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Model-based analysis for kinetic complexation study of Pizda and Cu(II) Spectrochimica Acta Part A M. Vosough , M. Maeder , M. Jalali-Heravi , S.E. Norman. Introduction. Chemical kinetic reactions : Model-based analysis kinetic model.
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Spectrochimica Acta Part A
M. Vosough , M. Maeder , M. Jalali-Heravi , S.E. Norman
Chemical kinetic reactions :
Many kinetic reactions in aqueous solutions are strongly pH-dependent and coupled to one or more protonation equilibria.
The kinetic reactions have to be studied at feasible pH ranges.
Recent developments in model-based methods :
incorporation of the effects of non-ideal experimental conditions into the fitting algorithm
The quantitative analysis of kinetic measurements that are not buffered is possible.
The complications due to addition of the buffers can be removed.
(C+ = (CtC)−1Ct)
R = Y − Ycalc = Y − CA
ssq =ΣR2i,j= f (Y, model,parameters)
knew = k +Δk
copper(II) and 1-(2-hydroxyl cyclohexyl)-3-[aminopropyl]-4-[3-aminopropyl]piperazine (Pizda)
in 50% ethanol–water
Pizda is a five-dentate ligand.
1:1 complex forms in a very fast second order reaction and strongly dependent on the initial pH.
Scheme 2. Cu(II)–Pizda complex in all possible complexation sites
Cu2+ + L (CuL’)2+
(CuL’)2+ + H+ CuLH3+
Cu2+ + LH+ CuLH3+
Cu2+ + LH22+ CuLH+ + H+
L + H+ LH+
LH+ + H+ LH22+
LH22+ + H+ LH33+
the kinetics of complex formation
Proton releases and therefore the pH decreases and finally would reach to its equilibrium value.
the kinetics of complex dissociation
The complex decomposes and therefore pH increases and finally would reach to its equilibrium value.
Ytot = CtotA + Rtot
Fig. 1. Concentration distribution diagram obtained for all chemical species in
complexation of Cu(II) by Pizda in equilibrium state as a function of pH
Protonation and complexation constants obtained from potentiometric study of
complex formation equilibria between Pizda and Cu2+ in 50% ethanol–water
solution and 25±0.5 ◦C and ionic strength of 0.1M (NaClO4)
forward pre-kinetic, backward pre-kinetic and equilibrium states.
Fig. 3. Two kinetic series in 21 wavelengths with stopped-flow measurements:
(a) in forward case withinitial concentration of acid 2.97×10−2 M
(b) in backward case with initial concentration of acid 4.70×10−2 M
Fig. 4. Some results obtained using global analysis. The results depicted for two kinetics traces in Fig. 3 in some selected wavelengths (520,565 and 655 nm) contains the calculated absorbance measurements and residual plots :
(a) forward and (b) backward reactions.
The possibility of working in unbuffered solutions.
The tedious measurements and analysis of buffer dependences is replaced by few reactions.
All pH ranges can be covered.
The main disadvantage :
The time consuming of the fitting process relative to the other methods which use the buffer solutions.
Rate constants and the protonation constants for complexation reaction between
Pizda andCu2+ in50%ethanol–water solution and 25±0.5 ◦C and ionic strength
of 0.1M (NaClO4)
Fig. 5. The calculated concentration profiles for the measurement series in Fig. 3: (a) forward and (b) backward reactions.
Fig. 6. pH profiles calculated with global analysis in the selected kinetic measurements (Fig. 3).
Fig. 7. Calculated absorption spectra in kinetic measurements for four absorbing species of Cu, CuL, CuL’ and CuLH in a global way.
M. Maeder, Y.M. Neuhold, G. Puxty, P. King, Phys. Chem. Chem. Phys. 5(2003) 2836.