the determined dissocition constnts by hving different counterions. The second dissocition constnts of turine were determined t (293.15, , nd ) K using the three different counterions. The results with sodium, potssium, nd lithium s counterions re given nd compred in Appendix B. It shows tht the results re ll within the experimentl uncertinties, nd there re no significnt effects using the different counterions. The experimentl equilibrium dt re given s Supporting Informtion. According to electrolyte thermodynmics, ctivity coefficients re only dependent on the ionic strength in dilute solutions (e.g., I m e 0.01 mol kg -1 ) nd not on individul ionic mollities or other solute properties. 39 Since the ionic strengths of the solutions under investigtion re low (e.g., mj N+,K +,Li + e 0.01 mol kg -1 ), it cn be concluded tht the type of counterion does not ffect the determined dissocition constnts. However, if mesurements were to be done t higher ionic strength, the type of counterion would most likely hve n effect on the determined dissocition constnts. Tble 12 shows r G m nd r H m of primry nd secondry mines used s commercil CO 2 bsorbents in comprison to the results from this work. The pk vlue of ech compound is lso listed for the convenience of the reder. The compounds re sorted with descending r G m nd pk vlues. The thermodynmic vlues from this work re relisted in Tble 12 for the convenience of the reder. Bluwhoff 40 nd Versteeg 41,42 relted the bsic strength of n bsorbent to the CO 2 rection rte constnt by Brønsted

9 2498 Journl of Chemicl nd Engineering Dt, Vol. 52, No. 6, 2007 Tble 16. Extrpolted Experimentl Results of the Dissocition Constnts run no. T/K ln(k) vg. rel. dev./% mx. rel. dev./% run no. T/K ln(k) vg. rel. dev./% mx. rel. dev./% Dissocition Constnts of Protonted MDEA Second Dissocition Constnts of β-alnine ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( Second Dissocition Constnts of Turine Second Dissociion Constnts of Srcosine ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( Second Dissocition Constnts of 6-Aminohexnoic Acid Second Dissocition Constnts of DL-Methionine ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (

10 Journl of Chemicl nd Engineering Dt, Vol. 52, No. 6, Tble 16. (Continued) run no. T/K ln(k) vg. rel. dev./% mx. rel. dev./% run no. T/K ln(k) vg. rel. dev./% mx. rel. dev./% Second Dissocition Constnts of Glycine Second Dissocition Constnts of L-Phenyllnine ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( Second Dissocition Constnts of L-Proline Third Dissocition Constnts of L-Glutmic Acid ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( Third Dissocition Constnts of L-Asprtic Acid ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( plot. According to the uthors, there re liner correltions between the logrithm of the forwrd second-order rection rte constnt nd the bsic strength. A higher bsic strength cn indicte fster rection between the bsorbent nd CO 2.A high vlue of r G m or pk cn indicte high rectivity of the bsorbent with CO 2 ; however, kinetic studies of the compounds hve to be crried out before definite conclusions cn be drwn. In commercil CO 2 cpture, n bsorber is operted round 313 K nd desorber round 393 K depending on operting conditions. 1 A high vlue of r H m will led to fvorble shift of the dissocition rection t bsorber nd desorber temperture conditions, given by eq 33. This might further led to cost sving

11 2500 Journl of Chemicl nd Engineering Dt, Vol. 52, No. 6, 2007 Tble 17. Comprison of the Effect of Different Counterions on the Determined Dissocition Constnts of Truine run no. T/K K + N ( ( ( Li + N ( ( ( opertions of CO 2 cpture plnts, since the temperture difference between the bsorber nd the desorber might be reduced. From Tble 12, 6-minohexnoic cid hs higher vlues of both r G m nd r H m thn the commercilly vilble mines listed. L-Proline, β-lnine, srcosine, L-sprtic cid, nd L-glutmic cid hve higher vlue of r G m thn the commercilly vilble mines but not necessrily higher vlue of r H m. 7. Conclusion The dissocition constnts of protonted MDEA, the second dissocition constnts of β-lnine, turine, srcosine, 6-minohexnoic cid, DL-methionine, glycine, L-phenyllnine, nd L-proline, nd the third dissocition constnts of L-glutmic cid nd L-sprtic cid hve been determined from electromotive force mesurements from (293 to 353) K. Prts of the results extend the temperture rnge of vilble literture dt of the investigted compounds. The experimentl results gree with results previously reported in the literture. The vlues of the dissocition constnts nd the thermodynmic reltions presented in this work give informtion bout the use of mino cid slts s possible bsorbents for CO 2 cpture. Appendix A: Outline of the Pitzer Model A very brief outline of the Pitzer model introduced by Pitzer 46 is given for 1:1 queous electrolyte solution. For n electrolyte solution contining w S kilogrms of solvent, with mollities m i, m j,..., of solute spices i, j,..., Pitzer 46 introduced the eqution for the excess Gibbs energy G ex RTw S ) f(i m ) + i Pitzer 46 further derived the expression for the ctivity coefficient. For the dissolved species i:j, the ctivity coefficients re estimted by ln(γ ij,m ) ) where I m is defined s m i m j B ij + j i nd b ) 1.2 kg 1/2 mol -1/2. The Debye-Hückel term is given A φ ) 1 3 e 2πN A F w ( (41) 4πɛ 0 ɛ w kt) j -A φ[ I m 1 + b I m + 2 b ln(1 + b I m )] I m ) 1 2 m x z x 2 x)i,j m i m j m k C ijk +... (38) k + mb ij + 3m2 C ij (39) (40) where the dielectric constnt of wter, ɛ w, ws tken from Brdley. 47 The second viril coefficient is given B ij ) 2β (0) ij + ( 2β (1) ij R 2 I m)[ 1 - (1 +RI 1/2 m - R2 I m 2 ) exp(-ri 1/2 m )] (42) where β (0) ij, β (1) ij, nd C ij re slt specific interction prmeters. For the cse considered here, R)2.0 kg 1/2 mol -1/2. In the cse where interction prmeters re neglected, the ctivity of wter follows from the Gibbs-Duhem eqution in the form of M w ln( w ) ) (43) I i} m {2A φ - m i*w 1 + b I m A.1. Interction Prmeters for KCl in the Pitzer Model. The following section reports the temperture dependence of the ion interction prmeters for KCl given by Pbln. 7 The interction prmeters re clculted from eq 44 nd Tble 13. T is the temperture in Kelvin; T R is K; P is the pressure in br; nd P R is 179 br. The density of wter, F w, ws tken from Sul. 20 The pressure, P, ws set to 1 br. The pressure dependence of the thermodynmic properties re clculted from eq 45 nd Tble 14. A complete description of the equtions below re given in Pbln s work. 7 f(t, P r ) ) u 1 T2 6 + u 2 T u 3 T 2( ln(t) ) u 4 T u 5 T u 6[ T 2 + 3(227)2 227(T - 227) ln(t - 227) 2T T ] - u 7[ 2(647 - T) ln(647 - T) + ln(647 - T T)] - K 1 T - f L (T r, P r ( ) T r T ) + K 2 + f G (T r, P R ) (44) ln γ ( (P 2 ) - ln γ ( (P 1 ) - 1/2 I [A φ (P 2 ) - A φ (P 1 )]( m 1 + bi + 2 1/2 b ln(1 + bi 1/2 m m )) + P P1 2{ ( 2m β (0) ij P )T + 2m R 2 I m( β (1) ij P )T[1 - (1 +RI 1/2 m - R2 I m 2 ) exp(-ri 1/2 m )] + 3m ( C 2 ij dp (45) where (( β (0) ij / P)) T ) B V ij nd B V ij is clculted from eq 46. Further, in this cse, (( β (1) ij / P)) T ) 0 nd (( C ij / P)) T ) 0. f V (T, P) ) q 1 + q 2 T + q 3 T + q 4 T2 + q T + P[ q 6 + q 7 A.2. Interction Prmeters for HCl in the Pitzer Model. The following section reports the density, pressure, nd temperture dependence on the ion interction prmeters for HCl given by Holmes. 6 The ion interction prmeters re given by eqution 48 nd Tble 15. T is the temperture in Kelvin; T* is 2 q 10 P ) T} T + q 8 T + q 9 T2 + (647 - T)] + P 2[ q 11 + q 12 q 15 T + q 13 T + q 14 T2 + (647 - T)] (46)

12 Journl of Chemicl nd Engineering Dt, Vol. 52, No. 6, K;P* is 1 MP; nd P* is1kg m -3. T R, P R, nd F R re the reference temperture, pressure, nd density nd re set to K, MP, nd kg m -3, respectively. The pressure nd density, P nd F, re set equl to the reference pressure nd the reference density. For HCl, the interction prmeter, C in eqution 39, is defined f(f,p,t) ) q 1 + q 2 ln( F F R ) + q (F -F R ) 3 F* C ) 1 2 C HCl (47) (T - T R ) (P - P R ) + q 4 + q T* 5 P* (48) B. Experimentl Results. Tbles 16 nd 17 contin the experimentl results. Acknowledgment The uthors wish to cknowledge H. F. G. Moed for the construction of the experimentl setup nd P. H. M. Feron nd F. Geuzebroek for their vluble discussions regrding the experimentl technique nd results. Nomenclture Nomenclture mj i /mol kg -1 ) overll mollity of component i nj i /mol ) overll number of moles of component i F/kg m -3 ) density i ) ctivity of component i A φ /kg 1/2 mol -1/2 ) Debye-Hückel constnt b/kg 1/2 mol -1/2 ) constnt in the modified Debye-Hückel term B i,j ) second viril coefficient in Pitzer s eqution c /1 mol L -1 ) reference molrity C P /J (mol kg) -1 ) het cpcity C ijk ) third viril coefficient in Pitzer s eqution E/mV ) electromotive force e/c ) chrge of electron E /mv ) stndrd potentil F/C mol -1 ) Frdys constnt f ) modified Debye-Hückel term G/kJ mol -1 ) Gibbs energy H/kJ mol -1 ) enthlpy I m /mol kg -1 ) ionic strength K ) dissocition constnt k/j K -1 ) Boltzmnn constnt K w ) dissocition constnt of wter m /1 mol kg -1 ) reference mollity N A /mol -1 ) Avogdro constnt n i /mol ) number of moles of component i S/J (mol K) -1 ) entropy T/K ) temperture w s /kg ) number of kilogrms of solvent z i ) number of chrges of component i Greek Letters R/kg 1/2 mol -1/2 ) constnt in Pitzer s eqution β (0) ij /kg mol -1 ) binry interction prmeter in Pitzer s eqution β (1) ij /kg mol -1 ) binry interction prmeter in Pitzer s eqution ) difference ɛ 0 /C 2 N -1 m -2 ) permittivity of vcuum ɛ w ) reltive dielectric constnt of wter γ (m) ) ctivity coefficient bsed on mollity γ ij ) men ctivity coefficient of i:j slt Susperscripts ) stndrd stte ex ) excess Subscripts I,II ) cell I, cell II 2 ) second dissocition constnt 3 ) third dissocition constnt c ) on molrity scle ij ) components i, j ijk ) components i, j, k m ) on mollity scle r ) rection AbbreVtions q ) in queous solution s ) solid exptl ) experimentl w ) wter Supporting Informtion Avilble: Experimentl equilibrium dt with Run no. corresponding to Appendix B. 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