Tom Meuzelaar Posted May 13, 2008 Share Posted May 13, 2008 [admin notice: the below is from the former GWB users group email distribution list. This message was originally posted 8/17/2006] Posted by: Joel Brugger Dear all, We have observed different outcomes for temperature extrapolation (between 2 temperatures for which LogKs are available on the grid) in rxn and act2. We're trying to model organic pH buffers. We have modified our database with a new "redox" basis species: DESPEN(aq) * formula= C12H28N2O6S2 charge= 0.0 ion size= 4.0 A mole wt.= 360.492 g 6 species in reaction -3.000 H2O -17.500 O2(aq) 16.0000 H+ 12.0000 HCO3- 2.000 NH3(aq) 2.000 SO4-- 2.7420 2.7420 2.7420 2.7420 2.7420 500.0000 500.0000 500.0000 * Added by Pascal Grundler, 16/8/2006 * source: Kandegedara, A. & Rorabacher, D.B. Anal. Chem. 1999, 71, 3140 * RANDOM VALUE; FOR USE IN DECOUPLED REDOX SYSTEMS Note that the LogK’s here are random values – we’re only interested in reaction among DESPEN species. Possibly, this if what causes the strange behaviour described below? The two deprotonated forms are entered as derived species: DESPEN- * formula= C12H27N2O6S2 charge= -1.0 ion size= 4.0 A mole wt.= 359.484 g 2 species in reaction 1.000 DESPEN(aq) -1.0000 H+ 5.956 5.62 5.176 4.796 4.421 -500.0000 -500.0000 -500.0000 * Added by Pascal Grundler, 16/8/2006 * Data from Kandegedara, A. & Rorabacher, D.B. Anal. Chem. 1999, 71, 3140 * values at 0 , 60 , 100 and 150 C obtained by extrapolation from the * variable temperature (15 to 45 C) data reported by Kandegedara & Rorabacher DESPEN-- * formula= C12H26N2O6S2 charge= -2.0 ion size= 4.0 A mole wt.= 358.476 g 2 species in reaction 1.000 DESPEN(aq) -2.0000 H+ 15.378 14.68 13.765 12.978 12.203 -500.0000 -500.0000 -500.0000 * Added by Pascal Grundler, 16/8/2006 * Data from Kandegedara, A. & Rorabacher, D.B. Anal. Chem. 1999, 71, 3140 * values at 0 , 60 , 100 and 150 C obtained by extrapolation from the * variable temperature (15 to 45 C) data reported by Kandegedara & Rorabacher Now if we calculate the pKa’s in rxn at 99 and 100C we get the correct values: DESPEN-- + 2 H+ = DESPEN(aq) Log K's: 0 C: 15.3780 150 C: 12.2030 25 C: 14.6800 200 C: ??? 60 C: 13.7650 250 C: ??? 100 C: 12.9780 300 C: ??? Polynomial fit: log K = 15.38 - .02695 T - 7.705e-5 T^2 + 1.654e-6 T^3 - 5.888e-9 T^4 Log K at 99 C = 12.9944 (Value interpolated using polynomial fit) Assumptions implicit in equilibrium equation: temperature = 99 C Equilibrium equation: 12.99 = - log a[DESPEN--] - 2 log a[H+] + log a[DESPEN(aq)] ------------------------------------------------------------------------------- DESPEN- + H+ = DESPEN(aq) Log K's: 0 C: 5.9560 150 C: 4.4210 25 C: 5.6200 200 C: ??? 60 C: 5.1760 250 C: ??? 100 C: 4.7960 300 C: ??? Polynomial fit: log K = 5.956 - .01285 T - 4.337e-5 T^2 + 8.658e-7 T^3 - 3.068e-9 T^4 Log K at 99 C = 4.8039 (Value interpolated using polynomial fit) Assumptions implicit in equilibrium equation: temperature = 99 C Equilibrium equation: 4.804 = - log a[DESPEN-] - log a[H+] + log a[DESPEN(aq)] ------------------------------------------------------------------------------- DESPEN-- + H+ = DESPEN- Log K's: 0 C: 9.4220 150 C: 7.7820 25 C: 9.0600 200 C: ??? 60 C: 8.5890 250 C: ??? 100 C: 8.1820 300 C: ??? Polynomial fit: log K = 9.422 - .01409 T - 3.368e-5 T^2 + 7.879e-7 T^3 - 2.82e-9 T^4 Log K at 99 C = 8.1905 (Value interpolated using polynomial fit) Assumptions implicit in equilibrium equation: temperature = 99 C Equilibrium equation: 8.191 = - log a[DESPEN--] - log a[H+] + log a[DESPEN-] However, calculations in act2 give the right answer at 100C, but not at 99C. So, why is the interpolation different in both programs? Is this a bug or a feature? What would react do? We’d appreciate some insider knowledge before we dive too deeply... Thanks! Joel & Pascal Posted by: Craig Bethke Hi Joel, Looks like you're using (positive) 500 as the flag for a missing log K in some of your entries, and -500 in others. The correct flag is 500. Hope this helps, Craig Link to comment Share on other sites More sharing options...

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