Chemical equilibria, spectrophotometric

Thus, any physical property that relates (preferably directly) to concentration, e.g. pH or spectrophotometric absorbance, may be used to investigate a chemical equilibrium and to measure the corresponding equilibrium constant, Kc. 

Experiment 32 Chemical Equilibrium 2 Spectrophotometric Determination of an Equilibrium Constant... 

This experiment demonstrates the chemical limitations to Beer s law using the NO2-N2O4 equilibrium as an example. Blanco, M. Iturriaga, H. Maspoch, S. et al. A Simple Method for Spectrophotometric Determination of Two-Components with Overlapped Spectra, /. Chem. Educ. 1989, 66, 178-180. 

Fig. 5. Spectrophotometric titration of the Fe3+/H3L 34 (11) equilibrium system over the pH range 0.69-6.95. Arrows indicate the direction of movement of the spectra (A) with increasing pH and (B) with decreasing pH. Conditions [Fe3+] = [H3LI34] = 2.0 x 10 4mol dm-3, T — 25°C, /j. — 0.10. Two equilibria appear over two distinct pH ranges, indicating two separate protonation/deprotonation events occurring. Reprinted with permission from Ref. (58). Copyright 2007 American Chemical Society.

The nitrosonium ion is a well-known chemical species and the equilibrium constant for its formation has been determined spectrophotometrically as 3 x 10-7 mol-1 dm3 (Bayliss et al., 1963).10 There is no spectroscopic evidence for the existence of the nitrous acidium ion. 

Sampling and Measurements. The determination of dissolved actinide concentration was started a week after the preparation of solutions and continued periodically for several months until the solubility equilibrium in each solution was attained. Some solutions, in which the solubilities of americium or plutonium were relatively high, were spectrophotometrically analyzed to ascertain the chemical state of dissolved species. For each sample, 0.2 to 1.0 mL of solution was filtered with a Millex-22 syringe filter (0.22 pm pore size) and the actinide concentration determined in a liquid scintillation counter. After filtration with a Millex-22, randomly chosen sample solutions were further filtered with various ultrafilters of different pore sizes in order to determine if different types of filtration would affect the measured concentration. The chemical stability of dissolved species was examined with respect to sorption on surfaces of experimental vials and of filters. The experiment was performed as follows the solution filtered by a Millex-22 was put into a polyethylene vial, stored one day, filtered with a new filter of the same pore size and put into another polyethylene vial. This procedure was repeated twice with two new polyethylene vials and the activities of filtrates were compared. The ultrafiltration was carried out by centrifugation with an appropriate filter holder. The results show that the dissolved species in solution after filtration with Millex-22 (0.22 ym) do not sorb on surfaces of experimental materials and that the actinide concentration is not appreciably changed with decreasing pore size of ultrafilters. The pore size of a filter is estimated from its given Dalton number on the basis of a hardsphere model used in the previous work (20). 

The overexpression and purification of the dimeric, 144 amino acid TnSOl MerR protein in large quantities has made detailed chemical studies possible (143, 145, 171). A variety of studies including gel filtration, equilibrium dialysis, nonequilibrium dialysis, and spectrophotometric titration have indicated that the MerR proteins isolated to date bind a single mercuric ion per protein dimer (82, 145, 171, 202). All of these studies have been performed in the presence of 10-10,(XX)-fold excess buffer thiol competitors, indicating that Hg(II) binds tightly to the protein. 

This chapter is intended to demonstrate some recent approaches to the quantitative determination of chemical processes based on the quantitative analysis of experimental spectrophotometric measurements. In this chapter we will discuss kinetic processes, equilibrium processes and also processes that include a combination of kinetic and equilibrium steps. 

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